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Malaria: Epidemiology, prevention, and control

Malaria: Epidemiology, prevention, and control
Author:
Joel G Breman, MD, DTPH
Section Editor:
Johanna Daily, MD, MSc
Deputy Editor:
Elinor L Baron, MD, DTMH
Literature review current through: Dec 2022. | This topic last updated: Nov 15, 2022.

INTRODUCTION — Malaria is endemic throughout most of the tropics; ongoing transmission occurs in 85 countries and territories [1]. The World Health Organization (WHO) reported 241 million cases and 627 thousand deaths from malaria in 2020 [2]; this is an increase from an estimated 227 million cases and 558 thousand deaths recorded in 2019.

The increase in cases and deaths is due to interruption of services due to the coronavirus disease 2019 (COVID-19) pandemic, as well as a revised method of computing the malaria burden [2]. With the revised WHO calculation, malaria accounts for 7.8 percent of the global disease burden (rather than 4.8 percent as reported previously).

The emergence of Plasmodium falciparum artemisinin partial resistance in the African region is of great concern, as are increasing reports of Anopheles spp resistance to pyrethroid insecticides. Licensure of the RTS,S malaria vaccine in 2021 is a positive development [2].

EPIDEMIOLOGY

Overview — The malaria parasite is transmitted via the bite of a female Anopheles spp mosquito, which occurs mainly between dusk and dawn. Other rare mechanisms for transmission include congenitally acquired disease, blood transfusion, sharing of contaminated needles, organ transplantation, and nosocomial transmission [3-5].

Over 95 percent of the burden occurs in the African region of the World Health Organization, followed by 2 percent each in the South East Asian and Eastern Mediterranean regions, with the American and Western Pacific regions contributing the remainder.

Twenty-nine countries account for 96 percent of cases; Nigeria (27 percent), the Democratic Republic of Congo (12 percent), Uganda (5 percent), Mozambique (4 percent), and Angola and Burkina Faso (each 3.4 percent) account for 55 percent of all cases globally.

While the increase in malaria is alarming, the Institute for Health Metrics and Epidemiology reported 1.82 million and 1.24 million deaths, respectively, in 2004 and 2010; deaths continue to occur mainly among African children <5 years of age [6,7].

Ascertainment of cases and deaths is the biggest challenge to understanding program success. Among 55 countries where the burden was estimated, 31 countries had a malaria reporting rate of less than 50 percent; this includes the high burden countries of Nigeria, the Democratic Republic of Congo, and India [2].

Malaria occurs throughout most of the tropical regions of the world, with P. falciparum causing the largest burden of disease, followed by Plasmodium vivax (figure 1A-B) [8]. P. falciparum predominates in sub-Saharan Africa, New Guinea, and Hispaniola (Haiti and the Dominican Republic); P. vivax is much more common in the Americas and the western Pacific. The prevalence of these two species was approximately equal in the Indian subcontinent, eastern Asia, and Oceania in the late 1900s but P. falciparum is now responsible for two-thirds of cases (table 1) [9-11]. Plasmodium malariae is uncommon and is found in most endemic areas, especially in sub-Saharan Africa. Plasmodium ovale, less common, is relatively unusual outside of Africa and comprises <1 percent of isolates. Plasmodium knowlesi, similar morphologically to P. malariae, has been identified by molecular methods in patients in Malaysia, the Philippines, Thailand, and Myanmar [12]; this species has not yet been proven to be transmitted from humans to mosquitoes (ie, a monkey reservoir may be required to infect mosquitoes). Plasmodium simium is a malaria species resembling P. vivax that occurs in primates; it has been isolated from humans in Brazil [13]. (See "Non-falciparum malaria: P. vivax, P. ovale, and P. malariae".)

In 2018, there were 1823 confirmed cases in the United States, 16 percent fewer than in 2017; 85 percent were from Africa, and 70 percent of them from West Africa. Overall, P. falciparum caused 70 percent of cases followed by P. vivax (10 percent) and P. ovale (5 percent). Virtually all patients became ill within three months of returning to the United States. Of the 95 percent who did not take prophylaxis properly, 25 percent took none at all. Severe illness occurred in 14 percent of patients, and seven persons died [14].

The epidemiology of malaria may vary considerably even within relatively small geographic areas. Endemicity traditionally has been defined in terms of parasitemia rates or palpable spleen rates in children two to nine years of age as hypoendemic (≤10 percent), mesoendemic (11 to 50 percent), hyperendemic (51 to 74 percent), and holoendemic (≥75 percent). In holo- and hyperendemic areas (eg, certain regions of tropical Africa or coastal New Guinea where there is intense P. falciparum transmission), people may receive more than one infectious mosquito bite per day and are infected repeatedly throughout their lives. In such settings with intense perennial transmission, morbidity and mortality due to malaria during childhood are considerable. Polymerase chain reaction (PCR) detection of parasitemia shows a higher prevalence of malaria than microscopy or rapid diagnosing testing; hence, some areas undoubtedly have a higher endemic status than previously thought or recorded. As elimination and eradication programs progress, molecular methods will become more important for diagnostic and certification purposes.

The entomologic inoculation rate (EIR; the number of infectious female anopheline bites per person per year) is a term used to indicate transmission intensity. While there are seasonal and geographic differences, an EIR of <10/year is low transmission, 10 to 49/year is intermediate, and ≥50/year is a high transmission area. In general, the higher the EIR, the greater the burden of malaria, particularly on young children [15].

Constant, year-round infection is termed stable transmission, generally occurring in areas with EIRs of ≥50/year; in such areas, most malaria infections in long-term resident adults are asymptomatic. In areas where transmission is low, highly seasonal, or focal, full protective immunity is not acquired, and symptomatic disease occurs at all ages. Malaria behaves like an epidemic disease in some areas, particularly those with seasonal unstable malaria such as northern India (Rajasthan), Afghanistan, Iraq, Turkey, Ethiopia, Eritrea, Burundi, southern Africa (Botswana, Mozambique, Namibia, South Africa, Swaziland, Zimbabwe), and Madagascar. An epidemic can develop when there are changes in environmental, economic, or social conditions, such as heavy rains following drought or migrations (usually of refugees or workers) from a non-malarious region to an area of high transmission; a breakdown in malaria control and prevention services can intensify epidemic conditions. This situation usually results in considerable morbidity and mortality among all age groups [16]. Political and economic turmoil in Venezuela has resulted in several hundred thousand cases of malaria in residents and refugees.

The principal determinants of the epidemiology of malaria are the number (density), the human-biting habits (indoors or outdoors), and the longevity of the female anopheline vectors. More specifically, the transmission of malaria is directly proportional to the density of the vector, the square of the number of human bites per day per mosquito, and the 10th power of the probability of the mosquito's survival for one day [17,18]. Mosquito longevity is particularly important, because the portion of the parasite's life cycle that takes place within the mosquito (sporogony; from gametocyte ingestion to subsequent inoculation) lasts for 8 to 30 days, depending on the species of Plasmodium and ambient temperature; thus, to transmit malaria, the mosquito must survive for >10 days. The most effective mosquito vectors of malaria are those such as Anopheles gambiae in Africa, which are long lived, occur in high densities in tropical climates, breed readily, rest and bite within dwellings, and bite humans in preference to other animals.

Maps of P. falciparum and P. vivax morbidity and mortality based on mathematic modeling (2000 to 2017) have been developed [19,20]. The utility of these maps is limited by a number of factors including limited data, statistical uncertainty, and heterogeneity of malaria transmission.

Surveillance — Clinical, epidemiologic, entomologic, and parasitologic surveillance of malaria is required to follow the progress of malaria control, elimination, and eradication programs. Depending on resources and control program progress, emphasis should be on clinical manifestations of illness in hospitals and health units initially (passive surveillance) in areas with public and private health care providers and then on rural communities. As elimination is undertaken, active detection of malaria infections house by house (active surveillance) will become essential [21]. Modern information technologies (eg, cell phones, hand-held data entry devices, computerized reporting, feedback bulletins) will be necessary to assure prompt and accurate sharing of information. Surveillance of morbidity, mortality, disability, drug, and insecticide resistance and of commodities including quality of drugs, nets, and insecticides will be the key to malaria elimination and ultimately eradication [22]; additionally, provider practices and patient compliance require monitoring and evaluation [23].

An article summarizing nearly 27,000 malaria parasite prevalence surveys in Africa indicated that there was a 40 percent reduction in the prevalence of malaria infection across most of sub-Saharan Africa in the decade 2000 to 2015 [24-26]. Interventions averted 663 million clinical cases (542 to 753 million credible interval) since 2000 [26]. Reductions in transmissions, while encouraging, are modest; more sensitive detection methods are needed, particularly of asymptomatic infections in areas targeted for elimination.

An analysis of deaths due to malaria in Africa noted highest mortality rates and lowest coverage of bed nets and antimalarial drug use in Nigeria, Angola, Cameroon, Central African Republic, Democratic Republic of Congo, Guinea, and Equatorial Guinea [27].

A review of antimalarial drug quality in Asia and Africa showed an alarming prevalence of counterfeit drugs on the shelves of pharmacies in multiple countries [28]; up to 36 percent of antimalarial drugs and 43 percent of the artemisinin combination drugs were falsified. This criminal behavior requires more attention, including definition of the problem locally, scientific and technological means to identify rapidly poor quality drugs and other commodities (eg, diagnostic tests), regulatory action, and national and international laws [29].

DEFINITIONS — To consider interventions for malaria control, it is important to define the goals of control, elimination, and eradication. Control is reduction of disease incidence and prevalence to levels that do not pose a threat to public health or that are acceptable to a community. Elimination is reduction of incidence and transmission to zero in humans in a defined geographic area. Eradication is global elimination of human disease. In all instances, a surveillance system that could detect, respond to, and report malaria cases and infections promptly is essential.

Despite progress, one problem with the Millennium Development Goals (MDGs; now Sustainable Development Goals [SDGs], which aimed to reduce malaria incidence and prevalence by 75 percent by 2015 compared with 2000) is that baseline epidemiologic information for malaria is incomplete in most endemic countries. Following successful elimination and eradication, ongoing rigorous surveillance is needed to identify imported cases, which must be detected, treated, and contained promptly to prevent reestablishment of endemic infection. Because there are no nonhuman reservoirs for the human plasmodia, malaria is considered by many to be a candidate for eradication.

In the past, clinical diagnosis alone has been used in most endemic African areas for recording malaria cases, contributing to imprecisions. Training and supervision of clinicians in proper diagnosis of presumed malaria remain the most important keys to effective management and surveillance [30]. Microscopic confirmation of malaria, the historic "gold standard," is frequently delayed and inaccurate; hence, the rapid diagnostic test (RDT) is being used with increasing frequency especially in Africa. Advantages of RDTs include improved diagnosis, rapidity, and more rational use of antimalarial drugs. Diagnosis of malaria is discussed in detail separately. (See "Laboratory tools for diagnosis of malaria".)

With the Bill and Melinda Gates challenge in 2007 to eradicate malaria, there is increasing enthusiasm to reassess older control methods and develop and evaluate new ones through research. The approach to malaria eradication proposed by the World Health Organization includes the preparatory, attack, consolidation, and maintenance phases (figure 2). Progression between these phases depends on reducing the incidence of parasitemia in the population, which requires an understanding of the intrinsic and extrinsic factors of malaria transmission. Intrinsic factors include the relationships between human, parasite, and mosquito; extrinsic factors include the environment, socioeconomic milieu, and control initiatives [31]. The most important epidemiological criterion for moving from the attack to the consolidation phase of malaria eradication (in which surveillance is the key) is annual incidence of <0.1 infections/1000 persons (1 case per 10,000 population) [17].

Research in support of malaria eradication is a major component of the current global effort. The malaria eradication research agenda has been defined by task forces addressing specific themes [32,33].

ELEMENTS FOR MALARIA CONTROL — Strategies to disrupt malaria transmission include effective deployment of antimalarial drugs, personal mosquito protection, mosquito vector control, and research (including vaccine development) [17]. Optimal malaria control (eg, leading to elimination) using these tools requires the following elements of the World Health Organization (WHO) Global Technical Strategy for Malaria 2016 to 2030 [34]. In addition to acceleration of elimination, country and community ownership, improved surveillance, equity in health services, and innovation in tools and implementation, the technical foci are:

Effective human, parasitologic, and entomologic surveillance at health facilities and in communities

An understanding of local Anopheline ecology (including breeding, biting, and resting habits)

Administrative, managerial, supervisory, and operational capacity (including trained leaders and staff)

An ongoing monitoring and evaluation system for deployment and use of drugs, long-lasting insecticidal nets, indoor residual spraying, and environmental modifications

Sustained high-level, long-term national and international commitment

In 2008, the WHO divided 104 malarious countries into three groups: 25 countries that have recently eliminated malaria or with conditions amenable to elimination, 32 countries with unstable malaria amenable to control and elimination with current tools, and 47 countries with stable transmission and poor infrastructure (eg, requiring further infrastructure development before additional measures can be implemented) [11,35]. This baseline scheme, while evolving, remains in use.

HOST PROTECTION

Overview — Tools for human protection from malaria infection include protection from mosquito bites, chemoprevention, and vaccination. It is uncertain whether combining these interventions may confer increased protection.

In one trial including more than 6000 children age 5 to 17 months in Burkina Faso and Mali (regions of seasonal transmission) randomly assigned to receive seasonal vaccination (primary three-dose series of RTS,S/ASO1 vaccine, followed by two annual boosters), chemoprevention (four monthly courses of sulfadoxine-pyrimethamine and amodiaquine each year), or both, a lower three-year incidence of uncomplicated malaria, severe malaria, and death from malaria was observed among those who received both interventions rather than either alone [36]. The protective efficacy of both interventions compared with chemoprevention alone against these outcomes (calculated as [1-hazard ratio] x 100) was 62.8 percent (95% CI 58.4-66.8), 70.5 percent (95% CI 41.9-85.0), and 72.9 percent (95% CI 2.9-92.4), respectively; the protective efficacy of both interventions compared with vaccine alone was 59.6 percent (95% CI 54.7-64.0), 70.6 percent (95% CI 42.3-85.0), and 75.3 percent (95% CI 12.5-93.0), respectively. These findings support a promising approach to reducing seasonal malaria; optimization of delivery to high-burden areas is needed.

Protection from mosquito bites

Repellants — Repellents applied to exposed skin may be used to protect against mosquito bites. Effective repellents include synthetic preparations such as DEET (N,N-diethyl-m-toluamide), Picardin (KBR3023), and IR 3535, as well as PMD (P-MENTHANE-3,8-DIOL), which is derived from lemon eucalyptus [37]. For maximal protection against mosquitoes, DEET is preferable over other agents. Selection and use of mosquito repellents are discussed separately. (See "Prevention of arthropod and insect bites: Repellents and other measures".)

The efficacy of mosquito repellents for protection against malaria is variable depending on the transmitting mosquito vector. Repellents are most effective in areas where vectors feed in the early evening (as with many South American and Asian vectors) rather than later in the night (as with African vectors such as An. gambiae). Repellents may also be more effective in regions with zoophilic vectors as in the Americas and south Asia (where mosquitoes may be diverted from a repellent-treated person to an animal) than in regions with anthropophilic vectors (Africa). This was illustrated in a cluster-randomized study of 4008 participants in rural Bolivia, where the principal vector is Anopheles darlingi [38]. All participants slept under treated nets; one group also used a plant-based insect repellent each evening, while a second group used placebo. Rates of P. vivax malaria episodes were reduced by 80 percent. A protective but not significant effect was observed for P. falciparum as the number of cases was small.

Repellents have also been protective against malaria in refugee camps on the Pakistan-Afghan border; in this region, the main vectors of malaria are Anopheles stephensi, An. culicifacies, An. pulcherrimus, and An. nigerrimus [39]. Among 127 families randomized to receive repellent soap (20% DEET/0.5% permethrin) or placebo, fewer cases of P. falciparum malaria were observed among those using repellent soap (3.7 versus 8.9 percent). There was no effect against P. vivax malaria.

In regions where vectors feed in the middle of the night, repellents might not increase the degree of protection afforded by use of insecticide-treated nets alone; this requires further evaluation.

Long-lasting insecticide-treated nets (LLINs)

Benefit − Use of LLINs is an important tool for malaria control. In a systematic review including 23 trials enrolling more than 275,000 adults and children between 1987 and 2001, use of LLINs was associated with child mortality from all causes by 17 percent (rate ratio 0.83, 95% CI 0.77 to 0.89) [40]. In addition, a 44 percent reduction in the incidence of severe malaria episodes was observed among those who used LLINs (rate ratio 0.56, 95% CI 0.38 to 0.82).

Thus far use of pyrethroid-impregnated nets does not appear to have toxicity for humans [40].

Approach to selection − Use of LLINs treated with pyrethroids has been an important tool for malaria control since the 1990s; however, pyrethroid resistance among mosquitoes has emerged. Use of LLINs treated with ≥2 insecticide compounds of different classes warrants further study, particularly in the context of other malaria control strategies.

The World Health Organization (WHO) continues to recommend use of pyrethroid-treated nets where effective [41]. In areas where the main malaria vector(s) have at least intermediate pyrethroid resistance (defined as mosquito mortality of 10 to 80 percent following pyrethroid exposure conferred via a monooxygenase-based mechanism), the WHO suggests use of piperonyl butoxide (PBO)-treated nets [41-43].

Insecticide formulations − Insecticides evaluated for LLIN use include pyrethroids, PBO, chlorfenapyr, pyriproxyfen, and others:

Pyrethroids − Pyrethroids have been the mainstay insecticide for LLIN treatment since the late 1990s. However, pyrethroid resistance among mosquitoes has emerged; failure of pyrethroid-based LLINs has been reported in West Africa and elsewhere [44-47].

Laboratory detection of resistance genes may or may not correlate with epidemiologic observations. Genes for pyrethroid resistance were first demonstrated among mosquito vectors in the Ivory Coast; however, pyrethroid-based LLINs remained effective for preventing malaria in this region [48].

Piperonyl butoxide (PBO) – Emergence of pyrethroid resistance has prompted study of LLINs incorporating a synergistic, PBO [49]. PBO-treated nets are not expected to confer any additional benefit in areas where the main malaria vectors are susceptible to pyrethroids.

In a 2020 cluster-randomized including more than 100 subdistricts in Uganda with high pyrethroid resistance, use of PBO-treated LLINs was associated with reduced parasite prevalence compared with conventional LLINs at 18 months (12 versus 14 percent; prevalence ratio 0.84, 95% CI 0.72-0.98) [50].

Similarly, in a 2018 cluster-randomized trial among more than 3600 children in Tanzania, the malaria prevalence after was lower among those who used PBO-treated nets than among those who used pyrethroid-treated nets (follow-up 9 months; 29 versus 42 percent; odds ratio 0.37, 95% CI 0.21-0.65); use of indoor residual spraying was not associated with additional benefit [51].

However, in a 2022 cluster-randomized study including more than 39,000 households in a Tanzania region with high prevalence of pyrethroid resistance, there was no difference in malaria prevalence among children who used PBO-treated nets and children who used pyrethroid-treated nets (adjusted odds ratio 0.99, 95% CI 0.67-1.45) [52].

Chlorfenapyr − In a 2022 cluster-randomized study including more than 39,000 households in a Tanzania region with high prevalence of pyrethroid resistance, lower malaria incidence among children age 6 months to 10 years was observed in clusters where nets treated with pyrethroid as well as chlorfenapyr were used (rather than nets treated with pyrethroid only; incidence rate ratio 0.56, 95% CI 0.37-0.86) [52]. Similarly, at two-year follow-up, malaria prevalence was lower among children who used pyrethroid-chlorfenapyr LLINs (25.6 versus 45.8 percent; odds ratio 0.45, 95% CI 0.30-0.67). The cost per disability-adjusted life-year averted with pyrethroid-chlorfenapyr LLINs was below plausible cost-effectiveness thresholds [52]. Before scale-up of pyrethroid-chlorfenapyr LLINs, resistance management strategies are needed to preserve their effectiveness.

Pyriproxyfen − In a 2018 randomized trial in Burkina Faso (an area of intense malaria transmission and high pyrethroid resistance) including more than 4000 children <5 years of age, the likelihood of malaria among children using LLINs treated with both permethrin (a pyrethroid) and pyriproxyfen (an insect growth regulator) was lower than among children using bed nets treated with permethrin only (1.5 versus 2.0 episodes per child-year; incidence rate ratio 0.88, 95% CI 0.77-0.99) [53].

However, in a 2022 cluster-randomized study including more than 39,000 households in a Tanzania region with high prevalence of pyrethroid resistance, there was no difference in malaria prevalence among children who used pyriproxyfen-treated nets and those who used pyrethroid-treated nets (adjusted odds ratio 0·79, 95% CI 0.54–1.17) [52].

Local LLIN implementation − Community health education is essential for correct LLIN installation, maintenance, and timing of use. Washproof nets may be used for about five years, after which they must be replaced because of wear (eg, holes) as well as diminished pyrethroid content.

LLIN use does not appear to delay acquisition of natural immunity against malaria − It has been postulated that LLIN use might delay acquisition of natural immunity against malaria, with increased risk for illness at older ages; however, this does not appear to be the case. In a long-term cohort study of LLIN use including more than 6700 individuals during early childhood in rural Tanzania between 1998 and 2003, follow-up on vital status in 2019 was obtained for 89 percent of participants [54]. Reported use of LLIN at more than half of the early-life study visits was associated with a reduced risk of death from early childhood to adulthood (hazard ratio 0.57, 95% CI 0.45-0.72). Although most deaths occurred prior to age 5, that level of LLIN use was still associated with a trend towards lower mortality between five years and adulthood (hazard ratio 0.93, 95% CI 0.58-1.49).

Use of LLINs with indoor residual spraying − Methods of interrupting malaria transmission must be considered carefully and evaluated both individually and in combination. In communities with variable LLIN coverage, the WHO has promoted adding IRS with DDT (dichloro-diphenyl-trichloroethane). However, decisions regarding deployment of such strategies should be guided by local malaria transmission; one study in the Gambia (where transmission is relatively low and seasonal) demonstrated no benefit to adding IRS [55]. Similar studies elsewhere where malaria transmission is higher (or lower) were not conclusive [56].

Chemoprophylaxis

Specific patient groups

Infants and children

Infants in regions with moderate to high transmission — We are in agreement with the WHO recommendation for coadministration of intermittent preventive treatment in infants (IPTi) with sulfadoxine-pyrimethamine (SP) to infants at the time of Tdap and measles vaccinations (second and third doses), in areas of sub-Saharan Africa with moderate to high malaria transmission and where malaria parasites are sensitive to these drugs [41,57].

IPTi with slowly eliminated antimalarials appears to be effective for reducing episodes of malaria in infants and does not appear to affect serologic response to routine vaccinations [58]. A 2021 Cochrane review of intermittent preventive treatment with SP in infants including 12 trials and more than 19,000 infants in sub-Saharan Africa between 1999 and 2013, use of IPT was associated with a 27 percent reduction in the incidence of clinical malaria infection (rate ratio 0.70, 95% CI 0.62-0.80) [59]. The effect of SP appeared to attenuate over time; trials conducted after 2009 demonstrated little or no effect.

The level of transmission intensity below which the risks of IPTi exceed the benefits is uncertain. As resistance increases to SP and other antimalarial drugs, the efficacy of IPTi will decrease where these drugs are used [60]. We are in agreement with the WHO, which recommends SP for IPTi in areas of moderate or intense malaria transmission but not if the frequency of the Pfdhps + Pfdhfr quintuple mutants (molecular markers of parasite resistance to SP) is ≥50 percent [61].

Infants with congenital HIV exposure — To reduce the likelihood of opportunistic infection among infants exposed to HIV, the WHO recommends that these individuals begin trimethoprim-sulfamethoxazole (cotrimoxazole) between four and six weeks of age and continue until at least six weeks after cessation of breastfeeding and until HIV infection has been ruled out [62]. Prophylaxis with trimethoprim-sulfamethoxazole protects against malaria as well as Pneumocystis jirovecii infection.

This approach is supported by findings from an observational cohort study of HIV-exposed infants between 6 and 36 weeks of age in Malawi in which use of trimethoprim-sulfamethoxazole preventive therapy was associated with a relative reduction in the risk of asymptomatic infection by 70 percent (95% CI 53-81 percent) [63]. Similar findings have been observed among older children; in a randomized trial among 541 children 1 to 14 years of age with HIV infection in Zambia randomly assigned to receive daily trimethoprim-sulfamethoxazole or placebo, the mortality rate after median follow-up of 19 months was lower among those who received cotrimoxazole (28 versus 42 percent; hazard ratio 0.57, 95% CI 0.43-0.77); a similar reduction in mortality was observed in a subgroup of children <5 years (RR 0.68, 95% CI 0.51-0.91) [64].

Children in areas with seasonal transmission

Intermittent preventive treatment in children <6 years (IPTc) has been shown to be effective in areas with seasonal malaria transmission; antimalarial drugs are administered to prevent malaria at regular intervals during the transmission (rainy) season regardless of whether malaria symptoms are present. Therefore, we are in agreement with the WHO recommendation for administration of seasonal malaria chemoprevention (SMC) in areas with highly seasonal transmission in Africa with monthly amodiaquine plus sulfadoxine-pyrimethamine (AQ-SP) for children <6 years, during transmission season [41,57].

Implementation of SMC among children in six West African countries with high malaria transmission has been associated with marked decrease in morbidity and mortality [65]. In a case-control study including 2185 cases of confirmed malaria and 4370 controls during the 2015 and 2016 transmission seasons in Burkina Faso, Chad, the Gambia, Guinea, Mali, Niger, and Nigeria, the pooled estimate of SMC effectiveness in reducing the incidence of clinical malaria within 28 days was 88.2 percent (95% CI 78.7-93.4). SMC was associated with reductions in the number of in-hospital malaria deaths during the high-transmission period in Burkina Faso by 42.4 percent (95% CI 5.9-64.7) and in the Gambia by 56.6 percent (95% CI 28.9-73.5). The estimated reduction in confirmed outpatient malaria cases ranged from 25.5 percent (95% CI 6.1-40.9) in Nigeria to 55.2 percent (95% CI 42.0-65.3) in the Gambia.

A review of seven trials including more than 12,000 participants noted that IPTc prevented 75 percent of malaria episodes [66]. A subsequent study of SMC in Senegal with AQ-SP among children <10 years noted reduction in malaria incidence (60 percent) and severe malaria (45 percent) [67].

Addition of azithromycin to antimalarial agents among children 3 to 59 months of age in Burkina Faso and Mali was not associated with a reduction in mortality [68].

IPT may be beneficial among school children [69,70]. In one trial in Uganda, 740 children aged 6 to 14 years were randomly assigned to receive dihydroartemisinin-piperaquine (once a month), dihydroartemisinin-piperaquine (once a school term; 4 treatments over 12 months), or placebo and were followed for 12 months [69]. In the placebo arm, the incidence of malaria was 0.34 episodes per person-year and the prevalence of parasitemia and anemia was 38 and 20 percent, respectively. Monthly IPT reduced the incidence of malaria by 96 percent (95% CI 88-99 percent), the prevalence of asymptomatic parasitemia by 94 percent (95% CI 92-96 percent), and the prevalence of anemia by 40 percent (95% CI 19-56 percent). IPT each school term had no significant effect on the incidence of symptomatic malaria or the prevalence of anemia but reduced the prevalence of asymptomatic parasitemia by 54 percent (95% CI 47-60 percent).

The role of IPT during the dry season is uncertain. It has been postulated that subclinical malaria infection during the nontransmission (dry) season is important in maintaining protective immunity and that administering treatment for asymptomatic infection may predispose to more severe infection during the following rainy season. However, a prospective study including more than 600 individuals in a region of seasonal malaria transmission in Mali demonstrated that treatment of asymptomatic infection with artemether-lumefantrine was not associated with increased risk of symptomatic malaria during the following two rainy seasons [71].

In areas with seasonal flooding, mass drug administration is very effective for reducing malaria. In one study including more than 550 Ugandan children ≤12 years of age treated with three monthly doses of dihydroartemisinin-piperaquine (with two neighboring villages as controls), lower malaria incidence was observed among those who received treatment (53.4 percent, 95% CI 0.34-0.62) after six months [72].

Pregnant women — Intermittent preventive treatment (IPT) is effective for reducing the risk of malaria infection among pregnant women (IPTp) [17,73]. This issue is discussed in detail separately. (See "Malaria in pregnancy: Prevention and treatment".)

Patients with HIV infection — We favor use of trimethoprim-sulfamethoxazole (TMP-SMX; cotrimoxazole) for prevention of malaria in HIV-infected patients with any CD4 count living in malaria-endemic areas.

Prophylaxis with TMP-SMX reduces morbidity and mortality rates due to malaria in HIV-infected individuals, even in areas with a high rate of malaria resistance [74-78].

Patients with sickle cell disease — Routine malaria prophylaxis is warranted for patients with sickle cell disease in areas where malaria is endemic; this issue is discussed further separately. (See "Sickle cell disease in sub-Saharan Africa".)

Mass drug administration — Issues related to prophylaxis for specific groups are discussed above. (See 'Specific patient groups' above.)

Antimalarial agents — Mass drug administration (MDA) campaigns can rapidly reduce malaria parasitemia in low-transmission areas, particularly in the setting of threatened P. falciparum drug resistance [79]. This was demonstrated by a randomized trial in Zambia in which patients received two doses of dihydroartemisinin-piperaquine or no treatment [80]. In low-transmission areas, an 87 percent reduction in parasite prevalence (from 7.71 to 0.54 percent) and 70 percent decrease in parasitemia was observed after five months; no such effect was observed initially in high-transmission areas.

However, in high-transmission areas, MDA may not be effective. In a cluster-randomized trial including three rounds of community-based mass testing and treatment with dihydroartemisinin-piperaquine in western Kenya over a two-year period, there was no reduction in parasite prevalence or clinical malaria [81]. This might have been attributable to inadequate RDT sensitivity for low-density infections [82].

Regionally targeted MDA may be beneficial in some circumstances:

A regional elimination program in Myanmar included identification of malarial "hotspots" (defined as presence of malaria in >40 percent of individuals, of which 20 percent was P. falciparum); hotspot villages had incidence of P. falciparum three times higher than neighboring villages [83]. MDA (dihydroartemisinin-piperaquine plus single-dose primaquine once per month for three consecutive months) was targeted to individuals in these areas; the intervention was associated with an 80 percent reduction in incidence of P. falciparum (adjusted incidence rate ratio 0.19, 95% CI 0.13-0.26).

In the Gambia (where there is seasonal falciparum malaria transmission), annual MDA with dihydroartemisinin-piperaquine in 2014-2015 in six village pairs prior to malaria season was associated with reduced infection and clinical disease during the first months. Reduction was maintained in low-transmission areas in eastern Gambia, but not in high-transmission areas [84].

Another approach to targeted MDA consists of identifying asymptomatic individuals with malaria followed by "focal" MDA:

In Zanzibar (an area of low falciparum transmission), drug treatment of all individuals residing within 300 to 1000 meters of individuals with positive rapid diagnostic tests enabled elimination of malaria in up to 66 percent of asymptomatic individuals [85].

In a Cambodian trial of MDA (three monthly rounds of a three-day course of dihydroartemisinin-piperaquine) in four malaria-endemic villages, the incidence of P. falciparum was lower in intervention villages than in the control villages during the subsequent 12 months (1.5 versus 37.1 cases per 1000 person years; incidence rate ratio 24) [86]. Artemisinin and incipient piperaquine resistance has emerged in the study area; the research team used artesunate-mefloquine for treatment failures.

In Indonesia, a trial of malaria screening (with microscopy) and targeted treatment (with dihydroartemisinin-piperaquine) in a low transmission area did not impact the level of parasitemia level among school children [87]. The observed poor impact was ascribed to untreated microscopically subpatent infections in the control group.

A study in Zambezi (Namibia) a low malaria-endemic setting compared "reactive case detection" (finding and treating patients only) with treating persons in nearby areas around the malarious patients with focal MDA and reactive focal vector control. The incidence of malaria was lower among those who received focal MDA plus reactive focal vector control than among those receiving reactive case detection only (adjusted incidence rate ratio 0.26, 95% CI 0.10-0.68) [88].

Systemic insecticides — Mass administration of systemic insecticides (also known as endectocides) to reduce malaria transmission is an area of growing interest.

Ivermectin is promising systemic insecticide; it is capable of killing anopheline mosquitoes when they blood feed on ivermectin-treated individuals [89]. In a cluster-randomized trial including more than 2700 children in Burkina Faso (median age 15 years, including 590 children ≤5 years) treated with single oral doses of ivermectin (150 to 200 mcg/kg) and albendazole (400 mg), those in the intervention group received five additional doses of ivermectin at three-week intervals over the 18-week treatment phase [90]. The cumulative malaria incidence was lower in the intervention group than the control group (2.0 versus 2.49 episodes per child; risk difference -0.49, 95% CI -0.79 to -0.21); there was no difference in adverse events between the groups.

Membrane feeding of mosquitoes on patients' venous blood (rather than direct skin feeding) has been proposed as an alternative method for assessment of ivermectin mosquitocidal efficacy. In one study including comparing these approaches, mosquitocidal effects of ivermectin were similar across a range of drug concentrations [91].

Further study is needed to use of ivermectin in other areas, establish the optimal dose or formulation, and evaluate the effects of repeated ivermectin treatment.

Vaccination — A successful malaria vaccine has potential to reduce the global disease burden due to malaria [92,93]. Many antigens have been identified as potential targets for malaria vaccine development.

RTS,S/ASO1 vaccine — The WHO approved the RTS,S vaccine in October 2021 for children in Sub-Sahara Africa and other regions with high transmission, based on studies of 830,000 children in Ghana, Kenya, and Malawi [2,94].

The RTS,S vaccine consists of a recombinant fusion protein created based on an antigen target consisting of a repetitive sequence of four amino acids in the circumsporozoite antigen on the surface of the P. falciparum sporozoite (figure 3) [95-100]. "RTS" stands for "repeat T epitopes" derived from the circumsporozoite protein, "S" stands for the S antigen derived from hepatitis B surface antigen (HBSAg), and AS01 is a proprietary adjuvant [101].

Phase III trial results of the RTS,S/AS01 vaccine among 15,459 infants demonstrated that the vaccine induced partial protection against clinical malaria among children ages 5 to 17 months over the follow-up period of the trial (median 48 months) and demonstrated benefit of the 20-month booster [100]. In the intention-to-treat population, vaccine efficacy among children ages 5 to 17 months who received three doses plus a booster (months 0, 1, 2, and 20) was 36 percent (95% CI 32-40). This finding is a decline from the 50 percent efficacy (95% CI 46-55) reported over the 14 months from the first dose [96]. No significant efficacy against severe malaria was noted in the 6 to 12 week age group over the duration of the trial, even with a booster.

Another RTS,S/AS01 vaccine trial including 447 children ages 5 to 17 months who received three doses of RTS,S/AS01 vaccine noted a decline of vaccine efficacy against malaria between the first and fourth years of follow-up, from 44 percent (95% CI 16-62) to zero [98]. Subsequent follow-up of the vaccine recipients in the intention-to-treat cohort at seven years was notable for overall efficacy of 4.4 percent; efficacy was 16.6 percent in the low-exposure group but was negative (-2.4 percent) in the high-exposure group [102]. Additional follow-up for those who received a four-dose vaccine regimen is pending.

The RTS,S/AS01 vaccine has been observed to afford greater protection against clinical malaria infection caused by parasites that match the vaccine in the circumsporozoite protein allele than malaria infection caused by parasites with a mismatched allele [103].

R21/MM vaccine — The R21 vaccine is a virus-like particle based on the circumsporozoite protein from P. falciparum strain NF54, fused to the N-terminus of HBsAg; it is manufactured using Matrix-M, a proprietary adjuvant.

In a phase 2 trial including 450 children (age 5 to 17 months) in Burkina Faso (a highly seasonal transmission setting) randomly assigned to receive three doses of R21/MM vaccine (with low- or high-dose adjuvant MM) or rabies vaccine, vaccine efficacy in the two treatment groups more than 6 months after vaccination was 74 percent (95% CI 63-82 percent) and 77 percent (95% CI 67-84 percent), respectively [104]. There were no serious adverse events related to vaccination. The trial was performed in the context of substantial bednet use, moderate seasonal malaria prophylaxis, and minimal indoor residual spraying.

Data on the durability of protection are particularly important for malaria vaccines, since, depending on the regional transmission intensity, the risk of death due to malaria may continue for many years. Thus far, efficacy data beyond 6 months are limited, since vaccinations were administered prior to the start of malaria season, and there were few cases of malaria 6 to 12 months after vaccination.

Based on available data, it is difficult to compare vaccine efficacy between R21 and RTS,S vaccines. In the R21 phase 2 trial, both passive and active case detection was used; however, purely passive case detection is favored for pivotal phase 3 trials since this method has greater relevance for public health.

Monoclonal antibodies — Use of monoclonal antibodies represents a novel approach to providing immune protection for malaria prevention.

L9LS – A phase 1 clinical trial evaluated L9LS, a monoclonal antibody against P. falciparum infection that targets the circumsporozoite protein on the parasite surface during the sporozoite stage (when infection is transmitted to establish infection in the liver); this is also the target of the RTS,S/ASO1 vaccine [105].

The trial enrolled healthy adults in the United States with no history of malaria infection or vaccination; 18 individuals received L9LS (serum half-life 56 days) and a control group of 9 individuals did not receive L9LS. Both groups underwent controlled malaria infection; protection against infection was greater among those who received L9LS than those who did not (88 versus 0 percent). No serious adverse events were reported. A subcutaneously delivered preparation also demonstrated good protection, and phase 2 studies are underway.

CIS43LS – A phase 2 clinical trial evaluated CIS43LS, a different monoclonal antibody against Plasmodium falciparum sporozoites [106].

The trial included more than 200 healthy adults in Mali who were randomly assigned to receive an infusion of CIS43LS dosed at 10 mg/kg, 40 mg/kg, or placebo. In addition, artemether-lumefantrine was administered prior to infusion to clear subclinical infection if present. The efficacy for the primary endpoint (first P. falciparum infection detected on blood smear examination performed at least every 2 weeks for 24 weeks) among those who received high-dose CIS43LS was 88 percent (adjusted 95% CI 79-93). Moderate headache occurred 3.3 times more frequently among those who received high-dose CIS43LS compared with placebo.

Study limitations include lack of information on the incidence of clinical malaria (since this was not a secondary endpoint) and limitation of study enrollment to healthy nonpregnant adults (whose lifelong malaria exposure confers substantial immunity). In addition, intravenous infusion is impractical; development of a more robust formulation would be needed for subcutaneous administration.

This tool for malaria prevention may be useful for individuals who are unable to mount an immune response to vaccination, as well as for individuals in regions with emerging antimalarial resistance and in settings of malaria outbreaks. Larger studies in endemic settings and among children and pregnant patients is needed.

MOSQUITO CONTROL — Important mosquito vectors in malarious areas are summarized in the table (table 1).

Mosquito vector control methods include use of long-lasting insecticide-treated nets (LLINs) household insecticide IRS, and larval control by environmental management. Genetic control methods remain an area of research. Since effective insecticides have become available for use against adult Anopheles, LLINs and IRS have become the most important measures for mosquito vector control.

Requirements for effective sustained implementation of vector control include [107]:

Continual monitoring for Anopheles resistance and cross-resistance to the insecticides deployed

Entomology expertise (including mosquito species identification and their biting and feeding habits) and vector control technology

Close supervision and monitoring of the effect of the intervention on the entomological inoculation rates and on clinical and parasitologic indices

The behavior of the vector is important for determining the choice of control method [108]. For mosquitoes that prefer to rest indoors (endophilic) and feed indoors (endophagic) at night, both IRS and LLINs are effective methods of control. For mosquitoes that prefer to rest outdoors (exophilic) and feed outdoors (exophagic) earlier in the day (eg, the South American vector Anopheles albimanus), the use of repellents may be the only feasible method of control. If the vector has clearly defined breeding sites (for example, Anopheles stephensi), then larval control measures can be applied [108]. However, an integrated control strategy is often the most successful approach [109].

Studies carried out prior to the implementation of a mosquito control program should survey the local mosquito population, identify vector species, and identify the most anthropophilic species [110]. The type and degree of insecticide resistance present in the mosquito population should also be investigated [111,112].

Mosquitoes progress through four distinct life stages: egg, larva, pupa, and adult [113]. The full life cycle usually takes approximately 10 days and varies according to species, season, temperature, and available nutrition.

Mosquito life cycle — Stages of the mosquito life cycle include egg, larva, pupa and adult.

Eggs – Female mosquitoes lay up to 200 cigar-shaped eggs per reproductive cycle (figure 4). Anopheles eggs have "floats" (sacs on the side of the egg). Anopheles and Aedes mosquitoes lay eggs individually; Culex mosquito lay eggs in groups ("egg rafts").

Anopheles mosquitoes lay eggs in clean water (such as rain puddles, water tanks, and irrigation ditches). Aedes mosquitoes (sometimes called "floodwater mosquitoes") lay eggs in moist environments with potential to be submerged; the eggs can withstand desiccation until a flooding event occurs.

Larva – Eggs hatch into larvae that live in the water and come to the surface to breathe (figure 4). Anopheles larvae lie in a horizontal position parallel to the water surface and do not have a siphon. Culex larvae maintain a position vertical to the water surface and breathe via a siphon, which extends to the water surface.

Pupa – Pupae float on the water surface (figure 4). In the pupa stage, the head and thorax are fused to form a comma-shaped cephalothorax; during this stage, there are no distinguishing characteristics between the genera. Pupae breathe but do not feed, so larvicide cannot be ingested during this stage, although surface oil can induce suffocation. Metamorphosis from pupa into an adult mosquito takes about two days.

Adult – The newly emerged adult female is able to mate immediately; the male mosquito is able to mate 24 hours after emergence. Only female mosquitoes bite humans or animals to obtain protein needed for producing eggs; both males and females feed on flower nectar for food. Female mosquitoes begin blood feeding approximately three days post-emergence.

Mosquito head appendages consist of a proboscis, a pair of antennae, and a pair of maxillary palps (figure 5). Female mosquitoes use the proboscis to pierce skin and suck blood. Male antennae are bushier than female antennae and are visible with the naked eye. The palp characteristics are the most reliable for differentiation between anopheline and culicine mosquitoes. Anopheline female palps are about the same length as the proboscis, while Anopheline male palps are club shaped at the ends. Culicine female palps are shorter than the proboscis, and Culicine male palps are long with a tapered point.

Live adult mosquitoes are recognizable by their stance at rest. Anopheline mosquitoes are oriented with head, thorax, and abdomen in a straight line at an acute angle to the surface, while Culicine mosquitoes rest with the head and body angled and the abdomen directed back to the surface (figure 6).

Residential interventions — Residential interventions include house screening, indoor residual spraying, and lethal house lures. The WHO recommends house screening [41], although there are no good data evaluating the effectiveness of this intervention. Data evaluating indoor residual spraying and lethal house lures are discussed below.  

Indoor residual spraying — Indoor residual spraying (IRS) involves spraying insecticide on indoor residential walls and ceilings; depending on the insecticide and surface, the effect can last for extended periods. The WHO recommends that effort be focused on optimal implementation of either LLINs or IRS, rather than deploying both in the same area. In settings where LLINs remain effective and optimal LLIN coverage has been achieved, additional implementing of IRS may have limited utility [41]. (See 'Long-lasting insecticide-treated nets (LLINs)' above.)

IRS is performed with hand-operated compression sprayers containing an aqueous suspension of insecticide. Spray teams must be trained and supervised closely in applying the correct dose to walls and ceilings and protecting themselves from toxicity [114]. Frequency of IRS application varies from once or twice yearly for organochlorine compounds (eg, DDT) to three or more times for organophosphates (eg, malathion) [115].

Mosquito contact with insecticide-treated surfaces is generally lethal; sublethal exposure drives mosquitoes outside the house. Ideally, the mosquito is killed or repelled before feeding on humans within the dwelling [116]. Widespread community IRS reduces the proportion of mosquitoes surviving long enough to transmit malaria since each mosquito attempt at a blood meal results in a potentially lethal insecticide exposure [108,115,117]. Community health education about IRS is essential; spraying may require furniture rearrangements, the walls may become streaked with chemical treatments, and there may be a residual odor from the IRS.

DDT is a widely used IRS insecticide based on its efficacy, low cost, low toxicity, and length of effect. Use of DDT is approved for indoor vector control by a global treaty to protect human health and the environment from chemicals (persistent organic pollutants [POPS]) that remain in the environment for long periods. The exemption for human health purposes requires that rigorous measures are taken to avoid outdoor agricultural use where DDT may cause harm to wildlife [118]. DDT was used for IRS programs that achieved malaria elimination in many areas in the 1940s and 1950s (including the southern United States, most Caribbean islands, southern Europe, most of the former USSR, and Taiwan). It has also been used successfully for reducing malaria prevalence in south Asia, southern Africa, South America, and Zanzibar [11,119,120].

In the mid-1990s, many regions switched from DDT to pyrethroids for IRS. This resulted from environmental concerns about toxicity and pressure against DDT use, and there has been a reduction in production of the compound. Subsequently, resistance to pyrethroids emerged rapidly among An. funestus mosquitoes; in Kwazulu-Natal, South Africa, the incidence of malaria increased greatly. A return to DDT (together with a switch in the first-line antimalarial drug to artemisinin-based compounds) reduced the malaria incidence rate by 91 percent [120].

In regions where Anopheles populations have developed resistance to DDT, organophosphates, carbamates, or pyrethroids have been used. In the early 2000s, the WHO again endorsed using DDT for IRS where the public health consequences (the malaria burden) outweighed the possible environmental and wildlife toxic effects. In mid-2009, the WHO modified its position, stating that DDT should be phased out over time to be replaced with other insecticides.

The combined effect of IRS and LLINs is uncertain. One study in Uganda noted that the combination of IRS and LLINs was associated with a marked decline in the burden of malaria; use of LLINs alone had a modest impact [121]. However, a study in Uganda (where high-level coverage of LLINs was achieved) showed that stopping IRS after four years of twice-yearly household spraying was associated with a rapid increase in malaria morbidity to pre-IRS levels [122]. Mosquito resistance to pyrethroids used in the LLINs is one of the possible reasons for resurgence of malaria; in one review, of communities using LLINs, the addition of IRS with 'non-pyrethroid-like' insecticides was associated with reduced malaria prevalence [123].

In some cases, use of IRS, even with LLINs and selective drug use, is not effective for elimination of transmission. In one study including more than 400 individuals in northern Uganda from 2014 to 2016, three rounds of IRS with carbamate bendiocarb reduced the incidence of clinical malaria and the prevalence of parasitemia; however, the parasitemia prevalence remained at 11.3 percent among children <5 years of age and at >15 percent in older age groups [124]. A subsequent study among more than 300 children between 6 months and 10 years of age in Uganda observed that multiple rounds of IRS (three rounds of carbamate bendiocarb followed by one round of an organophosphate, Actellic CS) was associated with decreased incidence of clinical malaria (by 83 percent) and increased hemoglobin level (by 10 percent) [125].

In areas with relatively low transmission and a strong surveillance program, targeted IRS (use of IRS only at the houses of individuals with malaria and their neighbors) may be more cost-effective than mass IRS. In a cluster-randomized study in South Africa, the incidence of malaria among clusters managed with mass IRS was similar to the incidence among clusters managed with targeted IRS (0.95 cases versus 1.05 cases per 1000 person-years) [126]. At this disease incidence, use of targeted IRS would have a 94 to 98 percent probability of being cost-effective, up to an incidence of 2.0 to 2.7 cases per 1000 person-years. The generalizability of the findings in higher transmission settings warrants further study.

Lethal house lures — A lethal house lure is a novel intervention for mosquito control. One approach consists of window screening in combination with installation of ventilation tubes in house eaves; the tubes funnel heat and odor cues (which attract mosquitoes) and contain powdered insecticide. In a cluster-randomized trial in Côte d’Ivoire including more than 8000 houses and 2000 children, a lower incidence of malaria was observed among the intervention group (use of a lethal house lure in addition to LLINs) than the control group (use of LLINs alone; 1.43 versus 2.29 per child-year; hazard ratio 0.62, 95% CI 0.51-0.76) [127]. No serious adverse events associated with the intervention were reported during follow-up. Further study is needed for additional optimization.

Vector control strategies — Tools for vector control include larval control and genetic control.

Larval control — For regions with ongoing malaria transmission and suboptimal coverage with LLINs or IRS, the WHO recommends larviciding as a supplementary intervention for malaria control in settings where aquatic habitats are few, fixed, and findable [41]. Larviciding should not regarding as a substitute for LLINs or IRS; larviciding reduces vector density, which does not have the same potential impact as LLINs and IRS (which reduce vector longevity and provide protection from biting vectors).

Prior to availability of insecticides for use against adult mosquitoes, a concerted effort was directed toward making potential breeding sites unsuitable for Anopheles larvae. Famous historical examples include fluctuation of water levels in the reservoirs of the Tennessee Valley Authority and the draining of the Pontine Marshes near Rome [109].

Larval control is challenging in that virtually all potential breeding places within mosquito flight range of the human community must be identified and addressed. To prevent anopheline population explosions, drainage and filling of water collection sites as well as careful management of large engineering projects such as dam and building construction are needed.

Larvivorous fish species (eg, Gambusia and Poecilia) are avid feeders on mosquito larvae and may be used to stock village wells and ponds (the breeding places of local mosquito vectors) [128]. This strategy was implemented in a series of villages in south India; from 1998 (over one year after release of fish) until 2003, no malaria cases were detected in the villages [109,129]. This method also allows villagers to continue their traditions of raising indoor silkworm moths without the threat of insecticide damage.

Insect growth regulators such as pyriproxyfen (in a sand granule formulation) may be used in water-filled pits to prevent metamorphosis from larva to pupa to adult. This technique was used in Sri Lanka, where malaria eradication was nearly achieved in the 1960s but disease persisted in areas where pits were dug in search of gems (subsequently, the pits filled with rainwater and became breeding sites for malaria vectors) [130]. Treatment of all the pits in four villages achieved reduction in incidence of malaria cases (compared with untreated villages). Retreatment two to three times a year was sufficient.

Use of larval control techniques in Africa has been nearly impossible since breeding sites of An. gambiae tend to be small and impermanent. However, a major reduction in the density of adult An. gambiae was achieved in a Kenyan village by use of the bacterial toxin Bacillus thuringiensis israelensis (Bti). This toxin is highly specific for mosquito larvae and targets very few other species [131]. Bti has a short half-life, and over 400 potential breeding sites were checked weekly and retreated, if necessary. An. funestus may be a more feasible target for antilarval control measures in Africa given that its breeding sites are larger and more permanent.

A problem with many of the biological control approaches to larval control has been that they are difficult to expand to large scale or national public usefulness. This is because of the site-specific nature of the research and other constraints. In some projects, the imported fish species did not adapt to local conditions and they were avidly consumed by the human populations they were to protect.

Genetic control — Genetic strategies for malaria control entail using male mosquitoes to introduce genetic factors that prevent the eggs from hatching, prevent the larvae from surviving, or produce adult insects incapable of transmitting human disease. Despite years of investigation, genetic control remains a promising research theme.

To prevent eggs from hatching or larvae from surviving, males carrying sterilizing factors (dominant lethal mutations introduced by irradiation of the adult male) compete for mates in a target population of females who have not already mated with fertile males [132]. This is known as the sterile insect technique (SIT). Experience with SIT for Anopheles mosquitoes is limited, although the technique has been successful for eradication of other pests [133].

To produce adult insects incapable of transmitting disease, a transgenic system has been developed that greatly reduces susceptibility of An. stephensi to infection with rodent malaria [134]. It is hoped that similar approaches will soon produce strains completely non-susceptible to P. falciparum [134,135]. However, the optimal approach for creating genetic factors that selectively incorporate themselves into later generations in a wild population is under intense study but remains uncertain [136-138].

PROGRESS TO ELIMINATION — In 2014, 16 countries reported zero indigenous cases (Argentina, Armenia, Azerbaijan, Costa Rica, Iraq, Georgia, Kyrgyzstan, Morocco, Oman, Paraguay, Sri Lanka, Tajikistan, Turkey, Turkmenistan, the United Arab Emirates, and Uzbekistan). Another three countries and territories reported fewer than 10 indigenous cases (Algeria, El Salvador, and Mayotte [France]). The World Health Organization (WHO) European Region reported zero indigenous cases for the first time in 2015, and Sri Lanka was declared free of malaria in 2016 [7].

Globally, the elimination net is widening; in 2017, 46 countries reported fewer than 10,000 cases (up from 37 countries in 2010). The number of countries with less than 100 indigenous cases increased from 15 to 26 countries between 2010 and 2017 [7]. Paraguay was certified free of malaria in 2018 and Algeria, Argentina, and Uzbekistan have requested certification of malaria-free status from the WHO [7,139]. Algeria and Argentina were certified malaria-free by the WHO in 2019.

Stringent environmental management and "integrated vector control" (concurrent use of multiple approaches) are required for eradication. Long-lasting insecticidal nets and indoor residual spraying are strategic components for all three groups; these and other strategies are discussed above. (See 'Elements for malaria control' above.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Malaria".)

SUMMARY

Malaria is the most important parasitic disease of humans, with transmission in 95 countries affecting over 3 billion people and likely causing over 400,000 deaths each year. Important challenges to malaria elimination include parasite drug resistance and mosquito insecticide resistance. (See 'Introduction' above.)

To consider interventions for malaria control, it is important to define goals of control, elimination, and eradication. Control is reduction of disease incidence and prevalence to levels that do not pose a threat to public health or that are acceptable to a community. Elimination is reduction to zero transmission in humans in a defined geographic area. Eradication is global elimination of human disease. (See 'Definitions' above.)

Intermittent preventive treatment (IPT) appears to be useful for reducing the risk of malaria infection among individuals at high risk, particularly pregnant women. IPT and seasonal chemoprophylaxis of target groups are important policies in the new World Health Organization Strategic Plan. (See 'Specific patient groups' above.)

Mosquito vector control methods include use of long-lasting insecticide-treated nets (LLINs), household insecticide residual spraying (IRS), larval control, and genetic control methods. Since effective insecticides for use against adult Anopheles have become available, LLINs and IRS have become the most important measures for mosquito vector control. (See 'Mosquito control' above.)

The efficacy of mosquito repellents for protection against malaria is variable depending on the transmitting mosquito vector. (See 'Repellants' above.)

Mass drug administration may be a useful tool in regions where elimination of falciparum malaria is a feasible goal. An effective drug, rapid diagnostics, and sensitive monitoring system for adverse events are required for such an intervention. (See 'Mass drug administration' above.)

A successful malaria vaccine used in conjunction with other control interventions could reduce the global disease burden of malaria. (See 'Vaccination' above.)

Measuring progress by continuous, timely, widely shared surveillance information is the key to control and eradication. Surveillance of the quality of antimalarial drugs (and other commodities) is essential to assure effective treatment and prevention. (See 'Surveillance' above.)

Continuing research on the issues noted above is required to achieve control, elimination, and eradication of malaria in the 21st century. (See 'Elements for malaria control' above.)

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