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Giardiasis: Treatment and prevention

Giardiasis: Treatment and prevention
Luther A Bartelt, MD
Section Editors:
Peter F Weller, MD, MACP
Sheldon L Kaplan, MD
Deputy Editor:
Milana Bogorodskaya, MD
Literature review current through: Dec 2022. | This topic last updated: Sep 06, 2022.

INTRODUCTION — Giardia lamblia (also known as Giardia duodenalis or Giardia intestinalis) is a flagellated protozoan parasite [1]. G. lamblia causes both epidemic and sporadic disease; it is an important etiology of waterborne and foodborne diarrhea and daycare center outbreaks [2-4].

Issues related to the treatment and prevention of giardiasis will be reviewed here. The life cycle, epidemiology, clinical manifestations, and diagnosis of giardiasis are discussed separately. (See "Giardiasis: Epidemiology, clinical manifestations, and diagnosis".)


Nonpregnant adults and children

Asymptomatic individuals — The goal of treatment in asymptomatic individuals is to prevent spread of infection to other, more vulnerable patient populations. We suggest treatment of asymptomatic individuals in the following circumstances (algorithm 1) [5-7]:

Individuals in group settings with risk for transmission to others (such as a child in a daycare setting, health care workers, or nursing facility worker)

Immunocompromised individuals

Household contacts of immunocompromised individuals (especially those with hypogammaglobulinemia or cystic fibrosis)

Household contacts of pregnant individuals

Food handlers

In the absence of risk for transmission, it is reasonable to forgo treatment of asymptomatic individuals. We discuss the lack of data demonstrating benefit in asymptomatic low-risk patients and the toxicities of therapy and then come to a decision whether to treat through shared decision making with the patient. When treating asymptomatic infection, the approach is the same as for symptomatic individuals. (See 'Symptomatic individuals' below.)

Symptomatic individuals — The approach to treatment of symptomatic individuals (eg, diarrhea, abdominal cramping, bloating) with giardiasis includes antimicrobial therapy and supportive care (eg, correction of fluid and electrolyte abnormalities resulting from the diarrhea). (See "Treatment of hypovolemia (dehydration) in children" and "Oral rehydration therapy".)

However, since some infections are self-limited, it is reasonable to forgo antimicrobial treatment for patients with mild symptoms and/or contraindications to antimicrobial therapy. Main agents for the treatment of giardiasis are tinidazole, nitazoxanide, and metronidazole. Our approach to selecting among them depends on age (algorithm 1).

Individuals ≥3 years old — For individuals ≥3 years old, we suggest tinidazole. A single dose is associated with high efficacy (>90 percent) and is better tolerated than other agents [8]. If tinidazole is not available, nitazoxanide is a highly effective alternative agent but requires a three-day course. Metronidazole is also effective, but may be associated with more side effects. Other alternatives are discussed elsewhere. (See 'Alternative agents' below.)

Dosing is summarized in the table (table 1) [9,10]. Tinidazole is available only in tablet form. For children unable to swallow tablets, the tablets may be crushed by a pharmacist and mixed with flavored syrup; the suspension should be shaken before use and is good for seven days at room temperature [11]. Side effects of tinidazole include metallic taste (17 percent), nausea, and headache (<3 percent) [12-14]. The drug has been associated with a disulfiram-like effect, so alcohol consumption should be avoided [5,15,16]. To minimize gastrointestinal side effects, tinidazole should be taken with food [7].

Tinidazole may be more effective than alternative agents, although there is substantial heterogeneity across studies [12]. In a network meta-analysis of 60 randomized trials that included over 6000 children and adults with giardiasis, tinidazole was associated with higher rates of parasitologic cure compared with metronidazole (relative risk [RR] 1.23, 95% CI 1.12-1.35) and albendazole (RR 1.35, 95% CI 1.21-1.50), although risk of bias among studies was high [8]. In another open-label, randomized trial including more than 160 children with giardiasis, parasitologic cure at 10 days was achieved more frequently with tinidazole 50 mg/kg as a single dose compared with nitazoxanide 7.5 mg/kg twice daily for three days (90 versus 78 percent) [13]. As a single-dose regimen, tinidazole is more effective and better tolerated than metronidazole [14].

Individuals between 1 and 3 years old — For individuals between ages one and three years, we favor nitazoxanide. The safety of tinidazole has not been established in patients less than three years of age. Metronidazole is an alternative, although it may be associated with more side effects. Additionally, in the United States, nitazoxanide is the only drug with US Food and Drug Administration approval for giardiasis for this age group. Other alternatives are discussed elsewhere. (See 'Alternative agents' below.)

Dosing is summarized in the table (table 1) [17]. Nitazoxanide is available in liquid and tablet form [7]. Side effects of nitazoxanide include nausea, anorexia, flatulence, increased appetite, enlarged salivary glands, yellow eyes, dysuria, and discolored (bright yellow) urine [16].

The efficacy of nitazoxanide in various trials ranges from 78 to 85 percent [13,16,18-21]. For example, in a randomized study of 110 children with giardiasis treated with a five-day course of metronidazole or three-day course of nitazoxanide, symptoms resolved within seven days in the majority of children (80 versus 85 percent; 5.4 percent difference, 95% CI -8.9 to 19.7 percent) [18]. In addition, nitazoxanide may be effective in treating other intestinal parasites (eg, cryptosporidiosis and amebiasis) and may shorten the duration of diarrhea attributed to infection even when no pathogen is detected [7,21-24].

Individuals <12 months old — For individuals less than 12 months of age, we suggest metronidazole due to lack of safety data with use of tinidazole and nitazoxanide in this patient population.  

Dosing is summarized in the table (table 1) [7]. Oral metronidazole is available only in tablet form; an oral suspension may be prepared by a compounding pharmacy. Side effects associated with metronidazole include metallic taste, nausea, gastrointestinal discomfort, and headache [14,25]. Less common effects include leukopenia, dark urine, paresthesias, and dizziness [5].

The efficacy of metronidazole for treatment of giardiasis is 75 to 100 percent [16,25,26]. For example, in a randomized study of 110 children with giardiasis treated with a five-day course of metronidazole or three-day course of nitazoxanide, symptoms resolved within seven days in the majority of children (80 versus 85 percent) [18]. Adverse events were common (in approximately 75 percent) but were mild and transient and did not result in treatment discontinuation.

Alternative agents — Alternative agents with activity against giardiasis are summarized below. Selection depends upon availability, need for concurrent treatment of other intestinal parasites, and drug intolerances.

Albendazole or mebendazoleAlbendazole and mebendazole have comparable efficacy (80 to 100 percent) for treatment of giardiasis but are not as effective as tinidazole [16,25,27-35]. However, since both drugs also have activity against other parasites that may occur as coinfections in patients with Giardia (such as hookworm and Ascaris), they may be a reasonable option for certain individuals [31].

Dosing is summarized in the table (table 1). Albendazole comes in tablets; mebendazole comes in tablets or suspension. Both drugs should be taken with food to increase bioavailability [5]. Albendazole or mebendazole have fewer side effects than tinidazole or metronidazole [15,32,33,36]; these include abdominal pain, nausea, vomiting, and increased hepatic transaminases.

Albendazole is not as effective in eradicating Giardia compared with tinidazole. In an open-label, randomized study of 113 children with giardiasis treated with a one-time dose of tinidazole or a three-day course of albendazole, parasitologic cure rate was higher in the tinidazole-treated group (96 versus 50 percent) [37]. In another randomized study of 122 children with giardiasis who were treated either with a one-time dose of tinidazole or three doses of mebendazole, parasitologic cure rate at seven days was higher in the tinidazole group (82 versus 64 percent) [15].

Paromomycin – The reported efficacy of paromomycin for treatment of giardiasis is 55 to 90 percent; dosing is summarized in the table (table 1) [5]. It is a reasonable alternative agent for treatment of mild giardiasis in settings where other agents are contraindicated, such as pregnancy (see 'Pregnant and lactating individuals' below). It is not recommended for treatment of severe giardiasis since the lack of systemic absorption limits its efficacy. Side effects include nausea, increased gastrointestinal motility, diarrhea, and abdominal pain [38].

Furazolidone – The efficacy of furazolidone for treatment of giardiasis is approximately 80 percent; dosing is summarized in the table (table 1) [5,7,16,39]. Furazolidone is no longer available in the United States. Side effects include gastrointestinal upset, headache, hemolysis in patients with glucose-6-phosphate dehydrogenase deficiency, and neuropathy [5,16].

Quinacrine – The efficacy of quinacrine for treatment of giardiasis is >90 percent; dosing is summarized in the table (table 1). However, use of quinacrine for treatment of giardiasis is reserved for refractory cases due to the drug’s adverse effects and the drug should not be used in pregnant individuals [38,40,41]. In a multicenter prospective study of 106 European travelers with nitroimidazole refractory giardiasis, quinacrine (100 mg three times a day for five days) yielded higher clinical (81 percent) and parasitologic (100 percent) cure rates compared with albendazole plus chloroquine combination therapy [41]. Availability of quinacrine is extremely limited (it is not commercially available) but can be compounded at some pharmacies. Side effects of quinacrine including nausea, vomiting, dizziness, headache, psychosis, yellow-orange discoloration of the skin and mucous membranes, hemolysis in patients with glucose-6-phosphate dehydrogenase deficiency, and increased risk for cancer of the reproductive tract [5,7,16,39,41]. (See 'Refractory giardiasis after repeated trials of preferred agents' below.)

SecnidazoleSecnidazole may be administered as a single dose for treatment of adults with giardiasis. Cure rates of 80 to 100 percent have been observed, but the drug has been associated with abdominal pain, nausea, anorexia, glossitis, and stomatitis [38].

Ornidazole – Ornidazole may be administered as a single daily dose in children and adults [42,43]. It has similar efficacy to tinidazole and is available in some countries outside the United States.

Pregnant and lactating individuals

Pregnant individuals in the first trimester – For treatment of pregnant individuals with mild giardiasis who are able to maintain hydration and nutrition, it is reasonable to delay therapy until at least the second trimester to minimize the risk of adverse drug effects for the fetus [5]. If treatment is necessary during the first trimester, we suggest paromomycin since it is associated with limited systemic absorption [5] (see 'Alternative agents' above). Tinidazole can cross the placenta and should not be used during first trimester.

Pregnant individuals in the second and third trimester – During the second and third trimesters, we suggest paromomycin for mild disease and metronidazole for severe disease due to more intrapartum safety data available for these agents compared with tinidazole or nitazoxanide. Paromomycin should not be used to treat severe disease due to lack of published efficacy in this setting. In cases of refractory disease or when metronidazole cannot be used, use of tinidazole or nitazoxanide may be appropriate. Clinicians should discuss the risks and benefits of treatment with tinidazole and nitazoxanide with the patient given the limited safety data with these agents. (See 'Symptomatic individuals' above.)

Lactating individuals – For treatment of lactating individuals with giardiasis, reasonable agents include paromomycin, tinidazole, nitazoxanide, or metronidazole, and the choice among them may be informed by need to interrupt breastfeeding. Women treated with paromomycin need not interrupt lactation; if tinidazole, nitazoxanide, or metronidazole is used, we suggest brief lactation interruption. The suggested duration of lactation interruption is 72 hours for tinidazole and 12 to 24 hours for metronidazole [44]. The optimal duration of breastfeeding cessation in women treated with nitazoxanide is uncertain [45]. Low levels of tizoxanide, a nitazoxanide metabolite, appear in breastmilk following a single dose. At the concentrations measured, tizoxanide is not expected to cause adverse effects in infants (especially >2 months of age).


Expected response to therapy — Among patients with acute giardiasis who are treated appropriately, symptoms typically resolve within five to seven days, and parasites usually clear from the stool in this time frame [5,46,47]. In low-prevalence settings, successful therapy also results in rapid clearance of Giardia DNA from stool [47]. In those who had chronic symptoms (>8 weeks) associated with Giardia prior to treatment, symptoms may take several months to resolve [48].

There is no need to repeat the stool examination to check for parasite clearance for individuals whose symptoms resolve within the expected time frame based on acute versus chronic presentation.

Evaluation — Persistent symptoms may be attributable to persistent infection related to treatment failure, ongoing exposure, inadequate adherence, or immunosuppression. Recurrent symptoms after initial response to therapy may be due to persistent infection or reinfection. More often, however, persistent or recurrent symptoms occur as a result of noninfectious causes; these include postinfectious lactose intolerance, other absorptive deficiencies, and irritable bowel syndrome.

Patients with recurrent or persistent symptoms should undergo the following:

Repeat stool examination (stool antigen or polymerase chain reaction [PCR]) for detection of Giardia [5]. A negative stool PCR is suggestive of parasite clearance [47]; in such cases, management should focus on postinfectious sequelae or pursuit of an alternative diagnosis. Stool microscopy is not recommended as it lacks adequate sensitivity to rule out ongoing infection in patients with persistent symptoms [46,47]. (See "Giardiasis: Epidemiology, clinical manifestations, and diagnosis", section on 'Diagnosis'.)

Evaluation for potential sources of reinfection (eg, review of the exposure history, risk factors, and hygiene practices). (See "Giardiasis: Epidemiology, clinical manifestations, and diagnosis", section on 'Routes of transmission'.)

For individuals with >1 recurrence or for those with history suggestive of immunocompromise (eg, other recurrent infections), we perform evaluation for underlying immunodeficiency by conducting a thorough history and exam, checking quantitative immunoglobulin (Ig) levels (eg, hypogammaglobulinemia, X-linked agammaglobulinemia, IgA deficiency), and reviewing the presence of immunosuppressive drug use and/or immunocompromising conditions (eg, hematologic malignancy, organ transplantation, or HIV). Patients with underlying immunodeficiency and/or epidemiologic exposure to parasitic infections also warrant evaluation for other pathogens including intestinal helminths [31,49]. (See "Approach to the child with recurrent infections" and "Ascariasis" and "Hookworm infection" and "Strongyloidiasis" and "Enterobiasis (pinworm) and trichuriasis (whipworm)".)

Individuals with negative repeat stool examination should undergo evaluation for noninfectious causes of symptoms. (See 'Persistent symptoms with negative stool assay' below.)

Management — Individuals with an established diagnosis of recurrent or persistent giardiasis (based on repeat diagnostic testing) warrant a repeat course of antimicrobial therapy.

Recurrence of disease (diagnostically proven) — In individuals in whom symptoms resolved weeks or months prior to recurrence, we favor treatment with tinidazole or nitazoxanide, even if they had received the same agent previously [7]. The dose and duration recommended for initial treatment are appropriate for recurrent disease. In patients initially treated with a short course of metronidazole (single-dose or five-day course) who cannot take tinidazole or nitazoxanide, it is reasonable to give a longer course of metronidazole (for example, extending the duration to 10 days).

In individuals with more than one recurrence, referral to an infectious diseases specialist for management is warranted.

Minimal or no response to initial antimicrobial therapy — In individuals who had minimal or no response to initial therapy, we favor treatment with an antimicrobial agent from a different class. As an example, patients treated initially with a nitroimidazole (tinidazole or metronidazole) may be treated with nitazoxanide, which has demonstrated activity against nitroimidazole-refractory infections [39,50,51].

Refractory giardiasis after repeated trials of preferred agents — In individuals who repeatedly fail therapy with nitroimidazoles (eg, tinidazole or metronidazole), nitazoxanide, or an alternative agent (eg, albendazole), we favor treatment with the following combination regimen [50,52,53]:

Albendazole (15 mg/kg per day orally, maximum dose 400 mg) PLUS metronidazole (15 mg/kg orally per day divided in three doses, with a maximum dose of 250 mg per dose) for five days [5,29,54-56]. In an observational study of 38 adults with metronidazole-refractory giardiasis, albendazole in combination with metronidazole was effective in achieving parasitologic cure in 79 percent of cases [54].

Quinacrine-containing regimens are potential alternative options, but these should only be used for refractory cases given the substantial side effects. Such regimens include the following (see 'Alternative agents' above):

Quinacrine (6 mg/kg per day orally divided in three doses; maximum dose 100 mg per dose) for five days

Tinidazole (50 mg/kg orally as a single dose; maximum dose of 2 g) PLUS quinacrine (6 mg/kg per day orally divided in three doses; maximum dose 100 mg per dose) for 14 to 21 days

Metronidazole (15 mg/kg orally per day divided in three doses; maximum dose of 250 mg per dose) PLUS quinacrine for 14 to 21 days [46]

A sequential approach with a trial of more than one regimen may be necessary if symptoms persist; in such cases, consultation with a specialist in infectious diseases and evaluation for underlying immunodeficiency is warranted [7,13]. (See 'Evaluation' above.)

Use of quinacrine has been shown to be effective for patients not responding to nitroimidazole monotherapy; however, its use is limited by side effects and availability [40,41,57,58] (see 'Alternative agents' above). Data on use of combination regimens for treatment of recurrent or persistent giardiasis are limited. Tinidazole or metronidazole combined with nitazoxanide has not been studied.

Persistent symptoms with negative stool assay — In some patients, poor weight gain and gastrointestinal symptoms (including bloating, diarrhea, and nausea) may persist after completion of treatment and documentation of negative stool assays. In most of these cases, Giardia infection has cleared and additional antimicrobial therapy is not warranted. We recommend a trial of a lactose-free diet given that lactase deficiency occurs in 20 to 40 percent of patients with giardiasis and may linger for several weeks after treatment [16].

If suspicion for persistent giardiasis remains and the sensitivity of the stool test used is low (eg, stool microscopy), gastroenterology consultation regarding endoscopy to evaluate for ongoing presence of trophozoites in the small intestine may be warranted. Such evaluation is also useful for evaluation of other small intestinal/malabsorption pathologies [59-62]. (See "Approach to the adult with chronic diarrhea in resource-abundant settings" and "Approach to chronic diarrhea in children >6 months in resource-rich countries" and "Persistent diarrhea in children in resource-limited countries".)

ANTIMICROBIAL RESISTANCE — Antimicrobial resistance testing is not routinely available in most clinical settings. Data regarding local antimicrobial resistance prevalence and its clinical relevance are thus too sparse to influence treatment recommendations. In general, presence of persistent or recurrent treatment-refractory symptoms should prompt suspicion for antimicrobial resistance; however, the management approach is largely empiric. The approach for such cases is discussed above. (See 'Persistent or recurrent symptoms' above.)

Giardia isolates vary in their in vitro susceptibility to tinidazole and metronidazole (both are nitroimidazoles) and resistance to these agents has been observed [25,38,63-68]. Increasing rates of nitroimidazole-refractory infection in some settings has raised concern regarding spread of nitroimidazole resistance [38,63-65,69]. However, in vitro drug resistance does not correlate consistently with clinical outcome [69]. Thus far, nitazoxanide resistance appears restricted to laboratory strains [70].

COUNSELING AND PREVENTION — Measures for prevention of giardiasis include counseling, infection control interventions, and water purification. There is no vaccine available for prevention of giardiasis.

Counseling on reducing transmission — Individuals with giardiasis should be counseled on avoiding settings with risk for transmission to others (such as childcare centers, school, or recreational water venues) until asymptomatic for 48 hours [1]. They should also be counseled to diligently adhere to hand hygiene with soap and water (preferred over hand hygiene with alcohol-based hand disinfection [AHD]) while symptomatic. Anal receptive and oral-anal sex should be avoided while actively infected.

Counseling to prevent infection — To avoid becoming infected with Giardia, patients should be counseled to avoid drinking water or eating food that may be contaminated and swallowing water while swimming in recreational water areas. Giardia cysts are eliminated by boiling tap water at least one minute [71,72]. Water purification tools for hikers and campers include water filtration iodination and chlorination; iodine-based treatments are more effective than chlorine-based treatments [73].

Patients should be counseled to avoid contact with stool during sex, to use a barrier during oral-anal sex, and to wash hands after handling a condom used during anal sex and after touching the anus or rectal area. These are general recommendations for prevention of gastrointestinal pathogens, regardless of whether partners are known to have infection.

The type of Giardia that infects humans is usually not the same type that infects dogs or cats. Issues related to prevention of transmission from pets are discussed on the United States Centers for Disease Control and Prevention website.

Infection control — In health care and other institutional settings, patients with giardiasis who are in diapers or are incontinent should be placed on contact precautions for the duration of illness (in addition to standard precautions) [7]. (See "Infection prevention: Precautions for preventing transmission of infection", section on 'Contact precautions'.)

Attention to hygiene is important for preventing person-to-person transmission of giardiasis. Strict handwashing, care with diaper disposal, and treatment of symptomatic children can prevent the spread of giardiasis. Hand hygiene with soap and water is preferred over hand hygiene with AHD. AHD is effective against trophozoites passed in the stool but is not effective against the cyst form that survives in the environment (figure 1).

Issues related to environmental cleaning and disinfection are discussed separately. (See "Infection prevention: General principles", section on 'Health care environment: Cleaning and disinfection'.)

Diagnosis of giardiasis should prompt notification of the local health department (this is usually performed by the laboratory) [2,74].

Resource-limited settings — Due to the low infectious dose necessary to cause Giardia infection, access to clean water, sanitation, and hygiene are critical to reduce Giardia community prevalence. In resource-limited settings, inadequate water, sanitation, and hygiene results in endemic pediatric Giardia infection. Individual sanitation and hygiene measures result in the greatest reduction in Giardia infection in these settings [75]. Other tools for prevention of giardiasis in resource-limited settings include breastfeeding and nutrient supplementation, although nutrient supplementation without improvements in sanitation and hygiene may be of limited benefit [75]. There is no role for mass drug administration to reduce Giardia carriage in resource-limited settings [76].

Breastfeeding is protective against giardiasis in nursing infants in endemic areas [77-79]. In one study including more than 150 infants in Egypt, exclusively breastfed infants had a lower incidence of giardiasis than infants who were not exclusively breastfed [79].

Nutrition should be optimized based on World Health Organization nutrition guidance for specific age groups [80,81]. Adequate nutrition can help protect against giardiasis. Zinc and vitamin A have been associated with a protective effect against giardiasis among children in resource-limited settings [78]. In one randomized study in Brazil (in a region where approximately one-third of children had at least mild vitamin A deficiency) of 79 children age 2 months to 9 years of age, those who received vitamin A had significantly fewer new Giardia infections compared with those treated with placebo over a three-year period (for example, 0 versus 19 percent at one month post-treatment) [82]. In another study in Mexico City, treatment of 707 children age 6 to 15 months with zinc, vitamin A, or both was associated with decreased incidence of giardiasis [83].

However, some nutrient supplements may adversely alter the course of illness. In a randomized study including more than 600 children age 6 to 60 months in Tanzania, administration of a multi-nutrient supplement increased diarrheal symptoms among children with giardiasis (hazard ratio 2.03, 95% CI 1.24-3.32) [84].

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: Acute diarrhea in adults".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

Basics topic (see "Patient education: Giardia (The Basics)")

Beyond the Basics topic (see "Patient education: Giardia (Beyond the Basics)")


Asymptomatic individuals – Treatment for most asymptomatic individuals is not necessary. We suggest treatment of asymptomatic infection in the following individuals to reduce transmission risk (algorithm 1) (Grade 2C):

Individuals in group settings with risk for transmission to others (such as a child in a daycare setting, health care workers, or nursing facility worker)

Immunocompromised individuals

Household contacts of immunocompromised individuals (especially those with hypogammaglobulinemia or cystic fibrosis)

Household contacts of pregnant individuals, and

Food handlers

Regimen selection is the same as in symptomatic individuals. (See 'Asymptomatic individuals' above.)

Initial treatment of symptomatic individuals – For individuals ≥3 years old, we suggest single-dose tinidazole over other agents (Grade 2C). If unavailable, nitazoxanide is similarly highly effective but warrants a three-day course. Metronidazole is less effective and may be associated with more side effects. For younger children, some options do not have sufficient safety data for use. Thus, for individuals between ages one and three years, we suggest nitazoxanide (Grade 2C) and for individuals less than 12 months of age, we suggest metronidazole (algorithm 1) (Grade 2C).

Other alternative agents include albendazole and mebendazole, paromomycin, furazolidone, quinacrine, secnidazole, and ornidazole (table 1). (See 'Symptomatic individuals' above.)

Pregnant and lactating individuals – During the first trimester, it is reasonable to defer therapy of mild giardiasis to minimize the risk of adverse drug effects for the fetus. For those whose symptoms warrant treatment during the first trimester and for those with mild disease in later trimesters, we suggest paromomycin, which has limited systemic absorption (Grade 2C). For more severe disease in the second and third trimesters, we suggest metronidazole (Grade 2C). Paromomycin is also an option for lactating individuals who do not want to interrupt breastfeeding for medication administration. (See 'Pregnant and lactating individuals' above.)

Expected response to therapy – Symptoms of acute giardiasis typically resolve within five to seven days of treatment, and parasites usually clear from the stool in this time frame. In those who had chronic symptoms (>8 weeks) prior to treatment, symptoms may take several months to resolve. There is no need to repeat the stool examination to check for parasite clearance after symptom resolution. (See 'Expected response to therapy' above.)

Persistent or recurrent symptoms

Evaluation – Patients with recurrent or persistent symptoms should undergo a repeat stool examination (stool antigen or polymerase chain reaction) for detection of giardiasis and evaluation for potential sources of reinfection (eg, review of the exposure history, risk factors, and hygiene practices). For those with history suggestive of immune deficits or who have >1 recurrence, evaluation for underlying immunodeficiency is also warranted. (See 'Evaluation' above.)

Management of persistent or recurrent symptoms For patients with persistent or recurrent symptoms, we reserve treatment for those with microbiologic confirmation of infection. Options include using the same agent, a different agent, or a combination, depending on prior treatment history. For patients with a negative stool assay, ongoing symptoms often have a noninfectious etiology. (See 'Management' above.)

Preventing transmission – This includes contact precautions for incontinent patients in institutional settings, strict adherence to hand hygiene with soap and water, and avoidance of recreational areas (eg, childcare centers, recreational water venues) until asymptomatic for 48 hours. (See 'Counseling and prevention' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Flor M Munoz, MD, MSc, who contributed to an earlier version of this topic review.

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