INTRODUCTION — Tapeworms (cestodes) are flat, hermaphroditic worms that can live as parasites in the human gastrointestinal tract. Some of these organisms are primarily human pathogens, while others have animals as their natural hosts but can also cause human infection.
Issues related to diseases associated with human intestinal tapeworms (taeniasis, diphyllobothriasis, and hymenolepiasis) will be reviewed here. Issues related to cysticercosis are discussed separately. (See "Cysticercosis: Epidemiology, transmission, and prevention" and "Cysticercosis: Clinical manifestations and diagnosis" and "Cysticercosis: Treatment".)
Issues related to human disease associated with animal tapeworms (sparganosis, coenurosis, and others) will be reviewed here. Issues related to echinococcus are discussed separately. (See "Epidemiology and control of echinococcosis" and "Echinococcosis: Clinical manifestations and diagnosis" and "Echinococcosis: Treatment".)
HUMAN INTESTINAL TAPEWORMS
Morphology — Adult tapeworms consist of a head, neck, and segmented body. The head (also known as a scolex) is equipped with suckers, hooks, or grooves (depending upon the species); the worm uses these to attach itself to the intestine of the host.
Each segment (known as a proglottid) has a complete set of reproductive organs. Depending on the species, the adult worm may have hundreds or thousands of proglottids. In some species (such as Taenia), the distal segments can separate from the rest of the body, and the proglottids (or the eggs they contain) are passed in the stool. In other species (such as Hymenolepis), proglottids degenerate within the host so only eggs are found in the stool.
Tapeworms are covered with a tegument through which they absorb nutrients and secrete waste. Different species of tapeworms can be differentiated by morphologic characteristics of the scolex, proglottids, and eggs [1].
Forms of disease
Taeniasis — Taenia species for which humans are the only definitive hosts include Taenia saginata (beef tapeworm), Taenia solium (pork tapeworm), and Taenia asiatica (pork tapeworm). T. saginata occurs worldwide but is most common in areas where consumption of undercooked beef is customary, such as Europe and parts of Asia. T. saginata is widely distributed in 21 of the 54 countries in the Americas [2]. T. asiatica is found among pigs in Taiwan, Korea, China, Vietnam, Indonesia, Thailand, the Philippines, Lao People’s Democratic Republic, and Japan [1,3-7]. Concurrent infections with more than one Taenia species have been described [8].
Issues related to T. saginata are discussed here. The epidemiology, transmission, clinical manifestations, diagnosis, treatment, and prevention of cysticercosis due to T. solium are discussed separately. (See "Cysticercosis: Epidemiology, transmission, and prevention" and "Cysticercosis: Clinical manifestations and diagnosis" and "Cysticercosis: Treatment".)
Life cycle — Humans are the only definitive hosts for T. saginata, T. solium, and T. asiatica (figure 1). Eggs or gravid proglottids are passed in human stool; eggs can survive for days to months in the environment. Cattle (T. saginata) and pigs (T. solium and T. asiatica) become infected by ingesting eggs or gravid proglottids (pigs ingest the ova or proglottids directly, whereas bovines ingest contaminated vegetation). In the animal's intestine, the invasive larvae (oncospheres) hatch, invade the intestinal wall, and migrate hematogenously to striated muscles (T. solium or T. saginata) or viscera (T. asiatica), where they develop into cysticerci. The cysticercus, which contains a single scolex, usually measures 5 by 10 mm and can survive for several years in the animal.
Humans become infected by ingesting raw or undercooked infected meat containing cysticerci. In the human intestine, the scolex vaginates and attaches to the intestinal wall via suckers and/or hooks. The adult tapeworm develops by forming proglottids that arise from the neck region of the scolex; this occurs over about two months. The adult tapeworm can survive for years in the small intestine. Most human infections are with only one or a few adult tapeworms.
The length of an adult worm is usually ≤5 m for T. saginata (however, it may reach up to 25 m) and 2 to 7 m for T. solium. The adult worms produce proglottids that mature, become gravid, detach from the tapeworm, and are passed in the stool (approximately six per day). T. saginata adult worms usually have 1000 to 2000 proglottids, while T. solium adult worms have an average of 1000 proglottids. The eggs in the gravid proglottids are released after the proglottids are passed in the stool. T. saginata may produce up to 100,000 eggs per proglottid, and T. solium may produce 50,000 eggs per proglottid.
Clinical manifestations — Most human carriers of adult tapeworms are asymptomatic. Intermittently, patients may pass proglottids in the stool (T. solium) or spontaneously (T. saginata) or may notice segments in their stool. T. saginata is motile; carriers may sense movement of proglottids through the anus. There may be associated symptoms including nausea, anorexia, or epigastric pain. Anxiety, headache, dizziness, and urticaria can also occur. A peripheral eosinophilia (up 15 percent) may be observed.
Occasionally, segments can enter the appendix, common bile duct, or pancreatic duct and cause obstruction [9,10]. Rarely, proglottids can be aspirated or regurgitated.
Diagnosis — The diagnosis is generally established by identifying eggs or proglottids in the stool. The eggs of Taenia species are morphologically indistinguishable (picture 1); they are round with a double-walled, radially striated membrane and measure 30 to 40 micrometers. T. saginata eggs have an acid-fast shell; T. solium eggs are not acid fast.
The proglottids and scolices of T. solium and T. saginata are morphologically distinguishable and can be used to establish a species diagnosis (picture 2 and picture 3). T. saginata and T. asiatica proglottids have 12 or more primary uterine branches; T. solium proglottids have ≤10. These branches can be seen on direct examination or by injecting India ink into the segment via its lateral genital opening.
The scolex usually remains in the intestine when proglottids are passed; it is rare for the entire worm to be eliminated spontaneously. Following antiparasitic therapy, however, the scolex of a fully evacuated worm may be identified in the stool. Both T. saginata and T. asiatica have a scolex with four lateral suckers and no hooks ("unarmed"). T. solium has a scolex with a well-developed rostellum (crown) that has four suckers and a double row of hooks ("armed").
The sensitivity of stool examination is limited since elimination of eggs and proglottids is intermittent. To increase the diagnostic yield, repeat specimens should be examined, and concentration techniques can be used. In addition, T. saginata eggs may be deposited on perianal areas and be detected by anal swabs.
Laboratory workers must exercise caution when performing stool examinations; T. solium eggs are infectious if ingested [11,12]. (See "Cysticercosis: Clinical manifestations and diagnosis".)
Immunologic and molecular methods have been developed to improve diagnostic sensitivity, including an enzyme-linked immunosorbent assay (ELISA) for the detection of T. solium antigens in fecal samples and DNA hybridization techniques for the detection of eggs in stools [13-19]. In addition, several polymerase chain reaction (PCR) assays targeting various genomic regions have been developed for distinguishing between species of human Taenia infections, including PCR-restriction fragment length polymorphism (RFLP) methods [20,21], species-specific DNA probes [16], loop-mediated isothermal amplification (LAMP) [22-24], and nested/multiplex PCR [25-29]. Primers targeting the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene of the three Taenia species have been developed and show promise for more routine use to distinguish between the three Taenia species [6]. In general, these assays are not yet suitable for routine diagnosis or field studies and are not widely available.
Diagnosis of taeniasis has also been made by visualizing the worm on capsule endoscopy [30].
Diphyllobothriasis — Diphyllobothriasis (fish tapeworm) is the largest human tapeworm. Common species worldwide include:
●Dibothriocephalus latus (formerly Diphyllobothrium latum), observed predominantly in northern and eastern Europe. D. latus infection can be transmitted by consumption of a variety of freshwater fish. Individuals who eat various forms of raw fish, such as sushi, sashimi, and ceviche, are at risk of infection.
●Dibothriocephalus nihonkaiensis, which is predominantly found in the northeast Asia including Japan and Korea. D. nihonkaiensis has mainly saltwater hosts, such as salmon [31].
●Adenocephalus pacificus (formerly Diphyllobothrium pacificum), found along the Pacific coast of South America [32,33].
Freshwater fish infected with diphyllobothriid larvae may be transported to (and consumed in) areas where active transmission does not occur. As an example, cases of D. latus infection associated with consumption of imported fish have been reported in Brazil [34].
Humans are the main definitive host for D. latus and the most important reservoir of infection; some other diphyllobothriid species primarily infect birds or mammals and are less common causes of human infection.
Life cycle — The adult parasites live in the human intestinal tract, and eggs and proglottids are passed in the stool (figure 2). Under appropriate conditions, the eggs embryonate in water (after approximately 18 to 20 days); coracidia hatch from eggs and are ingested by small crustaceans (the first intermediate host).
The coracidia develop into procercoid larvae in the body cavity of the infected crustacean, which is then ingested by small freshwater fish (the second intermediate host), typically minnows. The procercoid larvae are released from the crustacean and migrate into the fish flesh where they develop into a plerocercoid larvae (sparganum). The sparganum are the infective stage for humans. Since humans do not generally eat undercooked minnows or other small freshwater fish, these do not represent an important source of infection. These small freshwater fish are eaten by larger predator species (later intermediate host), such as trout, perch, or pike.
The sparganum can migrate from the small fish to the musculature of the larger predator fish; humans and other mammals can acquire infection by eating these predator fish raw or undercooked. After human ingestion of the infected fish, the sparganum develop into mature adult tapeworms that reside in the small intestine. The D. latus adult worms attach to the intestinal mucosa via the two bilateral grooves of their scolex. The adults can grow up to 12 m in length, with 3000 to 4000 proglottids. Immature eggs are released from the proglottids (up to one million eggs per day per worm) and are intermittently passed in the stool beginning five to six weeks after infection. The adult tapeworm survives for approximately 10 years in the human intestine.
Clinical manifestations — Most individuals with diphyllobothriasis are asymptomatic. Occasionally, a proglottid can be passed in the stool or regurgitated (picture 4), but this occurs less commonly than with Taenia species. Nonspecific symptoms that have been attributed to infection include fatigue, diarrhea, numbness, dizziness, and allergic symptoms. Occasionally, mechanical obstruction of the intestine can occur if several worms become entangled. A peripheral eosinophilia (5 to 10 percent) may be observed in some patients.
The classical manifestation of infection with D. latus is megaloblastic anemia due to vitamin B12 deficiency. D. latus has a unique affinity for vitamin B12 and therefore competes with the host for absorption. Vitamin B12 deficiency is more likely to develop if the host had marginal levels before acquiring the infection. It has been reported that approximately 40 percent of infected individuals have low vitamin B12 levels, but only about 2 percent actually develop anemia [1]. In the setting of severe vitamin B12 deficiency, pancytopenia, glossitis, dyspnea, and neurologic abnormalities (subacute combined spinal and peripheral nerve degeneration) can develop. (See "Treatment of vitamin B12 and folate deficiencies".)
Diagnosis — Megaloblastic anemia and vitamin B12 deficiency can be a clue to the diagnosis of diphyllobothriasis.
The diagnosis is generally established by identifying eggs or proglottids in the stool (picture 5). The eggs of diphyllobothriid species have an operculum, or lid-like opening, which is commonly seen in trematode eggs but is not seen with eggs of other cestodes (picture 6). The eggs measure 40 by 60 mcm and have a small knob on the end opposite the operculum. Concentration methods are not necessary for diagnosis in most cases, since there are typically large numbers of eggs present in the stool.
The scolex of the adult is spoon shaped and has characteristic ventral sucking grooves (known as "bothria") with which the worm attaches to the intestinal mucosa [35]. The proglottids can be differentiated from Taenia proglottids because their width is greater than length, and there is a characteristic egg-filled uterus that appears as a dark rosette in the center of mature proglottids (picture 7). It is generally difficult to distinguish among diphyllobothriid species based on morphology, but molecular methods may be used [36,37]. Occasionally, diphyllobothriid species may be visualized on colonoscopy or capsule endoscopy [38,39].
Hymenolepiasis — Hymenolepis nana (dwarf tapeworm) differs from most other tapeworms since it can complete its entire life cycle in a single host (figure 3). Its name derives from the fact that the adult parasite is much smaller than most other cestodes, measuring only 30 to 40 mm by 1 mm.
H. nana is the most common cestode globally and is nearly universal, especially in the tropics; infection is associated with poor sanitation and hygiene. Transmission can occur between humans in the absence of an intermediate host.
Life cycle — Eggs of H. nana are immediately infective when passed in the stool and cannot survive more than two weeks in the external environment. Humans may ingest these eggs via person-to-person spread or via contaminated food or water; thereafter, the oncospheres within the eggs are released and penetrate the intestinal villus where they develop into cysticercoid larvae. After five to six days, these develop into adult tapeworms that attach to the ileum via the scolex, which consists of four suckers and a retractable rostellum with a single circle of 20 to 30 hooks. Production of new eggs begins approximately 20 to 30 days after initial infection; eggs are passed in the stool when released from proglottids or when proglottids disintegrate in the small intestine.
Internal autoinfection is also possible; eggs can release their oncospheres within the human intestine, continuing the infective cycle without passage through the external environment. The lifespan of adult worms is four to six weeks, but internal autoinfection allows the infection to persist for years.
Alternatively, the eggs passed in the stool may be ingested by an arthropod intermediate host (various species of beetles and fleas may serve as intermediate hosts). Cysticercoid larvae develop within the insect, which are then ingested by humans or rodents and develop into adult worms in the small intestine. Humans can become infected by ingesting the infected insects accidentally, often in contaminated grain.
Clinical manifestations — Infection with H. nana is most common in children, since they are more prone to breaches in fecal-oral hygiene. Most infections are asymptomatic, but symptoms become more common as the parasite burden increases. Heavy infections with >1000 worms or >500 eggs/g of stool are more often associated with symptoms such as crampy abdominal pain, diarrhea, anorexia, weight loss, fatigue, and pruritus ani. Anemia, dizziness, irritability, sleep disturbance, seizures, and jaundice have also been described [40,41]. A peripheral blood eosinophilia of 5 to 10 percent may be observed.
Malignant transformation of H. nana has been described in an HIV-infected patient [42]. Further study of host-parasite interactions is needed to better understand the potential relationships between infection and cancer.
Diagnosis — The diagnosis is generally established by identifying eggs or proglottids in the stool (picture 8 and picture 9). Eggs are 30 to 50 mcm in diameter. They contain an oncosphere and are covered with a thin hyaline outer membrane and a thick inner membrane; four to eight hair-like filaments arise from the even-thicker polar ends of the membrane. Proglottids disintegrate in the intestine and are not found in the feces. The sensitivity of stool microscopy can be increased by using concentration techniques such as the FLOTAC method [43-45].
Diagnosis of hymenolepiasis should prompt household screening or empiric treatment, given the potential for person-to-person spread.
Treatment — Praziquantel is the treatment of choice for all of the tapeworm infections discussed above [46,47]. Dosing depends upon the species [48]:
●Dosing for taeniasis and diphyllobothriasis is 10 mg/kg orally (single dose); however, efficacy against T. saginata infections has been reported at doses as low as 2.5 mg/kg [49].
●Dosing for hymenolepiasis is 25 mg/kg orally (single dose), followed by repeat dose 10 days later.
Caution should be used when using praziquantel in areas endemic for T. solium, since administration of praziquantel can precipitate symptoms of occult neurocysticercosis; for this reason, it should be avoided in patients with prior neurologic symptoms. However, excluding individuals based on symptom screening does not fully eliminate the potential risk of precipitating neurologic symptoms in individuals with asymptomatic neurocysticercosis. When used for treatment of tapeworm infections, praziquantel should be given on an empty stomach (which reduces absorption) [50].
Praziquantel is a synthetic heterocyclic isoquinolone-pyrazine derivative that induces ultrastructural changes in the teguments of parasites, resulting in increased permeability to calcium ions. Calcium ions accumulate in the parasite cytosol, leading to muscular contractions and ultimate paralysis of adult worms [51]. By damaging the tegument membrane, praziquantel also exposes parasite antigens to host immune responses [52]. These effects lead to dislodgement of worms from their intestinal sites and subsequent expulsion by peristalsis.
Niclosamide is an acceptable alternative treatment for tapeworms if praziquantel is not available, and some experts use it as an agent of choice in areas endemic for T. solium. Niclosamide comes in 500 mg tablets that need to be chewed; it is not available in the United States. Dosing consists of four tablets (500 mg) in a single dose (2 g) for adults, two tablets (1 g) for children 11 to 34 kg, and three tablets (1.5 g) for children >34 kg.
Nitazoxanide has been used to treat hymenolepiasis with 75 to 85 percent efficacy [53,54]. Dosing consists of 500 mg twice daily for three days for patients >11 years of age, 200 mg twice daily for three days for patients 4 to 11 years of age, and 100 mg twice daily for three days for patients 1 to 3 years of age [53,54].
For successful treatment, the scolex must be destroyed and eliminated; a residual scolex can result in regrowth of the entire tapeworm. After treatment, the proximal parts of the tapeworms disintegrate, and gravid proglottids can release eggs as they are being destroyed. Because praziquantel kills adult worms but not eggs, precautions should be taken to prevent autoinfection, laboratory-acquired infection, or dissemination to others. With large tapeworms, intact or disintegrating segments and eggs may be passed for several days. Following therapy, stools should be rechecked for eggs at one month (hymenolepiasis or diphyllobothriasis) or three months (taeniasis) to document cure.
Prevention — Measures for prevention of intestinal tapeworm infection include avoiding ingestion of undercooked beef (T. saginata), undercooked pork (T. solium), raw freshwater fish (D. latus), or contaminated vegetables, water, or grains (H. nana).
Cooking meat at temperatures of at least 145°F (63°C) kills tapeworm eggs and larvae, as does freezing meat and fish at -4°F (-20°C) or below for 7 days, or deep freezing at -31°F (-35°C) or below for 12 to 24 hours.
Hand washing with soap and water before eating or handling food and after using the toilet or touching animals is also recommended to reduce the risks of tapeworm infection.
Mass drug administration has been evaluated; repeat dosing are likely to be needed for sustained control [55]. Guidelines suggest either praziquantel 10 mg/kg or niclosamide 2 g (dose adjusted for children as outlined above) [50]. Treatment should be avoided in pregnant women in their first trimester. In addition, people with a history of severe headache or seizures and people with subcutaneous cysticercosis should not receive praziquantel.
However, excluding individuals based on symptom screening does not fully eliminate the potential risk of precipitating neurologic symptoms in individuals with asymptomatic neurocysticercosis.
ANIMAL TAPEWORMS CAPABLE OF CAUSING HUMAN DISEASE
Causes of human intestinal disease — Rarely, animals tapeworms can cause human intestinal infection.
Hymenolepis diminuta is a species related to H. nana that is primarily a parasite of domestic rats (figure 4) [56]. Humans can be infected as accidental hosts by swallowing infected insects, such as grain beetles or mealworms (intermediate hosts), usually in flour or cereals. Infection occurs mainly in areas with heavy rodent infestation. The worms and the eggs are morphologically different from H. nana, but similar symptoms can develop. The management is as discussed above for human intestinal tapeworms. (See 'Treatment' above.)
The dog tapeworm, Dipylidium caninum, commonly infects domestic cats and dogs and may also accidentally infect humans (figure 5). Fleas usually serve as the intermediate hosts and contain cysticercoids. If an animal or human (usually a child) ingests cysticercoids, adult tapeworms can develop in the intestine. Infection is usually asymptomatic; abdominal pain, diarrhea, pruritus ani, and urticaria can develop. Parents may notice the passage of proglottids in their children's stool. The adult tapeworm is 15 to 20 cm in length. The management is as discussed above for human intestinal tapeworms. (See 'Treatment' above.)
Causes of human extraintestinal disease — T. solium can cause intestinal taeniasis and extraintestinal cysticercosis. (See "Cysticercosis: Epidemiology, transmission, and prevention" and "Cysticercosis: Clinical manifestations and diagnosis" and "Cysticercosis: Treatment".)
Issues related to human extraintestinal disease associated with animal tapeworms (sparganosis, coenurosis, and others) are discussed below. Issues related to echinococcus are discussed separately. (See "Epidemiology and control of echinococcosis" and "Echinococcosis: Clinical manifestations and diagnosis" and "Echinococcosis: Treatment".)
Sparganosis — Sparganosis is caused by invasion of human tissue by spargana (tapeworm larvae) of the genus Spirometra [57]. Species include Spirometra mansoni, Spirometra ranarum, Spirometra mansonoides, Spirometra erinacei, and Spirometra proliferum.
Epidemiology — The genus Spirometra occurs worldwide. Most human cases have been recognized in Southeast Asian countries. This observation reflects regional and cultural practices of consuming raw frogs, tadpoles, and snake meats, as well as application of raw frog meat as poultices to sooth sores and periocular inflammation. In addition, drinking contaminated, unfiltered water may be common among field workers, especially in rice paddies.
In one review including more than 400 cases of sparganosis in Korea, 76 percent of cases were in males and 63 percent were associated with consumption of frogs and snakes and 17 percent were due to drinking unfiltered water [58]. Among 81 snakes examined in Korea, 75 percent were found to be infected with spargana [58]. In Korea, application of contaminated frog flesh poultice is uncommon, and ocular sparganosis is relatively rare [58,59]. In contrast, in Thailand, use of periorbital poultice application is more common, one-third of reported sparganosis cases involved the eye [59].
Animal sparganosis is endemic in North America, but human cases are rare.
Life cycle — Spirometra species live in the intestines of dogs and cats (definitive hosts), which shed Spirometra eggs in stool (figure 6). The eggs hatch in water and release coracidia which are ingested by small crustaceans and develop into procercoid larvae. Second intermediate hosts (freshwater fish, amphibians, reptiles. rodents, and other mammals) ingest infected crustaceans; in these hosts, procercoid larvae develop into plerocercoid larvae (spargana). The life cycle is completed when second intermediate hosts are ingested by a dog or cat.
Spirometra larvae may be transmitted to humans in the following ways:
●By drinking water contaminated with crustaceans that contain procercoid larvae
●By eating raw or undercooked meat from second intermediate hosts containing plerocercoid larvae
●By using flesh from second intermediate hosts as a poultice applied to sores, ulcers, or inflamed eyes
The incubation period depends on the route of infection. For subcutaneous disease, the incubation period is one day to several months; for central nervous system disease, the incubation period may be one to thirty years [60].
Forms of disease and clinical approach — Spargana migrate to subcutaneous tissues and other locations, where they form a nodular mass. Forms of disease include subcutaneous nodules, ocular manifestations, central nervous system disease, and visceral involvement; these are discussed further below. Clinical manifestations of sparganosis can develop within weeks of infection or may be delayed for many years [57].
The diagnosis of sparganosis should be suspected in patients with relevant clinical manifestations (such as a migratory, painful subcutaneous nodule, ocular manifestations, central nervous system disease, or visceral involvement) in the setting of relevant epidemiologic exposure in Southeast Asian countries (eg, consumption of raw or undercooked meat from snakes or frogs, drinking untreated water, or using raw flesh in traditional poultices).
The diagnosis may be definitively established based on identification of the parasite in a tissue specimen or via surgical removal. Serum eosinophilia may be observed but is not uniformly present [57,61]. Serologic assays are available and used in endemic areas; however, development of serologic testing requires further validation to exclude cross-reactivity with other cestodes [57,61]. A presumptive clinical diagnosis may be made in the setting of a slowly migrating subcutaneous nodule or a mass in the setting of relevant epidemiologic risk factors.
In the setting of infection due to S. proliferum, larvae proliferate throughout the subcutaneous tissues with several spargana in a single site. In the setting of AIDS, disseminated infection due to this species has been described [62].
Subcutaneous nodules — Subcutaneous sparganosis often begins as a single nodular lesion (0.5 to 5 cm) that gradually increases in size and may slowly migrate. Lesions may develop on any area of the body, including the face, abdominal wall, chest wall, breast, labia, scrotum, and lower extremities [57,58]. Lesions may persist for months or years with no symptoms, or become tender suddenly. The lesion may be associated with pruritus or inflammation.
The diagnosis of subcutaneous sparganosis is made based on identification of the parasite in a tissue specimen or via surgical removal. A presumptive clinical diagnosis may be made in the setting of a slowly migrating subcutaneous nodule in the setting of relevant epidemiologic risk factors (including supportive geographic, dietary, and exposure histories).
In the setting of subcutaneous sparganosis, ultrasonography may demonstrate a coiled linear hypoechoic body in a clear ovoid mass [57]. Computed tomography (CT) and magnetic resonance imaging (MRI) may reveal multiple tubular and cystic lesions [63].
The differential diagnosis of subcutaneous nodules includes a variety of parasitic infections (table 1 and table 2), as well as noninfectious causes. (See "Skin lesions in the returning traveler", section on 'Nodules' and "Overview of benign lesions of the skin".)
Management consists of surgical extraction of the larva; administration of antiparasitic therapy in the absence of worm removal is not effective. Extracted larvae are white, opaque, ribbon-like worms, often three to seven centimeters in length. The entire larva should be removed; if the head and protoscolex are retained, they remain viable and may serve a source of recurrence [57,64].
Ocular manifestations — Ocular sparganosis may present with eyelid swelling, subconjunctival edema, or an intraorbital mass. Symptoms include lacrimation, itching, pain, and redness of the eye. Eosinophilia is often observed in patients with orbital sparganosis [64]. CT or MRI may demonstrate a mass-like structure mimicking tumor [65].
The diagnosis of ocular sparganosis is made based on identification of the parasite in a tissue specimen or via surgical removal.
The differential diagnosis of orbital sparganosis includes hordeolum, uveitis, and orbital cellulitis; these conditions are not associated with eosinophilia (see "Eyelid lesions", section on 'Hordeolum (stye)' and "Uveitis: Etiology, clinical manifestations, and diagnosis" and "Orbital cellulitis"). Onchocerciasis is associated with keratitis; the findings are intraorbital, not periorbital. Loiasis is associated with subconjunctival worm migration with no mass or periorbital involvement. (See "Onchocerciasis" and "Loiasis (Loa loa infection)".)
Management consists of surgical extraction of the larva; administration of antiparasitic therapy in the absence of worm removal is not effective. The entire larva should be removed; if the head and protoscolex are retained, they remain viable and may serve a source of recurrence [57,64].
Central nervous system disease — In cerebrospinal sparganosis, lesions may develop in any area of the brain or spinal cord, with consequent neurologic sequelae [66]. In one Korean series of 148 cases with central nervous system (CNS) involvement, 21 percent experienced seizures, 13 percent headache, and 7 percent hemiparesis [58].
In the setting of cerebral sparganosis, MRI findings may include aggregated ring-like enhancement (often three to six bead-shaped rings), "tunnel sign" with enhancement of lesions measuring approximately 4 cm in length and 0.8 cm wide, and migration of radiographic lesions as larvae migrate [66,67]. Analysis of CSF may demonstrate a lymphocytic or eosinophilic pleocytosis [68]. The diagnosis is definitively established based on histopathology of an excised lesion. The differential diagnosis is includes malignancy and other infectious inflammatory masses such cerebral paragonimiasis.
Management consists of surgical extraction of the larva, if feasible [57,64]. For patients with disease that precludes surgical extraction (via craniotomy or stereotactic CT-guided aspiration), administration of high-dose praziquantel (50 mg/kg/day for 10 days) with radiographic monitoring is warranted. Praziquantel may be repeated monthly until active lesions resolve radiographically.
The above approach is supported by a retrospective study from China including 54 patients treated with high-dose praziquantel (50 mg/kg/day for 10 days), repeated at monthly intervals until active lesions disappeared on MRI [67]. Complete radiographic response was observed in 94 percent of cases; a maximum of eight repeated praziquantel treatments were administered. Radiographic lesions persisted after eight courses of praziquantel in three patients; surgical removal was undertaken in these cases. Mild allergic reactions during praziquantel treatment were observed in six patients.
In another retrospective study from China including 10 patients with cerebral sparganosis not amenable to surgical excision, treatment with high-dose praziquantel (25 mg/kg three times daily for 10 days) was administered every three months until there was clinical improvement and serial MRI studies showed disappearance of the initial enhancing lesions or replacement by stable, chronic lesions [69]. Patients were treated with dexamethasone if the frequency of seizures increased or clinical symptoms worsened with initial therapy. One patient received one course of high-dose praziquantel, five patients received two courses, two received three courses, one received four courses, and one received five courses. Motile lesions were transformed into stable, chronic lesions in eight patients, and motile lesions were eliminated in two patients.
Visceral involvement — Visceral sparganosis is caused by migration and growth of spargana in the thoracic or abdominal cavity. Pulmonary sparganosis may include involvement of the airway, lung parenchyma, pleura, and pulmonary vessels. In one review including 40 patients with pulmonary sparganosis, symptoms included cough (60 percent), fever (58 percent), and chest pain (42 percent); pleural effusion was observed in 78 percent of cases [61]. Manifestations of abdominal involvement include intestinal obstruction or perforation [57]. Radiographic imaging may demonstrate complex and cystic tubular or round and poorly defined masses. A double hypoechoic ring surrounding a mass ("tubule in tubule" appearance) may be observed [58].
The diagnosis is often based on compatible clinical and radiographic appearance, supported by pertinent epidemiology and serology; definitive diagnosis is based on identification of a worm in surgically resected material.
Management consists of surgical extraction of the larva, if feasible [57,64]. For patients with visceral, pleuropulmonary, and pericardial involvement (in the absence of discrete excisable nodules), definitive surgical management may not be feasible. In such cases, administration of high-dose praziquantel is warranted. In one report including 12 patients with pulmonary sparganosis treated with three to seven days of praziquantel (total dose of 150 mg/kg), clinical improvement was observed, but long-term follow-up was limited [61].
Coenurosis — Coenurosis is a rare parasitic infection caused by the larval cyst (coenurus) of the canid tapeworms Taenia multiceps and Taenia serialis.
Epidemiology and life cycle — Human infections are more prevalent in Africa, but rarely are also found in sheep-raising areas of Europe, South America, Canada [70], and the United States [71,72].
T. multiceps is an intestinal parasite of dogs, foxes, coyotes, and other canids. T. serialis is an intestinal parasite of dogs and foxes. Humans acquire infection by ingestion of eggs passed in stool (figure 7); eggs hatch in the intestine; oncospheres are released that circulate via the bloodstream until they lodge in suitable organs.
Forms of disease — The clinical manifestations depend on sites of tissue localization.
The diagnosis of coenurosis should be suspected in patients with relevant clinical manifestations (such as a subcutaneous nodule, ocular manifestations, or CNS disease) in the setting of relevant epidemiologic exposure such as exposure to dogs. Eosinophilia is often absent [72].
Subcutaneous nodules — Coenuri of T. serialis may be found in subcutaneous tissues. The clinical presentation is usually a nodule on any area of the body; the lesion may be fluctuant and may or may not be tender. Involvement of the intercostal areas and the anterior abdominal wall have been described in Africa [73].
The diagnosis is established by surgical removal and pathologic examination of the excised coenurus. There are no serologic tests.
The differential diagnosis may include lipoma, neurofibroma, and lymphoma.
Ocular and central nervous system disease — Coenuri of T. multiceps may be found in the eyes and brain. Clinical manifestations may include severe headache, seizures, focal neurologic deficit leading to hemiparesis or other neurologic sequelae, or intraventricular obstruction with features of raised intracranial hypertension with papilledema [66]. Personality disturbances may be a presenting feature. Arachnoiditis, ependymitis, spinal cord and ocular coenurosis have also been described.
Radiographic findings may include complex multilocular cystic brain lesions, often with a juxtacortical distribution [74]. The cystic lesions typically suppress on fluid attenuated inversion recovery imaging with rim-like enhancement. Magnetic resonance spectroscopic signature of succinate and alanine, if present, strengthens the likelihood of coenurosis [74].
The diagnosis is established by surgical removal and pathologic examination of the excised coenurus. There are no serologic tests.
The differential diagnosis includes larval cysts that develop from the pork tapeworm T. solium to cause cysticercosis, and larval cysts from echinococcal species that develop to cause hydatid cysts. (See "Cysticercosis: Clinical manifestations and diagnosis" and "Echinococcosis: Clinical manifestations and diagnosis".)
Coenuri are larger than T. solium lesions but may be difficult to distinguish from racemose T. solium cysticercosis [66]. CNS coenuri cysts are rarely mistaken for hydatid cysts [75].
Treatment — Management consists of surgical excision of the coenuri. Praziquantel has been employed in isolated cases, but the dosing of this agent for coenurosis is undefined.
SUMMARY AND RECOMMENDATIONS
●Human intestinal tapeworms – Tapeworms (cestodes) are flat, hermaphroditic worms that can live as parasites in the human gastrointestinal tract. Some of these organisms are primarily human pathogens, while others have animals as their natural hosts but can also cause human infection. (See 'Introduction' above.)
•Taeniasis – Taenia saginata occurs worldwide but is most common in areas where consumption of undercooked beef is customary, such as Europe and parts of Asia (figure 1). Most human carriers of adult tapeworms are asymptomatic; there may be associated symptoms, including nausea, anorexia, or epigastric pain, and a peripheral eosinophilia may be observed. The diagnosis is generally established by identifying eggs or proglottids in the stool (picture 1). (See 'Taeniasis' above.)
•Diphyllobothriasis (see 'Diphyllobothriasis' above):
-Dibothriocephalus latus, Dibothriocephalus nihonkaiensis, and Adenocephalus pacificus are the most common species of fish tapeworm worldwide. The diagnosis is generally established by identifying eggs or proglottids in the stool (picture 5).
-D. latus infection can be transmitted by consumption of a variety of raw freshwater fish (figure 2). Megaloblastic anemia due to vitamin B12 deficiency is a classical manifestation of infection.
•Hymenolepiasis – Hymenolepis nana can be transmitted between humans in the absence of an intermediate host or via an alternative life cycle including rodents and arthropods (figure 4 and figure 3). It is associated with poor sanitation and is prevalent worldwide. Infection is most common in children. Most infections are asymptomatic, but heavy infection may be associated with crampy abdominal pain, diarrhea, anorexia, and pruritus ani. The diagnosis is generally established by identifying eggs or proglottids in the stool (picture 9 and picture 8). (See 'Hymenolepiasis' above.)
•Treatment – We suggest praziquantel for treatment of infection due to T. saginata, D. latus, and H. nana (Grade 2C); dosing depends on the species. Praziquantel should be used with caution in areas endemic for Taenia solium. Niclosamide is an acceptable alternative agent. Following treatment, stools should be rechecked for eggs to document cure. (See 'Treatment' above.)
●Animal tapeworms capable of causing human disease
•Intestinal disease – Rare causes of human intestinal disease caused by animal tapeworms include Hymenolepis diminuta (a rat tapeworm), and Dipylidium caninum (a dog tapeworm). The treatment approach is the same as for human intestinal tapeworms. (See 'Causes of human intestinal disease' above.)
•Extraintestinal disease – Animal tapeworms can cause human extraintestinal disease. Sparganosis and coenurosis are zoonotic infections and in humans often present as inflammatory masses. The diagnosis and treatment usually consist of surgical excision; praziquantel may also have efficacy. (See 'Causes of human extraintestinal disease' above.)
