Cysticercosis: Epidemiology, transmission, and prevention

INTRODUCTION — Cysticercosis is caused by the larval stage of the tapeworm Taenia solium; clinical syndromes include neurocysticercosis and extraneural cysticercosis [1]. In endemic areas, neurocysticercosis is an important cause of adult-onset seizures [2,3].

The epidemiology and transmission of cysticercosis will be reviewed here. The clinical manifestations, diagnosis, treatment, and prevention of cysticercosis and the life cycle of T. solium are discussed separately. (See "Cysticercosis: Clinical manifestations and diagnosis" and "Cysticercosis: Treatment" and "Tapeworm infections".)

EPIDEMIOLOGY — Cysticercosis is endemic in many regions of Central and South America, sub-Saharan Africa, India, and Asia [4-10]. Data on the global prevalence of neurocysticercosis are limited. There are a few population-based studies on prevalence incorporating neuroimaging studies [10-14]. In endemic villages, approximately 10 to 20 percent of individuals have evidence of neurocysticercosis on computed tomography scans, primarily in the form of calcified lesions [15-17]. In countries where cysticercosis is endemic, the prevalence is often higher in rural or periurban areas where pigs are raised and sanitary conditions are suboptimal [12,13]. In some such communities, the rate of epilepsy approaches 3 percent, approximately 30 percent of these cases have evidence of cysticercosis [12-14].

Individuals with cysticercosis also present for medical attention outside of endemic areas, particularly where there are significant numbers of immigrants [9,18-23]. A review of a national database estimated that there were more than 18,000 hospitalizations for neurocysticercosis in the United States between 2003 and 2012 [21]. The incidence of neurocysticercosis may be twice the estimates based on inpatient data; in a review of an Oregon state database including 125 patients with neurocysticercosis, 56 percent were seen as outpatients only [24]. In a prospective study including 1800 patients with seizures admitted to 11 emergency departments in the United States over a two-year period, neurocysticercosis was the etiologic agent in about 2 percent of cases [19]. Similar data were generated from a retrospective study in Houston [18].

Individuals with no history of pork consumption or travel to endemic areas can also develop neurocysticercosis. In a report of four cases in an Orthodox Jewish community (whose dietary laws strictly prohibit pork consumption), infection was transmitted by domestic workers from Latin American countries where T. solium is endemic [25]. Similarly, cases in Kuwait (a nonendemic country) have also been linked to tapeworm carriers from endemic regions [23]. Epidemiologic studies have demonstrated tight clustering in households; household contacts of patients with neurocysticercosis have a threefold higher risk of positive serology for cysticercosis compared with controls [26].

LIFE CYCLE AND TRANSMISSION — Cysticercosis is transmitted by ingestion of T. solium eggs shed in the stool of a human tapeworm carrier (figure 1) [5,6]. Pigs acquire infection through ingestion of proglottids, infected human feces, or food contaminated by infected human feces. Rates of transmission are high in rural communities where pigs roam free and human fecal contamination of soil is common.

Following ingestion, embryos (oncospheres) hatch in the small intestine, invade the bowel wall, and disseminate hematogenously to brain, striated muscles, liver, and/or other tissues. Over a period of three to eight weeks, tissue cysticerci develop; these consist of membranous walls filled with fluid and an invaginated scolex.

Humans develop T. solium tapeworms after ingesting undercooked pork containing cysticerci in muscle tissue [5,6]. Once ingested, the scolex evaginates and attaches to the human small intestine by its suckers and hooks. Proglottids (segments) arise from the base of the scolex, gradually enlarge and are displaced from the neck by new proglottids. The proglottids mature over a period of two to four months. Adult tapeworms can reside in the small intestine for years; they may develop up to 7 m in length with each proglottid segment containing 50,000 to 100,000 eggs. The proglottids or eggs are infectious for pigs.

Humans can also be infected by ingestion of the ova from a tapeworm carrier. After ingestion, the ova hatch and larvae are distributed via the blood stream to a variety of tissues. Cysts located in the brain result in neurocysticercosis; humans with cysticercosis are incidental dead-end hosts. For immigrants from an endemic to a nonendemic area, the median period between immigration and symptom onset is 3.5 years [18,27].

A common misconception is that cysticercosis is acquired directly by eating pork. However, as the life cycle illustrates, ingestion of infected pork only causes adult tapeworm infestation (taeniasis); infected pork contains the larval cysts that develop into adult worms in the human intestine, but does not contain the eggs that cause cysticercosis (figure 1). Some patients with cysticercosis are tapeworm carriers, but most individuals with intestinal tapeworm infection do not develop symptomatic cysticercosis [26]. However, human tapeworm carriers are at risk for fecal-oral autoinoculation of eggs and subsequent development of cysticercosis.

Transmission of cysticercosis was previously thought to occur by indirect means, such as by the ingestion of produce irrigated with water contaminated with human feces containing T. solium eggs. However, epidemiologic evidence suggests that the most common source of infective eggs is an asymptomatic household tapeworm carrier [5,6,26]. Therefore, cysticercosis should be viewed as a disease largely transmitted from person to person, with infected pigs as perpetuators of infection.

The Taenia life cycle is discussed in further detail separately. (See "Tapeworm infections", section on 'Taeniasis'.)

PREVENTION — There are several stages of transmission that can be targeted, and eradication programs likely require implementation of all of these strategies [6].

Tapeworms in humans — Preventing human tapeworm infection is important for reducing the reservoir of egg carriers and can be done by eliminating human consumption of pork contaminated with viable cysticerci. Methods include:

●Inspection of pork for cysticerci, which are visible in raw meat ("measly meat").

●Freezing or adequately cooking meat to destroy cysticerci; pickling and salting are not adequate techniques. These methods have been successful in countries with appropriate resources but not in resource-limited settings in which inspection can be bypassed.

Cysticercosis in pigs — Preventing the transmission of cysticercal infections to pigs can be accomplished by eliminating access of pigs to human fecal material. Methods include improving sanitary conditions for proper disposal of human waste (using latrines or sewage systems) and confining pigs so they are not allowed to roam freely in areas with human waste [28]. However, cost and local taboos are barriers to improving sanitation in the poorest areas [29,30].

Reducing transmission — Mechanisms for interrupting the transmission of eggs between humans include:

●Community education regarding routes of transmission

●Good personal hygiene and hand washing prior to food preparation

●Targeted treatment for human tapeworm carriers (perhaps identified by history of proglottid passage)

●Mass community anthelminthic programs to treat tapeworm carriers

The treatments of choice for mass chemotherapy of T. solium tapeworms are niclosamide or praziquantel [31]:

●Niclosamide dosing for adults consists of 2 g (single dose; four chewable tablets); dosing for children >34 kg consists of 1.5 g and dosing for children 11 and 34 kg consists of 1 g. Niclosamide is no longer marketed in the United States.

●Praziquantel dosing consists of 10 mg/kg. There is a low risk of seizures in patients with occult neurocysticercosis who are treated with praziquantel; this may be mitigated by screening [31,32].

Vaccination — No vaccine has been studied in humans.

A recombinant vaccine has proven highly effective in preventing porcine cysticercosis and is licensed in India. Vaccination has proved critical in programs to eliminate cysticercosis in Peru, Zambia, and Laos [32-34]. Since pigs may be infected before they can respond to the vaccine, vaccine is usually combined with treatment for pre-existing infection (usually with oxfendazole).

Mathematical models suggest that eradication of T. solium may be best achieved by a combination of pig vaccination, mass chemotherapy for pigs, and mass chemotherapy for human tapeworm carriers [6,35], which have been confirmed in field studies from South America, Africa, and Asia [32-34]. However, efforts focused around pigs may be as effective as untargeted approaches [36].

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: Cysticercosis".)

SUMMARY

●Cysticercosis is caused by the larval stage of the tapeworm Taenia solium. (See 'Introduction' above.)

●Cysticercosis is endemic in many regions of Central and South America, sub-Saharan Africa, India, and Asia. Among patients with seizures in endemic areas, it is estimated that approximately 30 percent of patients have cysticercosis infection. The prevalence of cysticercosis varies and is often higher in rural or periurban areas, especially where pigs are raised and where sanitary conditions are suboptimal. (See 'Epidemiology' above.)

●The most common source of infective eggs is an asymptomatic household tapeworm carrier. Household contacts of patients with neurocysticercosis have a threefold higher risk of positive serology for cysticercosis in comparison with controls. (See 'Epidemiology' above.)

●Cysticercosis is transmitted by ingestion of T. solium eggs shed in the stool of a human tapeworm carrier. Following ingestion, embryos (oncospheres) hatch in the small intestine, invade the bowel wall, and disseminate hematogenously to brain, muscles, and/or other tissues. (See 'Life cycle and transmission' above.)

●Humans become T. solium tapeworm carriers by ingesting undercooked pork containing cysticerci in muscle tissue. A small minority of patients with cysticercosis are tapeworm carriers, and most individuals with intestinal tapeworm infection do not develop symptomatic cysticercosis. (See 'Life cycle and transmission' above.)

●A common misconception is that one can acquire neurocysticercosis directly by eating pork. However, ingestion of infected pork causes adult tapeworm infestation (taeniasis) but not cysticercosis. This is because infected pork contains the larval cysts that develop into adult worms in human intestine but does not contain the eggs that cause cysticercosis. (See 'Life cycle and transmission' above.)

●Eradication programs will likely require implementation of strategies to reduce transmission at several stages. In endemic countries, the main tools include vaccination and mass treatment to eliminate human tapeworms. Other measures include preventing human tapeworm infection due to pork consumption, improving sanitary conditions to prevent transmission of cysticercal infections from humans to pigs, and measures to interrupt transmission of eggs between humans. (See 'Prevention' above.)