- Author: Sowmya Nanjappa, MD; Chief Editor: Russell W Steele, MD more...
Of the 32 recognized species of Taenia, only Taenia solium and Taenia saginata are medically important. However, recent epidemiologic studies in Southeast Asia have identified a third Taenia species in humans, known as T asiatica.[1, 2] Cysticercosis is the development of extraintestinal encysted larval forms of T solium in various organs (see Cysticercosis). The CNS is involved in 60-90% of cases; this condition is termed neurocysticercosis (NCC). For more information, see Neurocysticercosis.
Cysticercosis caused by T saginata (also called the cattle or beef tapeworm) is rare; T saginata has far lower impact on human health than T solium. Differentiating between T solium and T saginata infections is important because both infections are endemic in Southeast Asia, Africa, Europe, and Central and South America. Infection in children usually goes unrecognized.
Adult tapeworms live in the human small intestine. Humans pass gravid eggs in feces; these mature eggs contaminate pastures and barnyards, where cattle and pigs ingest them. Upon reaching the alimentary canal of infected animals, the embryos are released, penetrate the gut wall, and enter the circulation. The embryos filter from the circulation and encyst in muscular tissue. Larvae (ie, cysticerci) become infectious within 2-3 months. Humans develop a tapeworm infection by eating raw or undercooked beef or pork. The cysticercus becomes activated, attaches to the wall of the small intestine by the scolex, and becomes a mature tapeworm. This maturation process takes 10-12 weeks for T saginata and 5-12 weeks for T solium. A single tapeworm produces an average of 50,000 eggs per day and may live 25 years.
Humans can also act as an intermediate host for T solium. Cysticercosis results from human ingestion of T solium eggs through fecal contamination, reverse peristalsis of gravid proglottids, or autoinfection. The cysticerci may develop in any organ, and their effects depend entirely on the location of the cysticerci.
A coenurus is the larval stage of Taenia multiceps, Taenia serialis, and Taenia brauni. Adult tapeworms develop in dogs or other canids that ingest coenurus larvae in the tissues of various intermediate hosts. These hosts include sheep, goats, hares, rabbits, and other herbivores for T multiceps; hares, rabbits, and other rodents for T serialis; and gerbils for T brauni. Each protoscolex within a coenurus can mature into an adult tapeworm after ingestion by a canid host. Adult worms produce eggs, which are passed in feces; these eggs are morphologically similar to taeniid eggs. Ingestion of eggs by an appropriate intermediate host or by humans leads to development of coenurus. Coenuri are cysts that contain many protoscolices attached in rows on the internal membrane of the cyst.
Cysticercosis is primarily an imported disease. Approximately 1000 cases are diagnosed each year. Most occur in persons who have immigrated, primarily from Latin America. Cysticercosis has also developed following close contact with recently immigrated, infected individuals. Although some patients with NCC are born in the United States, many have traveled to rural areas in countries where the condition is endemic. Locally acquired infections have been confirmed in Los Angeles, New York, Chicago, Oregon,[5, 6] and elsewhere. Although T saginata infection occurs worldwide, prevalence in the United States is less than 1% because most US cattle are free of the parasite.
A study analyzed in-patient records using the Nationwide Inpatient Sample for 1998-2011 to estimate cysticercosis-related hospitalizations and patient/institutional characteristics in the United States. The study reported that there were an estimated 33,060 cysticercosis-related hospitalizations nationwide, representing a hospitalization rate of 8.03 per million population. The study further reported that the highest proportion of cases were male (54.8%), Hispanic (62.0%), aged 18-44 (58.8%), and occurred in the West (45.1%). An estimated 459 deaths occurred, representing an in-hospital case-fatality rate of 1.4%.
Approximately 50 million people worldwide are infected by T saginata or T solium. Approximately 50,000 people die annually of cysticercosis. T saginata is common in cattle-breeding regions. Areas with the highest (ie, >10%) prevalence are central Asia, the Near East, and central and eastern Africa.[8, 9, 10] Areas with low (ie, 1%) prevalence are Southeast Asia, Europe, and Central and South America.
T solium is endemic in Central and South America, Southeast Asia,[11, 12, 13] India, the Philippines, Africa, Eastern Europe, and China. Areas of highest prevalence include Latin America and Africa. In some regions of Mexico, prevalence may reach 3.6% of the general population.
T multiceps has been reported in the Americas and parts of Europe and Africa.
T serialis infections occur in the United States and Canada.
T brauni has been reported in Africa.
Most intestinal taeniid infections are asymptomatic. When symptoms occur, they are usually mild and involve abdominal pain, anorexia, weight loss, or malaise. Cysticercosis causes a mass effect in various vital organs (eg, brain, eye, heart). The mortality rate for cysticercosis is low and is generally caused by complications such as encephalitis, increased intracranial pressure secondary to edema and/or hydrocephalus, and stroke.
All races are equally affected.
Both genders are equally affected.
All ages are susceptible to infection. The age at which raw meat consumption begins is the primary determinant. T solium taeniasis has been reported in children older than 2 years in certain rural communities of Mexico.
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