Cysticercosis is an infection with the larval form of the pork tapeworm, Taenia solium
, which resides in the small intestine of humans (see Box 1
). Neurocysticercosis (NCC), invasion of the nervous system, is a major cause of adult acquired epilepsy and other neurological morbidity in many areas of the world ().[1
] Recent analyses calculated that 29% of epilepsy in endemic regions world wide is consistently caused by NCC, [3
] a value that closely approximates estimates from Latin America.[4
] Determination of prevalence of infection and disease has been estimated in relatively few regions and thus knowledge of the worldwide burden of disease is quite limited.[8
] Overall, 1.7 to 3 million persons are conservatively estimated to suffer from epilepsy due to NCC worldwide.[9
] Infections are common in Central and South America excluding Chile, Uruguay and Argentina, parts of the Caribbean, notably Haiti, India, Indonesia, most of Southeast Asia, part of China, many regions of non Muslim sub Saharan Africa, regions of Eastern Europe and residual transmission in Spain.[10
] Although transmission is absent or rare in the US and most of Europe, NCC is still frequently diagnosed in migrant populations from endemic areas.[16
] Seizures are still the most common manifestation, but a significant number present with complicated subarachnoid or ventricular disease requiring costly, sophisticated and/or prolonged treatments.
BOX 1 - LIFE CYCLE
- Agent: Taenia solium, cestode worm (“pork tapeworm”)
- Location - small intestine of humans, their sole host.
- Mature tapeworms are 2-4 meters long and consist of a head containing 4 suckers and a rostellum of hooklets for grasping, a neck and a series of increasingly mature proglottids. Liberated ova and/or segments of terminal mature proglottids are shed in feces.
- Taeniasis infection: The adult tapeworm develops only in the intestine of a human host, after ingestion of raw or poorly cooked infected pork. Cysts evaginate in the upper small intestine, attach to the mucosa, and develop into adult tapeworms.
- Cysticercosis (larval form) infection: After ingestion by the intermediate host, usually free roaming pigs, the ova hatch, invade the intestines and are then infective embryos are carried by the blood stream throughout the body. Larval cysts mature in 2-3 months. Although cysts may be found in any blood supplied tissue, they are most commonly found in muscles, subcutaneous tissue and brain. Humans develop cysticercosis, like the pig intermediate host, after the accidental ingestion of ova. Because each proglottid contains between 30,000-50,000 ova, the human tapeworm carrier is very infectious, the “typhoid Mary” of parasitology.
Despite impressive gains in diagnosis and availability of antiparasitic treatments, large gaps in both basic and practical knowledge about the parasite and host responses to it exist. Factors hindering further advances are the lack of clinically appropriate model infections, inability to maintain the life cycle experimentally, and the reliance on naturally infected humans as a source of infectious ova. Rodent model infections using other cestodes are useful but generally of limited usefulness. Even though pig infections are helpful, their failure to develop seizures limits their usefulness. Therefore, the most appropriate study of NCC is in humans. These studies are long and difficult, require sophisticated imaging and special tests, large numbers of personal and therefore considerable resources, which compared to other most other neglected diseases have not been forthcoming. It has been relegated along with echinococcosis as neglected neglected parasitic infections.
The tapeworm carrier is the sole source of infection and is likely most at risk and prone to high levels of exposure occasionally resulting in heavy infections including disseminated or encephalitic neurocysticercosis and complicated disease.[20
] Family members and close contacts are also at considerable risk. Contamination of the environment, food and water as a lower level of exposure could account for the high prevalence of single enhancing lesions (SEL) in India.[23