The larval form (cysticercus) of the pork tapeworm, Taenia solium.
Cysticercosis is an infection of both humans and pigs with the larval stages of the parasitic cestode, Taenia solium. This infection is caused by ingestion of eggs shed in the feces of a human tapeworm carrier. Pigs and humans become infected by ingesting eggs or gravid proglottids, . Humans are infected either by ingestion of food contaminated with feces, or by autoinfection. In the latter case, a human infected with adult T. solium can ingest eggs produced by that tapeworm, either through fecal contamination or, possibly, from proglottids carried into the stomach by reverse peristalsis. Once eggs are ingested, oncospheres hatch in the intestine, invade the intestinal wall, and migrate to striated muscles, as well as the brain, liver, and other tissues, where they develop into cysticerci . In humans, cysts can cause serious sequellae if they localize in the brain, resulting in neurocysticercosis. The parasite life cycle is completed, resulting in human tapeworm infection, when humans ingest undercooked pork containing cysticerci . Cysts evaginate and attach to the small intestine by their scolex. Adult tapeworms develop, (up to 2 to 7 m in length and produce less than 1000 proglottids, each with approximately 50,000 eggs) and reside in the small intestine for years.
Taenia solium is found worldwide. Because pigs are intermediate hosts of the parasite, completion of the life cycle occurs in regions where humans live in close contact with pigs and eat undercooked pork. Taeniasis and cysticercosis are very rare in Muslim countries. It is important to note that human cysticercosis is acquired by ingesting T. solium eggs shed in the feces of a human T. solium tapeworm carrier, and thus can occur in populations that neither eat pork nor share environments with pigs.
The symptoms of cysticercosis are caused by the development of cysticerci in various sites. Of greatest concern is cerebral cysticercosis (or neurocysticercosis), which can cause diverse manifestations including seizures, mental disturbances, focal neurologic deficits, and signs of space-occupying intracerebral lesions. Death can occur suddenly. Extracerebral cysticercosis can cause ocular, cardiac, or spinal lesions with associated symptoms. Asymptomatic subcutaneous nodules and calcified intramuscular nodules can be encountered.
Larval Taenia solium.
The definitive diagnosis consists of demonstrating the cysticercus in the tissue involved. Demonstration of Taenia solium eggs and proglottids in the feces diagnoses taeniasis and not cysticercosis. Persons who are found to have eggs or proglottids in their feces should be evaluated serologically since autoinfection, resulting in cysticercosis, can occur.
Typical antibody reactions in CDC’s immunoblot for cysticercosis. Individual sera from patients with either cysticercosis were analyzed using the CDC immunoblot for cysticercosis. Cysticercosis-specific antibodies react with glycoproteins derived from T. solium cysts. The relative molecular masses (Mr) of the seven diagnostic glycoproteins are expressed in kDa and marked with arrows. A positive result is a reaction with any one of the seven cysticercosis-specific proteins.
CDC’s immunoblot assay with purified Taenia solium antigens has been acknowledged by the World Health Organization and the Pan American Health Organization as the immunodiagnostic test of choice for confirming a clinical and radiologic presumptive diagnosis of neurocysticercosis. CDC’s immunoblot is based on detection of antibodies to one or more of 7 lentil-lectin glycoprotein antigens from the cysticerci of T. solium. In cases where two or more cysts are present, this assay is very sensitive, 100% and 95%, using serum or CSF, respectively, and is essentially 100% specific for either sample. The most important factors contributing to positive immunoblot reactions are the numbers and stage of cysticerci. Cumulative clinical experience has confirmed that in patients with multiple (more than two) lesions, the test has more than 95% sensitivity. Seropositivity in biopsy-confirmed patients with single, enhancing parenchymal cysts was < 50 to 70%%. Seropositivity of patients with multiple but only calcified cysts was 82% in serum and 77% in CSF. In all patients, regardless of their clinical presentation, the immunoblot assay is slightly more sensitive in serum than in CSF specimens; consequently, there is no need to obtain CSF solely for use in the immunoblot assay.
CDC’s immunoblot is both more specific and more sensitive than enzyme immunoassays (EIAs). Lack of specificity has been a major problem in most EIAs because of cross-reacting components in crude antigens derived from cysticerci; these components react with antibodies produced in other helminthic infections, especially echinococcosis and filariasis. All positives and any negative strongly suspected of cysticercosis should be confirmed by immunoblot. Currently available antibody detection tests for cysticercosis do not distinguish between active and inactive infections and thus have not been useful in evaluating the outcomes and prognoses of medically treated patients. The CDC immunoblot is commercially available in the United States.
Tests that detect circulating cysticercal antigens in serum and CSF have been developed and may prove to be most useful to follow response to therapy in in subarachnoid and ventricular forms of neurocysticercosis. Antigen levels drop quickly in cured NCC patients, so serum antigen monitoring is useful for assessing treatment and determining of clinical cases. Antigen detection testing is not as sensitive as antibody detection and should not be used to diagnose neurocysticercosis. Antigen testing is available at CDC in specific instances and can be ordered after consultation, which can be arranged by contacting CDC (DPDx).
PCR tests have been developed to detect T. solium DNA in CSF but these are not widely used for clinical laboratory diagnosis of neurocysticercosis.
Rodriguez S, Wilkins P, Dorny P. Immunological and molecular diagnosis of cysticercosis. Pathogens and Global Health 2012; 106:286-298.
DPDx is an educational resource designed for health professionals and laboratory scientists. For an overview including prevention, control, and treatment visit www.cdc.gov/parasites/.