Taenia

Taenia

Also called tapeworm or cestodes, three species cause human infections- Taenia saginata (beef tapeworm), Taenia solium (pork tapeworm) and Taenia asiatica (Asian tapeworm). Eating raw or undercooked beef or pork causes the infestation. In the USA T.solium is more common and is seen in Latin American immigrants.

Pathogenesis: Taenia in the cysticercus stage is ingested by humans. It develops in the human intestines into adult tapeworms. They may grow up to several metres long. The adults produce proglottids (segments containing eggs) which mature, become gravid, detach from the tapeworm and migrate to the anus or are passed in the stool. Eggs are released from the proglottids and they infect cattle or pigs.

Clinical features: Most people are asymptomatic. Tapeworms can cause digestive problems including abdominal pain, loss of appetite, weight loss, and upset stomach. The most visible symptom of taeniasis is the active passing of proglottids (tapeworm segments) through the anus and in the feces. In rare cases, tapeworm segments become lodged in the appendix or the bile and pancreatic ducts causing obstructive symptoms. T.solium (NOT T.saginata) can cause cysticercosis.

Cysticercosis: Caused by ingestion of T.solium eggs. [Very important to understand that the mode of infection in intestinal taeniasis was different …What was it?]. People do not get cysticercosis by eating undercooked pork. The eggs develop into larval cysts which then infect the brain (neurocysticercosis), muscle, eyes or other tissues. Symptoms will depend on the location of cysts. Neural form presents as seizures, headaches, confusion, stroke etc. Muscle disease may present as tender subcutaneous lumps.

Section of human brain tissue with a cysticercus (H&E stained). The scolex (arrow) and bladder wall (darts) are indicated.

Diagnosis of Taeniasis: Laboratory diagnosis is by demonstration of tapeworm eggs in stool samples. There may be a history of passing tapeworm segments in the stool. Antigen detection is by ELISA while antibody by ELISA, indirect haemagglutination and indirect immunofluorescence. PCR and DNA probes can be used for molecular diagnosis. Neurocysticercosis is diagnosed by ELISA and csf Immunoblot for antibodies as well as antigen in csf by ELISA. Tissue biopsy will show cysticerci. CT scan and MRI will show solitary or multiple calcified intracranial lesions/ cysts.

Unstained Taenia sp. egg, teased from a proglottid of an adult. Four hooks can easily be seen in this image.

Diphyllobothrium latum

It is the largest tapeworm infecting humans. It is related to fish tapeworm. Infection is acquired by the ingestion of raw or undercooked fish. It is killed by freezing and cooking. Most infections are asymptomatic. It may cause intestinal obstruction and gallbladder disease by migration of proglottids. Vitamin B 12 deficiency may occur. Diagnosis is by identification of eggs and tapeworm segments in stool samples.

Sections of diphyllobothriid gravid proglottids containing eggs (arrow) in intestinal tissue, stained with H&E.

Echinococcus

It is associated with the formation of unilocular or multilocular cysts in visceral organs. The cyst is filled with clear fluid, numerous brood capsules and protoscolices (which may settle at the bottom called hydatid sand). This tapeworm needs two hosts to complete its life cycle- mainly dogs and sheep. Infection with larval stage of E. granulosus causes cystic echinococcosis while alveolar echinococcosis is caused by the larval stage of E.multilocularis. Human infection is caused by the ingestion of food and water contaminated with tapeworm eggs. People who raise sheep are at high risk.

Close-up of the scolex of E. granulosus. In this focal plane, one of the suckers is clearly visible, as is the ring of rostellar hooks.

Echinococcus granulosus infections often remain asymptomatic for years before the cysts grow large enough to cause symptoms in the affected organs. Hydatid cysts develop in the liver, lungs, spleen, kidneys, CNs etc. Rupture of the cysts can produce a host reaction manifesting as fever, urticaria, eosinophilia and anaphylactic shock; rupture of the cyst may also lead to cyst dissemination. The presence of a cyst-like mass in a person with a history of exposure to sheep or dogs in an area where E. granulosus is endemic suggests a diagnosis of cystic echinococcosis. Imaging techniques, such as CT scans, ultrasonography, and MRIs, are used to detect cysts. After a cyst has been detected, serologic tests may be used to confirm the diagnosis. Remember do not pick stool exam as an answer for diagnosis of Echinococcosis!

Protoscolices liberated from a hydatid cyst. The two protoscolices on the right side of the image are evaginated.

Cross-section of an E. granulosus cyst, stained with H&E. Host tissue (A) encapsulates the hydatid cyst wall, which is composed of an acellular laminated layer (B) and a nucleate germinal layer © from which the brood capsule (D) arises. Inside the brood capsule are numerous protoscolices (E) with visible hooklets (F).

Echinococcus multilocularis affects the liver as a slow growing, destructive tumor, often with abdominal pain and biliary obstruction being the only manifestations evident in early infection. This may be misdiagnosed as liver cancer. Rarely, metastatic lesions into the lungs, spleen, and brain occur. Untreated infections have a high fatality rate. Alveolar echinococcosis is typically found in older people. Imaging techniques such as CT scans are used to visually confirm the parasitic vesicles and cyst-like structures and serologic tests can confirm the parasitic infection.