Human tapeworm infections

Tapeworms are flat, segmented worms belonging to the class Cestoda. The life cycle of this type of parasite involves two hosts. The definitive host carries the adult form of the worm in their intestines. This adult holds itself in place, within the intestines of its host by suckers and/or hooks present on its head or scolex. An intermediate host carries the larval form, which lodges in various tissues of the body including muscle and brain. Secondary hosts are infected with the larval form by accident but do not pass on the larval form to the definitive host and are therefore a dead end for the parasite’s life cycle. The definitive host normally acquires the infection by ingestion of meat from an infected intermediate host.

The smallest of the tapeworms using man as its definitive host is the dwarf tapeworm Hymenolepis species. Two species of Hymenolepis infect man H. nana and H. diminuta. These both use beetles as their intermediate host and man becomes infected by ingesting food or water contaminated by infected beetles. Rats also act as definitive hosts to H. species and beetles become infected by ingesting rat feces. H. nana is unusual in that, in addition to using beetles as an intermediate host, it can complete its life cycle within man without using an intermediate host.

Many cases of H. species infection are asymptomatic. A patient who develops symptoms may experience loss of appetite, nausea, diarrhea, weakness and abdominal pain.

Taenia saginata or the beef tapeworm uses cattle as intermediate hosts. Man becomes infected with T. saginata by eating raw or undercooked beef from an infected animal. Adult T. saginata worms grow to between 12 and 25 feet in length.  As with the dwarf tapeworm asymptomatic infections with T. saginata are common.  Symptomatic cases of beef tapeworm infection show a similar picture to Hymenolepis infections. In addition, the patient sometimes shows evidence of malnutrition and malabsorption.

The other Taenia species tapeworm that infects man is T. solium or the pork tapeworm. Spread by the ingesting of undercooked infected pork the adult worm grows from three to six feet in length. Symptoms of T. solium infection are similar to those of T. saginata. Asymptomatic cases are also common with T. solium.

In addition to infecting humans with the adult worm T. solium larvae also infect man. Infection with the larval form of T. solium occurs if a human ingests the eggs from a source contaminated by human feces. The larval form burrows through the intestinal wall and circulates around the body. Eventually the larvae lodge in a number of tissues where they form cysts. This type of T. solium infection is known as cysticercosis. Cyst formation in most tissues is asymptomatic. Where cysts develop in the brain the patient develops neurocysticercosis. While many cases of neurocysticercosis are asymptomatic, some patients do develop symptoms including seizure, stroke and hydrocephalus. Less dramatic symptoms of neurocysticercosis include headache, nausea, vomiting and vision disturbances. Patients with neurocysticercosis sometimes exhibit changes in their mental status.

The fish tapeworm Diphyllobothrium latum infects humans who eat raw or undercooked fish such as turbot, pike or salmon. This parasite can grow to 30 feet in length but normally measures three to seven feet in long. Like all the other tapeworms, asymptomatic infections are common with D. latum. In symptomatic patients symptoms resembling those of Taenia infections occur. The large tapeworm occasionally blocks the intestine. Such blockages are potentially fatal and require immediate medical assistance. D. latum consumes much of the vitamin B12 within its host’s diet. Vitamin B12 is important in the production of hemoglobin so infection with D. latum causes anemia.

Diagnosis of these tapeworm infections requires examination of a stool sample. Occasionally segments of the tapeworm are present in a stool; otherwise, a microscopic examination for the presence of tapeworm eggs is required. Cysticercosis diagnosis requires a blood test looking for specific antibodies.

Echinococcus species are tapeworms that infect a variety of carnivorous hosts such as foxes, wolves, coyotes and cats. Man if infected with the larval form, by eating food contaminated with fecal matter from a definitive host, develops echinococcosis also known as hydatid disease. The cysts, formed by the larval form of the parasite, lodge within human tissues and grow very slowly. Hydatid cysts are most common in the liver but other organs can be affected. While the condition remains asymptomatic for many years, the cyst continues to grow until symptoms start to occur. The most common symptom is pain the upper right quadrant of the abdomen. Rupture of the cyst leads to fatal anaphylactic shock in many undiagnosed cases. A blood test for the presence of antibodies to Echinococcus species provides a diagnosis for cases of echinococcosis.