Taenia saginata, solium, and asiatica are three related species of tapeworms, each with its own lifecycle, mode of transmission, and clinical implications. The adult tapeworms cause few symptoms, but their larvae can be more troublesome. These parasites have afflicted mankind for thousands of years, perhaps when switching from large carnivores to less dangerous hosts such as hominids and their domesticated animals.

Taenia saginata, the beef tapeworm, is the most common of the species worldwide, infecting an estimated 40–60 million humans. It lives in their small intestines for two to three years, growing to about four to twelve meters but occasionally to twenty-five meters in length. Infected people usually have no symptoms or only mild abdominal discomfort, nausea, and diarrhea, but some may lose weight or become malnourished. Rarely, the worms have caused pancreatitis or perforated the bowel or gallbladder. Infections with T. saginata are due to eating raw or undercooked beef. Such infections are common in Eastern Europe, Russia, East and South-Saharan Africa, and Latin America. In the United States infections with T. saginata are uncommon, but could be caused by eating rare steak, kebabs, and steak tartare.

An adult tapeworm consists of a head (scolex), a neck, and a strobila, which is a chain of as many as 1,000 to 2,000 segments called proglottids. The proglottids near the neck of the worm are immature, but those closer to the tail produce thousands of eggs. Proglottids released into feces are eaten by cattle and undergo a specific reproductive circle. They transform themselves into invasive larval creatures called oncospheres that have six hooks, penetrate their host’s intestinal wall, and migrate to their striated muscles. There, they develop into different creatures called cysticerci or bladder worms, these being a larval stage consisting of a fluid-filled sac and containing an invaginated scolex or head with hooks. These cysticerci in turn are eaten by humans, establish themselves in their intestines, growing into adult tapeworms and thus completing the reproductive cycle.

In northern Russia, a morphologically indistinguishable variant of T. saginata, is found in the cerebral meninges of reindeer. Its unique life cycle is dependent on a native custom of eating raw reindeer brain.¹

Taenia solium, the pork tapeworm, has a similar lifecycle but the pig is its intermediate host. Humans become infected by consuming undercooked pork containing cysticerci or through the fecal-oral route by ingesting T. solium eggs directly. Taeniasis due to T. solium is usually asymptomatic or mild. It can, however, be dangerous because its eggs, rather than being excreted, can hatch larvae that migrate to the host’s brain, muscles, and eyes, forming cysts so that the disease is now no longer called taeniasis but rather cysticercosis. In its most severe form, neurocysticercosis, the cysts form in the brain, causing headaches, seizures, or even more severe neurological problems.

Taenia asiatica is found mainly in Asia, Korea, China, Taiwan, Indonesia, and Thailand. Its lifecycle is similar to that of T. saginata and T. solium. Its intermediate host is usually the pig, and it infects humans who eat undercooked pig meat and its cysticerci. These develop into adult tapeworms in the intestine, and release in their feces eggs that are then eaten by pigs. Infections with T. asiatica are asymptomatic or cause only mild gastrointestinal symptoms and are less dangerous than those with T. solium.

While T. saginata, T. solium, and T. asiatica share similarities in their lifecycles and modes of transmission, they differ in their clinical implications and geographical distribution. T. saginata and T. solium are found worldwide, whereas T. asiatica is confined to Asia. T. solium poses the greatest health risk because it can cause cysticercosis, particularly neurocysticercosis. Controlling these diseases requires improved sanitation and hygiene, proper cooking of meat, and health education. In endemic areas, regular deworming programs and monitoring of livestock are crucial, as are ensuring safe meat handling practices.

Adult worms can be eradicated with specific drugs such as praziquantel or niclosamide. Treatment of symptomatic neurocysticercosis is more complicated; it includes corticosteroids, antiseizure drugs, and, in some situations, albendazole or praziquantel. In rare situations surgery may be required.

Reference

1. Konyaev SV, Nakao M, Ito A, and Lvikainen A. History of Taenia saginata Tapeworms in Northern Russia. Emerg Infect Dis. 2017;23(12):2030.