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Don't Blame the Pig for Tapeworms

Turns out humans gave it to domestic animals

TUESDAY, Nov. 20, 2001 (HealthDayNews) --Though scientists have long thought that domestic animals infected ancient humans with the tapeworm, new research shows it was the other way around.

A report in the British publication Proceedings of the Royal Society says African hominids, the ancestors of humans, first picked up tapeworms by sharing food with wild hyenas and cats and then, thousands of years later, passed them on to domesticated cattle and swine, the new main source of food for humans.

After cattle and swine became interim hosts for tapeworms in the larval stage, the parasites also began circulating among humans.

"The cycle was perpetuated simply because of the close proximity between humans and their domestic stock," says Eric Hoberg, head of the research team and curator of the U.S. National Parasite Collection at the Beltsville Agricultural Research Center, one of the largest collections of its kind in the world. "The association between these parasites and hominids is far older than we originally thought."

Based on evolutionary genetic evidence, Hoberg says, "We found certain species in humans more closely related to those in hyenas or large cats, and we suggested that the ancestors of ancient humans may have acquired these parasites as they made the shift from herbivorous to omnivorous, perhaps as early as 2 million years ago," says Hoberg.

Tapeworms of the genus Taenia are parasites ranging in length from 2 to 3 inches to 50 feet that can live in humans and other mammals. Three species infect humans as adult parasites, spending the beginning of their lives in herbivore hosts (animals that eat only plants).

Today, humans usually are infected with adult parasites by eating undercooked pork or beef, but these infections are not the major problem. Humans also can become infected with the larvae of the pork tapeworm (Taenia solium) through contact with the feces of other infected humans. Infections by the larvae (cysticerci) of Taenia solium killed about 50,000 people worldwide in 1992 and severely debilitated those who survived. The larvae of the pork tapeworm, for instance, are one of the most common causes of epilepsy in children in certain parts of the world. Because infected animals often have to be destroyed, tapeworms also cause tremendous economic losses.

The parasites are less of a problem in the United States than in other parts of the world. "We just don't have many of the tapeworms here," says Dr. A. Clinton White, associate professor in the infectious disease section of the department of medicine at Baylor College of Medicine, in Houston. "In a year, in all of the millions of the pigs that are butchered, maybe one to 10 are contaminated. Those are very small numbers, but there are over 1,000 clinical cases of this disease cysticercosis each year, and most of those are imported."

In areas with large immigrant populations, however, tapeworms are beginning to pose problems. One outbreak among Orthodox Jewish children in New York City about a decade ago was traced to housekeepers who had immigrated from other countries, says White.

Hoberg says he hopes his team's genetic findings will pave the way for a new generation of knowledge about the parasite. "The pork tapeworm is unique because it can infect a wide array of mammals as the intermediate host. "We may be able to apply the results of studies in comparative genomics to pinpoint why larvae of certain species of Taenia can infect a diversity of intermediate hosts."

For example, Hoberg says finding out why T. solium -- but not other species of Taenia -- causes debilitating infections in humans could lead to new treatment and prevention methods, though not right away.

White says, "It's an important question, but it's more an interesting biological and historical question. These biological questions often end up with some implications down the road, but they're not near term."

What To Do: Learn everything you ever wanted to know about tapeworms and other parasites at the U.S. National Parasite Collection. And here's a fact sheet from the Centers for Disease Control and Prevention.

SOURCES: Interviews with A. Clinton White, M.D., associate professor, infectious disease section, department of medicine, Baylor College of Medicine, Houston; Eric Hoberg, Ph.D., curator, U.S. National Parasite Collection, Beltsville Agricultural Research Center, Agricultural Research Service, Beltsville, Md.; 2001 Proceedings of the Royal Society, London
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