Scientific Sleuths Track the Origin of Tapeworms in Humans

An international scientific team has uncovered new genealogical evidence contradicting a long-held scientific theory about tapeworms. The new evidence shows that humans actually infected cattle and pigs with tapeworms, rather than the animals spreading the parasites to humans.

The finding was made by Agricultural Research Service zoologist Eric P. Hoberg of the Parasite Biology, Epidemiology and Systematics Laboratory at Beltsville, Md.; Alan de Queiroz and Nancy L. Alkire at the University of Colorado-Boulder; and Arlene Jones at The Natural History Museum in London. Their research was published earlier this year in theProceedings of the Royal Society, London.

The tapeworms, called Taenia, range in length from about 0.04 inch to more than 50 feet. They are internal parasites that infect humans and other mammals. The tapeworm’s complex life cycle requires it to live first in an herbivore, and then in a carnivore where it reproduces.

Tapeworms have global impact because of both the sickness and death they cause in humans and domestic livestock, and the loss of food resources incurred through condemnation of infected meat. But little has been known about the origin and history of tapeworms in humans, the parasite’s geographic distribution, its potential for causing disease and other factors.

Now, Hoberg’s team has produced scientific evidence indicating that before the origin of modern humans, African hominids became infected with tapeworms when they scavenged for meat--left behind by hyenas and large cats--that already harbored the parasite. So, those early hominids acquired Taenia tapeworms long before cattle and swine were domesticated. With the advent of animal domestication, these tapeworms were transferred to our primary food animals.

Eventually, evolution among different Taenia species associated with those ancestors of modern humans resulted in three distinct tapeworm species that infected only humans. These species then were shared with domesticated cattle and swine as their initial herbivore hosts, and with humans as their final host for reproduction.

ARS is the U.S. Department of Agriculture’s chief scientific research agency.