Sleuths Track the Origin of Tapeworms in Humans
October 22, 2001
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
The finding was made by Agricultural
Research Service zoologist Eric P. Hoberg of the
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 the Proceedings of the Royal
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 tapeworms 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 parasites geographic
distribution, its potential for causing disease and other factors.
Now, Hobergs 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
Agricultures chief scientific research agency.