Humans Prompted New Paths for Parasites
Perry November 25, 2008
Agricultural Research Service
(ARS) scientists are tracking how the dissemination of the parasite
Trichinella spiralis throughout Europe, North Africa and the Americas
was facilitated by human travel and the transportation of animals.
T. spiralis lurks in the muscle tissue of a wide range of
mammals and can infect humans who consume undercooked meat contaminated with
the parasite. It is no longer a major threat to the U.S. food supply, but it
does persist in some European countries.
Dunams work at the ARS
Parasitic Diseases Laboratory in Beltsville, Md. They used
Trichinella DNA collected from 28 countries on four continents to
evaluate potential links between parasite hosts, geographic distribution and
Over time, the genetic traits of a pathogen may shift as the pathogen
expands beyond its original range and becomes isolated. Geographic barriers
prevent contact between the new populations, and these barriers support the
development and maintenance of unique genetic mutations within each group.
These mutations, in turn, can be used to trace the links between
individuals in each population. They can also be compared with populations that
have dispersed to other areas.
Although T. spiralis is believed to be at least 20 million
years old, the scientists were surprised to find that parasite samples from
Europe, North Africa and the Americas had remarkably uniform DNAs. In fact,
statistical analyses grouped all 44 samples from all evaluated regions into a
single "Western" group of T. spiralis, due to the high degree of genetic
This evidence suggests that the T. spiralis found in Europe
first evolved after the domestication of swine. Settlers on their way to the
New World and elsewhere traveled with swine for food, and some of these pigs
were infected with T. spiralis.
The team concluded that human travel was the primary source of
disseminating T. spiralis throughout the New World. They also believe
that these migration patterns explain the limited range of genetic diversity
observed in the European, North African and the American isolates of T.
ARS is a scientific research agency of the
U.S. Department of