Humans Prompted New Paths For Parasites

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.

Ben Rosenthal, Dante Zarlenga and Detiger Dunams work at the ARS Animal 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 species diversity.

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 similarity.

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. spiralis.