Discerning Parasites Is Key to Risk Assessment

By Rosalie Bliss
May 10, 2002

The single-celled parasite Toxoplasma gondii and its cousin, Neospora caninum, are responsible for abortion and other health problems in livestock. Millions of infectious-stage oocysts are excreted in the feces of infected animals on or near farmland. When the hardy oocysts come into contact with farm feed, livestock can get infected.

The oocyst stages of the two protozoans are virtually indistinguishable. And until recently, researchers didn’t have the sensitive tests necessary to diagnose or distinguish N. caninum infections from those caused by T. gondii.

Agricultural Research Service scientists in the Parasite Biology, Epidemiology and Systematics Laboratory in Beltsville, Md., have been involved in developing both serological and genetic tests to help distinguish a host of insidious look-alike parasites. The ability to make such distinctions will help researchers define the distribution of single-celled parasites in domestic and wild animals, as well as in people.

Cats are the reservoir of infection for T. gondii. Dogs are the reservoir of infection for N. caninum. Infected dogs and cats excrete the environmentally resistant oocysts in their feces.

ARS scientists are working to discern how many cyst-forming parasitic species exist, how they are related to one another, and how to tell them apart. To do that, they look at variations in genes by genotyping strains of parasites taken from all classes of food animals. The research will lead to a better understanding of the epidemiology of such parasites, which form cysts in the muscles of domesticated animals, wildlife and primates. These include parasites belonging to the genera Neospora, Hammondia, Besnoittia and Sarcocystis.

Read more about defining risk from meatborne parasites in the May issue of Agricultural Research magazine.

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