Toxoplasmosis in Deer: Feral Cats Spread the Parasite



Hunters are often concerned about feral hogs impacting a local deer herd, but feral cats? Well… it turns out that free-ranging house cats are doing more out in the woods than just killing mice and raiding songbird nests. Feral cats are also spreading parasites to white-tailed deer, other animals and maybe even to you. The problem is toxoplasmosis, a parasitic disease caused by the protozoan Toxoplasma gondii. Will cats infected with the parasite impact deer hunting in your area? Probably not, for a number of different reasons, but it’s important to pay attention if you eat venison since this disease is prevalent in many areas.

It’s currently estimated that more than 60 million people in the U.S. have the Toxoplasma parasite in them. Most folks will not get sick, though the parasite can cause serious problems for some people, such as those with weakened immune systems and babies whose mothers become infected for the first time during pregnancy. Problems stemming from an infection can include damage to the brain, eyes, and other organs; serious stuff.

Parasites in White-tailed Deer: Disease Spread by Feral Cats

Source: “This study documents the widespread infection of deer populations in northeastern Ohio, most likely resulting from feral cats, and highlights the need for consumers of venison to make absolutely certain that any deer meat planned for consumption is thoroughly and properly cooked,” said Gregory Ballash of the Department of Veterinary Preventive Medicine at Ohio State University and lead author of the study.

Two hundred free-roaming cats and 444 white-tailed deer were tested for the parasite Toxoplasma gondii, which causes toxoplasmosis. Almost 60 percent (261) of the deer showed evidence of infection and more than 65 percent (164) of the studied cats tested positive.


According to the report, approximately 14 percent of the United States’ human population is infected with toxoplasmosis by the age of 40, with an estimated 1 million new cases diagnosed each year. Cats, both domestic and wild (such as bobcats), play a critical role in the spread of toxoplasmosis because they serve as
the definitive hosts, fulfilling the requirements needed for the parasite to sexually reproduce and complete its life cycle.

This parasitic disease is not just limited to deer in Ohio. Similar estimates for white-tailed deer infections have been found in Iowa (53.5 -64.2 percent), Pennsylvania (60 percent), and Mississippi (46.5 percent), suggesting Toxoplasma gondii is thriving entirely because of feral cats. The domestic cat (Felis catus) is not native to North America, but free-ranging feral populations are often abundant at the fringes of urban/suburban areas, where deer are found and hunting takes place.

The odds of deer from urban locations testing positive for toxoplasmosis were nearly three times those of deer from more rural areas. The study found that densities of human households, and likely cats, were a significant predictor of infection in deer. Since the parasite that causes this disease has the ability to impact a fetus during gestation, there is potential for this protozoan to impact a deer population on a local level.

If you hunt suburban properties or in places with high numbers of feral cats, consider cooking that steak well done next time you fire up the grill. Otherwise, the Center for Disease Control says you can freeze venison for several days at sub-zero (0° F) temperatures before cooking to greatly reduce chance of infection.

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