Northeastern Ohio Deer Populations Show High Rates of Toxoplasmosis Infection

White-tailed Deer, Paul Tessier/Shutterstock

(Washington, D.C., December 23, 2014) A new peer-reviewed study published in the journal EcoHealth has found that a large percentage of the Greater Cleveland area's white-tailed deer—a species widely hunted and harvested by thousands of sportsmen throughout the U.S.—is infected with a parasite associated with feral domestic cats.

“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, 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 one million new cases diagnosed each year. Cats, both domestic and wild (e.g., 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. Domestic cats are often infected at less than one year of age. Once infected, they can contaminate the environment by shedding hundreds of millions of infectious eggs (called “oocysts”) via their feces. Free-roaming cats—those that are allowed free access to the outdoors—are more likely to be exposed and infected, thereby contributing more frequently to environmental contamination than indoor cats.

Similar estimates for white-tailed deer infections have been found in Iowa (53.5 -64.2%), Pennsylvania (60%), Mississippi (46.5%), and Ohio (44%), suggesting widespread environmental contamination.

In the Ohio study, deer samples were collected from six reservations at the Cleveland Metroparks in northeastern Ohio, two suburban reservations (Brecksville and North Chagrin), and four urban reservations (Rocky River, Mill Stream Run, Bedford, and Bradley Woods).

Free-roaming cat samples were collected in conjunction with a local trap, neuter, release (TNR) program in the Greater Cleveland area.

The odds of deer from urban locations testing positive for toxoplasmosis were nearly three times those of deer from suburban reservations when adjusted for age and gender. The study found that densities of human households, and likely cats, were a significant predictor of infection in deer.

According to the study, the “difference between urban and suburban cat densities is the most likely reason for an increased [prevalence of infection] in urban white-tailed deer.” The authors went on to say that “these data have public health implications for individuals living near or visiting urban areas where outdoor cats are abundant as well as those individuals who may consume white-tailed deer venison.”

This latest investigation further validates the large number of studies already raising public health concerns over cat-spread T. gondii. A study published in the scientific journal Brain, Behavior, and Immunity documented what scientists described as “remarkable” working memory performance reductions in seniors 65 and older that test positive for T. gondii infection. A 2013 study published in the journal Trends in Parasitology concluded that “Because cats are now so ubiquitous in the environment, one may become infected by neighboring cats which defecate in one's garden or play area, or by playing in public areas such as parks or school grounds. Indeed, as cats increasingly contaminate public areas with T. gondii oocysts, it will become progressively more difficult to avoid exposure.” The parasite is believed to infect about one-third of the world's population.

Although human infection by the T. gondii parasite is often associated with improperly cooked meat, the staggering growth in the number of free-roaming cats is increasingly viewed as a key transmission vector in the U.S. According to Dr. George Fenwick, a Johns Hopkins-educated pathobiologist who is also President of American Bird Conservancy, “The number of domestic cats in the U.S.—both owned and un-owned—has increased to as many as 188 million. Studies have shown that up to 74 percent of those cats will test positive for this parasite during their lifetime…"

While a T. gondii infection may not show symptoms, dormant cysts in the human brain or elsewhere may still cause lasting, negative effects. T. gondii infections are potentially fatal to immuno-compromised people and may cause spontaneous abortion in pregnant women or birth defects in their children. Some preliminary studies suggest that chronic infection may be linked to schizophrenia or suicidal behavior.

The study was authored by Gregory A. Ballash, Department of Veterinary Preventive Medicine, Ohio State University; J. P. Dubey and O.C.H. Kwok, Animal Parasitic Diseases Laboratory, Beltsville Agricultural Research Center, United States Department of Agriculture, Agricultural Research Service; Abigail B. Shoben, Division of Biostatistics, College of Public Health, Ohio State University; Terry L. Robison and Tom J. Kraft, Department of Planning, Design, and Natural Resources, Cleveland Metroparks; and Patricia M. Dennis, Department of Veterinary Preventive Medicine, Ohio State University and Cleveland Metroparks Zoo.