Free-roaming domestic cats (Felis catus) are widely understood to have substantial negative impacts on wildlife. The International Union for the Conservation of Nature (IUCN) lists cats among the world’s worst non-native invasive species, and cats on islands worldwide have contributed to 33 species extinctions (Lowe et al. 2000, Medina et al. 2011). In the United States free-roaming cats are the top source of direct anthropogenic mortality to birds and mammals, killing approximately 2.4 billion birds and 12.3 billion mammals each year (Loss et al. 2013).

The indirect impacts of cats on wildlife are less obvious, but one of the greatest emerging threats from free-roaming cats is infection with Toxoplasma gondii. T. gondii is a parasitic protozoan that can infect all warm-blooded species but relies on felids to complete its life cycle. According to a new study published in EcoHealth, feral cats are likely driving white-tailed deer (Odocoileus virginianus) infections in northeastern Ohio (Ballash et al. 2014). Cats that host T. gondii excrete oocysts into the environment in their feces, and a single cat can deposit hundreds of millions of oocysts, which may remain infectious for up to 18 months (Tenter et al. 2000).

The study’s authors collected white-tailed deer samples at the Cleveland Metroparks as part of a deer management program. Cat serum samples were collected from cats in a trap, neuter, release (TNR) program in the Greater Cleveland area. TNR programs spay/neuter feral cats and then release them into the environment. Nearly 60% of white-tailed deer and 52% of feral cats tested positive for T. gondii. Older deer and deer in urban environments were more likely to be infected, suggesting horizontal transmission from environmental exposure.

The study’s findings have implications for people as well. Widespread environmental contamination increases the likelihood of human infections. In people, infection has been linked to schizophrenia and can lead to miscarriages, blindness, memory loss, and death (Torrey and Yolken 2013, Gajewski et al. 2014). Due to the creation of tissue cysts in infected deer, people that consume undercooked venison can also acquire T. gondii and the subsequent disease, toxoplasmosis.

The Wildlife Society actively supports the humane removal of feral cats from native ecosystems. See our position statement and fact sheet for more information on how feral and free-ranging domestic cats impact wildlife.

This article was written in cooperation with the American Bird Conservancy.