Free-Roaming Cats Spread Deadly Parasite to Wildlife

Keep cats indoors to limit toxoplasmosis infections, researchers suggest.

cat roaming outside
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When domestic cats roam outdoors, they can spread a potentially deadly parasite to wildlife.

New research suggests that free-roaming cats are likely infecting other animals with Toxoplasma gondii, the parasite responsible for toxoplasmosis. This disease is linked to nervous system disorders, respiratory and heart disease, and other chronic illnesses.

“For a long time, conservationists have emphasized the interconnectedness of human and wildlife health. Toxoplasma gondii is a perfect example of this shared fate, because it is one of the world’s most common parasites and infects both humans and wildlife,” lead researcher Amy Wilson, University of British Columbia faculty of forestry adjunct professor, tells Treehugger.

“It is important to understand the risk factors for this infection because toxoplasmosis can have severe impacts on susceptible individuals, but even in healthy individuals, hosts are infected for life.”

Because research in humans has shown that toxoplasmosis infections can have long-term health consequences with various serious neurological diseases, Wilson and her team wanted to use the vast amount of infection data available in wildlife to better understand what was driving these infections.

For their study, researchers analyzed more than 45,000 cases of toxoplasmosis in wild animals using data gathered from 202 studies. The studies included 238 different species in 981 locations around the world.

They studied the data, extraction information on species-specific ecological traits, as well as geographic information and human population density in the area where the infections occurred.

They found that wildlife living near areas of high human density were more likely to be infected.

“As increasing human densities are associated with increased densities of domestic cats, our study suggests that free-roaming domestic cats—whether pets or feral cats—are the most likely cause of these infections,” Wilson says.

“This finding is significant because by simply limiting free roaming of cats, we can reduce the impact of Toxoplasma on wildlife.”

The results were published in the journal Proceedings of the Royal Society B.

Why Domestic Cats Matter

Only wild and domestic cats (called felids) can spread the infectious form of toxoplasma into the environment through eggs called oocysts in their feces.

“There has been a growing recognition that domestic cats are the most likely felid to be driving wildlife toxoplasma infections,” Wilson says. “Domestic cats outnumber wild felids by several orders of magnitude so when you consider their population size and that they can shed millions of long-lived oocysts intermittently throughout their life; the potential for environmental contamination is considerable.”

An acutely infected cat can excrete as many as 500 million toxoplasma eggs in two weeks, and even one oocyst can cause an infection.

Field studies and DNA research also offered evidence that it’s domestic cats and not wild ones spreading the parasite.

“Our study further supports this role because wild felids avoid human environments and because we found that wildlife toxoplasma infections are higher in areas with greater human density, it suggests domestic cats are the link whereas it would be the opposite pattern if wild felids were the main source,” Wilson says.

A Healthy Environment

If an animal or person is healthy, Toxoplasma gondii rarely causes symptoms or harm. However, if the immune system is compromised, the parasite may cause serious illness or even be fatal.

Likewise, if the environment is healthy, then streams, forests, and other ecosystems can help filter out potentially dangerous pathogens like this.

“In the case of Toxoplasma gondii, ecosystems with healthy populations of native predators can deter domestic cats from roaming into ecologically important wildlife areas and reduce their pathogen inputs into those environments,” Wilson explains.

“For pathogens that are present, vegetation, healthy populations of soil bacteria and invertebrates increase the capacity of soil to filter out or inactivate pathogens. When you have bare soil or concrete, pathogens can sit on the surface or be taken up by run-off and transmitted straight into aquatic habitats.”

Protecting Wildlife

These study findings are important, the researchers say, because it’s a clear example of how human activity is raising the risk of a parasite in wildlife. And wild animals can be indicators of human risk, as well.

One way to decrease this risk is to limit outdoor exposure for pet cats.

“Free-roaming cats kill billions of wildlife in the U.S. every year. In the case of birds, the losses due to cats are three fold higher than all other direct causes combined,” Wilson says. “In the current extinction crisis, we cannot afford to lose wildlife to frivolous sources.”

The greatest risk is from cats that are allowed to roam freely and hunt wildlife, she says.

“The hunting instinct and ability to kill wildlife is present in both cats and dogs, but for dogs, owners are expected to provide alternative forms of enrichment, and the same responsibilities need to be extended to cat owners. There is a progressive movement amongst cat owners for supervised access through harness training and catios which is very encouraging for this issue and feline welfare,” Wilson says.

“It is crucial that people understand that the conservation of healthy intact ecosystems has benefits for not only wildlife health and resiliency, but also human health. Although we may not fully understand all the mechanisms of this benefit, it is imperative that we move quickly to protect whatever we can before it is lost.”

View Article Sources
  1. Wilson, Amy G., et al. "Human Density is Associated with the Increased Prevalence of a Generalist Zoonotic Parasite in Mammalian Wildlife." Proceedings of the Royal Society B: Biological Sciences, vol. 288, no. 1961, 2021, doi:10.1098/rspb.2021.1724

  2. "Toxoplasmosis FAQs." Center for Disease Control and Prevention.

  3. lead researcher Amy Wilson, University of British Columbia faculty of forestry adjunct professor

  4. Easton, John. "What Does it Mean When 2 Billion People Share Their Brain With a Parasite?" University of Chicago Medicine, 2014.