Scientists just reviewed more than 400 cat studies and found that outdoor-roaming pets had three to five times the odds of carrying germs that can infect people, turning a common habit into a quiet public health risk

If your cat spends time outside, it might be bringing home more than muddy paws. A sweeping new study poses some uncomfortable questions to the millions of American households that allow their cats to roam free, and the findings may make you think twice about how much freedom your pet really enjoys.

According to a study titled ‘Outdoor roaming of owned cats elevates risk of zoonotic pathogen exposure: A global synthesis,’ published in PLOS Pathogens and led by researchers at the University of British Columbia, pet cats that are allowed to roam freely outdoors have rates of infectious diseases similar to those of feral cats, even if they are provided with regular veterinary care, food and shelter. The analysis was based on data from 604 studies covering around 174,067 cats across 88 countries, making it one of the most extensive evaluations of feline disease risk ever conducted.

The numbers are harder to ignore than you'd think
According to the PLOS Pathogens study, researchers identified 124 species of pathogens in cats, 97 of which can infect humans. They are talking about things like rabies, Toxoplasma gondii, roundworms, Bartonella (the bacterium responsible for cat-scratch fever), and Salmonella. The same study also found that outdoor-roaming pet cats were 3 to 5 times more likely to carry a zoonotic pathogen than indoor-only cats.


The researchers were actually surprised by how much outdoor pet cats resembled feral cats in terms of disease risk. One of PLOS Pathogens study’s authors, Dr. Amy Wilson said, "We expected outdoor cats would have higher disease risk than indoor cats, because the range of diseases indoor cats are exposed to is much smaller. But we were surprised that owned outdoor cats were comparable to feral cats for overall infection risk."

The study found that approximately 60 percent of owned cats in the reviewed research had unsupervised outdoor access, with some areas reporting the number as high as 90 percent. Most owners think their vaccinated, well-fed house cat is a whole different league from a stray. This research suggests that the assumption may not be correct when your cat begins going outside on its own.

Your cat is hunting more than you realize
Perhaps the most surprising thing about the study is what it shows about how cats use their unsupervised time. According to the PLOS Pathogens study, cat owners underestimate hunting by about 80 percent. That means that a lot of prey captures and animal contacts are completely missed. Your cat could come home looking spotless, but it might have spent hours hunting rodents, birds, or bats before sauntering back through the door.

The study says estimates of wildlife killed by cats in some countries run into the billions, and more than 2,000 wildlife species have been identified as prey for domestic cats. These animals are often disease carriers. The same study reports documented cases of cats bringing rabies-positive bats into homes, and a cat bringing back prey provides a direct route for pathogens circulating in wildlife populations to reach people in the house.

The new, larger study is consistent with the findings of an earlier meta-analysis, ‘Who let the cats out? ,’ in Biology Letters, which showed that cats with outdoor access were 2.77 times more likely to be infected with parasites than indoor-only cats.

Your family is not the only one at risk
The health risks from outdoor cats don’t end at your front door. According to the PLOS Pathogens study, outdoor cats defecate in gardens, parks, playgrounds and other shared spaces. According to a study published in the Journal of the American Veterinary Medical Association, outdoor-owned cats in one California municipality deposited an estimated 77 tons of feces annually. According to a PLOS Pathogens study, depending on the parasite, feces can contain hundreds to hundreds of thousands of parasite eggs that can persist in soil or water for months to years, putting anyone who touches contaminated ground at risk.

One of the better-known examples is Toxoplasma gondii. According to Cornell University's College of Veterinary Medicine, the CDC has identified toxoplasmosis as one of five neglected parasitic infections of public health concern, and more than 60 million people in the US are thought to be infected. According to this source, pregnant women and those with compromised immune systems are among the groups most at risk, and between a third and half of babies born to mothers infected with Toxoplasma during pregnancy are themselves infected.

What you can actually do about it
The good news is that this is not an all-or-nothing proposition. According to the PLOS Pathogens study, supervised outdoor access through enclosed patios, cat-containment fencing or leash walks can provide enrichment and significantly reduce contact with wildlife and other animals.

Researchers keep returning to the analogy with dogs. We don’t let dogs run freely through the neighborhood unsupervised, not because we think dogs don’t deserve a good life, but because the risks are real. A similar shift may be warranted for cats, Dr. Wilson argues: “Unsupervised outdoor access is not essential to feline welfare or the human-animal bond. If we can improve how we manage our cats, we can protect cats, wildlife, and people.”

Indoor cats and cats with supervised outdoor time can live healthy, enriched, and longer lives. When cat owners see the whole picture, that's a trade-off they'd probably be willing to make.