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Bird flu. COVID-19. Monkeypox. These zoonotic diseases are all transmitted from animals to humans—though wild species aren’t the only ones responsible for such outbreaks. Your pets can also act as a vector of disease, especially if you let them roam freely.

Take outdoor cats, for instance. Veterinarians, ecologists, and disease experts largely agree that the wandering lifestyle not only hurts felines, but also poses a risk to the larger ecosystem and to public health.

“No veterinarian would say it’s safe to just let your cat outside,” says Peter Marra, a professor of biology and the environment at Georgetown University and author of Cat Wars. “We wouldn’t let our dogs freely roam around the neighborhood, so why would you do that with a cat?”

The many diseases spread by outdoor cats

Although originally bred from their wild counterparts, domestic cats (which include feral felines and pets) are not native to any ecosystem, making them an invasive species everywhere they exist, Marra says. At the same time, they are extremely common around the world, with an estimated 50 million to 100 million in the US alone.

The high densities and carnivorous diets of cats make them substantial predators that prey on everything from birds to rabbits. In a 2016 study looking at invasive predators and global biodiversity loss, researchers found that cats threaten 430 species of wildlife and are linked to the extinction of 40 birds, 21 mammals, and two reptiles. Another study estimated that cats in the US kill up to 4 billion birds and 22 billion mammals a year.

But as much as outdoor cats threaten biodiversity and the health of ecosystems, they also act as a vector for disease. When cats are given the chance to roam outside, they are more likely to come in contact with zoonotic diseases and pass on viruses, parasites, and bacteria from wildlife to humans, says Richard Gerhold, a professor at the University of Tennessee Knoxville who specializes in parasitic infections.

“There’s a spectrum of free-roaming cats,” he explains. “For those that are owned and well taken care of, the free-roaming lifestyle is probably more of a risk to the cat itself and to conservation than to public health. But feral cats that don’t have vet care can be a major source of disease, spreading everything from tick- and flea-borne illnesses to viruses such as rabies.”

For one, felines are the primary host for Toxoplasma gondii, a parasite that infects an estimated up to 50 percent of the human population, with certain communities recording even higher infection rates. Although most people who become infected do not experience symptoms, severe infection can cause damage to the brain, eyes, and other organs. A recent study found that an estimated one in 150 Australians have ocular toxoplasmosis, an eye infection caused by the parasite. Another study found that among those with ocular toxoplasmosis, about half experienced permanent vision loss and one-quarter went blind.

One of the ways that humans can become infected with Toxoplasma is by accidentally ingesting the parasite through contact with cat feces, whether by cleaning a litter box or gardening around scat. 

[Related: The animal kingdom is full of coronaviruses]

Rabies, an infectious disease that is basically 100 percent fatal once symptoms appear, is another common pathogen in kitties. The majority of human rabies cases in the US are attributable to bats, but cats have become the top source of human rabies exposure among domestic animals.

“Dogs used to be the primary way humans picked up rabies from domestic animals,” Marra says. “But we started licensing dogs and requiring leash laws and rabies vaccines. Now, dogs are not the problem at all—it’s primarily cats.”

In general, letting cats wander on their own outdoors can increase the chance that they bring home a disease. A global study of parasitic infection in the mammals found that those with outdoor access were 2.77 times more likely to be infected with parasites than indoor-only individuals.

“When we’re talking about feline-associated diseases, it’s in no way the threat of the cat—it’s the free-roaming lifestyle that’s causing all these problems,” says Amy Wilson, who co-authored a recent study on the public health impact of bat predation by cats and other domestic animals.

As a veterinarian and conservationist, Wilson emphasizes the importance of One Health—the concept that the well-being of people, animals, and the environment are all interconnected. When looking at the impact of outdoor cats from this perspective, it’s evident that the lifestyle is not beneficial to humans, wildlife, or pets themselves, she says. 

“Because cats are out predating wildlife, they’re perpetuating parasitic lifecycles and giving immunosuppressive diseases to wildlife, making it even harder for wildlife to persist in this world,” Wilson adds. Left to the elements, wandering cats also run the risk of getting hit by a car or predated on by species like coyotes or great horned owls.

How free-roaming cats challenge conservation

Gerhold says there is “no place that’s quote-on-quote immune to the effects of feral cats.” But certain geographies can be more susceptible to their impacts, including islands, where biodiversity is particularly fragile and vulnerable to extinction. On the Hawaiian archipelago, for instance, outdoor cats have been a major issue for the rapidly declining Hawaiian honeycreeper, a unique and colorful group of songbirds, Gerhold says.

Domestic cats have also been a big concern in Australia, where toxoplasma has been documented to cause significant mortality of marsupials like kangaroos, wallabies, possums, and wombats. 

“It always comes down to the question of why we should care [about species biodiversity],” Gerhold says. “Every time one of those species is lost, that is a detriment to us, one way or another. It’s becoming pretty obvious that the need to conserve wildlife overlaps with the need for keeping humans safe and protected. I think people are beginning to see the interconnectedness of humans with their environment a bit better.”

Wilson, on her end, thinks there needs to be a paradigm shift in the way we consider cat behaviors. She argues that we’ve done the same for dogs: It has become socially unacceptable for a domestic dog to kill another species. In fact, it communicates that the dog is not under control and reflects poorly on the owner.

[Related: The planet needs you to pick up your dog’s poop]

For the sake of feline welfare, public health, and conservation, cat lovers should only allow “supervised access” to the outdoors, Wilson says. Fresh-air enclosures like catios can be one effective solution; harness training is another for owners who want to take their companions on a walk or a small adventure. Finally, making sure pets are vaccinated against infectious diseases and cleaned after spending time outside can also help minimize the risk that they bring illnesses into the household.

At the core, the negative impacts of outdoor cats are a result of human negligence rather than the animal’s own misdeeds. Many people are simply not aware of the risks associated with letting their kitties explore the great outdoors. Responsible pet ownership can not only minimize the spread of disease and wildlife deaths, but also improve and extend the lives of our feline friends.

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