Those of us with domestic cats may not be aware of the effects of letting them roam. Outdoor cats can be a menace to birds and small mammals such as voles and ground squirrels. While these animals may seem insignificant, they truly are part of the GVM ecosystem. Small mammals are the mainstay of foxes and coyotes who deserve to have the prey in their territories left for them. Your adorable house cat is also a ruthless predator. Please keep pet cats indoors, and work to eliminate feral cat populations.

How Bad Is It to Let Your Cat Outside?

A quote from a recently published study: “In the lower 48 of the United States alone, “free-ranging domestic cats kill 1.3–4.0 billion birds and 6.3–22.3 billion mammals annually… This magnitude of mortality is far greater than previous estimates of cat predation on wildlife and may exceed all other sources of anthropogenic mortality of US birds and mammals.”

A conservation biologist makes the case for keeping cats indoors, or at least on leashes. Read more at

Another reason you may want to keep your cat indoors is to protect them. In a recent research study published in Biology Letters (Chalkowski, Wilson, et. al, 2019) comparing nearly two dozen studies from around the world, researchers found that outdoor cats were nearly three times more likely to contract a parasitic infection than indoor cats.

The findings indicate that to protect your pets, wildlife and even the health of your family, keep pet cats inside. Domestic animals can contract parasites from insects and ticks, prey, soil or other animals and can spread them to humans, pets or wildlife. Cats allowed to go out hunting on their own can return for a nap on their owner’s lap or bed and bring in several diseases. And the diseases cats spread can be highly virulent. The elderly, very young, and those with compromised immune systems are especially vulnerable to these diseases. Cat-scratch fever (Cat-scratch disease) is one such disease that is caused by the bacteria, Bortonella henselae. Those infected with this bacterium experience high fever, swollen axillary (arm-pit) lymph nodes, and pustules at the site of the scratch. Other bacterial infections spread by cats include Pasteurella multocida and Salmonella. Salmonella is primarily picked up by cats that feed on wild birds and mammals. Toxoplasma gondii, is a parasite commonly transmitted from domestic cats to people and is especially harmful to the unborn of pregnant women and immune-compromised people. Other parasitic diseases of cats that can be spread to people include Giardiasis (picked up from contaminated water) and Cryptosporidiosis. The fungal disease, ringworm, is a dermatophytosis of humans acquired from cats. Finally, cats are highly susceptible to rabies and a rabid cat can bring the virus home to its owners. It is recommended to vaccinate your cats for rabies, even if it is kept indoors because of the risk of it escaping to the outdoors.

Domestic cats also spread feline immunodeficiency virus, which makes them more susceptible to parasites and other diseases. They spread the virus to other cats, even mountain lions.

The team of researchers compared data from twenty-one studies that documented parasitic infections in pet cats living strictly indoors or with outdoor access. The study excluded feral cats. The lead researcher, Kayleigh Chalkowski, summarized by stating, “Over so many different studies, with so many different parasites, in so many different countries: No matter where you keep your cat indoors, it reduces risk of parasitic infection.”

Surprisingly, the risk of picking up parasites increased in higher geographic latitudes, where in general, there are fewer parasites. The researchers do not know why this might occur, although it is also known that rodents, common feline prey items, carry more diseases in northern latitudes too.

Some see depriving a cat of prey by keeping it indoors as cruel. But others say putting them outdoors to do whatever they want is cruel: Outdoor cats are frequently hit by cars. Some are eaten by predators. And, as predators themselves, cats can kill off birds or other species conservationists want to protect.

Kayleigh Chalkowski’s message from the results of this study is simple, “If you’re a responsible cat owner, you might really want to keep your cat indoors not just because of those things, but now we’re seeing an increase in parasitic risk.” But that doesn’t mean it will suffer, she adds: Cats can climb on pedestals and chase feather toys and lasers or hang out in a cat patio (a catio). Or maybe your cat can even go on walks with you, like dogs and their owners.

Kayleigh Chalkowski, Alan E. Wilson, Christopher A. Lepczyk, and Sarah Zohdy. Who let the cats out? A global meta-analysis on risk of parasitic infection in indoor versus outdoor domestic cats (Ferlis catus). Biology Letters, 17 April 2019.