Cats who are allowed outdoors are more likely than “indoor cats” to be infected by parasites, which may then be transmitted to humans and wildlife.

Domesticated animals—both those who share our homes and those we farm for food and other purposes—can contract diseases that may be transmitted to the humans and wild animals they encounter.

Domestic cats typically live in close contact with humans—many guardians even allow their cats to sleep on their beds. However, cats are unusual among most domesticated animals because they can be grouped into two categories: those whose guardians allow them to go outdoors and those who are “indoor-only.” In this study, the researchers set out to draw a comparison between the prevalence of parasitic infections in these two groups.

They looked at existing studies of 19 different pathogens (including bacteria and viruses, among others) in 16 countries and found that cats with outdoor access were 2.77 times as likely to be infected with parasites than indoor-only cats.

Some of the parasitic infections that can affect cats may be transmitted to the humans they have contact with. Many can also result in health issues for the cats themselves—including feline immunodeficiency virus (FIV), which can also be spread to wildlife in areas where they have contact with other feline species.

Interestingly, the authors found that for every degree increase in absolute latitude (i.e., the further from the equator you get), there was a 4% increase in the likelihood that cats with outdoor access would be infected by parasites. Although parasites exist in smaller numbers and with less diversity the further from the equator you are, this distance actually corresponds with higher infection rates among cats.

Since restricting outdoor access can reduce a cat’s risk of being infected by parasites—thereby decreasing the risk that they will suffer from resulting health conditions or pass these pathogens on to wildlife or humans—advocates concerned with the welfare of companion or wild animals should consider encouraging cat guardians to keep their animals indoors, especially if they live in temperate zones.

In certain countries, animal shelters and rescue groups that rehome cats are highly reluctant to place cats with potential guardians who can’t offer them animals outdoor access. Groups working in these areas could therefore help reduce homeless-cat populations by highlighting these findings—among other arguments against allowing cats to roam—to encourage shelters to change their policies.

Some animal protection organizations already raise awareness of the risks of allowing cats to go outdoors, but the authors suggest that stronger measures—such as legislation—are needed to persuade guardians to keep their companions inside, so political lobbying on this issue may be beneficial.

