Domestic cats have pretty much conquered the world, which is both good and bad. Cats bring joy to lots of people, and they can be great companion animals in the right context. The popularity of pet cats has also led to a global surge of feral cats, however, which are now obliterating native wildlife around the world, including some ecologically important endangered species.

In the U.S. alone, cats kill somewhere between 1.3 billion and 4 billion birds per year, according to one prominent study, from small adult songbirds to the chicks and eggs of much larger species. (This is mostly due to feral cats, the study’s authors noted, although free-roaming pet cats play a role in some places, too.) Cats have already driven some island birds to extinction, and they continue to threaten a wide range of vulnerable wildlife, a problem that has become especially salient in Australia and New Zealand.

But aside from preying on small animals, cats also pose a less obvious danger to larger wildlife. Cats are the definitive host for Toxoplasma gondii, a single-celled parasite behind the bizarre infection known as toxoplasmosis. By spreading infectious oocysts in their poop, cats can sicken or kill wild animals without ever getting near them. Not even aquatic animals are safe, since rain can wash cat poop into rivers, lakes and oceans, along with a horde of T. gondii oocysts that can remain stable in cold water for years.

The parasite’s effects vary by species and individual, but while it can infect almost any warm-blooded host, it can only reproduce inside the bodies of cats, which are therefore the main animals responsible for spreading it. A single cat with toxoplasmosis can poop out billions of infectious oocysts during its life. This includes native cat species like bobcats, lynxes or mountain lions, but since they rarely rival the population size and density of feral house-cat colonies, they’re less likely to fuel T. gondii outbreaks.

What the cat dragged in

cat overlooking the ocean in Monterey, California
A cat surveys the coast in Monterey, California, a hotspot for Toxoplasma infections in sea otters. (Photo: Chase Dekker/Shutterstock)

T. gondii has proven deadly for some marine mammals, including belugas and endangered Hawaiian monk seals. And as a new study reveals, infected poop from domestic cats has also become a significant threat to some of the planet’s most beloved marine mammals: sea otters. Scientists have known for years that T. gondii is infecting sea otters — with a prevalence as high as 70% in some high-risk locations — and that it can be fatal. But as Francie Diep reports in the New York Times, researchers have been reluctant to blame domestic cats until now, since it’s possible other feline species are spreading the parasite to sea otters.

The new study, however, demonstrates a strong genetic link between parasite strains in sea otters and those in domestic cats on nearby shores. “This is the ultimate proof that strains that are killing sea otters are coming from domestic cats,” lead author Karen Shapiro, a veterinarian and pathologist at the University of California-Davis, tells Diep.

Shapiro and her colleagues analyzed DNA from 135 sea otters with Toxoplasma infections that died between 1998 and 2015. Most of those otters showed no evidence of brain damage, they found, suggesting the parasite hadn’t been a factor in their deaths. But the researchers concluded that 12 of the otters had died primarily due to T. gondii, and all 12 of them were infected with a specific strain known as Type X. This strain seems to be more dangerous for sea otters than the more common Type II.

chart showing the spread of Toxoplasma gondii from cats
This chart shows how oocysts are key to T. gondii’s complex journey from prey species to cats and beyond. (Image: Karen C. Drayer Wildlife Health Center)

These 12 fatal infections were genetically identical to parasites collected from feral domestic cats on shore, as well as from one bobcat, the researchers report. The Type X strain is actually more common among wild cats in coastal California overall, they note, but they found that 22% of feral domestic cats in this area were infected with this strain. Plus, they add, there are a couple reasons why feral domestic cats are more likely to spread T. gondii to sea otters than wild species are.

“Population sizes of domestic cats in coastal California are much larger than those of wild felids. Domestic cats also inhabit developed landscapes with impervious surfaces (e.g. concrete) that facilitate pathogen run-off and they have higher relative contributions to environmental oocyst load along many areas of the sea otter range,” the researchers write.

This parasite alone may not doom sea otters, but it’s hardly the only problem they have. The charismatic furballs are still reeling from centuries of hunting and trapping by humans, and although they’re now protected by U.S. law, their population is still just a fraction of what it used to be. Sea otters face ongoing threats related to commercial fishing, offshore oil drilling and climate change, and are listed as an endangered species by the International Union for Conservation of Nature. Their plight is particularly troubling because sea otters are a keystone species, playing a key role in preserving the kelp forests where they live.

How to help

There are three main factors that lead to T. gondii in sea otters, according to the UC-Davis One Health Institute:

  • domestic cats, which contribute oocysts to coastal watersheds
  • loss of coastal wetlands, which could otherwise keep oocysts from washing into the sea
  • urban landscapes, where impermeable surfaces promote more runoff that carries oocysts out to sea

Even if you don’t have a cat, you can help with this problem by supporting the conservation and reconstruction of wetlands, the researchers say, as well as other natural ecosystems that border oceans. Reducing pavement and other impermeable in your landscaping can also help reduce the urban runoff that carries pathogens and pollutants into waterways.

Those who do have a cat should have it spayed or neutered, to help limit the growth of feral cat populations. Cat owners also shouldn’t let their pet roam freely outside, since that can expose it to parasites and other diseases, while also endangering birds and allowing any parasites in its poop to wash into aquatic habitats. If you do let your cat outdoors, Shapiro and her colleagues suggest keeping a litter box outside, or at least collecting the poop in a plastic bag before it goes in the trash.

That doesn’t mean cats have to stay indoors all the time, though. As one expert tells Diep, cat owners should see their pets more like dogs, which are commonly escorted outside under human supervision. And yes, cats can be trained to walk on a leash.





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