Let’s file this one under “here’s another reason to spay or neuter your cats.”

As pregnant women everywhere know, “kitty litter parasite” (a.k.a. Toxoplasma gondii), is a very real thing. The toxoplasmosis-causing parasite has long been the reason expecting mothers aren’t supposed to scoop the family cat’s litter box, but now it seems like “toxo” is making its mark elsewhere… Most notably in the ocean.

Thanks to the growing number of feral cats living near watersheds, toxo is becoming more frequent among marine creatures that are especially susceptible to the parasite.

Toxo has grown so problematic in Hawaii that marine mammal experts are now asking communities to keep their cat populations under control. The tipping point came last May, when two critically endangered female Hawaiian monk seals (one pregnant) were found dead on a beach in Oahu. Both animals died of toxo, increasing the tally of related Hawaiian monk seal deaths to eleven since 2001. And while that number may not seem significant at first glance, thanks to hunting, fishing, disease, and low genetic variability, there are roughly 1,300 such creatures left on earth—making them one of the most endangered marine mammals in the world.

“Females seem to be more likely to die from this disease, and from a conservation standpoint, that’s very concerning,” Claire Simeone, director of the Ke Kai Ola hospital for Hawaiian monk seals, told National Geographic.

According to scientists, toxo parasites affect different animals in various ways. While 30-50 per cent of the human population can contract the parasite at any given time, the majority of infected individuals are considered asymptomatic. In marine animals, the parasites work their way into the cells of the digestive tract and multiply until the cells burst. They then move on to the liver, brain and muscle tissues.

In monk seals particularly, toxo weakens the immune system and causes organ failure. In recent years scientists have also identified the parasites in whales, dolphins, manatees, seabirds, and otters–the latter of which also seems particularly susceptible to it.

According to one study at least half of the male California sea otters carry the disease, while according to researchers on another project at least 77 per cent of river otters in Southern Chile have it.

In general any warm-blooded animal can get contract toxo, but so far cats are the only creatures scientists know that can actually spread it. Infected cats will excrete oocysts (parasitic eggs), which in Hawaii are then being washed into rivers and streams that drain into the ocean, where they begin working their way up the marine food web.

Unfortunately there’s no known cure or prevention for these oocysts once they’re in the environment either. As Patricia Conrad, a professor of parasitology at the School of Veterinary Medicine at the University of California, told National Geographic, “They’re really tough” to maintain.

“You can store them in 10 per cent bleach for years, take them out, wash them off, and the parasites can still develop,” she said.

That makes it more important than ever to keep domestic cats indoors and to have them fixed—something that’s required by law in Hawaii. The state’s Humane Society estimates there are roughly 350,000 feral cats on Oahu alone, which are being fed by humans and adding to the overall problem. Given their impact on other mammals, birds, and reptiles in the area, it’s no surprise that governor David Ige has added feral cats to the list of Hawaii’s invasive species.

Besides, while it may seem cruel not to let your cat go outside, it’s actually better for them and the environment in the long run.

“Cats that are kept inside will live longer, healthier lives, there’s good data on that,” Conrad added. “If we lose these monk seals, we lose them forever.”