Cat Poop, Seaweed, and Snails Prove a Deadly Combination for Sea Otters

Scientists discover how sticky, invisible polymers on ocean kelp can transmit a deadly disease to endangered California sea otters.

(Photo: Michael L. Blaird/Getty Images)

Oct 8, 2014· 1 MIN READ
John R. Platt covers the environment, wildlife, and technology and for TakePart, Scientific American, Audubon, and other publications.

New research has finally revealed how parasites from cat poop are killing sea otters. The discovery not only may benefit the endangered marine mammals; it could also help humans.

Scientists have known for several years that the parasite Toxoplasma gondii, which only reproduces in felines, has been reaching the waters off California and causing severe brain inflammation in otters. It’s so bad that toxoplasmosis accounts for 17 percent of all sea otter deaths annually. (Some otters experience milder brain inflammations, leaving them more vulnerable to deadly shark attacks.)

Exactly how the parasite made its way into sea otters has, until now, remained a bit of a mystery. It’s not as though the otters were munching away on big piles of kitty litter. Previous research has shown that the otters most likely to contract toxoplasmosis were those that ate a diet high in three kinds of marine snails, as opposed to otters that ate abalone or other creatures. But why did the marine snails carry so many Toxoplasma parasites in the first place, given that creatures should have been spread out into the ocean?

Well, we finally have an answer. According to a study published Wednesday in the journal Proceedings of the Royal Society B, the marine snails feed off kelp (better known as seaweed), which is coated with sticky, invisible polymers. This “slime” collects the Toxoplasma parasite eggs (called oocysts) and concentrates them in one location, where they can be eaten by the snails. They then continue up the food chain into otters as the snails themselves are consumed.

This turned out to be the key. “If there was no mechanism to cause some kind of concentration, the likelihood of anybody, anything, encountering an oocyst would be very low,” said Karen Shapiro, an assistant research scientist in pathology, microbiology. and immunology at the University of California, Davis, and the lead author of the new paper.

The study belies the conventional wisdom about dumping garbage offshore: The solution to pollution is dilution. “We think we’ll never see it again,” Shapiro said. “These mechanisms concentrate our trash a lot closer to home than we’d think. It doesn’t go out into the great ocean.”

Shapiro said she hoped the paper can inspire people to change their behavior and manage what she called our “pathogen footprint.”

She suggests that people keep their cats indoors, put their kitty litter in the trash, and minimize the feeding of feral cats.

“Our trash and our pathogens don’t just dilute in the big blue sea,” she said. “They end up concentrating really close to us and being incorporated into the food web. If we can think of ways to minimize poop from getting to the ocean, or any body of water, we’re hoping that will help protect sea otters, other marine mammals, and obviously us too.”