Lafferty and Gerber analyzed data from previous studies by other researchers about the causes of California sea otter deaths. "Because existing data indicate that non-otter diseases are responsible for a large proportion of otter deaths, we wanted to see if there was a link between disease and sea otter population trends," says Lafferty.
A parasitic worm caused 14% of the otter deaths and the researchers found that more otters died in years when the worm was common, suggesting that it could be keeping the population from recovering. The worm (Polymorphus kenti) can penetrate the intestine and cause peritonitis, which is most likely to be lethal in pups and juveniles. The worm is a shorebird parasite that uses sand crabs as an intermediate host. Sea otters usually don't catch it because they prefer molluscs and sea urchins. But people also like molluscs and sea urchins and when these prey are scarce, an otter can eat several sand crabs in a row. "Humans may have increased the prevalence of this disease by competing with otters for food, causing them to seek alternative prey," say Lafferty and Gerber.
Toxoplasmosis caused up to 8% of the otter deaths. Toxoplasma gondii is a cat parasite, and the otters may catch Toxoplasma cysts from sea water contaminated by cat feces. This disease could be decreased by improving waste disposal.
Valley fever caused 4% of the otter deaths. This disease is caused by inhaling spores of a soil fungus (Coccidiodes immitis) in dust raised by construction and agriculture. "The emergence of valley fever as an otter disease corresponds with an increase in human cases in California," say Lafferty and Gerber. Dust control efforts that reduce human exposure to valley fever may also help protect sea otters from the disease.