Offering a partial explanation to a mysterious decline in Southern Sea Otter population, California Sea Grant researchers have established a strong body of circumstantial evidence linking cats to a lethal otter disease. University of California at Davis professor Patricia Conrad and doctoral student Melissa Miller, both in the School of Veterinary Medicine, have shown that otters near heavy freshwater flows are three times more likely to have been infected by Toxoplasma gondii - a potentially lethal parasitic protozoan that causes brain infections in otters - than otters from areas where runoff is light.
The scientists' best guess is that parasite eggs in cat droppings are washed into coastal-bound storm drains and creeks. Although many different kinds of animals, such as birds and rodents, can serve as intermediate hosts for the parasite, cats are the only animals known to shed the parasite's eggs in their droppings. Otters may be acquiring parasites directly through water contact, or they may be eating infected mussels or other bivalves.
Southern Sea Otters are listed as threatened under the federal Endangered Species Act. Once numbering more than 300,000, there were an estimated 2,100 otters off California this past spring. For more than a decade, otter numbers rose, hitting a peak in the spring of 1995. The recovery, for reasons still unexplained, appears to have stagnated or slid backward.
While the scientists are not certain how much of this decline can be attributed to the parasite infections, Miller's Sea Grant research suggests that about 60 percent of dead otters in her survey had been infected by the parasite. Further research suggests that many of these otters likely died of toxoplasma encephalitis.
To further investigate pathogens in storm water and runoff, a California Sea Grant study is looking at another parasite, Cryptosporidium, widely regarded as one of the most significant causes of diarrhea in humans. Conrad, and Rob Atwill, also at the School of Veterinary Medicine at Davis, are taking cues from the sea otters study, measuring pathogen levels in bivalves near outfalls of human and agricultural runoff, to track the upstream sources of pollution. Genetic tests are also being used to identify which animal species are the main sources of pathogen pollution. Wildlife, cattle, pets and people can spread Cryptosporidium.
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