Offering a partial explanation to a mysterious decline in the federally protected southern sea otter population, scientists funded by the NOAA National Sea Grant College Program have established a strong body of circumstantial evidence linking cats to a lethal otter disease.
 University of California at Davis professor Patricia Conrad and her 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 than otters from areas where runoff is light.
Toxoplasma gondii is a protozoan that causes convulsions, severe depression and death in otters. In people, toxoplasmosis is usually asymptomatic, though AIDS patients and others with compromised immune systems can develop hepatitis, pneumonia, blindness or severe neurological disorders. Toxoplasmosis can also be transmitted across the placenta, causing spontaneous abortions, still births or severe brain damage.
In a survey of 233 live and dead otters from Santa Barbara to Half Moon Bay, Calif., the scientists found that a staggering 76 percent of otters near heavy freshwater outflows — storm drains and river mouths — had antibodies to Toxoplasma gondii. There was also a surprisingly high rate of infection in the general otter population. Forty-two percent of live otters surveyed had antibodies to the parasite, an almost certain sign of infection.
How are Otters Becoming Infected?
 The scientists’ best guess is that parasite eggs in cat droppings are being washed by sprinklers and rains into coastal-bound storm drains and creeks. Although many animals — such as birds and rodents — can serve as intermediate hosts for Toxoplasma gondii, cats are the only animals known to shed the parasite's eggs in their droppings (This cat-parasite link is the reason pregnant women are advised against cleaning cat-litter boxes).
Otters may be acquiring parasites directly through water contact or from eating infected mussels or other bivalves. Parasite eggs have not yet been found in wild bivalves, but they are logically suspect since they filter huge amounts of water feeding on plankton and thus accumulate parasites.  The only marine mammal without an insulating blubber layer, otters have galloping metabolisms and snack voraciously on seafood treats, such as shellfish.
Otter Populations Past and Present
Once numbering more than 300,000, southern sea otters were hunted to near extinction in the 19th century for their luxurious pelts, which otters must constantly preen to keep filled with tiny insulating air bubbles. In 1977, otters were listed as threatened under the Endangered Species Act. Following restrictions on certain types of fishing gear, their numbers rose, hitting a peak in the spring of 1995, when there were an estimated 2,377 individuals. In the last seven years, however, their recovery has stagnated or slid backward. There were an estimated 2,100 otters off California in the spring of 2002 — roughly a 10 percent drop from the 1995 high.
Though scientists still do not know what has caused their decline or how much of it can be attributed to Toxoplasma gondii, otters originally thought to have been killed by boat strikes or shark bites have since been diagnosed with protozoal encephalitis. Miller's recent NOAA research has shown that about 60 percent of dead otters surveyed had been exposed to the parasite. New research led by a graduate student at the University of California at Davis suggests that for many of these otters infection proved lethal.
The National Sea Grant College Program, an arm of NOAA Research, awarded Conrad and Miller support for their otter research as part of a national strategic initiative to focus expertise on issues of national importance. The California Department of Fish and Game and the U.S. Geological Survey (which both co-manage the sea otter recovery program with the U.S. Fish and Wildlife Service) collaborated on the project, as did the California Regional Water Quality Control Board, the agency charged with enforcing the federal Clean Water Act and regulating runoff.
Other Risks from “Pathogen Pollution” Associated with Urban and Agricultural Runoff
The intense interest in unraveling the causes of these otter deaths, to a large extent, reflects the species’ endangered status and the federal mandate to develop recovery plans for listed species. Conrad and Miller’s research, however, also addresses other important emerging issues — that of understanding waterborne infections and “pathogen pollution” associated with urban and agricultural runoff. Coastal waters are the interface between what transpires on land and sea. Therefore, various forms of pathogen and chemical pollution may be contributing to declines in marine species and increasing the coast’s vulnerability to exotic species invasions.
Though NOAA Fisheries does not manage the southern sea otter, it does manage most federally protected marine species, including whales, dolphins, seals and sealions. “We are interested and following the otter research to see if it has any implications for marine mammal species under our jurisdiction,” said Joe Cordaro, coordinator of NOAA Fisheries’ California Marine Mammal Stranding Network in Long Beach, Calif. There is some concern, he said, about whether marine species besides otters could be suffering from parasites spread in runoff. "It is pretty reasonable to assume there are,” Miller said. Encephalitis has been documented in harbor seals, others report infections in spinner dolphins. “It is not a local California problem,” she said. It is also important to note the NOAA Ocean Service also works on related coastal issues (i.e., sustain coastal habitat, support coastal communities and mitigate coastal hazards).
Potential Risk to Humans
People are also potentially at risk, since they eat many of the same shellfish as otters. One documented outbreak of human toxoplasmosis in British Columbia was linked to contaminated drinking water, presumably from cat droppings. Though a potentially serious human health threat, Toxoplasma gondii is only one of many waterborne protozoans that may be entering beach waters via runoff.
A new study funded through the NOAA California Sea Grant program is looking at one of the more worrisome of these, Cryptosporidium, widely regarded as one of the most significant causes of diarrhea in humans. Leading the project are Rob Atwill, also at the School of Veterinary Medicine at Davis, and Conrad. Taking cues from sea otters, Atwill and Conrad are 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. The scientists are also working with dairies along the coast to test the degree to which management practices, such as planting vegetative buffer strips, can reduce pathogen pollution.