Photos courtesy of Jeff Foott

Something is killing sea otters that live along the northern California shoreline of the Pacific Ocean. Nearly 1,000 have been found dead along the coastline over the past five years. Given that the total otter population at any one time is probably well below 3,000 animals, this appears to be a high rate of mortality, especially considering that as many as half of the otters that die are probably never recovered. Veterinary pathologists, in their ongoing research, are finding that some of these marine mammals are dying from parasitic diseases for which the only known egg-shedding or definitive hosts are terrestrial mammals. They cannot yet say with absolute certainty, but they suspect the parasites may be coming through sewage or terrestrial animal waste streaming into their habitats, and perhaps from their prey as well."Here on the Pacific Coast, this seems to be a pretty...

Sea otters (Enhydra lutris) are more than just furry marine mammals that frolic along the shores of the North Pacific. They are considered by many people, says Jessup, to be "an almost ideal" sentinel species. "The otter is probably the one that's going to be signaling the most to us about problems that we may be the cause of in the environment," explains veterinarian Melissa A. Miller, who works with Jessup as a wildlife pathologist. "As a near-shore species with a high metabolic rate, they consume a lot of prey species, and they're right there where wastewater is coming out both rivers and streams and sewage outfalls. So when we start seeing problems in sea otters that are things you would not expect to see in a healthy marine animal population, it's a serious wake-up call for us."

Patricia A. Conrad, professor of parasitology at the University of California, Davis, who is contributing her expertise to the otter problem, agrees: "It's not true that what you don't know won't hurt you. Sea otters are the bioindicators who [appear now] to be giving us the first indication that something may be going wrong here."

From Land to Sea

Currently, researchers suspect at least two different parasites are involved in the infections that are killing the otters. "One is Toxoplasma gondii. If the parasite that we're isolating out of these sea otters and harbor seals1 is the same as that from terrestrial animals and humans, then the only recognized definitive host for that parasite today is felids, basically any type of cat--bobcats, cougars, domestic and feral cats," says Miller. The other isolated parasite and cause of disease and death in these marine animals is Sarcocystis neurona."This protozoal parasite was first recognized as a terrestrial disease in horses--equine protozoal myeloencephalitis--and to date, the only recognized definitive host for S. neurona is the opossum," notes Miller, who is currently completing her Ph.D. thesis on the pathology of these parasites and the development of techniques to further characterize them in wild populations.

"The really intriguing thing is that both these diseases have a previously recognized terrestrial animal life cycle--and in the case of Toxoplasma gondii, it was also recognized in people well in advance of these cases in marine mammals," adds Miller. "The thing about Toxoplasma is that it's insidious," explains Conrad. "You could become infected and have damage and not even know you're infected. If you were to get infected when you're healthy, these parasites would settle in your tissues and remain there, but if you were to become HIV infected or immunosuppressed, these parasites would come out of those tissues and go to your brain and then you can have fatal disease." However, S. nuerona has not been proved to be a human pathogen yet.

One of the reasons, Jessup believes, that these parasitic diseases may be relatively new in sea otters, and probably harbor seals, is because the clinical disease associated with them is "pretty striking," he says. "The animals are often found alive and are [suffering seizures], showing obvious signs of damage to their brains. They can't hold food and can't take care of themselves, and their eyes are dilated," he explains. That specific clinical disease syndrome hasn't been frequently recognized either in sea otters or harbor seals, even though the effort to care for sick sea otters or harbor seals began decades ago.

Jack Ames, senior sea otter biologist at CDFG, is one of the first environmental specialists who began conducting postmortem examinations on California sea otters some 30 years ago when he first joined the agency, using what equipment, time, and funding was available. In the early 1990s, the CDFG and the National Wildlife Health Laboratory in Madison, Wis., signed an agreement that sent most of the fresh dead animals to that lab, because, explains Jessup, "they had the facilities and wherewithal to do more thorough postmortems." There, the veterinary pathologists found about 40 percent of the fresh dead animals that came to them were dying of infectious diseases,3" recalls Jessup. "That was a jaw-dropper, because nobody really figured the percentage would be quite that high."

Adds Miller: "In conducting these systematic necropsies on the freshest of the dead southern sea otters, one of the problems they identified in those animals were protozoal infections of the brain, one of which was Toxoplasma gondii. They felt these were at least significant and, in many cases, were the cause of death in those animals--and that had never been reported in those animals before."

By late 1997, sea otter postmortem research returned to the home waterfront in part because of "funding generated by a fee on oil, designed to prevent and respond to oil spills," says Jessup. Some of the monies from those fees went to the creation of the Marine Wildlife Veterinary Care and Research Center, which Jessup designed and helped build, and where he and Miller are now based. However, most research on sea otter disease is funded by grants, contracts, and private donations. The center, which has been in operation for about four years now, is equipped to care for sea otters and other species should an oil spill occur, and also conducts other types of marine research, including their work on parasitic and other infections.

The veterinarians and researchers are now trying to determine whether new host-parasite interaction cycles that have never occurred before are killing sea otters. Miller and her colleagues have been developing serologic tests, confirming the infections in these mammals, and isolating the parasites in cell culture, using a variety of techniques to characterize them, including PCR and other technologies.

"Since we have little historical data on these infections in marine mammals, we really cannot say for sure what happened before this time point," confirms Conrad, who is Miller's major Ph.D. adviser and is also working on some of the various projects involving the otters. One reason for the lack of historical data, Miller notes, is that pathologists in the past didn't always look systematically at the brain of these marine mammals.

If T gondii and S. neurona weren't enough of a headache, it appears the problem has already spread beyond these two organisms. "We're getting the suspicion that there may be a number of organisms that are getting into the marine environment that are hard on sea otters," says Jessup. Researchers have recently discovered other organisms that are causing serious fatal infections in sea otters, which may or may not have anthropogenic origin as well. "We found one otter that died of Salmonella, another of [Escherichia] coli; both are pretty well recognized as being associated with fecal matter," says Jessup. "Another otter examined at the National Wildlife Health Center was found to have died of a Listeria septicemia."

Photo: Courtesy of Bryant Austin

Melissa A. Miller and David A. Jessup 

Seeking Routes

As the diseases striking the otters and seals are being isolated and identified, the focus is turning now to the specific routes of infection. "We're still kind of touching our toe in the water on this, ... but we're starting to suspect more and more that there may be a connection between some otter deaths and exposure to sewage and terrestrial animal waste," says Miller. "Any reasonable analysis of this would take a look at the potential connection between sewage and these diseases in sea otters," proffers Jessup. That hypothesis convinced the Regional Water Quality Control Board to request that the city of Pacific Grove, Calif., allocate a portion of the fines levied against the city for sewage spills to screen sea otter feces for specific bacteria and protozoa that may be related to sewage, Jessup adds.

While the researchers have spent quite a bit of time considering the exact routes of these infectious organisms, at the moment they are relegated to speculation. Some possibilities, however, appear to have some credibility. "For one thing, in the last 10 years or so, there's been a real increase in the use of what are called flushable cat litters, so people are dumping a lot more cat feces into their toilets than they used to," Jessup points out.

Then there's also the issue of feral cat colonies, pet cats, and cats in ocean areas. "For example, we look at an area called Moss Landing where we work--it's a little port between Monterey and Santa Cruz--and right around the wharf there, there are about 40 or 50 feral cats," Jessup explains. "There are also dairies in the area where there are a lot of cats, and so there are a lot of cat feces going directly into the oceans right in those two particular port areas, the areas where we have sea otters. We've had two cases of Sarcocystis and Toxoplasma encephalitis crop up in sea otter pups born and fed in that area during the first few months of their lives."

From the Sea Back to Land

Another route of entry may be through their food supply--the same shellfish humans eat. "In this part of the Pacific, sea otters eat a lot of shellfish, and when we look at their prey items down at Moss Landing and at Elkhorn Slough, we found chlorinated hydrocarbon pesticide-like DDE and DDD, the breakdown products of DDT [in them] left over from when those pesticides were being used in agriculture," Jessup says.

Shellfish are filter feeders that get their nutrients by filtering large volumes of water during a given time period, and they're also sessile, adds Miller. "If the quality of water they're in is poor and infectious organisms happen to fit the particle size range these shellfish would use for food selection, then it appears they're quite capable of taking up protozoal eggs and concentrating them in their tissues," she explains. "This has been demonstrated for well for well-known protozoal parasites, such as Giardia and Cryptosporidium. These organisms have been shown to be concentrated in shellfish, both in the lab and also under field conditions, and in a variety of shellfish--not just one type--but oysters, clams and mussels," says Miller.4

According to Jessup, this evidence is the sort "that just make us feel like there's something going on here that we're just seeing the tip of." Adds Conrad: "We don't have the scientific data yet to say this is the tip of an iceberg, but as a gut feeling I think it could be. This does mean that we have to evaluate how much of a risk is it for humans to be eating these same prey species, shellfish, but instead of putting the focus on 'don't eat oysters and mussels and clams'--I'd rather focus on if the shellfish are infected, how are they getting infected?"

Solutions: Expensive but Vital to Otters and Humans

The current research is critical to rendering solutions for the health of all concerned. "I do think we are entering a state of urgency here," says Conrad. "It's a high priority now to identify the source of protozoal infection for marine mammals of these parasites, for their sake and also for other animals, including humans, that might become infected by this route. And, if the source of that infection is from sewage or from feral cat feces, then we need the solid scientific data to show this is the case." Jessup adds: "Time is of the utmost essence, because before anyone signs onto this or get too supportive of what we're suggesting, they're going to want to see some pretty convincing proof that these things are, in fact, threatening the health of marine animals and potentially people."

One reason for needing such hard evidence, says Jessup, is that the solutions are going to be expensive, and to a large extent, will involve changing the way sewage, animal waste, and runoff are managed. "The engineers who work with sewage have been telling us that a lot of the sewage infrastructure built around our major cities, [Los Angeles, San Francisco], was all designed in the early part of the last century," he explains. "Not only are these systems old in design, but with the boom in our economy and population, these infrastructures can't hold up." The size of a standard sewage pipe is a mere eight inches, an arbitrary size chosen long ago and that isn't appropriate now, says Jessup. "When you double the population during that time period, how could you expect them to hold up?" he asks. And yet it's still the standard.

Another primary issue to deal with in terms of finding solutions is the method of sewage treatment. "Our waste matter goes into a sewage treatment plant, in either primary or secondary systems, or both," he continues. "Well, protozoal cysts are not killed by either primary or secondary treatment systems, so they can go through this process unharmed as far as we know. If there's more cat feces going into toilets, then it's reasonably likely more is going into the ocean via sewage."

Even though they have not yet secured a solid source of dedicated funds, those at the Marine Wildlife Veterinary Care and Research Center and the UC-Davis Wildlife Health Center are continuing their current investigations and starting work on a variety of pilot programs to uncover all the connections. In the coming months, Miller, Jessup, and Conrad will work with others at the UC-Davis Veterinary School of Medicine--including veterinary epidemiologist Ian Gardner, and marine biologist-parasitologist Kristen Arkush--examining shellfish from areas that seem to fairly high-risk areas for sea otters, screening both the otters and shellfish for evidence of T. gondii, and conducting a variety of lab-based experiments.

"We could certainly use some funding," sighs Jessup. "But even more important than that right now, I think, is the recognition that this is a really serious issue. Recognition on the part of the people and their elected officials would be a very good start."


A.J.S. Rayl ( is a freelance writer in Malibu, Calif.



1. M.A. Miller et al., "Isolation and characterization of parasitic protozoa from a pacific harbor seal (Phoca vitulina richardsi) with meningoencephalitis," Journal of Parasitology, in press.

2. M.A. Miller et al., "Isolation and characterization of Sarcocystis from brain tissue of a free-living southern sea otter (Enhydra lutris nereis) with fatal meningoencephalitis," Experimental Parasitology, in press.

3. N.J. Thomas et al., "Biology and status of the southern sea otter: the risk of disease and threats to the wild population," Endangered Species Update, 13:23-7, 1996.

4. T.K. Graczyk et al., "Detection of Cryptosporidium oocysts and Giardia cysts in the tissuesof eastern oysters carrying principal oyster infectious diseases," Journal of Parasitology, 84:1039-42, 1998.

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