Two parasites normally associated with land animals have been found in a wide array of marine species discovered dead on the beaches of the Pacific Northwest.
A joint Canada-U.S. study examined 151 marine mammals that had washed ashore - including seals, sea lions, sea otters, dolphin and porpoises - and another 10 sea lions that were healthy when killed in the Columbia River to protect salmon stocks.
All but four of the animals were infected with either Sarcocystis neurona or Toxoplasma gondii - parasites that are shed in the feces of infected cats or opossums, said Michael Grigg of the U.S. National Institute of Allergy and Infectious Diseases, who led the study.
While marine mammals can carry either parasite without becoming seriously ill, Dr. Grigg said 62 of the animals had both parasites, a combination that often proved fatal.
"When the marine mammals were co-infected with both parasites, that's when we saw massive inflammation, brain swelling and death," Dr. Grigg said.
Sarcocystis neurona is not known to infect people, but Toxoplasma gondii causes toxoplasmosis, an illness that is a threat to the fetuses of pregnant women and can be severe or even fatal in people who have compromised immune systems.
Dr. Grigg said the presence of the parasites in marine mammals does not raise any increased human health concerns, however.
"If you filter or boil your water, you have no concerns … [and]if you cook your food, you are fine … these parasites are completely inactivated at 50 degrees C for five minutes," he said.
Dr. Grigg said researchers can't say what route the parasites take to get into the marine mammals, most of which are high up the food chain, but the hypothesis is that the feces of infected animals wash into rivers, then flush into the ocean where they concentrate in filter feeders, such as mussels, or are transmitted by small bait fish.
Dr. Grigg said Toxoplasma gondii caused an outbreak of toxoplasmosis in 1995 in Greater Victoria, where people were infected by drinking unfiltered water from the city's reservoir.
"Epidemiological evidence suggests a couple of wild cats, cougars, had defecated in the environment and after a big storm those feces were washed into the reservoir. Over 4,000 people in the Victoria area were exposed to this parasite, toxoplasma, and … about 100 people ended up going to the hospital," he said.
Stephen Raverty, a veterinary pathologist with the British Columbia government's Animal Health Centre, said one big concern is the devastating impact the two parasites have in combination on marine mammals.
"It was really amazing in terms of how significant some of the lesions were in the brains of these animals that we examined. Very dramatic," Dr. Raverty said. "It really caused you to think of what the potential implications might be. … It's something that would pose a threat to the overall population and status of these animals."
During the five years the study was conducted, from 2004 to 2009, more than 5,000 dead marine mammals were reported on the coastal beaches of the Pacific Northwest. It is estimated only a small percentage of mammals that die at sea wash ashore.
Dr. Raverty said researchers focused only on dead animals that appeared to have been infected by parasites. He said a much wider sample of animals would need to be tested to determine how many mammals died of parasites over all.
The study, which involved researchers from the University of British Columbia, the Washington Department of Fish and Wildlife and Cascadia Research Collective, was published Tuesday in the journal PLoS Neglected Tropical Diseases.
A harbour porpoise that died recently at Vancouver Aquarium, after being rescued off Saltspring Island, had a parasitic infection thought to be caused by either Sarcocystis neurona or Toxoplasma gondii. That animal was not part of the study.