Two parasites, one carried by cats and the other by opossums, are teaming up in the Pacific Northwest to kill seals, otters and other marine mammals.
One of the parasites, Toxoplasma gondii, can infect people, though it's most often found in cat feces. That bug is a known contaminant along the Pacific coast.
The other parasite, Sarcocystis neurona, was a surprising find in the tissues of dead marine mammals, researchers report May 24 in the open-access journal PLoS Neglected Tropical Diseases. Even more surprising was the discovery that infection by S. neurona makes toxoplasmosis, the disease caused by T. gondii infection, worse. The result is brain swelling and death.
"The most remarkable finding of our study was the exacerbating role that S. neurona appears to play in causing more severe disease symptoms in those animals that are also infected with T. gondii," study researcher Michael Grigg of the National Institute of Allergy and Infectious Diseases said in a statement.
Grigg and his colleagues performed necropsies on 151 dead marine mammals who they suspected had parasitic encephalitis, the medical term for the brain swelling caused by parasite infection. They also examined 10 dead California sea lions that had been healthy when they were euthanized to protect fish stocks in the Columbia River.
The team found parasites in 147 of the animals, including all of the healthy sea lions. Thirty-two animals had T. gondii infections, while 37 carried S. neurona and 62 carried both parasites.
Among the animals with parasitic encephalitis as a likely cause of death, animals with both parasites were more likely than those with just one to have severe brain tissue swelling. Because the two parasites are similar, Grigg said, researchers suspected that infection with one would trigger an immune response protective against infection with the other. However, that did not seem to be the case, he said. Pregnant or nursing animals were particularly vulnerable to symptoms from the double infections.
T. gondii enters water via infected cat feces, while researchers suspect that S. neurona has been introduced by opossums, which have been moving northward from California. Rain in the area washes infected feces into waterways, where S. neurona can contaminate the food supply of dolphins, sea lions and other mammals.
"Identifying the threads that connect these parasites from wild and domestic land animals to marine mammals helps us to see ways that those threads might be cut, by, for example, managing feral cat and opossum populations, reducing runoff from urban areas near the coast, monitoring water quality and controlling erosion to prevent parasites from entering the marine food chain," Grigg said.
You can follow LiveScience senior writer Stephanie Pappas on Twitter. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on.