S. neurona Map
Map of the Western Washington region with the identical cases of S. neurona highlighted. Yellow represents the opossums and green represents the infected harbor porpoise. | Image: Courtesy Alice O'Byrne
Sixteen years ago researchers figured out in the lab that sea otters could be natural intermediate hosts for Sarcocystis neurona, the parasite best known for causing the neurologic disease equine protozoal myeloencephalitis (EPM). The question: Was this possible … much less practical in nature? It would first require the sea otter to be infected by S. neurona’s definitive host, an opossum. And opossums and sea otters aren’t commonly neighbors.

Alice O’Byrne, a graduate entry veterinary medicine student at University College Dublin, in Ireland, set out to solve of the mystery of how S. neurona gets from land to sea in Western Washington. She worked with epidemiologists at the University of California, Davis, and biologists with the Washington Department of Fish and Wildlife (WDFW) to gather marine mammal and opossum samples, process them, and figure out which S. neurona strains were at play in these creatures.

O’Byrne collected 32 fresh road-kill opossums around the region, and WDFW provided 27 samples from marine animals for which the cause of death was suspected to be protozoal encephalopathy. The team used molecular characterization techniques to confirm that the intestinal scrapings of three opossums (9.7%) and the brain samples of 12 marine mammals (40.7%) were positive for S. neurona.

The team further examined the S. neurona, comparing two genetic markers, sn7 and snSAG3, and found that “two opossums and one marine mammal were infected with genetically identical strains of S. neurona,” said O’Byrne, as she showed where the three animals were in relative to one other—the mammal at least 40 kilometers (about 25 miles) away from the opossums

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