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Dual Parasitic Infections Deadly to Marine Mammals



Toxoplasmosis, the illness caused by Toxoplasma gondii infection, is generally not serious in otherwise healthy people. However the parasites can cause severe or fatal disease in people with compromised immune systems and can also damage the fetuses of pregnant women. The parasites are globally distributed and enter water via infected cat feces. Chlorination does not kill T. gondii, but filtration eliminates them from the water supply. Although Sarcocystis neurona parasites do not infect people, other closely related species of Sarcocystis parasites do.


According to an article published online 24 May 2011in the open-access journal PLoS Neglected Tropical Diseases, a study of tissue samples from 161 marine mammals that died between 2004 and 2009 in the Pacific Northwest reveals an association between severe illness and co-infection with S. neurona and T. gondii. S. neurona, is new to the northwest coastal region of North America and is not known to infect people, there was a large outbreak of T. gondii in people in 1995.


For the study, over the course of the six-year evaluation period, more than 5,000 dead marine mammals were reported on the coastal beaches of the Pacific Northwest. Some deaths ascribed to parasitic encephalitis (brain swelling) were assumed to be caused by T. gondii, because the parasite can infect most mammals and was well established in the region. For the study, specimens were collected and animal autopsies (necropsies) were conducted by members of the Northwest and British Columbia Marine Mammal Stranding Network on 151 marine mammals with suspected cases of parasitic encephalitis. The mammals included several kinds of seals and sea lions, Northern sea otters, a Pacific white-sided dolphin, porpoises and three species of whale. An additional 10 animals, all healthy adult California sea lions that were euthanized in the Columbia River to protect fish stocks, were included in the study as controls. Brain tissue was examined from 108 animals positive for either S. neurona or T. gondii. The number of parasites in the tissues were measured and combined that with an assessment of the degree of brain inflammation to gauge whether the infection was likely to be the primary, contributing or incidental cause of death.


At NIAID, 494 brain, heart, lymph node and other tissue samples were screened with a variety of genetic techniques. The techniques were unbiased in that we do not directly search for any particular species of parasite. Rather, the screens simply reveal evidence of any parasite in the tissue being studied. The team then applied gene amplifying and gene sequencing methods to identify the species and, often, the subtype or lineage of the microbes.


Of the parasites found in 147 of the 161 of animals, 32 were infected with T. gondii, 37 with S. neurona and 62 with both parasites. The remaining 16 infections were caused by various other parasites, including several that had not been detected before in any kind of animal. Notably, all 10 healthy animals were infected with either one or both of the parasites.


According to the authors, the presence of T. gondii was not surprising, but the abundance of S. neurona infections was quite unexpected. The authors have theorized that S. neurona has been introduced into the Pacific Northwest by opossums, which gradually have been expanding their range northward from California and can shed an infectious form of the parasite in their feces. The ample rainfall in the region provides an easy route for infected feces to enter inland and coastal waterways and then contaminate shellfish and other foods eaten by marine mammals.


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