Parasite Spillover: Neospora caninum Infects Marine Mammals in the Pacific Northwest

A concerning trend of terrestrial parasite transmission to marine mammals is emerging in the northeastern Pacific Ocean, with recent research confirming the presence of Neospora caninum in six different marine mammal species. This parasite, known to cause reproductive issues in livestock, represents a third land-to-sea “pollutagen” impacting the health of these vulnerable populations. The findings, led by veterinary pathologist Dr. Stephen Raverty at the British Columbia Ministry of Agriculture, highlight the increasing interconnectedness of terrestrial and marine ecosystems and the potential for disease spillover.

What is Neospora caninum?

Neospora caninum is a protozoan parasite traditionally associated with cattle, where it can lead to abortion storms and reproductive loss . Dogs and wild canids, such as foxes, wolves, and coyotes, serve as both definitive and intermediate hosts. Unlike many coccidian parasites with limited host ranges, N. Caninum has been detected in a growing number of terrestrial and avian species .

The Discovery in Marine Mammals

Researchers investigated stranded marine mammals along the northeastern Pacific coast, analyzing tissue samples using immunohistochemistry and PCR-DNA sequencing. The study identified N. Caninum in eight animals representing six species: two sea otters (Enhydra lutris), two harbor seals (Phoca vitulina), one northern elephant seal (Mirounga angustirostris), one Guadalupe fur seal (Arctocephalus townsendi), and one Steller sea lion (Eumetopias jubatus) . Phylogenetic analysis confirmed that the parasite strain found in these marine mammals was identical to terrestrial strains of N. Caninum, distinct from N. Caninum-like parasites already circulating within marine mammal populations .

How is the Parasite Transmitted?

The transmission route remains under investigation, but several factors are likely contributing. Seasonal haul-out locations used for pupping and breeding, coupled with scavenging by wild canids, may facilitate parasite introduction. For sea otters, bioaccumulation through invertebrate prey is also a potential pathway, mirroring transmission dynamics observed with Toxoplasma gondii . The geographic distribution of infected animals suggests multiple points of exposure, rather than a single source.

Health Implications for Marine Mammals

While the direct impact of N. Caninum on marine mammal health is currently unknown, the parasite’s effects on reproductive fitness in livestock raise concerns. The detection of N. Caninum in a pregnant Steller sea lion, a harbor seal pup, and a northern elephant seal pup suggests the potential for vertical transmission – from mother to offspring – and reactivation of latent infections during pregnancy, similar to what is observed in cattle and dogs .

In the examined cases, N. Caninum infection was not identified as the primary cause of death. However, researchers noted that seven of the eight infected animals also had co-infections with other terrestrial parasites, Toxoplasma gondii and Sarcocystis neurona, suggesting polyparasitism may play a role in disease manifestation .

Broader Implications and Future Research

This discovery underscores the growing phenomenon of land-to-sea pathogen flow, driven by factors like climate change, environmental shifts, and increased human development . The evolutionary relationship between seals and sea lions and terrestrial canids may predispose these marine mammals to infections from canid-derived pathogens. Further research is needed to understand the life cycle of N. Caninum in the marine environment, the role of polyparasitism, and the potential pathogenicity of this parasite in susceptible marine mammal species. Molecular screening for N. Caninum should be considered in investigations of unusual marine mammal deaths, particularly those involving reproductive failures.

The Marine Mammal Research Unit at the University of British Columbia continues to monitor marine mammal populations and investigate emerging threats to their health . The British Columbia Ministry of Agriculture plays a crucial role in animal health surveillance and disease management , and Fisheries and Oceans Canada is responsible for the management of marine mammals in Canada’s Pacific Region .