Abstract

Parasitic helminths that are part of the normal flora of free-ranging pinnipeds can also cause morbidity and mortality.¹ Identifying helminth parasites and associated pathology is important for understanding population health, guiding treatment of infected animals in rehabilitation or captivity, and for establishing baselines for monitoring changes in population and ecosystem health. To fill an information gap regarding helminth parasitic infections of Pacific harbor seals (Phoca vitulina) in the Salish Sea, we performed a retrospective analysis of necropsy data and metazoan parasites collected from harbor seals that stranded in San Juan County, Washington (USA) between 2006–2016. The majority of parasitic infections in our study were detected in the gastrointestinal tract of adults (Table 1). Genera of nematodes identified in the gastrointestinal tract include Anisakis, Contracaecum, Pseudoterranova, and Phosascaris - all known gastrointestinal parasites of pinnipeds.¹ Relevant gastrointestinal pathology with Anisakidae infection included eosinophilic, lymphohistiocytic, and fibrogranulomatous gastritis, enteritis, and colitis. One case exhibited low-grade hepatitis, a potential sequela of ectopic parasite migration. Other gastrointestinal parasites detected were Pricetrema zalophi and Diphyllobothrium tetrapterum, neither of which contributed to pathology in any of the animals examined. Nematodes identified in the lungs include Anisakis sp., Anisakis sp. larvae, Contracaecum osculatum complex (trachea), Otostrongylus circumlitis, and Parafilaroides sp. Associated pathology includes eosinophilic bronchopneumonia and verminous bronchointerstitial pneumonia for all lungworms detected except Otostrongylus circumlitis and Contracaecum sp. Lungworms (Parafilaroides spp.) contribute to pneumonia in stranded pinnipeds, and have been linked to transmission of Brucella pinnipedialis in harbor seals.^2,3 While Otostrongylus did not cause significant pathology in these seals, this nematode has been reported as a cause of mortality in juvenile northern elephant seals in California.⁴ Acanthocheilonema spirocauda was detected in the heart of one adult animal that presented with endocarditis. Pseudoterranova decipiens complex is not a described heartworm in pinnipeds, but was identified in the left ventricle of another adult animal without any associated pathology. Most of the observed pathology associated with parasitic infection did not directly contribute to mortality, suggesting a degree of host-parasite adaptation. However, disruption of gastrointestinal or pulmonary mucosa secondary to helminthiasis may have provided portals for secondary bacterial invasion, which did lead to septicemia and death in a few cases. Laminar fibrosis of the small intestine was detected in some cases where Anisakis sp. and Contracaecum sp. were identified. There is a similar chronic eosinophilic gastrointestinal disease in cats with unknown pathogenesis,⁵ but it is possible that eosinophilic inflammation and subsequent fibrosis may be key in development of these lesions in both species. The known route of transmission for most helminth parasites infecting pinnipeds is through ingestion of an intermediate host. However, adult nematodes (Pseudoterranova decipiens complex and an unidentified nematode) were recovered from two nursing pups (Table 1), and vertical transmission of parasites has been hypothesized.⁶ This warrants further investigation. This retrospective analysis of helminth parasite infection and associated pathology is an important first step in understanding the role that parasites play in the health of individual harbor seals as well as in the population.

Table 1. Helminth parasites detected in dead stranded Pacific harbor seals (Phoca vitulina) in San Juan County, WA (2006–2016)

*A single animal may be represented more than once if >1 species of parasite was detected.

Acknowledgments

We thank the numerous volunteers, interns, and employees of the San Juan County Marine Mammal Stranding Network and The SeaDoc Society. Financial support was provided through several grants from the John H. Prescott Marine Mammal Rescue and Assistance grant program.

* Presenting author
+ Student presenter

Literature Cited

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