Abstract
An increasing human presence in coastal areas may facilitate transmission of pathogens of terrestrial origin to marine species, which may negatively impact the health and population dynamics of coastal marine species. This land-sea connection has been implicated as the transmission pathway for disease in several marine mammal species,1,2,3 either through direct contact with terrestrial species, or as a result of direct or indirect exposure through freshwater runoff. Here we report on the discovery of a facultative terrestrial parasite in the harbor seal (Phoca vitulina), a coastal marine mammal often found in urbanized and industrialized environments. We collected skin biopsies from free-ranging harbor seals from San Francisco Bay (n = 27) and Tomales Bay (n = 26) as part of a larger study on harbor seals from central California. Microscopic examination of hematoxylin and eosin-stained tissue sections revealed the presence of tightly coiled nematode larvae within the ostia of numerous hair follicles of four seals from Tomales Bay. Parasites were characterized by paired lateral alae, platymyarian musculature, and an indistinct, uninucleate digestive tract. Mild chronic superficial dermatitis and perifolliculitis were evident microscopically in association with the intrafollicular parasites. Histomorphological features of the larvae and their presence within hair follicles are consistent with previous reports of the facultative nematode parasite Pelodera strongyloides.4,5,6 Infection is most commonly reported in domestic dogs and livestock, and this is the first published report of P. strongyloides infection in any marine mammal. Because infection typically occurs when the skin comes into contact with infested soil or organic material, harbor seals likely acquired this parasite while resting ashore at or near their capture location. Differences in the prevalence of P. strongyloides infection between harbor seals in San Francisco Bay (0%) and Tomales Bay (15.4%) may be the result of differences in land use practices between these two locations. Land use practices in Tomales Bay are dominated (> 50%) by agriculture, primarily for livestock grazing and dairy farming, and conditions that facilitate proliferation and survival of these free-living nematodes can be associated with livestock agriculture.4,6 This preliminary study suggests the presence of another, previously unrecognized land-sea link for marine mammal disease, and indicates that P. strongyloides infection should be considered as a potential differential diagnosis for dermatitis in marine mammals that use terrestrial resting sites.
Acknowledgements
We thank the National Park Service, US Fish and Wildlife Service, Don Edwards National Wildlife Refuge, Moss Landing Marine Laboratories, and all of the volunteers that assisted with harbor seal captures. We also would like to acknowledge the following organizations that provided funding for the larger study under which these samples were originally collected: the PADI Foundation, Packard Foundation, Harvey Fellowship, and Earl and Ethel Myers Oceanographic and Marine Biology Trust.
* Presenting author
+ Student presenter
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