West Nile Flavivirus Polioencephalomyelitis in a harbor seal (Phoca vitulina).
Authors: Del Piero F, Stremme D W, Habecker P L, Cantile C
Journal: Veterinary pathology
Summary
West Nile virus (WNV) typically circulates through bird and mosquito populations, but this 2006 case report documented severe neurological disease in a 12-year-old harbour seal, expanding our understanding of which species can develop clinical infection from this flavivirus. The seal presented with progressive neurological signs including head tremors, muzzle twitching, and clonic spasms; pathological examination revealed polioencephalomyelitis with characteristic lesions including glial nodules, neuronophagia, and ring haemorrhages, with WNV antigens confirmed via immunohistochemistry, viral culture, and reverse transcriptase-polymerase chain reaction. The lesion pattern and clinical presentation mirrored those seen in naturally infected horses and other species, confirming that whilst seals cannot sustain WNV transmission cycles, they are susceptible to developing severe neuroinvasive disease. Although this remains a rare occurrence in marine mammals, the finding underscores that WNV continues to pose an unpredictable zoonotic threat with a broader host range than previously recognised—a consideration for equine practitioners monitoring disease trends and for those working in regions where WNV circulation is endemic or emerging. Clinicians should maintain awareness that flavivirus diseases affecting horses may reflect broader environmental exposure patterns affecting multiple species.
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Practical Takeaways
- •West Nile virus can infect marine mammals including harbor seals with clinical and pathologic presentations similar to equine cases; veterinarians should consider this zoonotic pathogen in neurologic cases
- •The identification of seals as susceptible dead-end hosts expands the known range of species affected by West Nile virus and reinforces the importance of mosquito vector control in endemic areas
- •Clinical signs of progressive neurologic dysfunction in marine mammals warrant investigation for flavivirus infections, particularly in geographic regions where West Nile virus is known to circulate
Key Findings
- •Harbor seal presented with progressive neurologic signs including head tremors, muzzle twitching, clonic spasms, and weakness consistent with West Nile virus infection
- •Histopathologic lesions included polioencephalomyelitis with glial nodules, spheroids, neuronophagia, and ring hemorrhages with intracytoplasmic West Nile flavivirus antigens demonstrated by immunohistochemistry
- •West Nile virus was successfully isolated from cultured cells and identified using monoclonal antibodies and reverse transcriptase-polymerase chain reaction
- •Lesion morphology and distribution in the seal were similar to those observed in equine West Nile virus infection, confirming seals as potential dead-end hosts