West Nile Virus

This is placeholder text. To change this content, double-click on the element and click Change Content.

West Nile Virus (WNV) is a member of the Flavivirus genus within the Flaviviridae family. It has a positive-sense, single-stranded RNA genome of approximately 11,000 nucleotides. WNV has significant genetic diversity, with two major genetic lineages, Lineage 1 and Lineage 2, and several proposed lineages.

Etiology:

WNV encodes ten proteins, including three structural (envelope, membrane, and nucleocapsid) and seven nonstructural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5). Lineage 1 is distributed globally, while Lineage 2 has been isolated in various regions, including Africa, Madagascar, the Middle East, and Europe.

Epidemiology and Transmission:

WNV is primarily maintained by ornithophilic Culex mosquitoes and wild birds. The specific mosquito species involved vary by region. In the United States, Culex tarsalis drives epidemic transmission in the western part, while Culex pipiens is the primary vector in the eastern part. Additional vectors include Culex quinquefasciatus and Culex nigripalpus.

Birds, particularly house sparrows and American robins, play a crucial role in the maintenance of WNV in nature. When preferred avian hosts become less abundant, ornithophilic mosquitoes shift their feeding to mammals, increasing the risk of human epidemics.

WNV can also be maintained in non-vector transmission mechanisms, including winter transmission in crow roosts, transmission in Bald Eagles through infected Eared Grebe carcasses, and direct transmission in geese. Chronic infection and vertical transmission from adult female mosquitoes to their offspring can contribute to overwintering of WNV.

Clinical Features:

WNV can cause various clinical features in different species:

- Birds: Avian infections can range from asymptomatic to fatal. Signs of neurologic disease in birds include recumbency, ataxia, tremors, seizures, and depression. American crows exhibit a high case fatality rate, and various pathological signs are observed in affected birds.

- Horses and Nonhuman Mammals: In horses, WNV infection can lead to depression, ataxia, muscle tremors, and recumbency. While dogs and cats typically show mild or no clinical signs, there have been reports of encephalitis, polyarthritis, and myocarditis in these species.

- Humans: WNV infections in humans can be neuroinvasive or non-neuroinvasive. Neuroinvasive cases manifest as meningitis or encephalitis, with symptoms like altered mental status, neurologic dysfunction, and pleocytosis. Complications may include acute flaccid paralysis, myocarditis, Guillain-Barré syndrome, and respiratory failure. Non-neuroinvasive cases present with fever, fatigue, headache, muscle weakness, difficulty concentrating, and sometimes a rash.

It's crucial for veterinary professionals to understand the complex transmission dynamics involving birds, mosquitoes, and mammals to manage and prevent WNV infections effectively. Additionally, knowing the diverse clinical features and complications in different species is vital for diagnosis and treatment.

References:

Trevejo RT. West Nile Virus. Zoonosis Update. JAVMA,Vol 232, No. 9, 2008.
Byas, AD. Comparative Pathology of West Nile Virus in Humans and Non-Human Animals. Pathogens, 2020, 9(1), 48.

Recommended Links:

West Nile Virus. UC Davis Veterinary Medicine. Center for Equine Health.
West Nile Virus (WNV). U.S. Department of Agriculture.
West Nile Virus. Zoonosis Update. Journal of the American Veterinary Medical Association.