Introduction

Classification

Taxonomy

• Family: Flaviviridae.
• Genus: Flavivirus.
• Group: Japanese Encephalitis.

Etymology

• Latin: flavus - yellow.

Active Forms

Clinical Effects

Epidemiology

Habitat

• High level viremia occurs in birds.
• Many species of mosquitoes have been shown to be adequate hosts.
• Virus has also been isolated from ticks; their role in transmission is not clear; transtadial transmission may be important for virus maintenance.
• Overwintering mechanism not known.

Lifecycle

• Virus circulates primarily between mosquitoes and birds.
• Inoculation into incidental host results in transient viremia followed by infection of CNS.
• Infection of CNS is enhanced by other factors that compromise the blood-brain barrier.
• Replication occurs in the cellular cytoplasm.

Transmission

• Arthropod-borne transmission.
• Peak of incidence in summer.
• Dogs and cats infected when bitten by infected mosquito.
• Virus can also be transmitted via contact with infected animals, their blood, or other tissues - although this form of transmission is extremely rare.

Pathological effects

• Neuronal and glial infection lead to destruction due to direct viral injury.
• Inflammation also leads to neuronal and astrocyte destruction.
• Dysfunction of neurons without cell death may occur.
• Subclinical infection or mild febrile disease may result in lieu of encephalitis.
• Most (80%) human infections appear to be asymptomatic. Neurologic disease is more likely to develop in older people.

Other Host Effects

• Humans, dogs and horses are incidental hosts, and not a source of virus spread.
• Pathogenicity and virulence may be strain-related.
• There is no specific treatment for WNV infection.
• Full recovery likely.

Control

Control via environment

• Mosquito and tick control, eg permethrins Permethrin.
• There are few commercial products safe to use on cats.
• Minimize time pet spends outdoors at dawn, dusk and evening, when mosquitoes are most active.

Diagnosis

Further Reading

Publications

Refereed papers

• Recent references from PubMed and VetMedResource.
• Hayes E B & Gubler D J (2006) West Nile Virus: epidemiology and clinical features of an emerging epidemic in the United States. Annu Rev Med 57, 181-194 PubMed.
• Karaca K, Bowen R, Austgen L E et al (2005) Recombinant canarypox vectored West Nile virus (WNV) vaccine protects dogs and cats against a mosquito WNV challenge. Vaccine 23 (29), 3808-3813 PubMed.
• Kile J C, Panella N A, Komar N et al (2005) Serologic survey of cats and dogs during an epidemic of West Nile virus infection in humans. JAVMA 226 (8), 1349-1353 PubMed.
• Read R W, Rodriguez D B & Summers BA (2005) West Nile virus encephalitis in a dog. Vet Pathol 42 (2), 219-222 PubMed.
• Austgen L E, Bowen R A, Bunning M L et al (2004) Experimental infection of cats and dogs with West Nile virus. Emerg Infect Dis 10 (1), 82-86 PubMed.
• Ebel G D, Carricaburu J, Young D et al (2004) Genetic and phenotypic variation of West Nile virus in New York, 2000-2003. Am J Trop Med Hyg 71 (4), 493-500 PubMed.
• Buckweitz S, Kleiboeker S, Marioni K et al (2003) Serological, reverse transcriptase-plymerase chain reaction, immunohistochemical detection of West Nile virus in a clinically affected dog. J Vet Diagn Invest 15 (4), 324-329 PubMed.
• Rappole J H & Hubálek Z (2003) Migratory birds and West Nile virus. J Appl Microbiol 94 (Suppl), 47S-58S PubMed.
• Komar N, Panella N A & Boyce E (2001) Exposure of domestic animals to West Nile virus during an outbreak of human encephalitis, New York City, 1999. Emerg Infect Dis 7 (4), 736-738 PubMed.
• Kramer L D & Bernard K A (2001) West Nile virus infection in birds and mammals. Ann N Y Acad Sci 951, 84-93 PubMed.
• Kulasekera V L, Kramer L, Nasci R S et al (2001) West Nile virus infection in mosquitoes, birds, horses and humans, Staten Island, New York, 2000. Emerg Infect Dis 7 (4), 722-725 PubMed.
• Marfin A A & Gubler D J (2001) West Nile encephalitis: an emerging disease in the United States. Clin Infect Dis 33 (10), 1713-1719 PubMed.
• Ostlund E N, Crom R L, Pedersen D D et al (2001) Equine West Nile encephalitis, United States. Emerg Infect Dis 7 (4), 665-669 PubMed.
• Nolen R S (2000) West Nile virus survives winter; no surprise, says CDC. JAVMA 216 (8), 1199-1200 PubMed.
• Rappole J H, Derrickson S R & Hubálek Z (2000) Migratory birds and spread of West Nile virus in the Western Hemisphere. Emerg Infect Dis 6 (4), 319-328 PubMed.
• Blackburn N K, Reyers F, Berry W L et al (1989) Susceptibility of dogs to West Nile virus: a survey and pathogenicity trial. J Comp Pathol 100 (1), 59-66 PubMed.
• Johnson B K, Chanas A C, Shockley P et al (1977) Arbovirus isolations from, and serological studies on, wild, and domestic vertebrates from Kano Plain, Kenya. Trans Roy Soc Trop Med Hyg 71 (6), 512-517 PubMed.
• Sixl W, Batiková M, Stünzner D et al (1973) Haemagglutination-inhibiting antibodies against arboviruses in animal sera, collected in some regions in Austria. II. Zentralbl Bakteriol Orig A​ 224 (3), 303-308 PubMed.
• Topciu V, Roşiu N, Arcan P et al (1971) Contributions to the study of arboviruses in Banat. Rev Roum Inframicriobiol 8 (2), 101-106 PubMed.

Other sources of information

• Hannoun C et al (1969) Epidemiology of West Nile infections in the South of France. In: Bardos Vet al Arboviruses of the California Complex and the Bunyamwera Group. pp 379-387. Vzdavatelstvo Slov. Akad. Vied, Bratislava.

Organization(s)

• ProMED Digest, 1999-2003 www.promedmail.org

Canada

• Health Canada: http://www.hc-sc.gc.ca/dc-ma/wnv-vno/index_e.html

UK

• Wildlife Information Network: www.wildlifeinformation.org

USA

• Center for Disease Control and Prevention: www.cdc.gov
• United States Department of Agriculture - APHIS: www.aphis.usda.gov
• United States Geological Survey, National Wildlife Health Center: www.nwhc.usgs.gov
• West Nile Virus: www.westnilefever.com
• West Nile Virus Maps: http://westnilemaps.usgs.gov