West Nile Virus: Fact Sheet

Cause:

• Virus family: Flaviviridae
• Virus genus: Flavivirus (arbovirus group B)
• West Nile Virus

Species Affected:

• Most mammals, except horses and humans, rarely develop clinical disease when infected naturally (not in an experimental setting)
• This virus has been found in 17 of the 23 orders of birds
• Since 1997 WNV has been associated with morbidity and mortality in an increasing number of avian species
• WNV infection has been confirmed in the USA associated with clinical signs and mortality in Florida Alligators and associated with nervous system symptoms in a Crocodile monitor
• WNV has also been isolated from frogs.

Source:

• Species that have the virus in their blood.

Transmission:

• Transmitted mainly by mosquitoes (Culicidae - Mosquitoes (Family)), but may also be transmitted by ticks (Argasidae - Soft ticks (Family) and Ixodidae - Soft ticks (Family)) and some other arthropods (insects). These insects bite or feed off of infected animals then carry the virus to uninfected animal.

Distribution:

• Current maps of distribution: http://www.cdc.gov/ncidod/dvbid/westnile/index.htm

Field Signs:

• Some infections may show no symptoms
• Clinical signs which may be seen include general signs such as depression, anorexia, weakness, recumbency and weight loss, and/or neurological signs such as abnormal posture of the head or neck, incoordination, inability to stand, tremors, circling, disorientation, impaired vision, and weakness (on one or both sides of the body
• Birds may be found dead without any clinical signs having been observed.

Diagnosis:

• In live birds: Diagnosis is made by testing the blood
• In dead birds: Diagnosis can be made by using animal tissues, preferrably kidney, heart and brain.

Prevention and Control:

• The most effective long-term control strategies are likely to involve the reduction of mosquito breeding habitat
• Quarantine animals that are moved from one area to another
• Vaccination has been developed for use in horses and has also been used in some birds

Human Heath Concerns:

• WNV infection may cause no symptoms
• May cause classical West Nile fever (fever and headache, with or without rash and/or enlarged lymph nodes, and sometimes other symptoms)
• May cause severe meningitis or encephalitis, sometimes with profound muscle weakness or sudden paralysis
• Hepatitis or respiratory signs, have been noted occasionally
• The signs and symptoms of WNV infection are not specific to this disease and are not diagnostic. West Nile fever cannot be distinguished clinically from other viral fevers and WNV meningitis or encephalitis cannot be distinguished clinically from similar syndromes caused by other viruses.

Domestic Animal Health Concerns:

• horses: WNV infection may cause a range of signs from severe neurological signs which may be fatal or require euthanasia or no clinical signs at all
• Neurological signs: stumbling, staggering, incoordination, wobbly gait, weakness or paralysis and recumbency, muscle twitching, blindness, disorientation, lip droop and tooth grinding, and fever may or may not be present
• Signs are non-specific and are not sufficient for diagnosis.