Current situation: In mid-June of 2002, West Nile virus (WNV) was first reported in Texas in blue jays in the eastern portion of the state. Since then, positive tests/cases have been reported in mosquitoes, birds (such as blue jays and crows), horses, and humans. There has also been a continued westward movement of the virus.
Species affected: Equines appear to be the only domestic animal adversely affected by WNV. The virus may infect other domestic animals, but they do not show any clinical signs. Some wild birds develop severe clinical signs and may have large-scale die offs.
Transmission: Equine species appear to be a dead-end host and are not involved in transmission of the virus; however, there is a vaccine available for equines. Migrating birds are the primary source of the virus, which is transmitted from them to other animals by mosquitoes.
Clinical signs: In most cases, equines infected with the virus will show no clinical signs. When they do show signs, they may be similar to those produced by other equine encephalitides. The most frequent signs are ataxia, weakness of limbs, recumbency, muscle fasciculation, and death.
Diagnosis: Diagnosis will usually be through serology and/or virus isolation. If WNV or another arboviral encephalitis is suspected, submit serum and/or the brain to the Texas Veterinary Medical Diagnostic Laboratory (TVMDL) for testing. The TVMDL will submit half of the brain to the Department of State Health Services (DSHS) Laboratory for testing.
Precautions: If the animal is showing CNS signs, always assume it may have rabies and exercise the appropriate precautions. When visiting locations that may be mosquito-infested, wear long-sleeves and use insect repellents containing DEET. Other repellent options include picaridin, oil of lemon eucalyptus, and permethrin (on clothing only).
Human disease: Like equines, most people infected with WNV will show no symptoms. Those that do typically develop mild symptoms of disease: fever, headache, body aches, skin rash, and lymphadenopathy. Of those individuals who become ill, a minority will develop more severe symptoms that include meningitis or encephalitis; death can result from infection with WNV.
Surveillance: State and local agencies are conducting active surveillance to detect WNV and determine its spread through Texas. Activities include collecting mosquitoes, plus sampling horses from geographic regions that are at greatest risk (east of the I-35 corridor). In addition, the DSHS attempts to isolate arbovirus from any equine brain that tests negative for rabies.
Contacts: For additional information or guidance on WNV, contact your Regional DSHS Zoonosis Control office, your area’s Texas Animal Health Commission (TAHC) office, or the TAHC headquarters at 1-800-550-8242. Information on WNV can also be obtained at http://www.dshs.state.tx.us/idcu/disease/arboviral/westNile/.