Mosquitoes and the Diseases They Share With Horses

Multiple viral diseases are carried by mosquitoes to horses and humans.
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Culex tarsalis mosquito

WEE is one of the viruses transmitted by the Culex tarsalis species of mosquito.

The news headlines have been filled with stories about the influx of Zika virus into southern Florida and the concern to pregnant mothers due to birth defects. Mosquitoes are the carriers for Zika and other viruses that affect humans, such as malaria, dengue and yellow fever. Horses are not exempt from similar concerns of mosquito-borne viruses; in fact, horses share some of the same viral risks as humans. The viruses that affect horses (and humans) wreak havoc on the neurologic system:

  • Eastern equine encephalitis (EEE)
  • Western equine encephalitis (WEE)
  • Venezuelan equine encephalitis (VEE)
  • West Nile virus (WNV)

In the United States, EEE usually occurs east of the Mississippi River. It is transmitted by Culiseta melanura mosquitoes, and it causes serious neurologic disease in horses, with 90% fatality. Horses with encephalitis appear sleepy, hence the nickname “sleeping sickness.” Besides having a fever, the horse develops an uncoordinated (ataxic) gait and often experiences involuntary muscle twitching. Eventually the horse goes down and is unable to get up. Most cases begin to appear in late summer on into the fall months.

WEE, a similar neurologic disease to EEE, is transmitted by the Culex tarsalis mosquito, which lives in the western part of the United States. Fatality rate is about 40-50%. In recent years, there has been a dramatic drop in equine cases, with none reported in the western United States since 2004. However, this virus is still found in birds and mosquitoes in this geographic area, so the virus is still around.

VEE causes death in 20-80% of horses infected by the virus. For now, it occurs primarily in states bordering Mexico, such as south Texas, New Mexico and Arizona. With climate change, there could be an influx of this virus farther north.

WNV infection does not always cause clinical signs, but it is quite serious when it does. Along with fever, many horses experience problems with their cranial nerves, which are responsible for functions of the head such as chewing, swallowing, blinking and facial muscle tone, to name a few. Infection with WNV also causes other neurologic problems such as hind limb weakness, ataxia, an inability to stand or rise, and paralysis. Other encephalitis signs such as seen with EEE, WEE and VEE are possible, including head pressing, aimless wandering, seizures, hyperexcitability and coma. Fatality rates for WNV are around 33%. Of the horses that recover, about 40% will experience a persistent neurologic deficit.

The interesting thing about these viral infections is that a horse cannot spread them to another horse or human because a mosquito is required as an intermediate vector to cause infection. Birds (and sometimes rodents) carry the viruses, but they don’t become infected themselves. Mosquitoes bite the birds, then those carrier mosquitoes bite a horse (or human). People and horses are considered dead-end hosts because they don’t have enough infective virus in their bloodstream to enable transfer of infection from mosquitoes or body fluids to another animal.

The four viruses listed above are effectively prevented with annual vaccinations. In areas of the country where the weather is mild year-round with persistent mosquito populations, twice annual boosters might be necessary.

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