Core Equine Vaccines for EEE and WEE

The two main equine encephalitis viruses in the United States have regional differences.
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mosquito Culex tarsalis

The Culex tarsalis mosquito transmits Western equine encephalitis.

The two main equine encephalitis viruses within the United States have regional differences. Eastern equine encephalitis (EEE) occurs east of the Mississippi River in the eastern USA. It is transmitted by Culiseta melanura mosquitoes and causes serious neurologic disease in horses, with a 90% fatality rate. 

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Horses with encephalitis appear sleepy, hence the moniker “sleeping sickness.” 

Besides having a fever, the affected horse develops an uncoordinated (ataxic) gait and often experiences involuntary muscle twitching. Eventually the horse goes to the ground, unable to get up. Most cases begin to appear in late summer on into the fall months.

Western equine encephalitis (WEE), a similar neurologic disease to EEE, is transmitted by the Culex tarsalis mosquito, which lives in the western part of the USA. The fatality rate is about 40-50%. 

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In recent years, there has been a dramatic drop in equine WEE 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 and annual vaccination should continue for the foreseeable future.

Birds 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 for transfer of infection from mosquitoes or body fluids to another animal.

Both EEE and WEE are considered diseases that every horse should be vaccinated against because of the high fatality rates.

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