aSF601.AlU5 



ANIMAL DISEASE ALERT 



Department of Agriculture • Animal and Plant Health Inspection Service 



West Nile Virus: 

 Protecting Your 



Horses 



How It Spreads 



West Nile virus (WNV) is a mosquito-borne virus that 

 was first detected in this country in 1999. Since then, 

 evidence of the virus has been found in all 48 of the 

 Continental United States. The virus causes 

 encephalitis or inflammation of the brain and has 

 previously been found in Africa, western Asia, the 

 Middle East, and the Mediterranean region of Europe. 

 Only birds are known to infect mosquitoes with West 

 Nile virus, and mosquitoes spread the disease to 

 horses and humans. 



West Nile Virus Transmission Cycle 



f 



Mosquitoes become 

 infected by feeding on 

 infected birds and pass 

 the virus to other birds, 

 animals, and people. 



During periods of blood 

 feeding there is 

 continual transmission 

 between mosquitoes 

 and their bird hosts. 

 Once a bird is infected, 

 it can transmit the 

 disease to mosquitoes 

 for 4 to 5 days. 

 Mosquitoes may feed on 

 the bird's blood, far from 

 the initial location, and 

 become infected with 

 the virus, furthering the 

 spread of the disease. 



The newly infected 

 mosquito may pass the 

 virus to humans or 

 horses. Both horses 

 and people are 

 considered "dead-end" 

 hosts, meaning that 

 while they become 

 infected, they do not 

 spread the infection. 

 Infected horses are not 

 a risk to other horses. 



Clinical Signs of West Nile Virus in Horses 



• Ataxia or stumbling and incoordination 



• Depression or apprehension 



• Weakness of limbs, partial paralysis, or the 

 inability to stand 



• Muscle twitching or 



• Death 



Horses may become infected without showing any 

 clinical signs. Fever is not a common sign. 



1 1 ^ 



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Figure 1 — Clinical signs of WNV include stumbling, incoor- 

 dination, and weakness of limbs. However, horses may 

 become infected without exhibiting any clinical signs. 

 (Photo by Maureen T. Long, College of Veterinary Medicine, 

 University of Florida, and used with permission.) 



Protecting Your Animals 



It is important to take preventive actions early, prior to 

 the time of the year when mosquitoes are likely to bite 

 and infect horses. 



Vaccinate Your Horses 



In November 2002, a vaccine intended to aid in 

 the prevention of WNV in horses was licensed by the 

 Veterinary Services division of the U.S. Department of 



APHIS 91-45-015 



Revised September 2005 



