West Nile Update 2015

West Nile is still active in Arizona. We have been fortunate to have fewer cases in horses due to preventative vaccinations. But, it is still out there. In 2014 , there have been over 200 positive mosquito pools in Maricopa County. In 2014, over 100 humans and 5 horse were infected with the disease. Most horses we now see that become affected by this virus were not vaccinated. We are recommending to our clients to vaccinate their horses with the West Nile Virus vaccine. The cost is $37 for the annual vaccine. We recommend vaccinating your horses against West Nile annually in the spring prior to mosquito season. We have had minimal reactions to the vaccination after thousands of shots. Foals must be at least 3 months old. Pregnant mares must be at least 45 days in foal.

As most of you are aware from recent news, West Nile Virus (WNV) has been very active in the U.S. since 1999. The mortality rate is about 30-40%. The reason for the decrease in cases in horses and increase in cases in humans is attributed to the extensive vaccination program of horses in theU.S. During the late summer and fall of 2003,Arizonahad its first positive cases of West Nile Virus in horses. As of May 2004, the virus has been proven to be prevalent in our bird and mosquito population. It is here to stay. We continue to have positive cases every year. Except for two cases, the only positive horses we have seen at our clinic have been in unvaccinated horses.

WNV is a mosquito- borne virus. It is maintained in nature by a bird-mosquito cycle. Thousands of birds nation-wide have died from this disease. Horses and humans can contract the disease from infected mosquitoes only. The disease is not transmissible from horse to human, or vice versa. Moreover, horses are not contagious to each other. Given that mosquitoes are associated with WNV transmission, the key to preventing or controlling outbreaks of WNV is to control mosquito populations and prevent exposure to mosquitoes. This can include dumping stagnate water, using mosquito repellants, keeping lawns and pastures mowed (they like to hide in tall grass) and the use of barn fans may help.

The disease affects the nervous system. The most common signs of WNV infection in horses have been incoordination, depression, weakness of the limbs, muscle tremors or twitching in the muzzle, hypersensitivity to touch & sound, and a horse down and unable to stand. Fever is only detected in 25% of positive cases. Diagnosis is based on clinical signs and a positive blood test. If a horse is positive with clinical signs, they are treated with supportive care and anti-inflammatories. There is now an anti-serum and hyper-immune plasma available for treatment as well.  70% of horses will fully recover; although some may take several months to completely recover.

The WNV vaccine is licensed. There are two companies who now produce the vaccine. We use Zoetis’s vaccine. Study results have indicated approximately 90% protection.  Please be sure to vaccinate your horse as this virus is still prevalent in our area and we see fatalities from this preventable disease.