West Nile virus, which is carried by mosquitoes, was first identified in 1999 and infected nearly 1,000 horses per year until 2006, when an effective vaccine was developed. In 2011, there were only 87 cases of West Nile, but the number of reported cases has been on the rise fairly rapidly, with nearly 500 cases reported in 2018. It's suspected that the actual number of cases is higher, but not all cases may be reported or a definitive diagnosis may not be reached, reported Dr. Kate Hepworth-Warren in DVM360.
One reason for the uptick in cases may be that owners are not as concerned with their horses contracting West Nile as they were when the disease was first recorded–meaning they may not be as diligent about vaccinating them. Peak season for West Nile is late September to early October, but the virus can occur any time mosquitoes are present. Ohio, Montana and Pennsylvania had more than 25 cases each of West Nile last year.
West Nile causes ataxia, abnormal gait, muscle twitches, depression, and potentially recumbency. The affected horse may also have a fever. An ELISA test will show if the virus is present. A spinal tap is rarely required, but West Nile can present similarly to Eastern and Western equine encephalitis, so a tap may be in the best interest of the horse to rule out other diagnoses.
There is no specific treatment for West Nile, though supportive care is commonly given. There is hyperimmune West Nile virus plasma available that is not approved by the FDA, but does seem to show promise. Anti-inflammatories, fluids and nutritional support are often given to the affected horse. Mortality rates for horses that have West Nile range from 28 to 33 percent, which is low compared to other viral encephalitides.
The American Association of Equine Practitioners (AAEP) recommends a horse be vaccinated for West Nile every 12 months, or more frequently if mosquitoes are present year-round. West Nile vaccines are extremely effective—horses that have not been vaccinated are twice as likely to die from the virus than those horses that have been vaccinate.
Read more at DVM360.
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