West Nile Virus/Eastern Equine Encephalitis: The Importance of Vaccination
Are your horses vaccinated appropriately against West Nile Virus and Eastern Equine Encephalitis? They should be! Both diseases have been diagnosed in horses in southern Louisiana, and while concern regarding the apparent “outbreak” of these diseases is understandable, horse owners should also know that both are very preventable.
West Nile Virus (WNV) and Eastern Equine Encephalitis (EEE) are viral diseases that are passed to horses by mosquitoes that have been infected following feeding on birds. As a result, these diseases thrive in areas that harbor mosquitoes and their breeding activity – warm, moist regions with stagnant water. While in many states mosquito survival is very seasonal and relatively short-lived, in southern Louisiana, they thrive almost all year round – putting horses at risk for extended periods of time. Hurricanes or other weather events that bring exceptional moisture perpetuate the problem.
After the virus is injected into the horse through a mosquito bite, it spends a few days replicating before it migrates to the nervous system (brain and spinal cord). It is during this time that the very first clinical sign appears: a fever. Fever (rectal temperature greater than 101.5°F; in these cases generally greater than 103°F) usually appears 2-3 days before the onset of neurologic symptoms, and may be accompanied by anorexia (off feed) or lethargy. Often times, the fever has resolved by the time other clinical signs are noticed.
Neurologic symptoms of WNV or EEE in horses are usually very sudden in onset, but otherwise vary widely. Since the WNV primarily affects the back of the brain and spinal cord, these horses often show ataxia (wobbling), stumbling, weakness, or behavior changes (depression or anxiety). A “classic” WNV case includes head and/or body tremors and extreme excitability. On the contrary, the EEE virus prefers forebrain tissue, so clinical signs usually include severe depression or coma, hence the laymen’s term “sleeping sickness”. Horses may go down and be unable to get up, due to severe weakness, incoordination, or depression.
There is no specific treatment for WNV or EEE. Aggressive use of anti-inflammatory medications such as Banamine, steroids, or DMSO may help alleviate some of the inflammation around the brain or spinal cord. Supportive care for horses that cannot eat, drink, or get up on their own include IV fluids, sling support, or feeding through a stomach tube. Complications may arise from being down, including colic, pressure sores, and muscle injury. Unfortunately, even with the state-of-the-art, round-the-clock care, horses that go down and cannot rise usually do not survive.
In general, WNV is less severe, and approximately 60% of horses will survive with appropriate veterinary care. However, almost all (greater than 90%) EEE horses will die or need to be euthanized due to coma, seizures, or an inability to rise. Veterinary care for either WNV or EEE can be very costly, depending on the severity of the disease, and horses that do survive may take several months to return to pre-disease function.
The good news is that these diseases are preventable! Several vaccinations are on the market that have been proven to significantly decrease the incidence of disease in the horse population. In fact, the American Association of Equine Practitioners lists WNV and EEE on their “core vaccines” list, meaning that every horse should receive them.
Vaccinations should be timed to best protect your horse during mosquito season. In southern Louisiana, it is recommended that horses be vaccinated for both WNV and EEE twice per year. The first vaccine should be given towards the end of the cooler weather, but before mosquitoes are present; the second one 6 months later. The very first time a horse receives a vaccine, a booster will also need to be given, approximately 1 month later, to ensure adequate immunity. Vaccines should always be administered in healthy horses, and be handled according to the manufacturer’s recommendations.
Is your horse protected? If not, talk to your veterinarian about an appropriate vaccination schedule. The cost of vaccination is certainly worth it, when you consider the potential outcome of disease.
***It is important to note that WNV and EEE are not contagious, in the sense that one horse cannot pass it to another. However, if a horse is affected, one can assume that the mosquitoes in the area are infected and therefore all other horses are at risk. Also, there are several other neurologic diseases that are contagious, and they are often difficult to differentiate based only on what the horse looks like, so isolation of any neurologic horse until special tests are performed by your veterinarian is recommended.
Written by Dr. Kristi Gran, Baronne Veterinary Clinic