By Julie McGwin, DVM, Wisconsin Department of Agriculture, Trade and Consumer Protection Division of Animal Health
Spring means warmer weather, which in turn brings mosquitoes. With mosquitoes come diseases that can affect both humans and animals.
Eastern Equine Encephalitis (EEE) and West Nile Virus (WNV) can cause neurologic symptoms and death in equines (horses, donkeys, mules, zebras) and humans alike. The good news is that there are effective vaccinations; fully-vaccinated equines rarely become ill from these diseases. To maximize effectiveness, these vaccinations should be administered in the spring, prior to the beginning of mosquito season. Most vaccines call for two injections initially and an annual booster, although some veterinarians recommend vaccinations more frequently in high-risk warmer climates. It takes at least two weeks to build up enough antibodies to protect the animal.
EEE and WNV are fatal in approximately 90% and 30% of clinical cases, respectively. Both are reportable diseases in Wisconsin, which means diagnosis should be reported to the Department of Agriculture, Trade and Consumer Protection (DATCP) office – EEE within one day of diagnosis and WNV within 10 days. DATCP reports positive equine EEE and WNV test results to the Equine Disease Communication Center (EDCC) and Wisconsin Division of Health Services (DHS). DHS forwards results to ArboNET, the Centers for Disease Control and Prevention (CDC) arboviral surveillance system. State reporting provides national surveillance for these viruses and other infectious diseases.
In 2020, Wisconsin confirmed 26 cases of EEE in equines, one of the highest numbers in the country. All but two of the equines either died or were euthanized, and all of the animals were unvaccinated or undervaccinated. Additionally, there were two confirmed human cases of EEE in 2020 in Chippewa and Eau Claire counties, one of which was fatal.
There were no confirmed equine cases of WNV in Wisconsin in 2020. According to the CDC, there was at least one confirmed human case in Ozaukee County.
Equines must meet specific criteria to be considered a confirmed positive for EEE or WNV for reporting purposes. Not only must the animals exhibit clinical signs of the diseases, they must also test positive using specific laboratory tests indicative of active acute disease process. A positive PCR test or IgM Capture ELISA test is required for diagnosis of acute disease. Although other tests for EEE or WNV are available, previous infection or vaccination may cause a positive test result. Please remember when submitting a sample for EEE or WNV testing, request PCR or IgM Capture ELISA tests.
Symptoms of EEE and WNV in equines include:
• Appetite loss
• Drooping eyelids and lower lip
• Paralysis or lack of coordination
• Aimless wandering
Equines may be recumbent, exhibit seizures or become unresponsive. This is especially true with EEE, which can be fatal within 24 hours.
These viruses are not contagious between equines. While humans can also be infected by WNV and EEE, the viruses do not pass directly between equines and people. Mosquitoes carry the viruses from infected birds, and the only route of transmission is from a mosquito bite.
The viruses require amplification within particular avian species before the concentration of virus particles is sufficient to allow disease transmission. Due to the amplification required, and because the viruses follow mosquito populations, the threat varies depending on the weather but normally starts in mid- to late summer in Wisconsin and remains until the first killing frost.
In addition to vaccination, mitigation steps that limit equine (and human) exposure to mosquitoes include:
• Removing items that could collect stagnant water such as old tires, tin cans and plastic containers;
• Keeping rain gutters clean and draining properly;
• Cleaning and chlorinating swimming pools, outdoor saunas and hot tubs, and draining water from pool covers;
• Turning wading pools and wheelbarrows upside down when not in use;
• Emptying and replacing water in birdbaths at least once a week;
• Keeping horses in the barn from dusk to dawn, when mosquitoes are most active; and
• Using equine mosquito repellents.
For more information about EEE and WNV, go to datcp.wi.gov and type “EEE” or “WNV” into the search bar.