Mosquitoes cause skin irritation and itching for horses and can also transmit several diseases. It is important to understand what diseases your horse might be at risk of contracting from mosquitos, how veterinarians identify and treat, how you can help prevent them in the first place.
Common Mosquito-Borne Diseases in Horses
The mosquito-borne diseases veterinarians commonly see in horses in the U.S. are Eastern equine encephalitis (EEE) and West Nile virus (WNV). While slightly less common, Western equine encephalitis can also be transmitted through mosquitoes. The seasons and climate tend to influence the disease transmission levels during the year because mosquitoes are more prevalent in warmer months and regions.
“EEE is primarily transmitted by mosquitoes who pick up the virus from birds, and possibly some mammals, who don’t experience symptoms of the disease themselves,” says Sarah F. Colmer, VMD, DACVIM-LAIM, Fellow at the University of Pennsylvania’s New Bolton Center, in Kennett Square. “However, there are many species that can experience EEE-associated disease after infection by mosquitoes, including horses, cattle, cats, dogs and more.”
Similarly, mosquitoes also transmit WNV after feeding on infected birds and small mammals, but can also spread via medical means. “Transmission of WNV may also occur by way of contaminated blood products (i.e. blood transfusions) or in utero via placental transmission,” says Colmer. WEE has a similar route of contraction to EEE and WNV, she adds.
Signs of Mosquito-Borne Disease in Horses
Clinical signs of mosquito-born disease in an infected horse can be somewhat vague and not evident right away. “Signs may not occur for one to two weeks after infection begins and often involves a transient fever during this time that may be missed,” says Colmer. “Signs are often rapidly progressive when they start, beginning with depression and decreased appetite. They progress to more severe and variable signs of neurologic dysfunction such as head pressing, inappropriate behavior ranging from hyperexcitability to stupor, trouble swallowing, weakness, inability to rise, among many others.”
When evaluating cerebrospinal fluid gathered via spinal tap from affected horses, practitioners might observe different cells depending on the disease, says Colmer. “This can help veterinarians distinguish one disease from another. Fortunately, WNV has a blood test available which can be helpful in some cases.”
Treating Mosquito-Borne Diseases in Horses
Clinically, it can also be difficult to distinguish one mosquito-borne disease from another. However, veterinarians generally treat them the same way—with supportive care, meaning they focus on safely helping the horse survive the effects of the infection.
“For some horses, this may mean hospitalization, particularly if the horse is unable to rise on his own, is severely uncoordinated, or unable to eat or drink,” says Colmer. Some horses might need to be hoisted to help them stand or require intravenous fluids and nutrition that can only be provided in a hospital setting, she adds.
“EEE is often incredibly severe with a high degree of mortality, unlike WEE which is often less severe with lower rates of mortality,” says Colmer. “Some horses with WNV can recover to varying degrees while others experience fatal infections.”
Preventing Mosquito-Borne Diseases in Horses
Horse owners and barn managers can take measures to control mosquitoes around their farms to help prevent horses from contracting these diseases. “Some efforts can be made, however, when possible and within reason,” says Colmer. Minimize standing water around your barn, regularly clean water troughs, and repair ditches that can collect rain.
“Rotating and resting pastures may help to give them an opportunity to dry out and grow,” she says. “Decreasing weeds and overgrowth around bodies of water may make these areas less desirable for mosquitoes as well. However, farms that are in coastal regions or below sea level may have more difficulty than others, particularly when dealing with influences of tide and rain.”
Vaccinating your horse is an important part of prevention—most horses that contract these mosquito-borne diseases do not have a history of regular vaccination, says Colmer. “It is best to discuss vaccination protocols for these diseases with your veterinarian, as some weather and conditions result in more intense mosquito populations. These may be endemic region and lend themselves to recommendations for more frequent vaccination.”
Be sure to minimize the opportunities mosquitoes have to bite horses. Use fly sheets, insect repellents, and fans in stalls (always position the fans so they bring in air from outside the barn) to deter mosquitoes, and be sure to remove trash or anything else from the barn that might attract them.
Take-Home Message
Preventing mosquito breeding by minimizing standing water; protecting your horse from mosquitoes; and keeping your horse up to date on vaccination are the best ways to avoid these potentially fatal conditions.
If you believe your horse might have a mosquito-borne disease, contact your veterinarian immediately for a workup so treatment can begin early.
