Michael Porter, DVM, owner of PHD Veterinary Services of Alachua, Fla., says this year's record rainfall in parts of Florida and Georgia may have awakened a "sleeping monster-equine encephalitis."
Depending on which virus is transmitted by mosquitoes, these horses can be infected with arboviruses West Nile or Eastern, Western or Venezuelan equine encephalomyelitis.
Encephalitides are transmitted between mosquitoes and avian hosts, with secondary transmission to the dead-end hosts horses and humans.
If a horse is bitten by an infected mosquito, the viruses can multiply in the blood system, cross the blood brain barrier, and cause inflammation of the brain and interfere with central nervous system functions.
"Due to the excessive rain in Florida," Dr. Porter says, "there are many areas with flooding or sitting water—ideal conditions for the mosquito population. Here in Alachua County, we've had a sentinel chicken already test positive for EEE."
In Florida, he says, EEE has been documented in every month of the year, while West Nile tends to have outbreaks primarily in the late summer or early fall.
In 2000, he was involved in a study at the University of Florida that indicated that "EEE cases are likely anytime of the year."
"EEE has a 90 percent mortality," he says. "It also has an effective, inexpensive vaccine that makes it 100 percent preventable. There is no excuse for a horse not to be vaccinated."
Porter says EEE symptoms present as neurological or central nervous system issues: wobbly gait or staggering, weakness, difficulty in rising after lying down, depression or listlessness, facial paralysis or twitching, teeth grinding, blindness, and fever in about one-third of the cases.
More clinical symptoms present in EEE than in West Nile, Porter says. "If you have a horse that has a high fever and central nervous system issues, it is likely to be encephalitis, though rabies is on the rise, too," he says.
West Nile has a 30 percent mortality rate, Porter says.
Tracy Norman, VMD, of College Station, Texas, says West Nile virus is the most common arboviral problem in horses in Texas, but "we occasionally see WEE, and VEE has also been reported."
Dr. Norman is clinical assistant professor at Texas A&M University's College of Veterinary Medicine & Biomedical Sciences, Large Animal Clinic.
"We have not had any outbreaks this year, but last year was the worst in a long time for WNV," Norman says.
"According to the USDA final report for 2012," says Frank M. Andrews, DVM, MS, Dipl. ACVIM, Baton Rouge, La., "Louisiana had 55 cases of EEE and 65 cases of WNV reported."
Dr. Andrews is board certified in Large Animal Internal Medicine and LVMA Equine Committee professor and director of the Equine Health Studies Program, Department of Veterinary Clinical Sciences, Louisiana State University School of Veterinary Medicine.
"We are most concerned with EEE and WNV," Andrews says. "We make sure that all horses are vaccinated against these viruses.
"West Nile virus is currently on the rise in Louisiana," he says, "but mainly in the human population.
State medical officials are seeing cases earlier than in previous years.
"We have not seen the same situation in horses, but are looking out for cases," Andrews says. "We'll see cases peaking in the September and October months. But last year there were more cases nationwide that the previous years."
Norman says most horses infected with WNV do not exhibit signs of the disease. For those that do, symptoms can include impairment of basic motor skills (loss of coordination or asymmetrical weakness, a change in behavior, or drowsiness).
Some horses with West Nile may have a fever early in the disease and show symptoms such as sensitivity to touch and sound, and muscle twitching in the face, muzzle and neck.
"Sometimes infected horses just appear colicky," she says.
Horses 6 months to 3 years are at highest risk of contracting EEE, Porter says.
They don't have built-up immunity, as most adult and geriatric horses usually have.
Horses that are not immunity-compromised are at lowest risk if they are exposed to the disease.
With West Nile, though, he says geriatric horses may be at more risk of infection and have more difficulty fighting the virus.
He says in cases he saw at UF, when vaccinated horses sickened, or "broke through," it was usually because they were only vaccinated once a year, not twice a year, or had not been vaccinated properly or consistently.
"We encourage veterinarians and clients in our area (Louisiana) to vaccinate against WNV in the spring and again in the fall," says Andrews, "especially if they're using one of the killed vaccine products. If using a vector or newer vaccine, then once per year is adequate."
"Vaccinated horses that do become sick with WNV are in general less sick, requiring less intensive treatment," Norman says. "They are sick for shorter periods of time, and have a better chance at making a full recovery than unvaccinated horses."
She explains that the main treatment for West Nile is supportive care. Anti-inflammatory drugs (Banamine, steroids, and DMSO) and intravenous fluids are often used.
If the horse is having difficulty balancing, a sling can be used for support as it recovers.
"The idea is to keep the horse healthy so it can fight the virus," Norman says. "The mortality rate for West Nile is about 30 to 40 percent.
"Many infected horses will survive, but many of those will have residual neurological impairment. Not all horses will regain their previous performance levels."
"Veterinarians should communicate with owners and use speaking opportunities (4-H, other horse clubs) to remind people of the importance of vaccination," Norman says. "We follow the AAEP guidelines."
Control and prevention of infection is critical. Barn management is just a matter of reminding owners to use "common sense," Porter says.
"Eliminating standing water is obvious," he says. "Treat water buckets with a bit of bleach, and clean them on a regular basis."
Norman reminds owners to spray horses with mosquito spray at dusk and dawn, and she encourages the use of mosquito nets on stall fronts.
"The use of fan on stalls and in barn breezeways creates turbulence that disrupts the mosquito's flight path," says Andrews. "Mosquitoes don't fly well in turbulent air."
"Horses should be kept in the barn during dawn and dusk," he says. "Keep lights off inside and on outside to encourage mosquitoes to stay away from stalled horses."
Andrews also recommends the use of mosquito traps and vacuums, which kill mosquitoes without pesticides.