OSU Internists Recommend Vaccination of Horses

As most people in Ohio are aware, the first positive West Nile Virus (WNV) bird was detected in July, 2001. This virus was detected in the tissues of a dead blue jay from Lake County, Ohio. That finding establishes that WNV has arrived in Ohio. Since the virus is present in Ohio, and the most at risk populations are crows, blue jays, horses and people, we need to work at preventing this virus from affecting these species.

According to the Ohio WNV Work Group, most of the efforts at preventing WNV infection are directed towards mosquito control. This can be accomplished by eliminating standing water: water may collect in old tires, tin cans, plastic containers, ceramic pots, or similar containers. Clean roof gutters so they drain properly in the spring and fall, clean and chlorinate swimming pools, outdoor saunas and hot tubs, and if not in use, keep empty and covered. Drain water from pool covers. Change the water in bird baths at least once a week, turn over plastic wading pools, and wheelbarrows, etc. when not in use. Non-toxic products such as mosquito dunks can be used to prevent maturation of the mosquito larvae in standing water that cannot be readily eliminated.

The USDA has recently approved a conditional licence for a WNV vaccine for equids, including horses. Dr. Dave Glauer, State Veterinarian, has approved the use of the vaccine in Ohio. The vaccine will be available starting in September of 2001. The vaccine is made with a killed virus and has limited efficacy data, however, a similar vaccine for Japanese Encephalitis virus, which is closely related to WNV, has been used for many years. The efficacy of the Japanese Encephalitis vaccine has been reported at ?80%. Cases of WNV in horses occurred as late as October in 2000 in the eastern US. Therefore, internists at The Ohio State University, College of Veterinary Medicine recommend that if the vaccine is available, horses should be vaccinated twice this fall. The vaccine requires an initial dose and a second dose 3 to 6 weeks later. Horse owners who are interested in vaccinating their horses for WNV should contact their local veterinarian. Veterinarians are being encouraged to place orders so the company will have some indication of the demand for the vaccine.

As part of the safety study of the vaccine, 649 horses and ponies were vaccinated. Systemic or local reactions occurred in very few of the vaccinated horses: 3 horses had localized site reactions (swelling and soreness), 2 horses were reported to have a fever following the initial vaccination only, and 2 horses had other reactions (mild diarrhea within 8 hours of injection in 1 horse and 1 horse was treated for colic within a few hours of the initial vaccination). According to the report filed by Fort Dodge Animal Health, the vaccine did not induce any severe local or systemic adverse reactions in 98.5% of the vaccinations, therefore, the vaccine appears to be safe.

Horse owners planning to export their animals should be aware that vaccinated animals may not meet the import requirements of receiving countries due to the presence of anti-WNV antibodies in blood. While these antibodies are present transiently, presence of the antibody when the horse is due to be exported could result in denial of permission to import.

In horses that do become clinically ill, the virus infects the central nervous system and causes signs of encephalitis. Clinical signs of encephalitis in horses include loss of appetite and depression, in addition to any combination of the following: fever, weakness or paralysis of hind limbs, impaired vision, ataxia (incoordination), head pressing, aimless wandering, convulsions, inability to swallow, circling, hyperexcitability, or coma. Differential diagnoses for neurologic disease in the horse include rabies, botulism, equine protozoal myeloencephalitis (EPM), cervical vertebral myelopathy (CVM), equine herpes (EHV1), equine degenerative myelopathy (EDM) and other encephalitides such as western equine encephalitis (WEE), eastern equine encephalitis (EEE-reportable in Ohio) and Venezuelan equine encephalitis (VEE-reportable in Ohio), should all be considered. Many of the reported cases have similar clinical signs to commonly diagnosed neurologic diseases, particularly EPM. See Ohio State University Web site for aids in differential diagnosis.

Horse owners are encouraged not to become complacent once their horses have been vaccinated. Elimination of mosquito breeding sites and personal protection are still important. Another important role the horse owner may play in our efforts to prevent WNV is to continue to report dead bird sightings to their local health department. The local health department will determine if those dead birds should be transported to the Ohio Department of Agriculture for pathologic examination.