The Outlook for West Nile Viral Encephalitis in Maryland Horses, 2001

Roger E. Olson, DVM, State Veterinarian
Maryland Department of Agriculture

July 2001

Since the West Nile virus (WNV) first appeared in the United States in the summer of 1999, Maryland state agencies concerned with public, animal, and wildlife health have followed its course among the states. A working group composed of representatives from the Departments of Agriculture (MDA), Health and Mental Hygiene (DHMH), and Natural Resources (DNR) has met at least monthly ever since. It is currently their consensus that Maryland, despite its good fortune in the past two years, is highly vulnerable to the introduction of WNV this summer. The virus has already been identified in three crows found dead in Baltimore this year. And, although many trapped mosquitoes have been tested and found negative, mosquito control experts expect a high population of these pests this season. Horses from Pennsylvania, Delaware, New Jersey, New York and New England were found infected last year. Several recovered but most died or were euthanatized.

Maryland citizens would be well advised, therefore, to protect themselves and their horses from exposure to mosquitoes, to be alert for any symptoms of neurologic disease, and to consult their physician or veterinarian if they suspect WNV or a similar disease such as eastern equine encephalitis (EEE) also likely to reappear in Maryland. What measures can be taken to protect our horses?

It is important that we know where the disease is likely to occur. You can help by reporting any dead crows, blue jays, or raptors (hawks, owls, etc.) to Maryland’s West Nile Virus Call Center at 1-866-866-CROW (2769) from 8 AM TO 8 PM, Monday-Friday, 10 AM to 5 PM on weekends. In most cases you will be contacted and such birds picked up for testing.

Do everything you can to eliminate and discourage breeding of mosquitoes. The most effective action is to eliminate any and all standing water near paddocks and stables. Feed tubs, buckets, old tires, and obstructed rain gutters are some of the most common sources of mosquito larvae. Consider screening of stables and stabling horses at night when most species of mosquitoes are more active. It may be impractical but in some locations may be necessary. Some horse owners report successful use of chemical repellents, but their effectiveness is unproven.

We highly recommend that all horses be vaccinated for EEE. An excellent vaccine is available, but it must be updated or boostered at least annually and preferably twice a year, in late spring or early summer. Consult your veterinarian. It is hoped that a similar vaccine for WNV will soon be available, possibly by late August. Rabies is another disease that can mimic WNV. Vaccination of horses for rabies is also recommended.

Finally, if a horse, pony, donkey, or mule shows any of the following signs or symptoms, consult your veterinarian promptly: apprehension, depression, listlessness, trembling, head shaking, lower lip paralysis, ataxia or staggering, weakness of the hind limbs, inability to stand, paralysis, or acute death. If your veterinarian confirms that there is reason to suspect a neurological disease and if treatment is unsuccessful, we recommend that it be referred to one of MDA’s Animal Health Laboratories for necropsy. Such cases will be examined, tissues collected and appropriately tested, carcasses disposed free of charge except for fees related to optional individual cremation and ash return. This procedure will enhance our surveillance program and provide you with an accurate diagnosis so other horses can be protected. Blood testing of living animals is also available through MDA laboratories which can forward samples submitted by veterinarians to appropriate labs.

These services are available at MDA Animal Labs in: Oakland - 301-334-2185, Frederick - 301-663-9528, College Park - 301-314-1870 (new number), Centreville, 410-758-0846, and Salisbury, 410-543-6610. Please call first. For more information contact Dr. Olson at 410-841-5810; FAX 410-841-5999; or olsonre@mda.state.md.us ; www.mda.state.md.us.geninf/genera9.htm