Equine West Nile Virus

What is West Nile Virus (WNV)? WNV is a mosquito-borne virus that may cause encephalitis in horses and humans. This virus has a normal life-cycle in birds and mosquitoes. Although this disease has been known in the Eastern Hemisphere (Mid-East, parts of Europe, North Africa), it surfaced on Long Island, NY in 1999 for the first time in the Western Hemisphere. It has since spread rapidly south and west and has now been documented in at least 45 states.

What is the disease cycle? This is one of many mosquito borne viral diseases that can cause serious disease in animals and man. Birds serve as the major host and reservoir. Horses, humans, and a number of other mammals (and even reptiles) are considered aberrant hosts. When a mosquito bites a bird that is carrying the virus, there may be enough virus in the blood meal to be infectious when the mosquito later bites another animal, human or bird. When a mosquito bites a horse or man that is sick with WNV, the amount of virus is the blood is so low that, under normal circumstances, there is not enough virus to be infectious when the mosquito next bites an animal/man. There has never been a documented case of transmission of WNV from horse to horse or horse to human. Horses and humans are therefore considered to be "dead end hosts".

What are the symptoms? Many horses that are exposed to WNV will have an adequate immune response and may show no clinical symptoms. Of the horses that become ill, symptoms may vary from mild to severe. Mild cases may just not feel well a few days or go off feed. In a low percentage of cases, perhaps 10% of sick horses, the virus may infect the central nervous system and cause mild to severe encephalitis. The most common symptoms reported in the South Carolina cases are rear leg weakness or paralysis, stumbling, general incoordination, muscle fasciculations (tremors) of the face, drooling and inability to eat or drink well, depression, and in severe cases, recumbency, seizures and death. Fever is not a commonly noted sign.

What are the treatments? Therapy is based on symptomatic treatment and supportive care. A majority of cases (60% to 65%) can be expected to recover, with total recovery from days to weeks, depending on the severity of the case. Anti-inflammatory medications (butezolidin, banamine, IV DMSO), antibiotics against secondary bacterial infections, and IV or stomach-tubed fluids are the most commonly used therapies. Treating veterinarians may use a number of other treatments and medications on a case-by-case basis. Caring for a horse with CNS disease may be difficult and dangerous. Euthanization is usually determined by a poor prognosis and humane considerations. What are other disease ruleouts? It is important to remember that WNV may look like EEE, Rabies and a number of other central nervous system diseases. Because WNV, EEE (WEE, VEE) and Rabies are considered public health disease threats and are reportable in South Carolina, it is important that veterinarians and owners report horses with this type of symptoms to the State Veterinarian and attempt to gain a laboratory diagnosis. Other non-reportable diseases that may look similar are Equine Protozoal Myelitis (EPM), Herpesvirus I (Rhino), or more rarely, cancer, bacterial abscesses, trauma, liver disease, moldy corn poisoning, and other. Serum, cerebral spinal fluid (CSF) and brain tissue are the samples usually recommended for testing. All persons handling a horse suspected of having an infectious CNS disease should take general sanitary precautions to avoid contact of horse's body fluids with eyes or breaks in skin and to wash and disinfect hands, clothing and boots as needed.

Will horses be quarantined with this disease? Because the horse is not considered to transmit the disease, there is no need to quarantine a horse or premise where WNV has been diagnosed. Certainly owners should be sure horses are vaccinated before putting them at facilities where WNV has been diagnosed, since that is evidence that birds and mosquitoes in that area are carrying virus levels high enough to cause disease. The same is true for EEE. Rabies, on the other hand, is cause for extreme caution in handling the sick or rapidly dying horse and any contact with body fluids, and cannot be ruled out except by laboratory testing of brain tissue after the death of the animal. Rabies usually kills within 3-7 days, so longer term illnesses are probably not rabies. In general, CNS cases in horses should be handled as potential public health risks until proven otherwise by time or testing, and definitive laboratory confirmation is important for both horse and public health plans and protocols.

What can a horse owner do to prevent the disease? USDA granted a conditional license for a Fort Dodge WNV Vaccine in the fall of 2000. This means the vaccine is considered safe, pure and to have a reasonable expectation of efficacy. USDA compilation of data from 2001-02, supports the vaccine is very safe and with better than 94% protection with proper vaccination protocol. Recommendations are for horses to receive an initial inoculation, a booster in 3-6 weeks, and repeat boosters in the face of an outbreak, or every 6 months, to best cover mosquito season. It is very important that horses receive the first two vaccinations at least 30 days prior to exposure to the virus for best protection. Although there is record of horses getting West Nile Virus after 1, 2, or even 3 vaccines, the disease is usually milder in those cases. As in most viral diseases, the very young, very old and stressed or immune compromised horses are at most risk. But because our horse population is totally naïve to this virus, WNV can cause serious disease even in healthy adult horses. All horses should receive vaccination in order to provide protection from WNV. What about mosquito control? In addition to vaccination, owners should take practical measures to control and protect from mosquitoes. Using insect repellants (especially those containing DEET), reducing standing/stagnant water sources, aerating ornamental ponds and keeping fish in them, keeping fans on in stalls and run-in sheds, and keeping horses in less mosquito laden areas during dusk until dawn times can all help reduce risk of mosquito transmission to the horse. Owners may check with their county Mosquito Control Officer about local mosquito control plans and activities. Owners should remember to protect themselves as well as the horses from this mosquito-borne disease.