What is the West Nile Virus?

“West Nile Virus” is a flavivirus commonly found in Africa, West Asia, and the Middle East. WNV is a mosquito-borne virus that can infect humans, birds, mosquitoes, horses and some other mammals. USDA has announced it considers WNV an endemic virus, much like EEE and other mosquito-borne diseases we are familiar with. Horse owners should take precautions to minimize the impact of WNV exposure and reduce the loss of life for horses from this serious neurological disease.

“West Nile fever” is a case of mild disease in people, characterized by flu-like symptoms. West Nile fever typically lasts only a few days and does not appear to cause any long-term health effects.

More severe disease can be “West Nile encephalitis,” “West Nile meningitis” or “West Nile meningoencephalitis.” Encephalitis refers to an inflammation of the brain, meningitis is an inflammation of the membrane around the brain and the spinal cord, and meningoencephalitis refers to inflammation of the brain and the membrane surrounding it.

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. In the temperate zone of the world (i.e., between latitudes 23.5° and 66.5° north and south), West Nile encephalitis cases occur primarily in the late summer or early fall. In the southern climates where temperatures are milder, West Nile virus can be transmitted year round

What is the disease cycle?

This virus has a normal life-cycle in birds and mosquitoes. 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 abcesses, trauma, liver disease, moldy corn poisoning, and other. Because USDA is now considering WNV an endemic disease (much like EEE), diagnostic testing costs will be more than when USDA was providing free testing. 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.

For More Information:
http://www.ruralheritage.com/vet_clinic/westnile.htm
http://westnilemaps.usgs.gov/
http://www.cdc.gov/ncidod/dvbid/westnile/