West Nile Virus (WNV)

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Introduction: West Nile virus (WNV) is a virus that can cause disease and death in horses, humans, and birds. The virus is common to areas of Africa, western Asia, the Middle East, and some parts of Europe. It was first diagnosed in the United States in New York in the Fall of 1999. Most cases in the United States have been found in the northeastern parts of the country. However, the outbreak in 2000 showed that the virus had spread to areas of Louisiana and Kentucky. During the year 2000 there were a total of 60 clinical equine cases in the United States. Out of the 60 cases, 37 survived and the remaining 23 (38%) died or were put down (euthanized). In 2001 there were 738 reported horse cases. In 2002 this number increased dramatically, with 14,717 cases throughout the United States. In 2002, 40 states (as far west as Washington state) had documented cases of WNV in horses. Experts say that it should spread to all the lower 48 states during 2003. This disease seems to affect animals of all ages and both sexes. Because the virus is spread by mosquitoes, other vertebrate animals such as livestock and poultry can be infected. Wild birds, particularly crows and blue jays, seem to be especially susceptible.

Causative Agent and Disease Transmission: Like the name indicates, this disease is caused by a virus. The virus is spread from place to place by vectors, particularly mosquitoes. Birds (both wild and domestic) harbor the virus and may be a significant means of spreading the disease across long distances. Mosquitoes spread the virus between different bird populations. The virus is then spread to horses and humans by mosquitoes that have taken a blood meal from an infected bird. Current studies suggest that the virus cannot be spread from horse to horse or from horse to human. Horses, humans, and livestock are considered incidental hosts that do not pass the virus on to vectors (mosquitoes) or to other animals.

Clinical Signs: The virus causes encephalitis or inflammation of the brain. Because of this, problems with coordination can be seen. Weakness, muscle trembling, lethargy, and recumbency (laying down) can also occur. Less than one-third of all cases have a fever. Up to 40% of all horses infected with the virus must be put down or die as a result of the infection.

Diagnosis: If WNV is suspected in a horse that is still alive, a blood sample should be collected and sent to the National Veterinary Services Laboratories (NVSL). Sometimes a history of having an increase in bird deaths in the area can also be a clue. If the animal has already died or has been euthanized, different tissue samples should be collected using the following guidelines:

  1. The person performing the necropsy should take precautions to avoid contaminating contact with the virus. However unlikely it may be to contract this disease directly from a horse, use the following suggestions:
    • Prevent any potential viral contact with open wounds and mucous membranes.
    • A face shield or goggles should be worn to protect mucous membranes of the eyes and mouth.
    • Use mechanical saws and sharp instruments only when necessary.
    • Wear disposable coveralls or a water-resistant, long-sleeve apron or gown.
    • Wear three pairs of gloves. The innermost pair should be latex or another type of disposable gloves. A thick pair of waterproof gloves should be worn over the latex. These gloves should be long enough for the gown sleeves to be tucked inside the gloves and duct tape may be used to keep the sleeves inside the gloves. To protect the hands from cuts and puncture wounds, the outermost pair of gloves should be metal or a filleting type glove.
    • A respirator is recommended to avoid breathing any aerosolized virus particles.

  2. The following samples should be collected and sent to NVSL or the state public health laboratory as indicated (these items are listed in order of priority):
    • A fresh brain tissue (for rabies testing) sample. Send this sample to a state public health laboratory. No testing will be performed by NVSL until the horse tests negative for rabies by a state laboratory.
    • A fresh and fixed brain tissue sample. Send this sample to NVSL.
    • A cerebral spinal fluid (CSF) sample, indicating where the sample was collected. Send this sample to NVSL.
    • A fresh and fixed sample of different spinal cord segments (cervical, thoracic, and lumbar). This sample should also be sent to NVSL.

For additional information and suggestions on how to collect and handle these samples, refer to the USDA’s website at www.aphis.usda.gov.

Treatment: Treating this disease involves providing supportive care, or in other words, helping the horse overcome the problems the virus causes in the body. Supportive care treatments for WNV include intravenous fluids, anti-inflammatory agents (DMSO, phenylbutazone), and potentially a sling. As mentioned previously, up to 40% of all horses infected with the virus must be put down or die as a result of the infection.

Prevention: Because mosquitoes are associated with WNV transmission, the first step to prevent outbreaks of WNV among horses is to control mosquito populations. The second step is to prevent mosquitoes from biting the horse.

Ways of reducing mosquito populations include removing areas where stagnant pools of water occur. This may involve draining ponds, aerating pools, cleaning ditches, and cleaning troughs. Realize that a mosquito can breed in stagnant water if it remains for more than 4 days.

To prevent mosquitoes from biting the horse, try the following:

  1. Use screened structures to prevent mosquito contact. Remember to control mosquito populations inside the structure.
  2. Use insect repellents in a wipe-on or spray formula. Products that contain permethrin as the active ingredient are often the safest and the easiest to use. The product Endure (page C202) is an example of an effective spray that can help prevent the transmission of WNV. Page A240 has some additional information about controlling biting insects.
  3. To lower their risk of infection, limit the amount of time the horse spends outside during hours of peak mosquito activity. Mosquitoes like the cool hours at dawn or dusk to feed.

Fort Dodge has produced a vaccine to help prevent WNV. The vaccine is marketed under the name West Nile - Innovator. Fort Dodge has gained a full license for the vaccine. Studies indicate that the vaccine is protective to 95% of the horses that have previously received the initial two dose series of the vaccine. To remain protected, each horse requires an annual booster, thereafter. Studies also indicate that the vaccine is safe. See page C950 for additional details about the vaccine.

Human Health Risks: Similar precautions that are listed under prevention in horses should also be followed by people during a WNV outbreak. Anything that will help prevent mosquito bites can be of benefit. This disease can cause death in people and should be regarded as a serious health risk in certain areas. During 2002, there were 4,156 reported human cases of WNV, including 284 deaths in the United States.

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