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West Nile Fever

By: Dr. Melissa Mazan

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What Are the Symptoms of West Nile Fever?

West Nile Fever causes encephalitis (inflammation of the brain) or encephalomyelitis (inflammation of the brain and other portions of the central nervous system). It can be very difficult to distinguish from other causes of encephalomyelitis.

Some of the diseases that can be confused with it include:

  • Equine Protozoal Myeloencephalitis
  • Rabies
  • Eastern, Western, or Venezuelan Equine Encephalomyelitis
  • Equine herpesvirus
  • The signs of West Nile Fever can develop weeks after the initial infection and fever, so it is easy to miss the acute period of infection.

  • Horses are often first described as having a hind limb lameness or unsteadiness before other signs are noticed. This is because the virus seems to have a predilection for the portion of the spinal cord that sends messages to the hind limbs. Horses that are severely affected will then go on to become progressively weaker, ataxic (unsteady gait due to nervous system malfunction), and may eventually become recumbent. It is very difficult to give adequate nursing care to a recumbent horse – simply due to his size. Eventually, some horses that fail to get up within a few days may be euthanized. Affected horses usually maintain normal mentatation until the disease is very far advanced.

  • On the other hand, horses may be lightly affected or the infection may not even be noticed. Horses that survive seem not to have any long-term problems stemming from the infection.

    What Is the Treatment Regimen?

  • There is no specific treatment for West Nile Fever. Your veterinarian may try to reduce brain inflammation by giving DMSO or mannitol. Your veterinarian may give your horse fluids intravenously or via nasogastric tube if he is not drinking adequately.

  • There is little that you need to do for your horse if he is lightly affected. Make sure that his food is palatable, that he has plenty of fresh water available, and keep him separated from other horses so that they don't take advantage of his weakened state to bully him away from his food.

    If your horse is so severely affected that he is down in his stall, then you have your work cut out for you. Horses are by nature difficult to care for when they are recumbent.

  • It will be important to keep him well-padded so that he does not develop sores over bony areas that are in contact with the ground.

  • Your horse should be rolled from one side to the other at least every 6 hours. This will help to avoid skin sores, and will help to prevent his lungs from collapsing – this can happen in a down horse just because of his weight. It is best to try to keep him up as much as possible.

  • It is difficult, but very important to keep your horse's bedding clean and dry.

  • Your horse should be offered frequent, small meals.

  • Your horse should be offered water at least every 4 hours – more frequently in hot, humid weather.

  • Some horses have extreme difficulties urinating or defecating when they are recumbent. Your veterinarian may need to give your horse mineral oil with a nasogastric tube to keep him from getting an impaction colic. It may also be necessary for your veterinarian to pass a urinary catheter to help keep his bladder empty.

    How Does My Veterinarian Diagnose West Nile Fever?

    If your veterinarian suspects West Nile Fever, he will be in touch with the state veterinarian. He will probably take serum (blood) to look for antibodies to West Nile Fever. In some situations, your veterinarian may choose to take a sample of the cerebrospinal fluid – antibodies in the CSF is good evidence of West Nile Fever.


    West Nile Fever is an uncommon cause of neurologic disease in the horse, but we still know relatively little about how this disease will progress and spread in the United States. Use the procedures discussed above to keep down your mosquito population – but don't panic. Remember that the odds of you or your horse being bitten by an infected mosquito are low, and the odds of becoming infected are even lower. Call your veterinarian immediately if you see signs of abnormalities in your horse – weak gait, especially in the hind end, recumbency, or generalized weakness may be the first signs. Watch for large-scale die-offs of birds, especially crows, in your area. This may be a warning that West Nile Fever is in your area.

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