West Nile Virus

West Nile Virus

Mar 01, 2002

Just what we need - another disease with possible vague symptoms of depression, mild lameness and subtle neurological signs. Rabies, equine protozoal myelitis (EPM), botulism and encephalitis were not enough.

Now we have to add West Nile Virus.

Lost in all the hype and hysteria of infected birds and mosquitoes, the announcement and subsequent use of West Nile Virus vaccine, and the near daily reporting of cases creeping across the country, has been the fact that veterinarians must try to recognize and diagnose a condition that few practitioners have ever seen.

The initial description of West Nile Virus has proven to be superficial and the actual diversity of presentation of confirmed cases has only pointed out that this is yet another disease to add to the oftentimes confusing list facing today's practitioner.

Some areas of Florida have seen many cases and certain other "hot-spots" have been identified throughout the country. Veterinarians in these areas have become skilled in diagnosing this new disease.

The majority of equine practitioners, however, are looking for signs of a disease that they have only read about. And the bad news is that cases of West Nile Virus vary greatly and often do not follow the brief description of the disease that was first reported.

More complete picture

By looking at some of the clinical signs from cases presented to the University of Florida (UF) College of Veterinary Medicine, practitioners in other areas may begin to develop a more complete picture of this disease and may make it slightly easier to recognize.

Florida has been one of the areas hardest hit by the West Nile Virus.

In the late summer and early fall, numerous cases were arriving weekly at UF's veterinary college. Michael Porter DVM, Ph.D., a resident at the veterinary college has seen his share of such horses-more than 60 cases to date, and is surprised by the wide diversity of clinical signs.

"We have seen hind limb weakness, fore limb weakness and generalized weakness," he says.

Porter adds that some cases exhibited blindness, some were unable to use their tongue to eat, and others showed behavioral changes such as marked aggression, hypersensitization and more. Some horses become very head shy, one exhibited no abnormalities other than a "dog-sitting" posture, and some were clearly neurological.

Some Florida West Nile cases were noted to bite at themselves, some knuckled over on the forelegs when bending down to attempt to eat and some horses fell down and were unable to get up at all.

"Surprisingly enough," according to Porter, "The most commonly noted clinical sign in our West Nile Virus cases was small muscle fascicultions of the lips and muzzle."

He further explained that these small muscle twitches and contractions went along with signs of dysfunction of other cranial nerves.

Many horses showed degrees of vision problems ranging from photosensitization to blindness. Other horses showed paralysis of the hypoglossal nerve and could not use their tongues. These horses had serious difficulty eating. High fever, a sign previously reported as a common finding in this disease, was noted only if the episode was caught early. Many horses did not exhibit such an elevated temperature, and those that did showed it only in the first or second day of disease.

Vaccinated horses

Horses that had been vaccinated with at least one dose of the West Nile vaccine had a higher survival rate than unvaccinated horses. But Porter is quick to add that many other factors may have influenced that statistic since these were field cases and not controlled research cases.

Still, his information shows a 30 percent mortality rate for West Nile Virus cases during 1999-2000 and a 15 percent mortality rate for such cases during 2000-2001. Perhaps the vaccine is helping some horses, and perhaps recognition and treatment has also improved during this time span.

Lab info

Laboratory information on these West Nile cases does not really help to clarify things greatly. Most horses have elevated levels of creatine kinase, usually due to muscle soreness from struggling to avoid tripping or falling, due to weakness or neurological problems. These values can be in the 5,000 range. Complete blood counts are usually within normal limits. Cerebral spinal fluid (CSF) analysis generally shows increased protein and a cell pattern consistent with viral encephalitis.

Almost all horses treated for West Nile Virus were also positive for EPM based on spinal tap results. This is thought to be due to the fact that West Nile Virus damages the blood brain barrier. Once this damage has occurred, antibodies in the blood can cross-contaminate the spinal fluid in affected horses and, more seriously, circulating EPM organisms can gain access to the spinal cord tissue.

West Nile Virus and EPM cause diseases that can be very similar clinically. The fact that both diseases will likely show similar blood chemistry results and will both be possible diagnoses based on spinal fluid analysis does not make it any easier for the practitioner.

Relapse a problem

Porter noted that relapses were a significant problem in the cases of West Nile Virus that were treated at the University of Florida. Treatment for this disease was mostly supportive and included fluids and nutritional supplementation if needed. Many of these horses had some involvement of the hypoglossal nerve and could not eat adequately. Intravenous fluid therapy, soft mashes and even parental nutrition was used in certain cases. Flunixin (Banamine) was given twice daily and DMSO was given intravenously once daily for the first three days of treatment.

Antibiotics were often added to the treatment protocol to help with the many scrapes and abrasions these horses suffered due to their weakness and/or ataxia.

Deep cushioned footing in safe (preferably padded) stalls was optimal, and feet and legs were wrapped for protection as a minimal step. Many horses showed an initial positive response to such supportive care but relapsed in four to seven days. Relapse was even more likely if aggressive supportive care was withdrawn too soon. Porter urged practitioners to "not be afraid to use steroids," especially if an affected horse was to become recumbent. He recommended doses as high as 80 mg per day in some cases.

The average hospital stay for these horses was seven to 10 days, though recovery can take up to six months. A yet unknown percentage of horses with West Nile Virus probably will not return to full function and horses with hind limb weakness tend to take longer to recover.

Differential diagnosis

The differential diagnosis list for cases similar to West Nile Virus contains many diseases with vague and related signs. Infection with herpes virus must be considered. These horses can show a fever and neurological signs along with muscle weakness.

The neurological form of herpes virus infection is more commonly associated with bladder paralysis and may also cause a flaccid tail.

Fortunately, West Nile Virus does not usually cause these specific signs and can be differentiated from herpes on the basis of viral blood titers. The signs of behavioral changes, photosensitization to blindness and aggression in horses may also be seen with rabies.

Rabid horses can also exhibit gait abnormalities in some cases. Rabies is more rapidly progressive than West Nile Virus but, because of its exposure risk and extremely high mortality, it is not a disease to be taken lightly and should be considered on the differential list. Appropriate samples of brain tissue should be collected from horses that die of suspected West Nile Virus and these samples should be tested for rabies as well.


Horses with cases of hepatic encephalopathy can show behavioral changes, aggression and a variety of signs that mimic those seen with West Nile Virus infection.

These horses are generally icteric and blood chemistry values will show a number of abnormalities related to liver function in these horses.

Eastern, Western, Venezuelan and other types of encephalities can present with fever, depression, weakness to ataxia and related signs.

Viral titers to these diseases will confirm a diagnosis but they will certainly be confused with cases of West Nile Virus on the basis of appearance only.

Botulism can cause a horse to present with severe depression, weakness and an inability to use its tongue and eat food. Cervical vertebral malformation in a young horse can show signs of weakness and/or ataxia. These horses will have a normal spinal tap and abnormal findings on cervical radiographs.

Most difficult

EPM will often be the most difficult disease to differentiate from West Nile Virus cases.

EPM horses rarely develop a fever and can show profound muscle loss while cases of West Nile Virus infection rarely show muscle loss.

Because these two diseases can both progress rapidly and be very serious, it is inevitable that some horses will be treated for both infections pending lab results. And, as Porter has already mentioned, most West Nile cases will also be positive for EPM as well.

It is hoped that more specific and rapid testing will be developed that will allow veterinarians to more quickly differentiate these two potentially very similar diseases.

Until then, supportive therapy will benefit horses suffering from either problem and anti-protozoal medication is not associated with significant side effects as to make it a problem to non-EPM horses that are treated prior to obtaining lab confirmation.

Sorting it all out

All this current confusion over diseases with similar clinical signs may find a way of sorting itself out, however.

The report that many horses are being identified with positive serum for West Nile Virus antibodies means that some horses can successfully fight off the virus without developing any clinical signs.

Add to this the fact that outbreaks of West Nile Virus in other countries such as Egypt, Italy and France have quieted down and all but disappeared within two to three years of occurrence and it does seem as if some type of developed natural immunity may be possible for this disease.

This possible natural immunity, along with the use of West Nile Virus vaccine, may drastically reduce the number of cases seen in the future.

Dr. Marcella, a 1983 graduate of Cornell University's veterinary college, was a professor of comparative medicine at the University of Virginia. His interests include muscle problems in sport horses, rehabilitation and other performance issues.