West Nile Virus


Vague signs, similar clinical presentation to other diseases confound practitioners

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, theannouncement and subsequent use of West Nile Virus vaccine, and the neardaily reporting of cases creeping across the country, has been the factthat veterinarians must try to recognize and diagnose a condition that fewpractitioners have ever seen.

The initial description of West Nile Virus has proven to be superficialand the actual diversity of presentation of confirmed cases has only pointedout that this is yet another disease to add to the oftentimes confusinglist 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 areashave become skilled in diagnosing this new disease.

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

More complete picture

By looking at some of the clinical signs from cases presented to theUniversity of Florida (UF) College of Veterinary Medicine, practitionersin other areas may begin to develop a more complete picture of this diseaseand 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 weeklyat UF's veterinary college. Michael Porter DVM, Ph.D., a resident at theveterinary college has seen his share of such horses-more than 60 casesto date, and is surprised by the wide diversity of clinical signs.

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

Porter adds that some cases exhibited blindness, some were unable touse their tongue to eat, and others showed behavioral changes such as markedaggression, 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 knuckledover on the forelegs when bending down to attempt to eat and some horsesfell down and were unable to get up at all.

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

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

Many horses showed degrees of vision problems ranging from photosensitizationto blindness. Other horses showed paralysis of the hypoglossal nerve andcould 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 exhibitsuch an elevated temperature, and those that did showed it only in the firstor second day of disease.

Vaccinated horses

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

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

Lab info

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

Almost all horses treated for West Nile Virus were also positive forEPM based on spinal tap results. This is thought to be due to the fact thatWest Nile Virus damages the blood brain barrier. Once this damage has occurred,antibodies in the blood can cross-contaminate the spinal fluid in affectedhorses and, more seriously, circulating EPM organisms can gain access tothe 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 resultsand will both be possible diagnoses based on spinal fluid analysis doesnot make it any easier for the practitioner.

Relapse a problem

Porter noted that relapses were a significant problem in the cases ofWest Nile Virus that were treated at the University of Florida. Treatmentfor this disease was mostly supportive and included fluids and nutritionalsupplementation if needed. Many of these horses had some involvement ofthe 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 dailyfor the first three days of treatment.

Antibiotics were often added to the treatment protocol to help withthe many scrapes and abrasions these horses suffered due to their weaknessand/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 horsesshowed an initial positive response to such supportive care but relapsedin four to seven days. Relapse was even more likely if aggressive supportivecare was withdrawn too soon. Porter urged practitioners to "not beafraid to use steroids," especially if an affected horse was to becomerecumbent. 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, thoughrecovery can take up to six months. A yet unknown percentage of horses withWest Nile Virus probably will not return to full function and horses withhind limb weakness tend to take longer to recover.

Differential diagnosis

The differential diagnosis list for cases similar to West Nile Viruscontains many diseases with vague and related signs. Infection with herpesvirus must be considered. These horses can show a fever and neurologicalsigns along with muscle weakness.

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

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

Rabid horses can also exhibit gait abnormalities in some cases. Rabiesis more rapidly progressive than West Nile Virus but, because of its exposurerisk and extremely high mortality, it is not a disease to be taken lightlyand should be considered on the differential list. Appropriate samples ofbrain tissue should be collected from horses that die of suspected WestNile 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 Virusinfection.

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

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

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

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

Most difficult

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

EPM horses rarely develop a fever and can show profound muscle loss whilecases 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 pendinglab results. And, as Porter has already mentioned, most West Nile caseswill also be positive for EPM as well.

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

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

Sorting it all out

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

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

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

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

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

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