Recent clinical case highlights importance of equine vaccination against West Nile

Virus and Rabies… 

Case Information:

Signalment: 4 year old, Quarter Horse, filly

History: recently moved to the area from California approximately 2 months ago, vaccination history  –  

owner reports horse was vaccinated in May 2013 with a combination vaccine containing WNV and given

a WNV booster in August upon arrival in Kansas (product information, dates administered, and

documentation unavailable at the time of this posting), no known Rabies vaccination history, owner

noticed significant muscle tremors, walking and stumbling as if drunk, and at times looked as if she was

going to fall down at home, appeared normal the previous day

Upon arrival at Strain Equine Services:

Physical exam  –   rectal temperature, pulse, respiratory rate, and gastrointestinal sounds within normal

limits, mucous membranes were pink, slightly tacky with a mildly delayed capillary refill time

 Neurological exam  –  mild facial paralysis, difficulty with prehension of food (the act of getting feed

material into mouth using lips), normal tongue movement and swallow reflex upon arrival, normal

 pupillary light response and menace response in both eyes, muscle fasciculations of muzzle, chest, and

forelimb musculature, severe forelimb weakness with buckling at the knees, limb placement tests delayed,

difficulty backing (would almost sit down in hindend and drag forelimbs back), minimal hindlimb ataxia

noted with and without head elevation, circling in either direction resulted in abnormal foot placement

and horse losing balance almost to the point of falling

Bloodwork  –   CBC/Chemistry/Fibrinogen: mild elevations of bilirubin (likely related to anorexia from

inability to eat properly) and fibrinogen (inflammatory protein that goes up with any inflammatory insult

within the body)

West Nile Virus and EPM testing was submitted to an outside laboratory.

Differential Diagnoses: West Nile Virus, EPM (Equine Protozoal Myeloencephalopathy), EHV-1

(Equine Herpes Virus-1 neurologic strain), Micronema deletrix, and Rabies Virus

The horse was hospitalized and treated with anti-inflammatories (banamine, dexamethasone, and DMSO),

Vitamin E supplementation, EPM medications, and supportive care including intravenous fluids. Despite

aggressive treatment, the horse’s symptoms rapidly declined and she became recumbent, unable to

swallow, and was humanely euthanized to eliminate any further pain and suffering.

Due to unknown Rabies vaccination status and potential for human exposure, this horse was rabies tested

at Kansas State University. There is no antemortem (while horse is alive) test available for Rabies virus.

The test can only be performed on fresh brain tissue that is harvested upon necropsy examination.

Further testing results:

 

Rabies testing –  Negative

EPM combined SAG 2,3,4 titer  –  1:250 (A majority of horses in our area have been exposed at some

 point to EPM and will have a positive titer, however, in this case the titer is quite low considering the

extreme clinical signs leaving us to believe EPM was not the causative agent of this horse’s neurological

disease.)

West Nile Virus IgM Capture ELISA  –   positive (the laboratory reported it was one of the strongest

 positives they have seen recently) This is a blood test performed using the horse’s serum. Vaccination

does not interfere with the results of this test, as vaccination does not create an IgM response within the

horse’s body. 

Complete vaccination history regarding this particular horse is still pending however there may have been

a break in immunity if the horse never received and a primary vaccination series, involving an initial

vaccination with either a killed or modified-live vaccine followed by a booster given 3-6 weeks later. The

 primary series must occur in order to ellicit optimal antibody production. This primary series does not

 provide longterm protection and therefore it is recommended to booster West Nile vaccinations annually

or biannually depending on exposure risk.

What do we recommend for your horses?

If you live close to a body of standing water or have noticed a high population of mosquitoes on your

 property please consider having a West Nile booster included in your vaccination visit this fall.

We also strongly urge you to include Rabies vaccination as part of your annual vaccination protocol.

Rabies is a fatal disease with zoonotic potential, which means people can acquire the disease from

infected animals. It is easily prevented with a relatively inexpensive annual vaccine.

Read more about West Nile Virus at the following link :

http://www.aaep.org/pdfs/control_guidelines/West%20Nile%20Virus.pdf

Read more about Rabies Virus at the following link:

http://www.aaep.org/pdfs/control_guidelines/Rabies.pdf