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West Nile Virus Resistance/Susceptibility

Resistance/Susceptibility to West Nile Virus Symptoms


Gene or region: OAS1

Reference allele: C

Mutant allele: T

Affected Breeds: many


Research Confidence:                             Preliminary, reproduced in studies of other mammals


What it does: 


West Nile virus is a mosquito-borne virus that can infect multiple types of mammals, including horses. Infection can lead to a series of severe symptoms, most commonly encephalitis – acute inflammation of the brain. Severe neurological symptoms develop in ~8% of exposed horses, and once symptoms manifest, the fatality rate ranges from 20-40% and is higher in older horses. Originally found in Africa, it spread to the United States in 1999 and equine cases are currently reported on all continents. Cases of WNV peaked in the USA in 2002, with over fifteen thousands equine cases reported; it has since declined due to the introduction of an equine vaccine.


OAS1 encodes an antiviral protein that activates virus-degrading enzymes within the cell. The particular SNP assessed here occurs in the promoter of OAS1, a DNA sequence that modulates expression of the OAS1 gene.


This testing indicates which genetic variant your horse may have and the resulting "susceptibility" or symptom severity that your horse may experience if he/she contracts West Nile Virus.  It does not indicate presence of the disease or any form of titer indicating past infection.  If your horse is negative (-/-) then he/she is assumed to be in the lowest-risk category for symptom severity.  The presence of one variant (+/-) is detected, then the risk for symptom severity in your horse is moderate.  If two variants are detected (+/+) these results correlated with the most severe symptom onset and your horse is possibly in the highest risk category for symptom onset and severity.

How should risk variant information be used?  For any disease, disorders or medical questions, you should always consult your veterinarian so they may assess this information along with the current knowledge and health history of your horse.  While it is always advisable to inoculate your horse, this information may be helpful in further assessing how often you inoculate based on your horse's disease susceptibility, your situation, and geographical location.  Be sure to consult your veterinarian for the best possible practice when it concerns the health of your horse.



Angenvoort et al., “West Nile viral infection of equids.” (2013) Veterinary Microbiology 167: 168-170. PMID: 24035480


Rios et al., “OAS1 polymorphisms are associated with susceptibility to West Nile encephalitis in horses.” (2010) PLoS One 5: e10537. PMID: 20479874

What might your results indicate?

-/-    Two normal alleles detected may mean your horse has the lowest susceptibility to WNV symptoms.

-/+   One variant allele detected may mean your horse has an increased risk for susceptibility to WNV symptoms.

+/+  Two variant alleles detected may mean your horse has a higher risk for susceptibility to WNV symptoms.

** This is an ongoing study and continued research is needed.  Should you find your horse has contracted and been diagnosed with West Nile Virus, please do contact us to discuss your horse's genetics and resulting degree of illness.  By working together to track and review this disease, we will discover the key to helping our horses in the future!


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