Equine Herpes Virus
Equine Herpes Virus–1 Alert
In 2007, raceways, horse shows, farms, and clinics in several states including California were impacted by multiple cases of Equine Herpes Virus–1 (EHV–1) related illness. While 6 horses statewide in 2007 were reported to have died from EHV–1, it is important to review the following:
- EHV–1 Update February 6, 2008:
- EHV–1 has multiple strains that may cause respiratory and neurological disease in horses, as well as abortion in mares. Both a mutant neurotropic strain of EHV–1 and existing latent strains were reported to be involved. Infection may result in no clinical illness or clinical illness ranging from mild clinical manifestations of respiratory disease to occasional cases of severe neurogenic disease. Many times when horses are incubating the virus, fever in excess of 102° F may be the only observable sign of infection. However, fever may not be present in all neurogenic clinical cases.
- While EHV–1 causes a distinct neurological disease, there is no clear evidence that this is the result of “neuronal invasion” (the virus directly invading the nerve cells). Rather, it has been noted that the virus directly damages the walls (endothelium) of the small blood vessels of the neurological system resulting in a loss of blood flow and destruction to the neurological tissue in varying degrees. This process has been termed as the “spinal stroke” phenomenon with the degree of clinical signs correlating to the amount of destruction done to the neurological system.
- It appears that at least 80% of adult horses are latent carriers (have already been exposed to, infected with, and still carry the virus) of EHV–1, and that 1) stress, 2) compromised immune systems, and 3) lack of previous exposure to the virus tend to make the horse more susceptible to becoming clinically ill.
- EHV–1 infected horses, whether clinically ill or not, may periodically shed both active and non–replicating virus in their nasal secretions. If the virus being shed is active, these horses have the potential to spread the virus to susceptible horses.
- Horse–to–horse contact, contaminated hands, equipment, tack, feed, and aerosol transmission may all play a role in its spread.
- It is felt that horses with severe clinical signs of neurological EHV–1 illness often have very large viral loads in both their blood and nasal secretions. These high viral loads are thought to significantly increase the potential for transmission of the disease to exposed horses. Therefore, rapid separation and isolation of identified suspect cases and biosecurity are key elements for disease control.
- As with all contagious diseases, if horses are commingled with strange horses, an unknown degree of inherent risk exists for exposure to EHV–1. Many factors may enhance or reduce the amount of risk. If the choice is made to commingle with a population that has the potential to harbor EHV–1 infected individuals, there is no foolproof way to completely eliminate the risk of exposure.
- The good news is that to date most horses recently known to have had exposure to incubating EHV–1 horses have not developed clinical disease.
- If horses are exposed to new horses, especially in stressful competitive environments or following long distance travel, it is helpful to establish a disease–monitoring plan under the advice of a veterinary practitioner. Temperature monitoring (2x / day) is a tool to be used for a differential diagnosis that could include EHV–1.
- There is realistically no predictive value or significance to PCR diagnostic testing results in the absence of clinical disease consistent with EHV–1 is. However, this testing is quite useful as an adjunct to diagnosis in the presence of clinical disease consistent with EHV–1.
- Currently, there is no equine vaccine that has a label claim for protection against neurological EHV–1 clinical disease.
- EHV–1 is not a regulatory disease in many states. In California, diagnostic facilities are required to report EHV–1 cases on a monthly basis to CDFA. It is crucial to repeat that owners and trainers are advised to work very closely with their veterinary practitioner to develop a preventative plan that is right for their horses.
- Questions regarding California Horse Racing Board (CHRB) requirements for race horses should be directed to the CHRB Equine Medical Director at (916) 263–6000 or visit "http://www.chrb.ca.gov".
- If you have further questions about EHV–1, refer to Web sites for CAHFS Laboratory and also the UC Davis Center for Equine Health:
Contact us for more information:
California Department of Food and Agriculture
Animal Health and Food Safety Services, Animal Health Branch
1220 N Street, Room A–107
Sacramento, California 95814
Telephone: (916) 654–1447
Fax: (916) 653–2215
or send an email to: ahbfeedback@cdfa.ca.gov