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March 10, 2022: Health & Safety News

Equine Health Alert: Santa Clara and San Mateo Counties

Equineherpesvirus myeloencephalopathy (EHM) has been detected in multiple horses in several counties in California. If not controlled, it can have significant consequences to the horse industry, infecting thousands of horses due to the extensive mixing of horses at statewide, national, and international horse events.


On March 10th, CDFA issued an updated Equine Health Alert for Santa Clara County and San Mateo County.


• CDFA Alert with link to map of affected areas in California...


To reduce disease spread, on March 3rd, the California State Veterinarian recommended that all hunter/jumper events (the most affected group) be postponed for 28 days, all equine events be postponed for 14 days, and all non-essential horse movements postponed for at least 14 days.


• Read the letter from the CA State Veterinarian...


EHV-1 is easily spread so a 21-day isolation period of confirmed positive EHM cases is recommended along with stringent biosecurity protocols.

Equine Herpesvirus (EHV) — also known as Equine Rhinopneumonitis* — is a highly infectious Alphaherpesviridae found virtually worldwide. There are currently 9 known EHVs. EHV-1, EHV-3 and EHV-4 pose the highest disease risk in the U.S. horse population. EHV-1 and EHV-4 can cause upper respiratory disease, neurological disease, abortions, and/or neonatal death. EHV-3 causes a venereal disease called coital exanthema. EHV has not been shown to be zoonotic.


In recent years there has been an increase in the number of EHV-1 neurologic cases, also referred to as Equine Herpes Myeloencephalopathy (EHM), which may be caused by damage to blood vessels of the brain and spinal cord associated with EHV-1 infection. The virus incubation period is highly variable and can be as long as 14 days. Prognosis is good for horses not affected with the neurologic form and varies for those with neurologic signs.

The goal any time an infectious disease is identified in a group of horses is to reduce the chance of horse-to-horse transmission. This is best achieved by keeping potentially exposed horses isolated and identifying any affected horses as early as possible to treat and further quarantine them. Because fever is commonly the first symptom in EHV, monitoring horses' temperatures twice daily is helpful in early detection. The normal temperature for horses is 99.5-101.5 degrees Fahrenheit at rest. If your horse is on Equioxx or another NSAID, 101.0 degrees Fahrenheit should be the upper limit. If your horse spikes a fever, call your regular veterinarian.


Regular biosecurity measures are always a good idea and become very important during times when the goal is to prevent an outbreak.


Horses that were in close contact with the affected horse should be especially closely monitored and should not have contact with other horses for 7 days.


Biosecurity: Please review and follow the American Association of Equine Practitioners (AAEP Biosecurity Guidelines.


Clinically normal horses housed within the primary perimeter may be permitted segregated exercise periods outside the perimeter. Precautions should be taken, and may include the following:

  • Exercise scheduled after general population’s exercise period to avoid potential virus transfer to unaffected horses/barns

  • Limit the use of shared areas like wash racks or cross ties

  • Shared equipment, if permitted, should be thoroughly disinfected between horses.

  • Direct horse-to-horse contact is to be avoided

  • Prompt post-contact use of alcohol hand sanitizer by individuals having contact with horses during exercise. Contaminated clothing can also be a source of transmission.


Vaccination: There are currently no licensed vaccine products with label claims for prevention or control of EHM. Some EHV-1 vaccines have been shown to reduce nasal shedding of EHV and in some cases, reduce viremia. These products may therefore have some theoretical value against EHM (by reducing viremia), and certainly against spread of the virus by reducing viral shedding in the environment. Booster vaccination of healthy animals in both primary and secondary contagion control perimeters may have some value. Vaccination in these circumstances is controversial, as some authorities speculate that certain aspects of the immune response in recently vaccinated horses with subsequent exposure to EHV-1 may play a role in the development of EHM. While this remains unproven, it is a possibility. The use of vaccination is therefore a risk-based decision. If animals are unvaccinated prior to an outbreak there is unlikely to be time to administer an effective vaccination series in time to provide protection during the at-risk time period. Furthermore, if vaccination is implemented in the face of an outbreak, some horses may develop a slightly elevated temperature post vaccination, which can complicate the implementation of outbreak response. Do not vaccinate clinically ill animals.

Please note that Equine Rhinopneumonitis is not the same as the Equine Rhinitis Virus (A and B strains), which also causes upper respiratory infections. Confusion can arise from because EHV 1/4 vaccines are often referred to as "rhino vaccines." 

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