Horse in Santa Clara County, California tests positive for neurologic EHV-1

An undetermined number of other horses have been potentially exposed to the virus; local authorities encourage temperature monitoring and enhanced biosecurity measures.

A 21-year-old Quarter Horse from Santa Clara County, California, has been confirmed positive for the neurologic form of equine herpesvirus, type-1 (EHV-1), and an undetermined number of other horses in the area have potentially been exposed to the virus, according to information from the Equine Disease Communication Center (EDCC).

Microscopic image of EHV virus

EHV-1 most often causes mild-to-moderate respiratory illness (rhinopneumonitis), but the infection occasionally leads to the life-threatening neurologic disease equine herpesvirus myeloencephalopathy (EHM). The mechanisms through which EHV-1 produces neurologic disease are not yet understood.

Learn more: Read “When EHV-1 Turns Deadly.” 

An outbreak of a particularly aggressive strain of neurological EHV-1 in Europe, associated with the show jumping competition circuit there, along with EHV-1 cases in show horses in Florida have veterinary officials on high alert for further spread of the virus.

The California mare began displaying neurological signs, including ataxia and weakness in her hind end, and was confirmed positive on March 23rd, according to the EDCC. The mare is alive and being treated in isolation at a veterinary hospital. The boarding stable she was kept at has been quarantined with enhanced biosecurity measures in place and twice-daily temperature monitoring.

Although the affected horse had no recent travel or links to other EHV-1 cases, the boarding stable did host an event on March 21 and another horse from that facility competed at an event from March 19 to 21, potentially exposing many horses to the virus. 

“Event managers have been notified, and are encouraging all owners of potentially exposed horses to perform twice-daily temperature monitoring, limit movement, and practice enhanced biosecurity measures for the next 14 days,” according to the EDCC. 

There is no vaccine that specifically protects against EHM, so biosecurity is a crucial part of prevention. EHV-1 spreads from horse to horse through nasal discharge or aerosol droplets. Humans can spread the virus via contaminated hands, clothing and equipment.




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