San Mateo County EHV Cases Rise to 28

California officials confirm a total of three horses with EHM and 25 with EHV-1.
Please login

No account yet? Register


San Mateo County, CA
As of February 25, California officials have confirmed a total of three horses with EHM and 25 with EHV-1. | Wikimedia Commons
On Feb. 25, California Department of Food and Agriculture (CDFA) officials confirmed two more horses from the San Mateo County index premises where an outbreak began in late January with equine herpesvirus-1 (EHV-1). The new cases exhibited fevers but no neurologic signs and are isolated at the index premises.

As of that date, three cases of EHM (equine herpesvirus myeloencephalopathy, the neurologic form of EHV-1) have been confirmed in San Mateo County along with 25 cases of EHV-1 that exhibited fevers only.

The quarantine will remain in effect until all positive horses have received two consecutive negative tests seven days apart. CDFA continues to monitor the outbreak.

EHV 101

Herpesvirus is highly contagious among horses and can cause a variety of ailments in equids, including rhinopneumonitis (a respiratory disease usually found in young horses), abortion in broodmares, and equine herpesvirus myeloencephalitis (EHM, the neurologic form).

equine herpesvirus
VIDEO | Health Alert: Equine Herpesvirus

In many horses, the first or only sign of EHV-1 infection is fever, which can go undetected. In addition to fever, other common signs of EHV-1 infection in young horses include cough, decreased appetite, depression, and a nasal discharge. Pregnant mares typically show no signs of infection before they abort, and abortions usually occur late in gestation (around eight months) but can be earlier. Abortions can occur anywhere from two weeks to several months following infection with EHV-1.

Horses with EHM usually have a fever at the onset of the disease and might show signs of a respiratory infection. A few days later, neurologic signs such as ataxia (incoordination), weakness or paralysis of the fore- and hind limbs, urine retention and dribbling, loss of tail tone, and recumbency (inability to rise) develop.

Herpesvirus is easily spread by nose-to-nose or close contact with an infectious horse; sharing contaminated equipment including bits, buckets, and towels; or clothing, hands, or equipment of people who have recently had contact with an infectious horse. Routine biosecurity measures, including hygiene and basic cleaning and disinfection practices, should be in place at all times to help prevent disease spread.

Current EHV-1 vaccines might reduce viral shedding but are not protective against the neurologic form of the disease. Implementing routine biosecurity practices is the best way to minimize viral spread, and the best method of disease control is disease prevention.


Written by:

Related Articles

Stay on top of the most recent Horse Health news with

FREE weekly newsletters from

Sponsored Content

Weekly Poll

sponsored by:

What signs does your horse show when he has gastric ulcers? Please check all that apply.
56 votes · 138 answers

Readers’ Most Popular

Sign In

Don’t have an account? Register for a FREE account here.

Need to update your account?

You need to be logged in to fill out this form

Create a free account with!