Third San Mateo, California, Horse Confirmed with Neurologic Equine Herpesvirus
A third mare in San Mateo County, California, tested positive for neurologic equine herpesvirus (equine herpesvirus myeloencephalopathy).
San Mateo County California map
A third mare in San Mateo County, California, has tested positive for neurologic equine herpesvirus type 1 (EHV-1, also known as equine herpesvirus myeloencephalopathy or EHM). There is no known link between the cases. iStock

On June 6, a 7-year-old Holsteiner mare in San Mateo County, California, tested positive for neurologic equine herpesvirus type 1 (also known as equine herpesvirus myeloencephalopathy or EHM). She was referred to a veterinary hospital after developing a fever and mild neurologic signs and is currently isolated and quarantined.

The mare participated at an event in Sacramento County the previous weekend (while asymptomatic), leaving on May 28 with five other horses from the same barn.

Event management is aware of the situation and has instructed other show attendees to observe biosecurity measures and isolate any horses that attended the show for 14 days once they leave the show property.

Nine other horses at the diagnosed mare’s facility were potentially exposed and are quarantined.

This is the third mare to test positive in San Mateo County this week; however, there’s no known link between this case and the previous cases in the county.

The first mare was confirmed in early June.

The second mare was confirmed on June 6 by the California Department of Food and Agriculture (CDFA) at a boarding facility in San Mateo County,. The 15-year-old Irish Sport Horse mare presented with ataxia (incoordination), dribbling urine and lack of anal tone. She was euthanized. Three hundred horses at the facility were exposed

EDCC Health Watch is an Equine Network marketing program that utilizes information from the Equine Disease Communication Center (EDCC) to create and disseminate verified equine disease reports. The EDCC is an independent nonprofit organization that is supported by industry donations in order to provide open access to infectious disease information.

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 EHM.

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.

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