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Deadly Virus Spreads from Horse to Horse

18 horses infected in California, including one in Ventura County.

A potentially deadly equine virus is spreading through the West beyond horses which originally contracted the disease in Ogden, UT. California has been particularly hard-hit, and a new case has turned up in Ventura County.

National media have started to inform the public about the outbreak which was first reported locally by Chatsworth Patch.

For the first time, equine herpes myeloencephalopathy (EHM), a lethal virus caused by equine herpes virus -1 (EHV-1), has been confirmed in two horses that did not attend the cutting in Ogden.  Those horses attended a cutting in Bakersfield on May 13th and are now located in Placer and Sacramento Counties.

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Cutting is an equestrian event that originated from cattle ranches where a cow or steer was separated from the herd for vaccinating, castrating, or sorting.

Closer to home, EHM was also confirmed in a horse located in Ventura County this past weekend.  On Saturday, May 21, the California Department of Food and Agriculture (CDFA) reported that the horse had attended the cutting in Ogden.

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A suspected case of EHM was investigated by the CDFA in a mare that had attended the Rancho Vistadores trail ride in Santa Ynez, May 5 - 12. That horse had not attended the Ogden event or the cutting in Bakersfield.  The CDFA explained, “This mare is exhibiting neurological signs compatible with a number of equine diseases or conditions.”  Although the mare was positive for EHV-1, after three swabs, she tested negative for EHM.

Despite efforts to contain the virus, such as quarantining all horses that attended the Ogden event, EHM has now spread beyond that primary group of horses.  This new development has horse owners throughout the western United States concerned about containment of the potentially deadly strain. 

“I’m concerned about the virus and I’m keeping my horse, Coda, home until the danger passes.”  said Samantha Kahn, a local competitor with the National Barrel Horse Association who boards her horse in Granada Hills.  “No one wants to take take their horses anywhere because a horse can be contagious without showing any signs of being sick.”

The number of horses testing positive in California has risen to 18 and all horses are under State quarantine. The positive confirmed cases are located in the following 12 counties:

Amador County
1 Glenn County 2 Kern County 2 Los Angeles County 1 Marin County 1 Napa County 1 Placer County 3 Plumas County 1 Sacramento County 1 Shasta County 1 Stanislaus County 3 Ventura County 1

On a national scale, it seems that California has been the hardest hit by reporting 18 of the 33 known cases in 8 States.  On May 19, the United States Department of Agriculture (USDA) released a Situation Report in response to the outbreak.  The report indicated that of the 33 positive cases nationally, 32 were contracted at the cutting event in Ogden.  To date, seven horses have been euthanized, one of those horses was at the Bakersfield cutting and died in California.

The USDA estimates a total of 308 horses exposed at Ogden, and 689 secondary exposures nationally.  The report did not include secondary exposure information for California, Minnesota, New Mexico, South Dakota or Utah.

One of the reasons that California may not have reported secondary exposure information is that number may be unknown.  At the Sundance Feedlot in Bakersfield, sick horses arrived from Ogden on May 11th for a cutting to take place on May 13th.  One of the sick horses from Ogden was euthanized on May 13th on site in Bakersfield and the other is being treated at UC Davis.

When other horse owners at Sundance learned that there were sick horses on the premises, they loaded their horses into horse trailers and scattered.  A worker on-site at Sundance described the scene, “It was like someone had yelled ‘Fire!’”  250 horses with potential exposure fled Sundance, and now one of those horses is testing positive for EHM in Placer County.

In an interview with thehorse.com, Tom Lenz, DVM, MS, Dipl. ACT, senior director of equine veterinary services for Pfizer Animal Health warned "EHV is extremely contagious, and many horses may become latently infected-infecting other horses but may never display symptoms themselves.”  This means that a contagious horse may not appear to be sick, but is just a carrier of EHM that can infect other horses.

Twenty five horses remained at Sundance and the facility is now under State quarantine.  It is unknown as to whether or not the State has been able to contact all of the owners of the horses who were on-site when the sick horses arrived from Ogden.  Steve Lyle, Director of Public Affairs at CDFA told Chatsworth Patch, “All participants with complete contact information, as provided by the show, were contacted.”  When asked whether or not all horses from Sundance were quarantined, the CDFA did not respond.

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