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The neurotropathogenic strain of equine herpes virus was confirmed in a 15-year-old Oldenburg mare in Sonoma County on Aug. 23, according to the California Department of Food and Agriculture.
CDFA said the mare subsequently as isolated, quarantined and undergoing treatment at a veterinary hospital.
Horses at the same premises where the mare initially was located had their temperatures monitored twice daily, and on Aug. 24 two horses with the neuropathogenic strain of EHV-1 were confirmed there, the state said.
The two affected horses showed fever as the only clinical sign, state officials said.
Besides a fever of 102 degrees Fahrenheit or greater, other clinical signs the CDFA identified include nasal discharge, lack of coordination, hindquarter weakness and a diminished tail bone, lethargy, urine dribbling, and the horse lying down more than normal.
According to a CDFA brochure on the disease, it spreads from horse-to-horse contact, short distance aerosol transmission – 30 feet or less – and hands, equipment, tack and feed that have been contaminated.
EHV-1 and equine herpes myeloencephalopathy – which is most often caused by the neuropathogenic strain of EHV-1 – cannot be transmitted to humans, CDFA said.
There currently is no US Department of Agriculture-licensed vaccine to protect horses against EHV-1's neurological strain.
The Sonoma County property where all of the horses had been housed was under quarantine, with 13 other horses at the site being monitored, CDFA said. The state's epidemiologic investigation findings indicated there was minimal risk of the disease spreading from the closed premises.
CDFA said the horses affected in Sonoma County were determined to have no direct link to the May outbreak of EHV-1 in cutting horses which resulted in 22 positive cases in California. The following month California declared the EHV-1 outbreak contained.
The May outbreak in California was traced to the National Cutting Horse Association Western National Championship in Ogden, Utah, according to a CDFA report.
Of the 400 competitors at the Utah event, approximately 54 California horses competed, with one of those animals later being euthanized at a Kern County event after showing severe neurological signs, CDFA reported. That horse's final diagnosis was equine herpes myeloencephalopathy.
The 22 California horses – from 12 counties and 14 locations – confirmed to have the disease were diagnosed with EHV-1's neuropathogenic strain, CDFA said.
Of the total cases, eight had neurological signs, 13 had fever only and one horse showed only respiratory signs. The state reported that two of the horses died from the virus.
Juliana Vidich, secretary of the Lake County Horse Council, said the May outbreak caused some local horse-related events to be canceled or delayed, including a gymkhana and a horse expo.
Likewise, an appearance by the Clear Lake Junior Horsemen that had been planned for the May 20 Lake County Sesquicentennial celebration in downtown Lakeport was canceled out of concerns of exposing the horses to the virus, officials reported.
Vidich said she hasn't yet seen such a response to this latest series of cases in neighboring Sonoma County.
“This may the same effect. I hope not,” she said, adding that it's a little early to tell.
Vidich said she will need to consider the possible implications that the new cases might have on a horse event she's planning in October.
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