California Updates Sonoma County EHV Outbreak Numbers

The CDFA reports one neurologic, one febrile, four suspected cases that tested negative, and one euthanized horse.

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California Updates Sonoma County EHV Outbreak Numbers
CDFA recommends that after returning to their home premises, all horses shown at the Sonoma County facility be monitored for fever twice daily for at least seven days and isolated at least 30 feet from other horses. | Photo: Wikimedia Commons
Following the euthanasia of a 6-year-old Warmblood mare that showed neurologic signs and subsequently tested positive for equine herpesvirus-1 (EHV-1) at a Sonoma Horse Park show in Petaluma on July 24, officials from the California Department of Food and Agriculture (CDFA) visited the mare’s home premises in Sacramento County on July 26.

The 18 surviving horses from that premises that attended the Sonoma County show returned to the home premises on Sunday, July 25, and are quarantined there. One horse in this group that exhibited fever on July 24 was confirmed positive for EHV-1 and remains under quarantine at the home premises. The horse has been afebrile (without fever) since July 24.

One horse at the home premises that did not attend the show and previously tested positive for EHV-1 began showing neurologic signs on Monday, July 26, but tested negative for EHV-1. He is being treated at an offsite veterinary facility and is in stable condition.

One additional horse at the home premises that attended the Sonoma County show and has not shown fever also became neurologic on July 26. She was also sent to an offsite veterinary facility for treatment and was confirmed negative for EHV-1 on July 26. She remains alive in stable condition.

Another horse, which was housed in the same Sonoma County show barn as the index case, became febrile with mild neurologic signs on July 24. He was removed to an offsite veterinary hospital for treatment and on July 26, was confirmed negative for EHV-1. He remains alive and in stable condition.

Three remaining horses in the same show barn returned to their home premises on July 25, where they remain isolated with twice-daily temperature monitoring. None have exhibited fever or neurologic signs.

CDFA lists the total number of cases in this incident as one confirmed with equine herpesvirus myeloencephalopathy (EHM, which means EHV-1 positive with neurologic signs); one new confirmed case of EHV-1 (fever and/or edema only) that has subsequently retested negative; and one confirmed case that was euthanized. All other affected horses are under veterinary care.

As of Monday, July 26, following the removal of all known affected and exposed horses from the Sonoma County show grounds, CDFA found no evidence of ongoing EHV-1 spread there. Unless ongoing disease spread is confirmed, a complete facility quarantine will not be enacted there. However, officials urge horse owners and exhibitors to consult with their veterinarians about the risk and participate at their own discretion.

CDFA recommends that after returning to their home premises, all horses shown at the Sonoma County facility be monitored for fever twice daily for at least seven days and isolated at least 30 feet from other horses.

As of July 27, CDFA continued to monitor the situation on the Sonoma County show premises.

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.


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