Equine Herpes Virus (EHV-1) infection in horses can cause respiratory disease, abortion in mares, neonatal foal death and neurological disease.

The neurological form of the disease is known as Equine Herpes Myeloencephalopathy (EHM) and may be caused by damage to blood vessels of the brain and spinal cord associated with EHV-1 infection. EHM is most often due to the neuropathogenic strain of EHV-1, but may occasionally be caused by the non-neuropathogenic strain of the virus.

EHV-1 is easily spread and typically has an incubation period between 2-10 days. Respiratory shedding of the virus generally occurs for 7-10 days, but may persist longer in infected horses.

For this reason, a twenty-one day isolation period of confirmed positive EHM cases is recommended along with stringent biosecurity protocols. Similar to herpes viruses in other species, the latent form of EHV-1 can reactivate at a later date, but generally with a low viral load posing a low risk of infecting other horses. Humans are not at risk of contracting the virus, however humans can act as an indirect mode of transmission.

Recommendations for Participants at Equine Events

CDFA Animal Health Officials are continually monitoring for EHV-1 cases. CDFA reminds horse owners traveling with horses to participate in an equine event, that there is always disease risk when horses of unknown health status are commingled for a show or competition.

CDFA strongly recommends that horse owners practice proper biosecurity when attending an equine event. Compliance with basic biosecurity practices is an important factor in reducing risk of exposure to all contagious equine diseases. Basic biosecurity measures to follow to decrease potential disease spread at equine events include:

• Limit horse-to-horse contact
• Limit horse-to-human-to-horse contact
• Avoid use of communal water sources
• Avoid sharing of equipment unless thoroughly cleaned and disinfected between uses
• Monitor your horse for clinical signs of disease and report any temperature over 102°F to a veterinarian

Here are the latest cases reported by California Animal Health officials:

EHV-1 Quarantines in Riverside County

February 15, 2022: Two (2) additional horses in the quarantined barn at the event premises displaying fever only with no neurological signs were confirmed positive for EHV-1. CDFA continues to be on site at the premises and is actively monitoring the situation.

February 11, 2022: A 12-year-old Zangersheide gelding displaying neurological signs on February 10^th was confirmed positive for Equine Herpesvirus Myeloencephalopathy (EHM) secondary to Equine herpesvirus-1 (EHV-1 non-neuropathogenic strain) on February 11^th. The gelding is currently attending an equestrian event in Riverside County and has been isolated at the event premises. Two (2) additional horses displaying fever only that were housed in the same barn on the premises have also been confirmed positive for EHV-1 and isolated.  The three (3) affected horses in addition to 45 horses housed in the exposed barn on the premises have been quarantined. CDFA staff was onsite at the event premises on February 11^th to conduct a site assessment and assist in implementation of enhanced biosecurity measures including twice daily temperature monitoring and prompt isolation of any horses displaying fever or neurological signs.  CDFA will continue to actively monitor the situation.

EHV-1 Quarantines in San Mateo County

February 16, 2022: One (1) additional horse, a 19-year-old Draft cross gelding, from the index premises displaying fever only with no neurological signs was confirmed positive for EHV-1 on February 16^th, and has been isolated at the index premises. To date, two (2)  EHM cases (EHV-1 positive with neurologic signs) and seventeen (17) EHV-1 febrile only cases have been confirmed. The quarantine will be released when all positive horses have had two (2) consecutive negative tests seven (7) days apart.  CDFA continues to actively monitor the situation.

February 15, 2022: Four (4) additional horses from the index premises displaying fever only with no neurological signs were confirmed positive for EHV-1 on February 15^th, and have been isolated at the index premises. To date, two (2)  EHM cases (EHV-1 positive with neurologic signs) and sixteen (16) EHV-1 febrile only cases have been confirmed. The quarantine will be released when all positive horses have had two (2) consecutive negative tests seven (7) days apart.  CDFA continues to actively monitor the situation.

February 10, 2022: One additional horse from the index premises, a 15-year-old Thoroughbred gelding, displaying fever only with no neurological signs was confirmed positive for EHV-1 on February 9^th, and has been isolated at the index premises. To date, two (2)  EHM cases (EHV-1 positive with neurologic signs) and twelve (12) EHV-1 febrile only cases have been confirmed. The quarantine will be released when all positive horses have had two (2) consecutive negative tests seven (7) days apart.  CDFA continues to actively monitor the situation.

February 8, 2022: Two additional horses from the index premises, an 11-year-old Warmblood gelding and a 20-year-old Warmblood mare, displaying fever only with no neurological signs were confirmed positive for EHV-1 on February 8^th, and have been isolated at the index premises. To date, two (2)  EHM cases (EHV-1 positive with neurologic signs) and eleven (11) EHV-1 febrile only cases have been confirmed. The quarantine will be released when all positive horses have had two (2) consecutive negative tests seven (7) days apart.  CDFA continues to actively monitor the situation.

February 7, 2022: One additional horse from the index premises, a 10-year-old Quarter Horse gelding displaying fever with no neurological signs on February 5th was confirmed positive for EHV-1 on February 7th and has been isolated at the index premises. The quarantine will be released when all positive horses have had two (2) consecutive negative tests seven (7) days apart.  CDFA continues to actively monitor the situation.

February 4, 2022: Two additional horses from the index premises, a 12-year-old Warmblood gelding and a 25-year-old Appaloosa gelding, displaying fever on January 30th and February 3rd respectively with no neurological signs, were confirmed positive for EHV-1 on February 4th.  The Warmblood gelding was transported to a veterinary clinic and has been isolated there, and the Appaloosa gelding has been isolated at the index premises. CDFA continues to actively monitor the situation.

February 2, 2022: One additional horse from the index premises, a 15-year-old Quarter Horse mare, displaying fever only on January 27th was confirmed positive for EHV-1 on February 2nd, and has been isolated on the premises. CDFA was onsite at the premises, and continues to actively monitor the situation. 

January 31, 2022: One additional horse from the index premises displaying neurological signs on January 29th, a 17-year-old Arabian/Quarter Horse gelding, was confirmed positive for EHV-1 on January 31st. He was transported to a veterinary referral clinic on January 29th and remains isolated under quarantine at the veterinary clinic. Three additional horses from the index premises displaying fever only, a 15-year-old Quarter Horse gelding, a 17-year-old Quarter Horse gelding, and a 7-year-old Warmblood/draft gelding have also been confirmed positive for EHV-1. CDFA continues to actively monitor the situation.

January 29, 2022: A 22-year-old Paint gelding with acute onset of neurological signs on January 28^th was confirmed positive for EHV-1, non-neuropathogenic strain, on January 28^th.  The horse was afebrile and isolated at his home premises, and was quarantined along with 300 additional horses on the property.  He was euthanized due to poor prognosis. Two additional horses at the property, a 19-year-old Arabian gelding that was transiently febrile the week prior and sent to a referral hospital on January 24^th, and a 15-year-old Thoroughbred gelding that was transiently febrile on January 24^th, were both tested on January 24^th and confirmed positive for EHV-1.  The Arabian gelding remains isolated and quarantined offsite at the referral hospital.  The Thoroughbred gelding traveled to and from an equine event on January 23^rd prior to clinical signs, and event management was notified.  CDFA was onsite at the premises to conduct a site assessment and assist in implementing enhanced biosecurity measures, including twice daily temperature monitoring. CDFA continues to closely monitor the situation.

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Press release by CDFA