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Thirty-eight burros from Riverside County have died after an equine influenza outbreak, and more are expected to succumb to the virus, officials announced Friday.

The first deaths occurred about two weeks ago with many in the Reche Canyon area, while six additional deaths were reported in the Moreno Valley foothills along Pigeon Pass Road, Heacock Street and Redlands Boulevard, the Riverside County Department of Animal Services reported.

The equine influenza, which is specific to horses, mules and donkeys, is highly contagious and spreads rapidly through groups of horses in aerosolized droplets dispersed by coughing or through fomite transmission, county officials said.

“It’s important we emphasize to the public regarding the importance of preventing contact between sick burros and healthy horses and donkeys, and avoiding all shared waterers, feeders, and equipment, as well as limiting fence line exposure to the greatest extent possible,” Emily Nietrzeba, an equine specialist veterinarian with the California Department of Food and Agriculture, said in a statement.

Riverside County, Moreno Valley and San Bernardino County officers are assisting with transporting some of the sick burros to the nonprofit organization DonkeyLand for isolation and care. The burros will be vaccinated and monitored there before being released back to the wild, officials said.

Veterinarians from the California Department of Food and Agriculture have also stepped in to help Riverside County’s sick burros.

While humans do not contract the disease, officials said people can act as fomites and transmit the virus between horses. Horse owners specifically are urged to stay away from the burros and consult veterinarians about booster vaccines for previously vaccinated animals, avoid traveling with their animals if they have been exposed to a sick burro and are advised to avoid having visitors with horses come to their properties.

The equine influenza can cause fever, edema and enlarged lymph nodes. Officials said cases are more severe in younger horses, while older horses usually have milder symptoms.

It is estimated approximately 500 burros are in the hills between Riverside and San Bernardino counties.