ANNAPOLIS, Md. – The Maryland Department of Agriculture’s Animal Health program strongly encourages all horse owners to have their equines vaccinated for West Nile virus and other mosquito-borne diseases. This reminder comes after three unvaccinated horses tested positive for West Nile virus in August. Mosquito populations have increased significantly this year due to sustained wet weather, which has lead to an increased risk of mosquito-borne diseases like West Nile virus and eastern equine encephalitis.

“With record rainfall this summer and high mosquito numbers, it is important that horse owners are being proactive and vaccinating their equines against mosquito-borne diseases,” said State Veterinarian Dr. Michael Radebaugh. “We urge horse owners to speak with their veterinarians about vaccinations, and also remind veterinarians throughout Maryland to report any cases of equine arboviruses to the department.”

At the end of August, three horses tested positive for West Nile virus in the following counties: Anne Arundel, Carroll and Washington. The equines stabled in Anne Arundel and Washington counties are responding to treatment and are expected to recover. The Carroll County horse had shown severe neurologic signs and was euthanized.

All three of the horses that tested positive for the virus had not been vaccinated. West Nile virus vaccination is recommended as a core vaccine and is considered an essential standard of care for all horses by the American Association of Equine Practitioners. 

Signs of West Nile virus and eastern equine encephalomyelitis include fever (though not always with West Nile virus), anorexia, head pressing, depression or personality change, wobbling or staggering, weakness, blindness, convulsions, muscle spasms in the head and neck, or hind-limb weakness.

Horse owners can also help during mosquito season by keeping horses inside during dawn and dusk, which are peak mosquito hours, and using topical insect repellents labeled for use on horses.

West Nile virus is a serious and, at times, deadly disease that affects humans and horses. Both humans and horses can contract West Nile virus and eastern equine encephalomyelitis if bitten by a mosquito carrying the virus, but the viruses cannot be transmitted between horses or from horses to people. The viruses normally exist in a cycle between mosquitoes and birds, but occasionally eastern equine encephalomyelitis can be transmitted from mosquitoes to mammals.

Because animals and humans both contract the virus from the bite of an infected mosquito, the department works closely with the Maryland Department of Health every year to monitor virus activity in equines. Veterinarians should use standard infection control precautions when caring for any animal suspected to have West Nile virus or eastern equine encephalomyelitis.

Resources for arboviral and other reportable infectious diseases in equines can be obtained from the Animal Health section of the department’s website. Equine arbovirus testing information can be found on the Maryland Department of Health’s website. More information about the Maryland Department of Agriculture’s mosquito control activities can be found here.