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Minnesota Identifies 3 Different Diseases Affecting Horses
Minnesota Ag Connection - 08/13/2019

Recent disease surveillance testing reveals a trio of rare equine diseases in three different Minnesota counties. Two of the diseases, Eastern equine encephalitis (EEE) and West Nile virus (WNV), are spread by mosquitoes and pose a risk to horses and people. The third disease, Equine infectious anemia (EIA), is not known to affect people, but requires infected horses to be euthanized or quarantined for life because there is no treatment or vaccine. Details surrounding the cases can be found below.

"Mid-July through early September is the highest risk time for EEE, WNV and other mosquito-transmitted diseases in Minnesota," said Minnesota Department of Health Epidemiologist Dave Neitzel. "Minnesotans can protect themselves by wearing mosquito repellents and taking other precautions outlined on the MDH website: www.health.state.mn.us/mosquitoes."

"This is also a high-risk time of year for horses to become infected with these diseases," said Board of Animal Health Equine Program Director, Dr. Courtney Wheeler. "We strongly encourage all horses be vaccinated against EEE and WNV. Horse owners can take additional precautions by working with their veterinarian to evaluate their biosecurity practices, screening for disease in sick horses, and reducing pest populations on their property with simple measures like draining stagnant water and applying screens to barns."

A Minnesota veterinarian reported a case of Eastern equine encephalitis (EEE), a viral disease that causes inflammation of a horse's brain and spinal cord, in a 14-year-old Belgian mare in Otter Tail County. Distribution of the disease in horses has historically been restricted to eastern, southern and southeastern states, although cases have been recently confirmed in Wisconsin. There were also three cases in Minnesota horses in 2001 in Blue Earth, Kanabec and Anoka counties.

The EEE positive horse was euthanized Thursday, August 1 after displaying symptoms of neurologic disease including uncontrolled leg movements, a limp tongue, and inability to rise on her own. Tissues from the horse were submitted to the University of Minnesota Veterinary Diagnostic Laboratory and the Minnesota Department of Health's Public Health Laboratory, which detected EEE virus in the horse's brain tissue. The mare was also tested for West Nile virus, Western equine encephalitis and rabies, which were not detected. The owner of the horse reported she had no significant history of travel off the farm over the past 11 years and no history of vaccination against EEE. At least twenty other horses remaining on the premises appear healthy at this time.

Although the disease is thought to be rare, EEE can cause fatal infections in horses and people. The virus circulates between mosquitoes and birds, and when horses are bitten by mosquitoes carrying the virus, they're considered a "dead-end host" meaning they're unable to transmit the disease to other horses or people. In horses, EEE is fatal in more than 90 percent of cases.

"Even though people cannot contract the disease from horses, cases in horses are a clear indication that infected mosquitoes are in the area and could infect humans," said Neitzel. "Current EEE risk to people is likely highest in the area around the property in Otter Tail County where the horse case was reported."

Clinical illness and fatality in horses can be limited through EEE vaccines and decreased exposure to insects. The Minnesota Board of Animal Health encourages all horse owners to work with their veterinarians to get their horses vaccinated and booster vaccinated horses, especially if housed near the area where the disease was confirmed.

Equine infectious anemia (EIA), a viral disease affecting horses, donkeys, and mules was confirmed by a Minnesota veterinarian in Pine County Aug. 8. The veterinarian reported the affected horse was tested as part of routine screening shortly after being purchased from another Minnesota premises. Animal health officials quarantined the premises and the horse was euthanized. They are investigating to identify any additional potentially exposed horses. EIA poses no known risk to people.

The Minnesota Board of Animal Health requires EIA infected horses be euthanized or quarantined for life because there is no vaccine or treatment, and once a horse is infected it remains a carrier for life and may not display any clinical signs. Horses must have a negative blood test for EIA (Coggins test) within 12 months of importation into Minnesota or before being exhibited at a public event.

"The Board's testing rules are in place to survey for this disease and prevent it from spreading," said Equine Program Director Dr. Courtney Wheeler. "Even if you own a horse that never leaves your property, we encourage testing for EIA routinely and avoiding shows or events where there are no testing requirements."

EIA is primarily spread through horse and deer fly bites. People can also spread it between horses by using contaminated needles or other equipment not sanitized after working with infected horses, mules or donkeys. Biosecurity is the best way to prevent many diseases like EIA. Some best practices include insect and pest control to reduce potentially infected flies from biting animals. Other measures, like cleaning and disinfecting equipment and supplies, and isolating new horses until they're tested for EIA are recommended. Minimizing or eliminating contact between non-exposed and infected horses is instrumental in preventing spread of the disease.

A Minnesota veterinarian reported suspected West Nile virus (WNV) in a 25-year old mare in Swift county. Clinical signs of neurologic illness progressed very quickly. The mare's owners reported the horse fell ill in the evening and was unable to rise the next morning. The horse was euthanized by the attending veterinarian who submitted a blood sample to the University of Minnesota Veterinary Diagnostic Laboratory for WNV testing.

Test results confirmed exposure to WNV (a Plaque Reduction Neutralization Test (PRNT) was positive). The horse had no documented history of vaccination against WNV. Three weeks prior to this case, another horse on the property presented with similar clinical signs and was euthanized but not tested for WNV. Three horses remaining on the premises appear healthy at this time and the owner plans to vaccinate them.

West Nile virus is regularly found in the United States, and birds serve as the primary host of the disease. Similar to EEE, this virus circulates between infected birds and mosquitoes. Once infected, mosquitoes can transmit the virus to horses or people. The virus can cause encephalitis, an inflammation of the brain and spinal cord. Infected horses may or may not show neurological symptoms and many recover completely, especially those who have a history of annual vaccination. While human WNV cases are reported in Minnesota every year, no human cases of WNV have been confirmed in the state in 2019 as of this release.

Vaccines for horses are widely available and have been proven to be effective in preventing infection.

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