EHM Outbreak in Montana
Horses at three different barns in Flathead County, Montana, are affected by EHM after attending events in the area.
Horses at three different barns in Flathead County, Montana, are affected by EHM after attending events in the area. | Wikimedia Commons

Three horses in Flathead County, Montana, tested positive for equine herpesvirus myeloencephalopathy (EHM), and two additional cases are suspected. The horses are spread across three different barns and developed neurological signs. So far, four of the confirmed and suspected cases have been euthanized due to the severity of disease. The fifth is displaying mild clinical signs and is under quarantine. Exposed horses from the affected premises are also subject to quarantine for 21 days beyond the last date of potential exposure.

Several of the affected horses had no travel history. However, herd mates, who are not displaying clinical signs, had attended events at two or more event centers in the area where they were exposed. Those horses are likely contributing to the disease spread at their home locations and other equine events.

Now, the Montana Department of Livestock is recommending that horse owners and event coordinators in the Flathead Valley limit travel and cancel events that bring horses from multiple sources together through February 6. If additional cases are detected, this date will be extended.

Horse owners who have attended events in the Flathead Valley in recent weeks should monitor their animals for fever or development of neurological signs for two weeks after their last travel.

EDCC Health Watch is an Equine Network marketing program that utilizes information from the Equine Disease Communication Center (EDCC) to create and disseminate verified equine disease reports. The EDCC is an independent nonprofit organization that is supported by industry donations in order to provide open access to infectious disease information.

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 EHM.

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.

Brought to you by Boehringer Ingelheim, The Art of the Horse

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