Disease Outbreaks in Horses

Published Nov. 7, 2011

Are any of you out there horse owners? I am, and when we had an equine herpes virus type 1 (EHV-1) outbreak here in the western states this summer, let me tell you, things got pretty interesting.

EHV-1 is a common pathogen, usually producing flu-like symptoms, but it can cause a potentially fatal neurologic disease as well. This summer’s strain of the virus seemed to cause a greater than normal percentage of animals to come down with neurologic symptoms. Vaccines for EHV are widely used but don’t protect well against the neurologic form of the disease.

I board my horse at a small barn just up the road from my house. It is a very laid back place; just a few horse lovers and their "pets" for the most part. When the EHV news hit the fan, we went into full lockdown. Vets, farriers, and other professionals that move from farm to farm were banned from the property for routine care. Horses could leave the premises, but they wouldn’t be allowed back on … you get the idea. I can only imagine how crazy things got at the bigger barns in the region.

The state veterinarians and other powers-that-be did a great job. Thankfully the outbreak was quickly contained and ran its course fairly quickly. In total, 57 horses were confirmed to be infected with EHV-1 and of these 33 developed the neurologic form of the disease; 13 horses died or were euthanized because of EHV-1 during this outbreak.

After my experiences this summer, I took notice of a first-of-its-kind study that was published in the July 2011 issue of Veterinary Record. U.S. veterinarians collected samples from 761 horses, mules and donkeys that were suffering from infectious upper respiratory tract disease and/or neurologic disease over a two year period. These samples were analyzed to determine the frequency with which four common pathogens – equine influenza virus (EIV), equine herpesvirus type 1 and type 4 (EHV-1and EHV-4), and Streptococcus equi subspecies equi – were responsible for the horses’ symptoms.

Of the horses tested, 201 (26.4 per cent) tested positive for one or more of the four pathogens. The highest detection rate was for EHV-4 (82 cases), followed by EIV (60 cases), S. equi subspecies equi (49 cases) and EHV-1 (23 cases). There were 15 horses with double infections and one horse with a triple infection.

The study is continuing with a major goal of determining what other less well-recognized pathogens might be responsible for the diseases seen in the 73.6 percent of cases that did not have EHV-4, EHV-1, EIV or S. equi subspecies equi. I look forward to learning what the veterinarians and other researchers associated with this study find.