Disease spreads invisibly, often thanks to horses shedding disease but not showing signs. In equine disease outbreaks a veterinarian often wears the hats of sleuth, adviser, and sometimes unpopular gatekeeper/rule enforcer.

At the 2016 American Association of Equine Practitioners Convention, held Dec. 3-7 in Orlando, Florida, Peter Morresey, BVSc, Dipl. ACT, Dipl. ACVIM, of Rood & Riddle Equine Hospital in Lexington, Kentucky, described how to investigate and control infectious respiratory disease outbreaks.

Leading causes of these outbreaks in equine populations include equine herpesvirus-1 and -4 and equine influenza virus, as well as equine arteritis virus and strangles (Streptococcus equi subspecies equi).

Morresey reviewed what questions to ask when faced with an outbreak:

  • What is the cause of the respiratory disease?
  • What is the timeline of the appearance of infectious and clinically affected cases?
  • How long will it take for the disease to spread through the exposed population (given the timeline of the initial cases)?
  • How long will naive populations—that is, horses that have never been exposed to the disease-remain at risk once sick horses have recovered?
  • Will any horses become “reservoirs” for disease, intermittently shedding the disease-causing organism at random points in the future?
  • When is it safe to transport, breed, show, or sell recovered and/or exposed horses without risk of infecting other horses and disseminating disease?
  • Will the disease-causing pathogen persist in the environment, lying low until a future date to sow chaos?

Realistically, few answers ever present themselves during an outbreak investigation. So instead of philosophizing about, discussing, and debating them, Morresey advocates action.

“Controlling an infectious respiratory disease involves two main strategies: isolation of affected individuals and identifying then segregating horses exposed to clinical cases,” he said.

Unfortunately, the success of these tactics depends on many factors beyond any veterinarian’s control. One such example pertains to the proportion of disease transmission prior to the onset of the first clinical case.

“The speed at which an outbreak occurs depends on the interval between (horses’) contact with the first clinical cases and signs of disease in the subsequent contacts,” noted Morresey.

Respiratory disease can spread slowly within a population. An infection might spread considerably before anyone even recognizes the first case. This limits the veterinarian’s ability to stop the spread of infection, and it can make isolation protocols ineffective.

A tendency to “shoot the messenger” in such situations makes a veterinarian’s job difficult, Morresey said, especially if it involves suspending horse movement and curtailing an equine facility’s normal day-to-day operations. Nonetheless, appropriate biosecurity protocols must be instituted, including barrier precautions, containment procedures, and decontamination/disinfection when indicated.