Emerging Equine Diseases: What You Should Know

Timoney encouraged owners to undertake a thorough import-risk analysis prior to bringing horses, semen, embryos, and other related items into the country and to utilize pre-export and post-import quarantines and testing.

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Not long ago, we didn't know that some now-common equine diseases even existed. Potomac horse fever, hendra virus infection, and contagious equine metritis, among others, were all once considered emergent diseases. And while we no longer think of them as new conditions, there are likely many more just waiting to make their first appearance or spread across an international border into a previously unaffected country.

At the recent 2015 University of Kentucky Equine Showcase, held in Lexington, Peter J. Timoney, FRCVS, PhD, shared some important insight into emerging equine diseases. Timoney is a professor and former department chair and director of the University of Kentucky Gluck Equine Research Center, also in Lexington.

What is an emergent disease?

Emergent diseases are those that are recorded for the first time in a population (such as Potomac horse fever or hendra virus) or those that might have been around for a period of time, but had not been diagnosed (like mare reproductive loss syndrome or contagious equine metritis), Timoney said.

Factors contributing to disease emergence include:

  • Microbial change and adaptation;
  • Host susceptibility to infection;
  • Climate change;
  • Altered ecosystems;
  • Changing population demographics,
  • International movement and trade; and 
  • Land use and economic development.

And, of course, Timoney said, "horses are not exempt when it comes to emergent diseases."

Emergent Infectious Diseases in Horses

Since 1969, numerous diseases—most of which are commonplace today—have been documented for the first time in horses around the world:


Causative Agent(s)

Where and When Reported

Equine granulocytic ehrlichiosis Anaplasma phagocytophilum United States in 1969
Equine encephalosis Encephalosis virus South Africa in 1970
Contagious equine metritis Taylorella equigenitalis and possibly T. asinigenitalis Ireland and the United Kingdom in 1977; United States in 1978
Getah virus infection Getah virus Japan in 1978
Potomac horse fever (equine neorickettsiosis) Neorickettsia risticii United States in 1984
Enterocolitis Clostridium difficile and C. perfringens A, B, C, and D United States in 1987; Europe and the United States (re-emergence) in 1993
Nocardioform placentitis Crossiella equi, Amycolatopsis kentuckyensis, A. lexingtonensis, and Cellulomonas cellulans United States in 1986 and (re-emergence) in 2003
Acute equine respiratory syndrome Hendra virus Australia in 1994
Proliferative enteropathy Lawsonia intracellularis Canada in 2000
Mare reproductive loss syndrome Eastern tent caterpillar United States in 2001
Airway disease Nicoletella semolina Europe in 2004
Equine amionitis and foetal loss Processionary caterpillar Australia in 2004
Cryptococcal infections Cryptococcus gattii Canada in 2005
Equine coronavirus enteritis Coronavirus Japan in 2011
Theiler's disease Theiler's disease associated virus United States in 2013


And, Timoney said, "the list will continue to grow. Of that, there's no question."

Some of the emergent diseases identified since the late 1960s only affect breeding animals (such as mare reproductive loss syndrome and contagious equine metritis), while others can affect any horse, young or old (such as Potomac horse fever and coronavirus).

More importantly, some emergent diseases are zoonotic, meaning they can be transmitted between horses and people and, thus, are important to public health. Such conditions include acute equine respiratory syndrome and equine granulocytic ehrlichiosis, Timoney said.

Mitigating Emergent Disease Risk

"The horse is a global traveler and, consequently, can be responsible for spreading a disease over a much larger geographic area than was initially the case," Timoney said. "The threat posed by an emergent disease being introduced into a country's equine population is significant."

He explained that market forces have contributed to horses becoming an international commodity and, thus, have increased the risk of disease spreading over international borders.

"Countries at greatest risk, obviously, are those with a significant import/export trade in semen as well as live animals," said Timoney, and that includes the United States.

So how can we reduce the risk of a new disease being imported into our home nation?

"Monitoring, surveillance, and prompt reporting of suspect cases of foreign animal, emergent, or re-emergent diseases to the appropriate authorities is of critical significance in safeguarding the health integrity of a nation's equine industry," Timoney said.

The equine practitioner plays a key role in any surveillance program. He said field veterinarians are on the "front line" when it comes to identifying suspect emergent disease cases and are critically important as "first-responders" when faced with such cases.

Other factors crucial to a successful disease surveillance program include:

  • A good understanding of endemic and transboundary diseases;
  • Prompt reporting of suspected cases of emergent diseases;
  • Laboratory confirmation of an emergent disease case as soon as possible—don't wait, Timoney stressed; and
  • Constant vigilance for the presence of an emergent disease.

He also noted that it's important for horse owners to "become educated (on transboundary diseases). Don't expect someone else to be your sole source of information."

Timoney encouraged owners to undertake a thorough import-risk analysis prior to bringing horses, semen, embryos, and other related items into the country and to utilize pre-export and post-import quarantines and testing.

Take-Home Message

Numerous diseases have emerged since the late 1960s, and it's probable that scientists will discover many more in the future. While we might not necessarily be able to prevent their spread, we can take steps to reduce the likelihood of them jumping international borders.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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