Published July, 2012

Hindsight is often 20/20, but foresight is a far more valuable skill when trying to protect our horse population from infectious and even noninfectious diseases.

"Emerging and re-emerging diseases are an omnipresent threat to horse industries throughout the world," said Peter J. Timoney, FRCVS, PhD, professor and former department chair and director of the Gluck Equine Research Center at the University of Kentucky.

Both new (emerging) diseases and older diseases rearing their heads (re-emerging) to strike again can devastate equine populations and cost the industry millions of dollars. This is why veterinarians and researchers throughout the world carefully monitor and track disease spread in equids worldwide.

By definition, an emerging disease is one that is either (1) new to a specific population (like West Nile virus, or WNV, was to the United States in 1999), or (2) a disease that has not been recognized previously (e.g., mare reproductive loss syndrome in 2001).

When attempting to determine whether a disease is truly emerging, there are several factors to consider.

"We first need to decide whether a disease is truly novel or not," said Timoney. "It is possible that the disease has been around for a while, but that we hadn't recognized it until improving our surveillance and diagnostic tests. Further, we also need to confirm that the infective agents we suspect are causing disease are really the primary pathogen (disease-causing organism) and not simply opportunistic organisms."

For example, the bacterium Proteus mirabiliswas mistakenly reported to be the cause of contagious equine metritis in 1977, but it turned out that P. mirabilis was actually a secondary or opportunistic pathogen that flourished only after a primary infection withTaylorella equigenitalis, the true causative bacteria.

Countless factors are believed to contribute to emerging disease spread, such as environmental factors (e.g., drought, El Niño) and the international movement of people, horses, and insect vectors that transmit disease.

"It is also worthwhile considering whether the overuse or abuse of antibiotics has contributed to the emergence of certain diseases, such as methicillin-resistantStaphylococcus aureus (MRSA)," Timoney proposed.

Several diseases are currently emerging in horses, contributing to loss of equine life and economic implications (for example, regarding cost of treatment, testing, vaccine development). Luckily, only few emerging equine diseases recognized since 1969 have any public health significance and are therefore not a threat to human health.

Once Bitten, Twice Shy
As if the worry about a new disease bearing its fangs and wreaking emotional and economic havoc on the North American horse industry isn't enough, experts advise that re-emerging diseases are an equally important threat.

"Re-emerging diseases are those that have been experienced before but have reappeared in a more virulent (severe) form or whose epidemiology has changed in some significant respect," Timoney explained.

One of the best examples of a re-emerging disease of concern to the U.S. horse industry is the neurologic form of equine herpesvirus type 1 (EHV-1), called equine herpesvirus myeloencephalopathy (EHM).

"Since 2000 there has been an increase in the number of outbreaks and cases of EHM--the vast majority of which are believed to be caused by specific 'neuropathogenic' strains of the virus," noted Timoney.

Neuropathogenicity of one particular EHV-1 strain is linked to a single change in one of the virus' genes controlling its replication.

Based on data complied by the late George Allen, PhD, from the University of Kentucky, the total number of confirmed EHM outbreaks in both the U.S. and U.K. from the '70s and '80s was only 14, but from 2001 to 2006 there were 33 confirmed outbreaks. Most of those in the U.S. (26), involved the "mutant" form of EHV-1 virus that causes EHM.

According to Timoney, even though more updated statistics are unavailable on the overall number of EHM outbreaks that have occurred over the past seven years, there is no doubt that disease incidence has continued to increase in the interim.

"EHM is not the only re-emerging disease of importance in horses," Timoney added. "Nocardioform placentitis and abortion can also be a disease of economic significance in breeding populations in some years."

A Worldwide Problem
"As evidenced by the West Nile virus, no longer is any country remote from the risk of incursion of any pathogen," Timoney concluded. "Fewer and fewer infectious diseases are currently considered restricted in their worldwide distribution."

According to Timoney, this makes risk analysis, communication, surveillance, and prompt reporting of disease outbreaks valuable tools to "safeguard the health integrity of a nation's equine industry."

Stacey Oke, DVM, MSc, is a freelance medical writer based out of Canada.