What will be the next disease to invade North America "West Nile style"?

According to some scientists, global warming is anticipated to be accompanied by altered patterns of precipitation, changes in the frequency and intensity of extreme weather events, and continued species extinction. In the face of these events, pathogenic microscopic organisms appear to be on a mission to impact the surviving species. Bacteria, viruses, fungi, and parasites are rapidly adapting to--and in some cases, thriving in--Earth's changing environment.

Pathogenic microorganisms pose an important health threat to our horses as new diseases invade North America and previously eradicated diseases reappear.

"By definition, an emerging (infectious) disease is an infectious disease that has newly appeared in a population, and a re-emerging disease is one that has been known for some time but the number of cases is rapidly increasing in a certain population or geographic range," explains Scott Weese, DVM, DVSc, Dipl. ACVIM, an associate professor at the University of Guelph's Ontario Veterinary College.

Likely the best-known example of a recently emerged disease in North American horses is West Nile virus, which was first diagnosed in the United States in 1999. Since then, the disease has spread across North America and continues to extend into other geographical areas. In the United States, re-emerging diseases of interest include the venereal disease contagious equine metritis (CEM), and equine piroplasmosis. These diseases were considered eradicated from the United States (in the late 1970s and the late 1980s, respectively), but both have resurfaced in recent outbreaks in the new millennium. Every horse owner and his or her veterinarian should be aware of emerging and re-emerging diseases to defend against their spread.

"Front-line veterinarians play a critical role in identification of emerging diseases," relays Weese. "For example, when West Nile virus first emerged in North America, equine veterinarians reported strange neurological disease in horses in Long Island, N.Y. These observations were critical for identifying the emerging outbreak."

Environmental Impact on Disease

A number of factors contribute to the emergence of a new disease, including human-induced changes, modifications of the disease-causing organism itself, and environmental alterations. Consider insect or tick-borne diseases such as West Nile virus, Eastern equine encephalitis, and Lyme disease, for example. The population of insects and ticks that help spread these diseases can increase and they might change or extend their habitats due to environmental changes (e.g., global warming) and urban sprawl or fusion between rural and urban communities. These changes can mean increased exposure of horses and humans to these vectors.

"Global climate change is an important consideration in emerging and re-emerging diseases," says Jack Trevors, MSc, PhD, professor at the University of Guelph's School of Environmental Sciences and adjunct professor at the University of Waterloo. Trevors and colleagues recently reviewed the environment's impact on the bacterium Escherichia coli in a manuscript published in the June 2010 edition of The ISME Journal.

"E. coli and mammals have evolved together over millions of years," says Trevors. "E. coli is an important microorganism in our gastrointestinal tracts. With global warming, E. coli may live for longer periods of time in water, particularly if the water is contaminated with fecal material or manure. These strains may become environmentally adapted."

He adds, "One concern is that E. coli in the environment can pick up genes from other strains of E. coli or even different types of bacteria. These new genes can allow the E. coli to produce toxins or become resistant to one or more antibiotics and more acidic conditions."

In other words, the normal, helpful strains of E. coli that help break down organic food material in mammals' gastrointestinal tracts have the potential to acquire additional genes and become toxin-producing strains. Luckily, at this time, the risk of E. coli mutating in such a way as to threaten the health of horses is fairly low.

Weese says, "I think the biggest risk of climate change for horses is expansion of ranges of insect (and tick) vector populations, which could allow establishment of various tick- and mosquito-borne diseases in new ranges."

This could include expansion of the ranges of diseases such as Lyme disease and West Nile virus (among others) and make it easier for some foreign diseases to establish themselves if they reach North American horses.

"There's also the potential that some organisms that usually survive poorly over winter could start sticking around better in some regions that have borderline winters," says Weese. "That's maybe most relevant for internal parasites such as ascarids and small strongyles that might be killed on pasture with a reasonable winter, but which could accumulate with warmer weather."

Know Thine Enemy

"The list of diseases that have the potential to infect horses in North America is extensive," says Weese. "Some are known entities like West Nile virus that already cause problems and could spread further, and some are foreign diseases that may already be emerging in some areas, such as the unexplained outbreaks of equine piroplasmosis in the U.S."

According to Weese, other threats are diseases just waiting for the initial "spark," as occurred with the introduction of West Nile virus. "Of equal or greater concern are diseases that we can't even predict," he adds. "Who would have thought West Nile virus would become an important disease in North America? Who would have thought fruit bats in Australia could be the source of a disease (Hendra virus) that can kill both horses and people that work with horses? We need to be concerned about the unknown threat that will certainly emerge."

The Real Deal: MRSA

Of all the potential diseases able to reach and subsequently invade North American soil and threaten the health of our horses, methicillin-resistant Staphylococcus aureus (MRSA) has many officials on high alert.

When strains of S. aureus become resistant to not only the antibiotic methicillin but also to all of the antibiotics classified as ß-lactams, including penicillins and cephalosporins, MRSA emerges. This means MRSA is resistant to the majority of first-line antibiotics used to treat infections in horses.

"MRSA is important not only because it is resistant to a large number of antibiotics, but also because it is an opportunistic pathogen," explains Weese. "That is, the bacterium does not normally cause infections in healthy animals but can cause infections in certain situations, such as when the body's normal barriers are compromised or in animals that have a com-promised or weakened immune system."

Most commonly, MRSA causes infections in soft tissues and skin at the site of wounds or surgical incisions. Less ¬commonly, MRSA causes cellulitis (infection of the skin and subcutaneous tissues), pneumonia, and septic arthritis (arthritis caused by infection).

MRSA was first identified in horses in the late 1990s and has since spread ¬internationally. According to Weese, the number of MRSA infections in horses continue to grow. Only an estimated 1-4% of apparently healthy North American horses are colonized by MRSA; however, MRSA is still important.

"The top three reasons why horse owners should care about MRSA are the fact that it can cause serious infections in horses, can spread silently in the horse population because healthy horses can be carriers for the bacterium, and because it can be spread to people," explains Weese.

Broader risks, particularly transmission in the environment, are probably not as relevant with MRSA as with E. coli, but they can't be completely dismissed. So while the environment doesn't appear to play a huge role in the emergence of MRSA, changes in the environment can potentially impact the disease just like any other microbial infection.

More information on MRSA is available on the University of Guelph's equIDblog (www.equidblog.com).

Atypical Myopathies: MADD and Pasture Associated Myopathy

Of the extensive list of diseases that researchers, veterinarians, and government officials keep their eyes on, atypical myopathy, or "MADD," is one of the more interesting and dramatic. (For recent studies about MADD, see www.TheHorse.com/16830.)

Equine atypical myopathy is a painful and often-fatal disease that destroys skeletal muscles. The condition occurs in horses on pasture in the United Kingdom and other European countries, including Denmark and Switzerland. Research has shown the myopathy is due to a biochemical defect called "multiple acyl-CoA dehydrogenase deficiency," which is why the disease is also referred to as MADD. The underlying cause of the biochemical defect, however, remains to be determined.

The disease is important for two main reasons. First, the mortality rate of MADD is as high as 90%, and second, it appears to be an emerging disease in Europe.

"MADD is emerging rapidly with periodical and ever-larger outbreaks with many affected horses. Most of them are dying and/or suffering, and the disease therefore has important welfare implications," explains Dutch researcher Han van der Kolk, DVM, PhD, Dipl. ECEIM, an assistant professor in equine internal medicine in the department of equine sciences, faculty of veterinary medicine at Utrecht University. MADD outbreaks occurred previously in 2004 and 2006. In the fall of 2009 MADD was diagnosed in 371 horses from 10 different European countries, killing 265 of the affected horses.

"Based on our research, MADD appears to be associated with the ingestion of maple leaves that are covered with the European tar spot fungus Rhytisma acerinum," shares van der Kolk. It is possible that environmental changes have permitted or facilitated the growth and spread of the fungus. This could explain the sudden increase in the number of affected horses over the past couple of years.

Stephanie Valberg, DVM, PhD, Dipl. ACVIM, professor of large animal medicine and director of the University of Minnesota Equine Center, adds, "There are several proposed toxins for the disease but as of yet we do not know for sure if the tar spot fungus or another toxin like the lethal toxin produced by the bacterium Clostridium sordellii is to blame."

According to Valberg, cases such as this also have appeared in the Midwestern United States. "In 2009 we had several cases of what we call 'pasture myopathy' in our area that strongly resembles MADD," she says.

The similarity between MADD and pasture myopathy makes the condition "of interest" to North American owners and veterinarians. In addition, the fungus R. acerinum, which van der Kolk and colleagues hypothesize could cause MADD, has been identified on Norway maple trees in Eastern Canada and appears to be adapting to Canadian soil (at least in part) due to less acid rain.

Take-Home Message

Not only do emerging and re-emerging diseases threaten our horses but they also cost the industry a considerable amount of money annually due to the research required to understand the development, frequency, and distribution of a disease, all so veterinarians can develop appropriate diagnostic techniques, treatments, and control strategies. Some of the emerging and re-emerging diseases that can affect horses are also zoonotic, meaning they have the ability to infect humans.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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