A four-step plan to keeping equine infectious diseases in check

The Equine Disease Communication Center’s near-real-time disease outbreak tracking site (EquineDiseaseCC.org) has been keeping the horse industry informed about the recent equine herpesvirus outbreaks in California, Kentucky, and Louisiana; incidents of equine infectious anemia (EIA) in Illinois and Utah; and strangles cases in Florida—one of the few states where strangles is reported to state animal health officials.

“Knowing what diseases your horses are at risk for now, what diseases we should be keeping an eye out for, and how those diseases are spread empowers horse owners to take steps to intercept pathogens (disease-causing organisms), thereby minimizing the chances of infection,” says Scott Weese, DVM, MSc, Dipl. ACVIM, a professor in the Department of Pathobiology at the University of Guelph’s Ontario Veterinary College and author of the Worms and Germs blog.

Here, Weese and the rest of our sources suggest practical ways to prevent an outbreak and be ready to respond accordingly.

1. Know What Infectious Diseases Threaten Horse Health

Wouldn’t it be fantastic to go online and search for a list of infectious diseases that pose a risk to your horse based on where you live and where you’re traveling? If you reside in Florida, for example, what are the top three infectious diseases to be worried about, and how worried should you really be? How many cases of those diseases occurred in your neck of the woods in the past year? If you move to Utah, does the list of diseases to be concerned about change?

Unfortunately, equine infectious disease resarchers agree this approach is simply not yet feasible.

“We do not currently have a system for collecting reliable and valid data to accurately define the two or three most important infectious diseases in each geographical area,” says Noah Cohen, VMD, MPH, PhD, Dipl. ACVIM, professor of large animal clinical sciences at Texas A&M University’s College of Veterinary Medicine & Biomedical Sciences, in College Station. “Moreover, ‘importance’ of a disease is influenced by factors other than frequency, such as regulatory, financial, and other social/societal impacts of the disease.”

Nonetheless, we know that some diseases occur more commonly in certain geographic areas than others. If you live in New England, for instance, Lyme disease poses an important risk. Certain tick species can transmit the disease’s causative agent, Borrelia burgdorferi, when feeding on horses, humans, or other animals. Clinical signs of Lyme disease in horses include lameness, swollen joints, increased sensitivity to touch, and other nonspecific symptoms. Or, along the East Coast and the southern states, the mosquito-borne Eastern equine encephalomyelitis (EEE) virus can infect horses, resulting in disease of the central nervous system and, in most cases, death.    

These diseases tend to be limited to specific geographic regions because of their vectors’ limited range: the tick and mosquito, respectively. As we learned from the rapid spread of West Nile virus (WNV) in the early part of this new millennium, however, viruses don’t always remain in a single vector (e.g., only one species of mosquito), and vectors and reservoir hosts (i.e., birds) in a single region. Also, viruses can resist change to cause more severe forms of disease. International travel, continued encroachment into wildlife-occupied areas, potential climate change, and even—in the case of bacterial disease—microbial resistance mean horses throughout North America and beyond will continue to be exposed to novel pathogens.

The American Association of Equine Practitioners (AAEP) recommends all U.S. horses, irrespective of their use or geographic location, receive the following five “core” vaccines: rabies, tetanus, EEE, WEE (Western equine encephalomyelitis), and WNV. The AAEP considers the remaining vaccines licensed for administration in horses as “risk-based,” meaning only horses with a risk of exposure to the pathogen are likely to benefit from vaccination. Risk-based vaccines include those protecting against anthrax, botulism, equine herpesvirus, equine influenza, equine viral arteritis, leptospirosis, Potomac horse fever, rotaviral diarrhea, strangles, and even a conditionally licensed vaccine against pigeon fever/lymphangitis caused by Corynebacterium pseudotuberculosis. More information on these infectious diseases and associated vaccinations is available at AAEP.org/info/vaccination-guidelines.

A plethora of other equine pathogens and diseases exists for which no vaccines are available. Some examples include coronavirus, equine herpesvirus myeloencephalopathy (EHM) caused by an equine herpesvirus-1 (EHV-1) mutation or a wild strain, and Lawsonia intracellularis, the causative agent of equine proliferative enteropathy.  

2. Go Local: Talk to a Veterinarian

The list of equine disease-­causing pathogens is clearly much longer than the one in this article. An individual horse’s disease risks depend on a variety of factors, including management tactics, the horse’s use, his travel history, environmental factors, his individual ability to ward off infection, and response to infection if it occurs.

“It’s very important to appreciate that how a particular horse or group of horses are managed and what the horses are used for greatly impacts infectious disease control,” says Weese. “One of the best ways to determine what infectious diseases your horses are at risk for in a specific region is to speak with a veterinarian in that area.”

Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, professor of equine medicine at Colorado State University’s College of Veterinary Medicine & Biomedical Sciences, suggests owners use the AAEP’s free Get-A-DVM service to connect with a knowledgeable veterinarian if moving or traveling to a new region.

“Directing equine owners and managers to contact a local veterinarian in an area they plan to move to or travel to with their horse(s) in advance of having a problem is ideal,” she says. “Owners are also encouraged to question the veterinarian regarding the need for any additional precautions they should be taking prior to and while traveling. For example, when taking their horse into an endemic Lyme disease location, they may want to do some tick mitigation not only for their horse but for themselves.”

In other words, take the phrase “know before you go” to heart and learn about the preventive disease practices veterinarians recommend in an area prior to traveling or moving your horses. Preventing a problem is always easier than resolving one.

3. Respond Appropriately in an Outbreak

Pretend for a moment you are at a large equine event. The horse next to yours in the barn is just not right (dull, depressed, not eating), and another horse a few stalls down has suddenly developed a fever. What should you do?

“First and foremost, contact your veterinarian or the event management team and veterinarian,” says Weese. If they determine a contagious disease is, in fact, at hand, “they will decide whether they can handle the outbreak themselves or if state and federal personnel need to be contacted to contain a reportable disease, as we saw in 2008 through 2010 when contagious equine metritis, a foreign-animal disease, was diagnosed in almost 30 horses.”

Typically, the next step is cohorting—grouping horses based on whether they are actively showing signs of infection, were potentially exposed to the pathogen, or are healthy and did not likely come into contact with the pathogen. Next, the veterinarian can perform diagnostics to identify the causative agent and treat affected horses. At the same time, horse show management institutes infection control practices on the property, including setting up an isolation area to minimize the chances of infected horses exposing yet-unexposed horses at the event.   

All said, when you notice a possibly contagious illness at a facility, even if your horse doesn’t currently look sick, it’s important to stay put to minimize the chances you spread the infection.

4. Don’t Get Complacent, Do Communicate

Even if we’re vigilant about preparing, preventing, and mitigating, life is full of surprises. As we’ve seen in the past few years, it’s important that horse owners not become complacent about diseases we haven’t seen for a while—such as large numbers of West Nile virus cases, which have turned up from time to time in unvaccinated horses, simply because owners didn’t perceive the disease as a risk.

There also will be new diseases that throw us for a loop; the only questions that remain are where and when they will occur.

To complicate matters, it might not even be a new disease to North America, such as African horse sickness or Australia’s Hendra virus. Instead, it could be an old disease identified in a new area of the country, like EEE being diagnosed in Idaho or Oregon. The possibility of more viruses mutating and causing diseases in unexpected hosts or geographic locations also exists. Remember in 2005 when the H3N8 equine flu virus jumped species and infected dogs? The same thing could happen in reverse, with viruses that normally do not cause disease in horses infecting them.

Predicting when or what the new disease will be poses challenges for infectious disease control researchers. Attempting to plan for a disease outbreak without prior knowledge of the pathogen or mode of transmission would be like a reality TV version of “Mission: Impossible.” Nonetheless, following basic biosecurity procedures even in times of health (i.e., washing hands between caring for different groups of horses), will allow horse owners to swiftly swing into action in the case of an outbreak.

“Another critical aspect of successfully managing an infectious disease outbreak is communication,” Weese says.

Veterinarians must communicate effectively with all relevant facility personnel in a timely manner. They must present all available information, along with solutions to the problem, detailing the steps to take to achieve outbreak resolution. Further, owners can help their veterinarians by curtailing the spread of rumors and false information. In the age of social media, this is increasingly challenging, yet imperative, during an outbreak.

Take-Home Message

Avoid the 15 minutes of internet fame associated with being a victim of an equine infectious disease outbreak. Vaccinate against preventable diseases as recommended by your veterinarian, institute appropriate biosecurity measures—­including basic hand hygiene—and talk to your veterinarian about constructing a comprehensive disease control plan. If faced with a sick horse, your veterinarian should be the first one you call; he or she can suggest first steps, and you can work together to implement a comprehensive plan, especially in a herd situation.

“Your veterinarian can help you tailor a biosecurity plan to meet your equine operation’s individual needs to best protect your horses,” says Weese.