The best way to keep your horses healthy is prevent them from getting sick. Sounds absurdly simple, right? In theory, this works. But in the real world of trying to protect horses from diseases such as equine protozoal myeloencephalitis (EPM), it isn't always that simple.
Sharon Witonsky, DVM, PhD, Dipl. ACVIM, associate professor in the Department of Large Animal Clinical Sciences at the Virginia-Maryland Regional College of Veterinary Medicine, describes the critical role immune response plays for a horse to mount effective protection against an invader: "We know, based on mouse models, that cell-mediated immunity is critical for protection. An optimal immune response would be one that stimulates both antibodies (in the serum) as well as cell-mediated immunity."
So how does this relate to EPM? Witonsky explains, "Antibodies can potentially coat S. neurona (the protozoan parasite Sarcocystis neurona that causes EPM) and stimulate complement- mediated killing," which is one method the immune response has of eliminating foreign proteins and antigens. "The cell-mediated immune response also results in killing of S. neurona. Ideally, S. neurona would be killed before entering the central nervous system."
Debra Sellon, DVM, PhD, Dipl. ACVIM, professor of equine medicine in Washington State University's Department of Veterinary Clinical Sciences, further explains this arm of the immune system that influences blockade of infection: "Currently, most investigators believe that production of gamma interferon is especially important for protection against disease. Gamma interferon can be produced by many different cell types, and it is unclear which of these cell types are most important for protection of horses against EPM. Most likely, a cell-mediated response from lymphocytes (specialized white blood cells) is crucial, while production of antibody is considered of less importance. But remember, all immune functions may be suppressed when horses are stressed."
Therefore, a primary objective in disease prevention should be to minimize stress so a horse's immune system can operate at maximal capacity. Witonsky comments, "At this time, we still don't know why some horses develop disease, although based on studies and on my clinical impression, stress from showing, shipping, training, etc. seems to be a risk factor for increased incidence of disease. As a trainer or owner, it is important to be sensitive to what one's horse believes is stressful, and try to be observant for subtle changes in behavior and performance which could be due to EPM. If a horse does develop disease, hopefully it will be detected early in the onset of disease. In that way, an infected horse can be started on treatment as early as possible to minimize and prevent horse losses and to improve overall outcome with regard to return to overall health and performance."
Bill Saville, DVM, PhD, Dipl. ACVIM, professor and chair in the Department of Veterinary Preventive Medicine at The Ohio State University, and his colleagues investigated risk factors for development of EPM. In this study they acknowledged the important role of the immune system in fending off disease.
"When animals are stressed, suppressive proteins produced by the central nervous system are released and lead to suppression of lymphocyte production and function," said Saville.
This, coupled with elevated cortisol levels related to stress, might increase a horse's risk of developing EPM.
Control Measures to Reduce Risk
Saville's comprehensive study (Journal of the American Veterinary Medical Association, 2000) revealed the following findings that tell us how we can more effectively prevent EPM in our horses:
Age The highest risk of infection occurred in horses aged 1-5 years. This could be due to the use of young horses in competitive situations and the associated stress.
Opossums Presence of opossums on a farm poses an increased risk.
Location Horses on farms with previously EPM-infected horses had a higher risk of developing EPM, likely due to the presence of protozoa in the feed or water and increased likelihood of exposure.
Seasonal effects More EPM cases occur in spring, summer, and fall, possibly related to hot weather acting as a stressor, as well as this being a time of increased travel to competitions with accompanying transport stress affecting the immune system.
Stress An association of stressful events (such as injury, accidents, foaling, surgery, transport, and illness) with increased risk might be related to suppression of a horse's immune system.
Natural water source Presence of water sources (creek or river) on the farm provided a preferred habitat for opossums away from the horse barns, thereby decreasing exposure and risk.
Food storage Securing feed and water sources from opossum fecal contamination is important in limiting exposure and risk.
It is important to limit opossum presence since sporocysts (the infective stage of the protozoon) are able to survive for as much as a year in the environment. Additionally, birds feed on insects and plant material in the feces of opossums, thereby serving as a vehicle to disseminate sporocysts in the environment. David Granstrom, DVM, PhD, one of the pioneer researchers of EPM when he worked at the University of Kentucky, emphasizes how environmental management can go a long way toward limiting infection.
Saville says it isn't easy to kill the parasites in the environment, and sporocysts are resistant to even the most intense disinfectants.
Granstrom adds, "It looks like the only way to clean barns that is effective and will not destroy the barn is by the heat of steam cleaning."
Because disinfectant foot baths will not impact sporocysts, it is suggested to change boots or use disposable boot covers in areas where there is the potential for barn contamination.
Attempts have been made to utilize prophylactic (preventive) antiprotozoal medications for horses stressed by rigorous training. Favorable reports have been noted for prophylaxis using either ponazuril or nitazoxanide, but this strategy has not received FDA approval. There is also concern that suboptimal dosing or long-term exclusive use of anti-protozoal drugs might increase development of drug resistance as well as the potential for untoward effects on the equine gastrointestinal tract.
An EPM vaccine from Fort Dodge Animal Health was given a conditional license for five years. This vaccine was taken off the market and is no longer available. Research for a new vaccine continues as reported by Siobhan Ellison, DVM, PhD, who has an interest in S. neurona infections in horses and is founder and CEO of Pathogenes Inc.
She is working on an EPM vaccine, which has been tested in a small number of horses using an intravenous challenge model. Her research involves using the recombinant SAG1 subunit antigen in an attempt to induce a protective immune response in horses. Recombinant subunit vaccine technology relies on putting a protein gene from the EPM disease- producing organism into another similar (but not disease-producing) organism, which then expresses the protein without presenting a risk to the horse. In this way, a subunit vaccine triggers the immune system to respond without introducing the entire disease-producing pathogen. There is heated controversy among research veterinarians as to the role of SAG1 antigen in some strains of S. neurona, and the research continues.
Statistically, it seems the incidence of EPM has been on a steady decline since the turn of this century. Granstrom notes, "Certainly far fewer cases are being admitted to the large referral centers. I believe a number of factors have contributed to the decline. Veterinarians have become more comfortable making the diagnosis in the field and initiating treatment rapidly with many more treatment options. Effective preventive measures were developed and publicized widely. In addition, West Nile virus and neurologic EHV-1 (equine herpesvirus-1) infections have increased dramatically and overshadowed EPM in recent years.
"It's not apparent that the vaccine was effective in preventing clinical disease, but I don't believe it was used widely enough to impact the incidence of EPM, even if it had been effective," he added.
Preventive management strategies (as described in the chart below) are clearly effective in limiting risk and incidence of EPM infection.
|Prevention of EPM|
|Secure horse feed and water sources||Limits fecal contamination by opossums|
|Limit wildlife access to garbage and feed||Makes the area less attractive to scavenging opossums|
|Install wire mesh on stalls||Limits access of scavenging opossums to horses, stalls|
|Confine horses in protected stall at night||Provides controlled area preventing opossum access to areas where horse is eating and drinking|
|Close stall and barn doors even when horses are not present in the barn||Prevents opossum access to stabling areas where horses eat and drink|
• Arrange comfortable herd dynamics and social interactions when possible
|Allows optimal function of immune system|
Minimize barn contamination:
• Steam clean the barn
|Minimizes cross-contamination in barn areas where horses congregate|
|Work with your veterinarian to evaluate neurologic or musculoskeletal concerns||Ensures early testing, diagnosis, and appropriate treatment when necessary|
|Research is still in process for a vaccine that works||Vaccine against EPM is a future possibility|
|Prophylactic use of antiprotozoal drugs might be a future possibility||More research must be done to substantiate safety and efficacy and to obtain FDA approval for these drugs|
About the Author
Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.
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