Fifty years ago, researchers identified EPM — equine protozoal encephalitis — but it wasn’t until the 1990s that the microscopic parasite Sarcocystis neurona was identified as the culprit. Today we know that there are actually two EPM-causing parasites: S. neurona and Neospora hughesi.
We also know that the definitive host for S. neurona and N. hughesi and their sexual reproduction is the opossum, but intermediate hosts include cats, skunks, raccoons, armadillos and sea otters.
According to the American Association of Equine Practitioners (AAEP), “more than 50 percent (and in some areas as high as 90%) of horses in the US may have been exposed to the organism that causes EPM. … Not all horses exposed to the protozoa will develop the disease and show clinical signs of EPM. Some horses seem to mount an effective immune response and are able to combat the disease before it gains a foothold. Other horses, especially those under stress, can succumb rapidly to the debilitating effects of EPM.”
Clinical EPM disease reported in animals
EPM has been reported not only in dogs, cats, and horses but in zebras, raccoons, red pandas, ferrets, skunks, lynx, sea otters, sea lions, and the Pacific harbor seal.
Infection doesn’t mean disease
The presence of antibodies against S. neurona and N. hughesi in the horse’s blood does not mean the horse has EPM. According to research at UC Davis, less than 1% of seropositive animals will go on to develop EPM.
The role of stress
Factors researchers have identified as stressful to the horse include shipping, showing, training, and pregnancy. In a 2016 study published in Journal of Veterinary Internal Medicine, the preventive approaches to EPM include “reducing stress along with reducing exposure to scat from opossums. Practical approaches such as not feeding off the ground, providing separate sources of fresh water for horses and preventing wildlife access to horse pastures, paddocks, and stalls may also help reduce the incidence of protozoal infections in horses.”
The role of the immune system
Horses have two types of immune responses when exposed to pathogens: humoral and cell-mediated.
- The humoral response is systemic, and involves the production of antibodies to counteract specific antigens.
- The cell-mediated response is the production of T-cells, which protect the body against intracellular organisms. The thymus gland provides the development of T-cells. Unlike generalized white blood cells, like macrophages, which attack a wide range of antigen-producing pathogens, the T-cells respond to a single antigen, such as a specific virus or bacteria. In humans it is estimated that the thymus gland produces 25 million to a billion different T-cells.
According to Sharon Witonsky, associate professor of Equine Field Service in the Department of Large Animal Clinical Sciences at the Virginia-Maryland College of Veterinary Medicine, researchers have data to support that horses need to develop a cell-mediated response to protect against S. neuroma.
The immune system and the GI tract
Throughout the intestinal tract there are immune system cells, lymphocytes, and macrophages. This immune system of the intestinal tract is called gut-associated lymphoid tissue, or GALT. Research has shown that interactions between the immune system and the bacteria colonizing the gut can have profound effects. We know, for instance, that active, live probiotics can direct immune cell activity, reducing inflammation and aiding in the production of specific antibodies.
Some researchers estimate that 70% of the body’s immune cells originate in the gut.
Support for EPM horses
Providing immune support for horses with EPM is essential, along with the FDA-approved treatments: sulfadiazine/pyrimethamine (ReBalance), ponazuril (Marquis), and diclazuril (Protazil).
1 – Immune Support
Bovine colostrum contains high levels of immunoglobulins, specifically IgG, IgM, and IgA. These antibodies help neutralize pathogens. Keep in mind that commercial colostrum products vary greatly in quality and method of preparation. Look for high-potency, ethically obtained formulations with an especially high IgG content, such as BioStar’s Colostrum-38 EQ.
Bovine colostrum is an important food for horses with EPM because it uniquely provides proline-rich peptides (PRP) that act as a regulator of the thymus gland, bringing it to homeostasis. It can help keep the thymus gland from overreacting, as in the case of hives, which is an overreaction of the immune system, or kick it into gear if it’s underperforming. In the case of EPM, the immune system is underperforming, and so the proline-rich peptides can stimulate the thymus for more T-cell production. Bovine colostrum’s PRP content can help support the cell-mediate response against S. neuroma.
2 – Probiotics
Active, live probiotics are important to the immune system tissues in the GI tract. Because there are literally billions of friendly and non-friendly bacteria of various strains that live in the gut, it is important to use probiotics that are multi-strain, providing a variety of healthy bacteria to the gut. The GI tract of horses is not a monoculture; it is diverse, and a probiotic supplement should reflect that.
It’s also important to use probiotics that have a high colony-forming unit (CFU) count on the label, which is the standard measurement of active probiotics. For a horse dealing with EPM, you need a minimum of 200 billion CFUs per day, and in some cases as high as 400 billion per day. BioStar’s BioFlora EQ probiotic-prebiotic blend provides 100 billion CFUs per serving, making it ideal for EPM support.
For EPM horses, I recommend the the same cooling probiotic strains Lactobacillus and Bifidus used in BioFlora EQ. Yeast probiotics are heating to the GI tract. If an EPM horse is suffering from weight loss or anorexia, then adding some yeast probiotics to the cooling probiotics would be beneficial.
3 – Managing stress
Stress plays a significant role in gut health and the immune system. It also plays a large role in EPM relapses.
Although the most common equine stressors are shipping, competing and training, there are also others: moving to a different barn or a different stall; losing an equine friend; human stress and drama in and around the barn; a new stall neighbor; a saddle, bit, or bridle that doesn’t fit well; sudden changes in weather; extreme heat; the farrier; the vet; lay-up; a change in hay; barn construction; fireworks; hot air balloons, the arrival of a new horse; changes in barn help, grooms, or owners.
When in situations that you know will stress your horse, or when you see signs of stress in your horse, you want to reduce cortisol. You can supplement with holy basil or ashwaganda for cortisol reduction. A supplement that focuses on the brain/gut/adrenal axis is BioStar’s Thera Calm EQ, which you can give as needed.
Keep opossums out of the barn and pastures. Keep the feed room door closed so no errant opossum can decide to leave his or her urine or feces there. Good horse-keeping management includes maintaining clean outside water troughs.
Remember that cats can also be an intermediate host for the micro-parasites, as can armadillos, skunks, and raccoons. The AAEP recommends that, “Picking up dead skunks, armadillos, raccoons or cats on your property and disposing the carcasses to prevent opossums from eating them may prevent many sporocysts from contaminating the environment and hence reduce the incidence of the disease.”
Fallen seed from bird feeders and fruit-bearing trees also attract opossums. Last year in Wellington, at the house I rented in a residential area, an opossum used the backyard privacy fence as a kind of highway around the neighborhood, going from yard to yard and never having to touch ground. When my two Australian shepherds caught site of the opossum one night, they sent that animal streaking away. Never saw the opossum again. But it shows that even in suburbia, the opossum can adapt.
As we learn more about EPM and how the equine immune system can fight off the protozoans that cause it, it’s important to be mindful of your horse’s level of stress, and of safeguarding pastures, water sources, barns and feed rooms.