We typically think of the intestinal tract as a digestive organ, but it has a very important role in the immune system as well.
The immune system of the intestinal tract is called GALT – gut associated lymphoid tissue. It begins with the rich system of lymphocyte nodules in the back of the horse's throat. Throughout the intestinal tract there are macrophages and lymphocytes positioned immediately under the intestinal lining cells as well as deeper in the intestinal tissue. Structures called Peyer's patches are very similar to lymph nodes and are located in the small intestine. Specialized immune cells also line the liver.
Microfold cells, or M cells, are located over collections of lymphoid tissue along the intestinal tract. The M cells reach out to engulf bacteria and other material from the intestinal tract then pull it inside and present it to immune system cells. This material is processed through T and B lymphocytes, eventually resulting in the production of IgA antibodies, antibodies restricted to the local tissues. However, it doesn't stop there.
The lymphocytes sensitized/primed to produce specific antibodies are released into the bloodstream before they come back to home in the intestinal lining again. In the process, they can share this information with the entire body's immune system. A good example of the body-wide, systemic effects is oral vaccines. Several human oral vaccines have been developed, including against polio and malaria. Oral rabies vaccines in bait have been used to successfully battle rabies in wild animals .
Rhodococcus equi is a bacterial infection of young foals that causes an often fatal pneumonia and may also invade other organs. It is similar to tuberculosis. Treatment is often ineffective but under experimental conditions oral vaccination with live organisms could result in protection while intramuscular vaccination failed.
The production of antibodies is only one isolated process in the immune system activity in the gut. Antibodies and cells that target other cells infected with organisms are the effects of the more sophisticated arm of the immune system. The other arm is called innate immunity. This is a “shoot first and ask questions later” approach.
Inflammation and free radical stress is the hallmark of the innate immune response. In addition to white cells engulfing invading organisms, they release cytokines. Cytokines are chemical messengers that direct cellular activity and cause inflammation. This is an important part of immune defenses but left unchecked it can damage the tissues.
Research has found that interactions between the immune system and the bacteria colonizing it can have profound effects. Probiotics are defined as bacterial strains capable of colonizing and surviving in the gut which have a beneficial effect on their host by suppressing the growth of harmful bacteria and interacting with the immune system. Documented effects of probiotics to date include:
> regulate cytokines and regulatory lymphocytes
> encourage production of protective mucus by the intestinal cells
> reduce inflammation
> increase production of IgA while decreasing the risk of allergy
Detailed information is available for humans regarding types and specific strains of bacteria that fit the definition of probiotics. Unfortunately, that information is not available for horses. Involvement of Lactobacilli is a safe guess since the upper intestinal tract of people and horses function similarly and we already know the upper intestine and stomach of the horse are colonized by Lactobacilli . Beyond that, we don't know. For example, the Bifidobacteria so beneficial in humans are not found in horses.
A 2008 study from Slovakia identified three strains (out of 25) of Enterococcus faecium from manure of horses that on preliminary investigation may have probiotic potential. However, this needs further work to confirm.
Live yeast cultures of Saccharomyces cerevisiae improve fiber digestibility and buffer acid pH but do not have other proven probiotic activity. A 2005 study documented that treatment with Saccharomyces boulardii can decrease the severity and duration of symptoms in horses with enterocolitis, an infection of the intestinal tract.
While we cannot necessarily reliably predict probiotic strains for horses based on human data because of their vastly different conditions in the colon, prebiotic substances should be the same. Prebiotics are complex carbohydrates or soluble fiber which cannot be digested in the small intestine. They are fermented in the cecum and colon. All work by supporting the proliferation of probiotic strains of bacteria. However, some seem to also have direct immune system effects.
Arabinogalactan, usually isolated from the bark of the Larch tree, has well documented prebiotic effects on probiotic organisms. It also has a stimulatory effect on GALT. A 2010 article documented more far-reaching effects in humans. The study looked at titer responses to the Strep pneumoniae vaccine in people who were supplemented compared to a control group that was not. There was a statistically significant difference between the groups, with the supplemented group showing a better vaccine response.
Short chain fructooligosaccharides (aka fructans), scFOS, is a documented prebiotic in people. A 2008 French study tested whether scFOS supplementation could protect against the gut organism disruptions caused by a sudden increase in grain in the equine diet. It was effective for this but interaction with the immune system has not been confirmed.
Beta-1,3/1,6- glucans are complex carbohydrates from the cell wall of yeasts such as Saccharomyces.
A 1999 study in broodmares confirmed an immunostimulatory effect in broodmares injected with these glucans. They documented higher antibody levels in the colostrum. Studies on effects when given orally are not available for horses but this substance reliably stimulates the immune system in other species when supplemented orally.
L-glutamine is an amino acid, precursor to the antioxidant compound glutathione. Many studies have documented that supplementation with L-glutathione has an antiinflammatory effect in the intestinal tract. Absorbed L-glutamine can also be used to produce glutathione in the liver and other tissues.
L-glutamine levels can also be boosted by supplementation with bioactive whey, which is a highly concentrated protein extract of raw milk. In addition, bioactive whey supports normal immune system function, quiets inflammation and autoimmune reactions.
In summary, the intestinal tract is a major player in the immune system. It has been estimated that 70% of the immune systems cells originate there. Immune activity in the gut has a bodywide effect. One of the best ways to boost and balance the immune system is through the oral administration of probiotics, prebiotics and other substances documented to support and balance immune function.
Holistic Horse: your guide to natural horse health