The Harry M. Zweig Memorial Fund for Equine Research

The Role of Cecal Histamine in the Pathogenesis of Acute, Alimentary Laminitis

Dr. J. Brett Woodie and Dr. Brad Njaa

For as long as people have owned horses, founder or laminitis has been identified as a problem most commonly associated with excessive intake of diets high in grain, starch or concentrate. The proposed mechanisms that have been discussed and investigated over the years have largely focused on vascular alterations at the level of the digits including decreased perfusion of the digital vessels with blood, local edema, shunting of blood away from the digit or blockage of small vascular capillaries with small clots known as microthrombi. Specifically, the alteration to the blood flow affects the lamina of the hoof wall as the hoof attaches to the underlying bone and soft tissue. However, the exact mechanism of this devastating disease largely remains a mystery.

Most current dogma is focused on the fact that during episodes of excess ingestion of grain or concentrate, the pH of the cecum and colon decrease due to preferential proliferation of bacteria known as Lactobacilli spp. These organisms proliferate rapidly in the energy rich environment and produce, as a by-product elevated levels of lactic acid. The elevation in the cecal and colonic acid concentration results in the lowered pH. How this alteration in pH translates to digital pain, warm feet, pulsing digital arteries and eventual laminitis continues to remain speculative.

A common theory suggests that the acidic environment or markedly decreased pH of the cecal and colonic luminal contents does two things - it firstly destroys many of the common bacteria normally found in the large intestine and secondly damages the mucosal epithelial lining of the intestine. In concert, the damaged mucosal barrier allows the release and absorption of the damaging bacterial cell wall components from the destroyed bacteria to enter the general circulation. These cell wall components are referred to as endotoxins. Once these endotoxins leave the bowel, they are free to circulate throughout the body and cause the greatest damage in the smallest blood vessels, capillaries. Therefore, the capillaries of the digits are extremely small and the end of the circulatory path to the limbs prior to venous return of the circulating blood. Once this endotoxin is in close contact with the cells lining the blood vessels, the endothelium, a host of tissue reactions can occur including formation of small blood clots that can block blood flow, increase leakiness to the blood vessels resulting in production of excess fluid outside the blood vessels (edema) and actual destruction of the endothelial cells further promoting clot formation. However, injections of endotoxin alone in clinically normal horses do not predictably induce laminitis in all cases.

Another theory is that the local vascular changes are due to release of chemicals locally and from the gastrointestinal tract following ingestion of a high carbohydrate diet. Some of these chemicals are referred to as vasoactive amines because of their profound effect on blood vessels. As early as the 1940s, investigators found that horses with clinical laminitis had elevated levels of histamine in both digesta contents and serum. Many of the researchers in the mid 1940s to the 1960s found strong associations between feeding horses and ruminants diets high in carbohydrate and lowered pH levels in the ceca/rumen, respectively, the generation of histamine in the intestinal tract and parallel increases in serum histamine. One of these publications even demonstrated protection from laminitis with the use of injectable antihistamines. However, the source of this elevated histamine has never been discovered until very recently.

Most of the researchers to date have focused on the Lactobacillus population in the ceca as the source of the histamine production. Histamine is a by product of a chemical reaction known as a decarboxylation reaction whereby a carboxy group is removed from an amino acid histidine to histamine. However, this enzymatic reaction does not occur with a high enough activity to generate the amount of histamine that has been recorded in the experimental cases of laminitis following feeding of high volumes of grain or concentrate.

Researchers at Cornell University have discovered a new bacterium Allisonella histabacter that is capable of producing histamine at a faster rate (10X) than most other bacteria. This new genus of bacterium has been isolated from both cattle and horses. Initial characterization of this bacterium has been done using the cow as the animal model. From these studies, much is known about this bacterium including the antibiotics that it is most susceptible to. Using fistulated cattle, initial experiments have shown strong correlation between feeding diets high in concentrate and the appearance of increased numbers of Allisonella histabacter (Refer to Figure 1 in the Appendix). However, this study has never been done using horses.

Given the emotional and economic impact that is felt when companion, athletic or work horses succumb to laminitis, it is imperative that the mechanisms involved in the induction and development of laminitis be better understood. Therefore, we propose a pilot study in which we obtain clinically healthy horses and surgically fistulate the cecum to the body wall in the right flank. Following recovery from the surgery and after allowance is made for acclimating to the animal housing environment, these horses will be given diets with in which portions of the roughage in the diet will be gradually replaced by incremental increases in grain or concentrate. During this time period, cecal contents and blood will be collected for enumeration of A. histabacter and histamine levels. Once a correlation has been established between level of concentrate in the diet with numbers of A. histabacter and concentration of histamine in the ingesta and serum, it is anticipated that at least one animal will be exposed to an abrupt increase in dietary concentrate in order to determine how quickly these parameters change relative to the development of laminitis. Assuming that this animal recovers from the acute episode of laminitis with supportive therapy, this animal or an additionally fistulated animal will be dosed with a diet high in concentrate but prior to and post feeding, this animal will be supplemented with oral and/or injectable antihistamines in order to inhibit the effects of histamine and prevent the development of acute laminitis.

We anticipate that the results will show a strong association between proliferation of Allisonella histabacter, cecal histamine production and serum histamine concentrations with the onset and development of laminitis. We also anticipate that following this pilot study, other studies will investigate the efficacy of antihistamine therapy for preventing or minimizing the effects of laminitis in animals known to be exposed to excess levels of dietary concentrate. We also project that an antimicrobial therapeutic may be developed that may specifically inhibit the effects of this bacterium.