Microbiomes: It's All About Balance

You know the drill: Kill the bacterium that's attacking the immune system then prepare yourself for the relentless fungus to come out and play in its wake. This bug balance upheaval is the reason we reach for yogurt when we go on antibiotics or we pursue a probiotic supplement if we start feeling "off." Some owners apply similar strategies when caring for horses. But one veterinarian believes it's not just a matter of wrangling fluctuations of one or two bugs at a time; diversity among our horses' microflora could be far greater than we've imagined, and addressing that balance might prove a bit more complex.

Scott Weese, DVM, DVSc, Dipl. ACVIM, a professor in the Department of Pathobiology at the University of Guelph's Ontario Veterinary College, presented on the concept of microbiomes and their function at the Ninth International Conference on Equine Infectious Diseases, held in October 2012 in Lexington, Ky. [VIDEO]

He defined a microbiome as the totality of microbes, their genomes, and interactions in a particular environment. A microbiome comprises different microbial "communities" on different sites of an organism's body--for example, nasal passages, oral cavities, skin, and various organs--and researchers are examining these communities' function in health and disease in a number of species. Weese and his team are focusing on horses.

Veterinarians know very little about the equine microbiome, he admitted, but with new technology they're starting to understand its complexity and diversity. When his team examined the microflora in the hindguts of normal, healthy horses and those with idiopathic colitis (unexplained inflammation of the colon), they noticed "rather remarkable differences between our normal and our diarrheic horses."

Specifically, the scientists noted that 11% of the healthy horses had different types of Clostridium in their feces, compared to only 5.5% of the horses with diarrhea. Common theory has usually been that clostridia are "bad" bacteria.

"While some (such as Clostridium difficile) can cause disease, most clostridia do not and a healthy clostridial population might actually be important to have a healthy horse," Weese said. "Maybe we need to start to think about the wide range of bacteria that are present in the gut and that we haven't paid much attention to in the past, instead of getting tunnel vision on a few high-profile individual bacteria."

Weese and his colleagues found 123 OTUs (operational taxonomic units; for all intents and purposes, species) that all horses, six healthy and 10 diarrheic, shared. Only six OTUs were present at least 25 times in each horse, meaning that while the team found a lot of bacteria in all the horses, many of those bacteria were quite rare. This suggests that those six OTUs are probably important. They noticed some unusual suspects, detecting Shigella spp (foodborne pathogenic microorganisms) in one horse with colitis, but not in any healthy horses, and high levels of Fusobacterium in some horses with diarrhea.

"We looked at data after treatment (of GI disease) ... and we noted a significantly different composition after antimicrobials," he explained, namely decreased bacterial diversity. "There were decreases in Spirochaetes, Actinobacteria, and Lactobacillus."

What became abundantly clear through the team's research was that the horses' guts were far different than other species, reflecting a wide variety of microbes; he said it's very likely that age, diet, geography, gender, season, etc., also impact the microbiota in a horse's belly. So, as for seeking a balance and promoting horse health, Weese posed the questions, "Do we know enough about 'normal' and its variations to define 'abnormal'? How much diversity do we want? How much species richness do we want?

"Can we therapeutically manipulate the microbiome with antimicrobials, prebiotics, and probiotics?" he continued. Sure we can, he suggested, but perhaps in additional ways than current approaches. "We need to keep our mind open about probiotic use in horses. Don't just look at Lactobacillus because everyone else is. We should think about some of these less well-known but probably more important bacteria that are widely present in healthy horses.

"We need to think about some diseases and syndromes--including some traditionally considered noninfectious--from a population standpoint, not a one-bug/one-disease standpoint," he concluded. "To maintain a healthy horse, we need a healthy gut. To maintain a healthy gut, the horse needs a healthy bacterial population. Figuring out ways to help with that, whether it's a proper diet, avoiding antibiotics except when they are truly needed, and other good management practices, is an area we need to pay attention to."

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.