The equine gastrointestinal microbiome is a fascinating entity that is present within the digestive tract. The balance of the hundreds of bacterial microorganisms within the horse’s digestive tract can either dictate health or disease, contributing on many levels outside of pure digestion.  In past research, we have demonstrated how an overgrowth of lactic acid bacteria within the hindgut of the horse can contribute to many potential health and lameness conditions, which is also reinforced by other research outside of our own. However, what is truly amazing is how you can assess a small group of horses in the same setting, but yet discover different levels of microbiome disturbances.  In this paper, I will highlight 4 horses out of a larger population, demonstrating the connection between microbial overgrowth and potential health complaints.


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Here is a tale of 4 horses out of a herd of 6, that had recent fecal cultures down for a health evaluation. In this study, you can see the differences between the horses and hopefully make some connections as to how the bacterial populations can shift, higher lactic acid bacterial counts, and then health is impacted.

As a guide to culture interpretation, we have two methods by which we are evaluating the levels of lactic acid bacteria in a set dilution of fecal material.  The first culture plate is used to help us visually appreciate lactic acid bacterial numbers based on a color change.  Initially, this plate starts off with a red color, which then is altered by the lactic acid bacteria, creating a color change to yellow.  The more yellow, the higher the lactic acid bacterial counts.  We grade this plate, based on color change, on a scale of 1-4, with a 4 indicating higher growth and more yellow color change. The second culture plate, teal color,  that is used helps us to quantify the actual lactic acid bacteria number, in regards to colony formation.  Ideally, the two plates are used together.  If we see more yellow in the initial plate, this should coincide with a higher count on the teal colored plate seen as black dots or colonies.

All four of these horses are under identical conditions, feeding, pasture, and supplement regimens.

Horse Number One:

This horse was cultured out of curiosity and is part of the herd, but does not demonstrate any major health concerns to his owner, nor lameness issues.

In this photo, you can see the two plates used during his culture.  On the left is the color enumeration plate, which is almost completely red in color, giving him a low growth score of 1+.  The teal colored plate indicates a very low level of colony formation as well, very few black dots are noted.

Horse Number One Lactic Acid Bacteria Culture

Horse Number One Lactic Acid Bacteria Culture

Horse Number Two:

This horse was also cultured as part of the herd and also does not demonstrate any major health or lameness concerns to his owner.

In the photos below, you can see almost the same results as noted in horse number one.  Very low counts on both plates, visually seen by little yellow coloration to the left plate and very few black dots or colony formation on the teal plate.

Horse Number Two Lactic Acid Bacteria Culture

Horse Number Two Lactic Acid Bacteria Culture

Horse Number Three:

This horse was also cultured as part of the herd project, exhibiting no major health or lameness concerns to his owner.

In the photos below, we are seeing a higher level of lactic acid bacteria, compared to the first 2 horses.  This horse was given a grade of 2+ on the color change, more yellow spots noted, and a higher number of black colonies on the teal plate.

Horse Number Four Lactic Acid Culture

Horse Number Three Lactic Acid Culture

Horse Number Four:

This horse was included primarily in the study due to ongoing concerns of metabolic related issues, over-weight body condition, chronic laminitis, and a touch of intermittent gas concerns.

In the photos below, this horse clearly has much higher levels than the prior three.  The red plate is now completely yellow in color, eluding to the fact that the lactic acid bacterial counts are higher.  This is then confirmed when we look at the teal plate, which has numerous black dots or bacterial colonies present. This horse was given a score of 4+, with very high levels of lactic acid bacteria present in the fecal samples.

Horse Number Four Lactic Acid Culture

Horse Number Four Lactic Acid Culture

Discussion and Relevance of the Cultures

In most cases, when fecal samples are submitted for culture analysis, they are focusing on a problem horse, which often demonstrate overgrowth as noted in the horse above.  However, some do not.  The problem that is present with these cultures is that often we can see the problem horse, as they are focused upon, but we fail to see others that may or may not have a problem.  It is almost like having fecal egg counts down on a group of horses, rather than just focusing on one in the group.  We get a better feel for the situation when we look at the population as a whole.

In this particular group of horses, 3 of them demonstrate no clinical problems to their owner and all 3 horses have lower counts of lactic acid bacteria in their fecal specimens.  However, the fourth horse not only demonstrated clinical problems, but also had a higher count.  Given this, we can associate the higher count with the clinical problems.  The question is regarding whether the higher lactic acid count is causing the clinical problems, or is the higher count a by-product of the health issues and ongoing stress that the horse is enduring?

The other thing that we can see here is that the third horse demonstrates lower levels, which is good, but may need to be monitored in the future.  If health problems develop, it could reflect that the once low count has now risen for some reason, whether if that is dietary, environmental or otherwise.

The final thing that is demonstrated here is that despite all of the horses being on the identical diet, supplement regimen, and environmental factors, one is exhibiting high levels and health issues.  So, this tells us that maybe that supplement and diet regimen is not ideal for all of the horses, or that maybe there is another factor, internal or external, that is impacting that one horse and needs to be addressed.

The fecal cultures are certainly enlightening and can yield a good amount of information, which can then guide us with a personalized approach to hopefully better the individual horse, and the entire herd.

For further information on fecal cultures, click here.

For further information on the digestive microbiome and influence on horse health, click here.


Author:  Tom Schell, D.V.M., CVCH, CHN


1 Comment

  1. Lynell Abbott on September 25, 2018 at 11:48 am

    Hey, Doc. This is fascinating. I ALMOST understand what you are saying. I would like to know how the salmonella and coliform counts are a part of this. Still struggling to connect the dots here.

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