Through the Looking-Glass

a bug's-eye view of the equine gut
and what it can tell us about feeding horses

© Christine M. King

 

Chapter 2

"There's no use trying," she said;
"one can't believe impossible things."
"I daresay you haven't had much practice," said the Queen.

 

We usually interpret these breathtaking bacterial numbers to mean that the microbiota of the equine gut makes important, even essential, contributions to the horse's digestion and health. While that certainly is true, in our ‘looking-glass’ view it may be equally true that these numbers indicate that the equine gut is an especially good habitat for many different bacteria — a great place to live.

'For Rent'

Given what we know of conditions within the equine gut, the small intestine and hindgut would be particularly well suited to bacteria of an anaerobic* persuasion that thrive in a dark, wet or frankly liquid environment, at a fairly constant temperature of around 38 degrees Celsius (100 degrees Fahrenheit), and a relatively neutral pH.

* In this context, anaerobic refers to organisms that thrive under conditions of low oxygen concentration (facultative anaerobes) or that can survive only in the absence of free oxygen (obligate anaerobes). Another way of putting it is that free oxygen is toxic to obligate anaerobes but less so to facultative anaerobes.

Absent human interference, the nutrient supply is more-or-less constant and its composition is sufficiently diverse that it creates a degree of adaptability in the gut microbiota which makes the community quite resilient in the face of inevitable seasonal variations.

Within the larger habitat of the gut, there are the options of the liquid portion of the gut contents, floating rafts of solid material (food and cell debris), and the layer of mucus that coats the gut lining. The latter may be particularly important for maintaining a stable gut microbiota, as the mucus layer is relatively more fixed and consistent in composition than the gut contents themselves.

In addition, there is ample opportunity for 'recycling' through the host’s external environment, as bacteria expelled from the gut in the faeces can continue to survive within the manure, on/in the top layer of the soil, and on/in the grass and other pasture plants, eventually returning to the gut as the horse grazes.

So, from the gut microbe’s perspective, the necessities of life are well supplied by this habitat: shelter, food, water, ‘air’ (i.e., one’s preferred blend of gases), light (i.e., one’s preferred intensity and spectrum of photons), optimal temperature... In other words, it’s very suitable accommodation.

However, successful inhabitants would need to have run the gauntlet of the stomach, where the pH* in the lower portion can drop to a blistering 2 on average (and to less than 1 in some situations), and the proteolytic enzyme, pepsin,** is released. They would also need to have survived the fairly dramatic rise in pH (to more than 6) and the wash of soapy bile salts and digestive enzymes in the duodenum and the remaining 20-plus metres of small intestine.

* The pH scale ranges from 0 (extremely acidic) to 14 (extremely alkaline or ‘basic’). Neutral is a pH of 7. As a point of reference, the pH of healthy blood is around 7.4 (slightly alkaline).

** Pepsin is released as a pro-enzyme, pepsinogen, which is then activated by contact with gastric acid. This enzyme begins the breakdown of proteins.

By the end of the small intestine, the pH of the gut contents will have risen to about 7.5 (slightly alkaline). Each 1 unit change in pH represents a ten-fold change in hydrogen ion activity (e.g., a solution with a pH of 1 is 10 times more acidic than a solution with a pH of 2), so there are enormous swings in acidity between the mouth and the caecum. The changes in acidity from mouth (alkaline, pH of 8.5–9) to stomach to caecum represent swings of between 100,000-fold and 1-million-fold.

The microbes would then have to be able to compete for space and nutrients with the other resident or transient gut microbes, withstand or recover from inhospitable conditions created by some of the other inhabitants, and have a sufficiently rapid replication rate* or successful attachment mechanism that they can withstand the constant flow-through of material which continually empties their environment — not to mention the variety of antibacterial defenses in constant use by the host cells which define and maintain the space!

* Bacteria multiply by simple cell division (one cell splits into two). Under ideal conditions, the replication rate or doubling time for most culturable bacteria ranges from 15 minutes to 1 hour. For example, the doubling time for E. coli is 15–20 minutes in the laboratory; but in the gut it’s estimated to be 12–24 hours.

So, while this habitat may be ideal for the gut microbes, it is far from idyllic. Life in the equine gut is never dull.

A Bug's Life

So then, what would it take to keep the gut microbiota, and thus its equine host, healthy? Following Lewis Carroll’s lead, if I were a bacterial community, what would I want? In no particular order, the list might include the following.

1. Food—a fairly constant supply of suitable food.

The constant supply part is taken care of, as long as we allow horses to follow their natural eating pattern, which, to quote the horsemen’s adage, is “little and often.” Depending on the season and on forage availability and quality, both domesticated horses kept on pasture and feral or wild horses allowed free range spend 60% to 75% of their time grazing. Factoring-in that the horse’s stomach is not fully emptied for several hours after a high-fibre meal (e.g., grass hay), that’s near enough to constant food supply for the gut microbes.

In further support of the constancy of food supply for the gut microbes is the curious case of the equine liver and pancreas. Unlike most other domestic animals, the horse does not have a gall bladder, which is a small sac attached to the liver for the storage and timed release of bile (i.e., in response to a meal). Instead, horses secrete bile into the duodenum continuously, and the rate of secretion does not change with feeding.

A lesser known fact is that the equine pancreas does the same thing: it secretes its bicarbonate-rich fluid, along with relatively low concentrations of digestive enzymes, into the duodenum continuously — and in impressively large volumes. In a seminal study, the total volume of bile secreted in a 100-kilogram (220-pound) pony was approximately 4 litres (1 gallon) per day, and the total volume of pancreatic fluid was 10 to 12 litres (approximately 3 gallons) per day. Most of this fluid is resorbed in the small and large intestines, so it is effectively recycled in the healthy horse. The point here is that the continuous production of these digestive juices indicates that the horse’s digestive system is designed for fairly continuous intake of food.

In horses with little or no pasture access, or when there's little or no pasture in the field, this natural feeding pattern can be emulated by ensuring a fairly continuous supply of good quality grass hay — i.e., hay available at all times, allowing the horse to ‘graze’ at will (ad libitum). For horses needing calorie-restricted diets, the daily forage ration (a dry matter intake of 1.5% to 2% of ideal body weight, or 7 to 9 kilograms of ‘carb-safe’ hay for a 450-kg horse) can be divided and fed in at least 3 meals over the course of the day, from early morning to late evening, so that the horse is never without food for more than about 8 hours.

[To be continued...]

intro

chapter 3