Rabbits (and Horses) Experience Fatal Abdominal Pain from Gastrointestinal Microbial Hydrogen Sulfide Production!

Rabbits (and Horses) Experience Fatal Abdominal Pain from Gastrointestinal Microbial Hydrogen Sulfide Production!
By Assoc. Prof. Dr. Sompote Weerakul (Doctor Kaew)


An academic topic that most people are unaware of, but should know: gastrointestinal problems in rabbits have consistently been the most common issue. Almost every case presented to veterinarians involves pain, ranging from mild to severe enough to be fatal. What causes these events? I will explain the details by tracing the disease mechanism from the symptoms back to the root cause. While this topic might be difficult for the general public to fully grasp, it is essential knowledge that specialized veterinarians should further understand.

The Nature of the Pain
First, it is important to understand that the pain experienced by rabbits during gastrointestinal stasis primarily occurs in the large intestine. It is a colicky pain associated with gastrointestinal contractions and morphological or anatomical changes in the large intestine, which can be observed on X-rays from the early stages. Changes in other parts of the digestive tract follow later. This is similar to colic in horses and elephants. One major, yet often overlooked, root cause is hydrogen sulfide.

The Role of Hydrogen Sulfide
The production of hydrogen sulfide, or "rotten egg gas," comes from a group of bacteria known as sulfate-reducing bacteria, which convert sulfate into sulfide. Normally, the digestive tract naturally produces this gas in appropriate amounts. Therefore, normal ranges can be found in healthy humans and animals (hydrogen sulfide concentration in healthy adults ranges between 0.3 and 3.4 mmol/L, as cited by Dordevic et al., 2021, Journal of Advanced Research). In normal amounts, it causes no harm to the body and even plays a role in physiological mechanisms, including chemical alterations, utilization, and elimination.

However, excessive amounts cause inflammation and colicky pain in the large intestine. In other animals (like horses, cows, and buffaloes), this might cause temporary colic followed by projectile feces with a sulfurous odor. In most rabbits, however, it causes severe colic without defecation, similar to what is seen in horses. In humans, it is believed to be a contributing factor to inflammatory bowel diseases (IBD), specifically ulcerative colitis, as reports link hydrogen sulfide to large intestinal inflammation. It damages the intestinal mucosa and disrupts gastrointestinal cell function by reducing butyrate oxidation; since colon cells derive over 70% of their energy from butyrate, this reduction leads to energy depletion and cell death.

Bacterial Mechanisms and Diet
These bacteria naturally exist in the environment and the digestive tract. In anaerobic (low-oxygen) conditions, these microbes can even be found in our drinking and tap water (a sulfur smell indicates low oxygen). Hydrogen sulfide is highly toxic and a primary culprit in gastrointestinal inflammation. These bacteria thrive by utilizing metabolic byproducts from other bacteria as energy sources—such as lactate, pyruvate, malate, succinate, and acetate—which are natural products of herbivore fermentation.

This process begins with lactic acid bacteria producing large amounts of lactic acid, along with some acetic acid, ethanol, and carbon dioxide. Lactate and acetate act as the initial substrates for sulfate-reducing bacteria. Because these bacteria can grow on various free fatty acids, it is highly likely that the normal baseline values for herbivores are higher than those for humans and other animals.

Abnormally high levels usually stem from the excessive production of lactic acid, lactate, and acetate. This typically originates from a diet containing unacceptably high levels of starch, such as inappropriate food, snacks, or fruits. This causes lactic acid bacteria to rapidly multiply in the initial stages, subsequently producing large amounts of the aforementioned byproducts, which become a source of sulfate. It can also be caused by feeding diets containing sulfate and sulfite ingredients, often found in human food products.

Summary for Pet Owners
To summarize simply for everyone, including interested pet owners: feeding inappropriate diets—such as young plants, snacks, starches, fruits, high-sugar plants, poor-quality concentrated pellets unsuitable for herbivores, and foods mixed with inappropriate amounts of grains (muesli) (concentrates derived from starch should not exceed 0.06% of body weight)—results in a massive production of lactic acid.

This triggers a cascade of negative health effects:

Dysbiosis: It increases acidity (lowers the pH) to the point where beneficial microbes die off. Opportunistic pathogens then attack, leading to a microbial imbalance.
Inflammation: Feeding these diets leads to excessive production of free fatty acids and numerous hormones, causing an imbalance. The production of inflammatory mediators, such as histamine, increases abnormally.
While beneficial in proper amounts, an excess negatively impacts health. I highlight hydrogen sulfide here as an often-overlooked example to emphasize the need for understanding. In excess, it becomes toxic, acts as a major inflammatory agent, and most importantly, is a primary cause of severe, potentially fatal colic in rabbits.

Conclusion
The starting point is the same root cause that most people are already aware of: improper dietary management. The difference lies in having a deep versus a superficial understanding. As a veterinarian, a key role is to provide comprehensive knowledge to those eager to learn, raising awareness about the dangers of inappropriate diets. Beyond this gas, there are many other factors that cause gastrointestinal stasis and enteritis, such as histamine, pH levels, and the stimulation of PGE, aldosterone, and motilin. This understanding helps everyone, from veterinarians to pet owners, recognize effective treatment approaches and preventative measures for a long life, strategies that have been continually developing over time.