Last updated: 24 Sep 2025 | 115 Views |
**“Rabbits (and horses) may experience fatal abdominal pain caused by hydrogen sulfide production by gastrointestinal microorganisms!
By Assoc. Prof. Dr. Somphot Veerakul (Dr. Kaew)
This is a scholarly topic that most people seldom know, but ought to understand. Gastrointestinal disorders in rabbits have long been one of the most commonly encountered problems. In nearly every case presented to veterinarians, pain is observed at levels from mild to severe, even potentially leading to death. What causes these events? I will go into detail later, tracing back to the underlying mechanisms and etiologies. This subject may be difficult for the general public to grasp, but it is one that veterinary specialists should seek deeper understanding of.
One point to understand from the outset is that the pain experienced by rabbits in ileus (intestinal stasis or gas accumulation) mainly occurs in the large intestine. It is a cramping pain associated with spasmodic contractions of the gastrointestinal tract, along with anatomical or morphological changes in the large intestine—changes that can often be seen in the first radiographic images. Other parts of the digestive tract manifest secondary changes thereafter. It is analogous to colic in horses and elephants. One often overlooked but central causative factor is hydrogen sulfide.
The by‑product hydrogen sulfide (the “rotten egg” gas) is produced by a group of bacteria called sulfate‑reducing bacteria (SRB). These bacteria convert sulfate into sulfide. Under normal conditions in the gut, this gas is produced at moderate levels. In healthy humans and animals, values can fall within a normal range (in healthy adults, hydrogen sulfide concentration is between 0.3 and 3.4 mmol/L, as cited in Dordevic et al., 2021 in Journal of Advanced Research). In such amounts it causes no overt harm, and participates in physiological functions such as chemical modulation, utilization, and clearance. But when levels become excessive, inflammation and colicky pain of the colon may occur. In other species, one might also observe explosive diarrhea, with feces that smell of sulfur (e.g. in horses, cattle, buffalo). However, in rabbits the typical presentation is a cramping pain and inability to pass feces, much like colic in horses.
In humans, elevated hydrogen sulfide has been proposed as one contributing factor in inflammatory bowel diseases such as ulcerative colitis. Reports have shown associations between hydrogen sulfide and colonic inflammation: it can damage the intestinal epithelium and interfere with function of enteric cells. A key mechanism is that hydrogen sulfide impairs butyrate oxidation: since colonocytes derive over 70 % of their energy from butyrate, this impairment leads to energy deficit and cell death.
These sulfate‑reducing bacteria are naturally present in the environment and in the gut. Under low-oxygen—or anaerobic—conditions, they can thrive. Even drinking or utility water may contain traces, so when oxygen levels are low, a sulfurous odor may develop. Hydrogen sulfide is a potent toxin and a major factor that can drive gastrointestinal inflammation. The SRB feed on substrates generated by other bacteria—such as lactate, pyruvate, malate, succinate, and acetate—which arise from plant fermentation in herbivores. Initially lactic acid bacteria ferment dietary carbohydrates into lactic acid, acetate, ethanol, and carbon dioxide. The lactate and acetate thereafter become substrates for sulfate‑reducing bacteria. Some SRB strains can grow directly on these fatty acids, so these substrates serve as a starting point. Hence, in herbivores, the “normal” baseline of hydrogen sulfide may even be higher than in humans or other mammals.
Excessive hydrogen sulfide generally originates from overproduction of lactic acid, lactate, and acetate—often due to feeding of diets too rich in starch, sweets, or fruit. Such feeding encourages overgrowth of lactic acid bacteria, leading to high concentrations of the above substrates, which in turn fuel SRB activity. Dietary components containing sulfate or sulfite (found in some human food products) can also exacerbate hydrogen sulfide production.
To summarize for laypersons or interested rabbit keepers: inappropriate diet—such as young easily digestible plants, sweets, starchy feeds, high-sugar fruits, or concentrates containing excessive grains (e.g. muesli) in unsuitable amounts (concentrated feeds should not exceed ~0.06 % of body weight)—promotes excessive lactic acid production. This yields abundant substrates that drive SRB overgrowth, leading to excess hydrogen sulfide, which is toxic, inflammatory, and can cause severe cramping pain—even death in rabbits.
The root cause is thus usually dietary mismanagement. Experts and novices differ in depth of insight, but veterinarians also bear the duty of educating informed owners about dietary dangers. There are many other factors that contribute to intestinal stasis and inflammation beyond hydrogen sulfide—such as histamine, acid-base balance, effects on prostaglandins (e.g. PGE), aldosterone, motilin, etc. This deeper understanding helps veterinarians and owners see effective treatment and preventive strategies that extend lifespan—concepts which have been developed over a long time.”**
