Heat stress in livestock: how gut damage may drive heat stroke

Why the old explanations fell short#

For much of the twentieth century, heat stroke was put down to direct damage to the brain from heat, or to a simple collapse of the circulatory system. Cronjé's review notes that these ideas fit the sudden rise in temperature, the drop in blood pressure, and the delirium and collapse seen in severe cases, but they leave other observations unexplained.

Findings such as widespread clotting inside blood vessels, failing kidneys, and damage to the liver and the pancreas look more like a picture of multiple organs shutting down together, resembling sepsis more than straightforward brain injury or circulatory failure. Bleeding from the gut lining and from the lining of the abdominal cavity, along with death of liver tissue, were also recorded in earlier case studies and sat awkwardly with the older explanations.

Timing added to the puzzle. Tissue injury and abnormal blood clotting can keep progressing even once body temperature is back to normal, and in livestock a gap of three to four days has been observed between a period of heat and a fall in production or the onset of heat stroke. A newer framework drawn from human and animal medicine tries to account for this lag by tracing an injury pathway that starts in the gut.

Tracing the path from blood flow to organ failure#

The review sets out a sequence that begins with the body's first response to heat: skin blood vessels widen to shed heat, and to stop blood pressure falling too far, the arteries supplying the gut and related organs narrow in compensation. This starves gut tissue of oxygen and nutrients, a state called ischemia, and the gut is singled out as especially sensitive to this because it normally has high metabolic demands. The paper reports that gut blood supply can fall by 40 to 50% during periods of heat exposure.

Deprived of oxygen, the cells lining the gut lose function, and the barrier they normally form against gut bacteria begins to fail. This is described as allowing endotoxin, a toxic component from the outer wall of certain gut bacteria, to cross into the bloodstream. Endotoxin has been measured in the portal blood of heat-stressed monkeys and rabbits, in people suffering heat stroke, and in marathon runners, and giving oral antibiotics stopped the rise, which points to the gut as the source.

Once in circulation, endotoxin sets off cytokines, the chemical signals of the immune system, producing inflammation that can turn destructive rather than protective. The review then outlines reperfusion injury: when blood flow to the gut returns, an enzyme called xanthine oxidase reacts with the newly available oxygen to generate reactive oxygen species, and this second wave of damage is described as potentially more harmful than the original oxygen shortage because it can spread injury well beyond the gut itself.

Does this apply to sheep, cattle and pigs#

Most of the detailed evidence for this pathway comes from medical research using rodents, so the review asks whether it plausibly extends to farm animals. Species differ in how they shed heat: horses rely most on sweating, with cattle, sheep, pigs and poultry following in that order of reliance. Pigs sweat little and pant poorly, depending instead on wallowing to cool down, while sheep combine modest sweating with heavy panting, during which breathing rate can climb from 50 to 320 breaths per minute as heat load increases.

In sheep exposed to heat, blood supply to the stomach and ileum, and more broadly the abdominal organs, falls sharply, with the figures given in the review comparable to the reduction linked to endotoxin uptake in pigs, where a 50% drop in blood flow was enough to let endotoxin through. Visible changes in cattle that died of heatstroke, including congestion of the gut's mucous membranes and clusters of small round cells in the gut wall, are compared in the review with the cell swelling and internal vacuole formation reported in heat-exposed rat gut tissue.

The review also cites work in which calves were injected with rumen bacterial endotoxin, producing severe shock together with bleeding and widespread clotting disorders, symptoms also seen in cattle and sheep that died of heatstroke. Direct measurement of gut permeability or endotoxin concentrations in heat-stressed livestock does not appear to have been carried out, something the review flags openly as a gap in the evidence rather than a settled conclusion.

A delayed stress protein response measured in heated cattle#

The review reports unpublished work from Cronjé and colleagues in which concentrations of heat shock protein-70 were measured in white blood cells from cattle subjected to a pattern of heat loading repeated daily over five days. Heat shock protein-70 is one of the molecular chaperones the body produces to stabilise and repair damaged proteins, and other studies cited in the review show it accumulates strongly in gut and liver tissue after heat exposure.

Only a modest rise in this protein occurred while the heat was being applied, but a much larger rise followed once the heat loading had finished. The review describes this as the first time a delayed rise of this kind has been shown directly in a living animal, and links it to separate evidence that concentrations of this protein increase once blood flow is restored after a period of oxygen shortage, suggesting that much of the gut injury may occur once normal circulation resumes rather than during the heat exposure itself.

This single unpublished dataset is the only livestock-specific stress-protein evidence offered in the review, and the authors present it as supportive rather than conclusive. We would treat it as a useful pointer rather than firm proof, since it has not been reported in detail or repeated elsewhere within the paper.

Feedlot diets, acidosis and heat stress in livestock#

The review links this gut-centred model to common feeding practices. Energy-dense diets are already known to harm the gut independently of heat: finely ground pelleted rations are linked to stomach ulcers in pigs, and grain-heavy diets raise the concentration of dissolved particles in rumen fluid, drawing water across the rumen wall and separating its lining cells from the tissue beneath. That damage is described as the route through which endotoxin and bacteria reach the bloodstream and cause liver abscesses, reported at a 22% incidence rate in feedlot cattle.

Lower feed intake in hot weather is accompanied by slower movement of material through the gut, so acid production per unit of feed rises and rumen acidity increases, a pattern the review ties to a known seasonal rise in acidosis cases during warmer months and to digestive-related feedlot deaths that have been reported to peak over summer.

A specific feedlot death event is discussed, in which cattle died during a second, milder spell of heat that followed a cooler period, having apparently eaten heavily once temperatures fell. The review questions the usual explanation, that stored heat from that extra feed intake overwhelmed the animals' cooling capacity, given that five days passed before the deaths occurred, and instead suggests that injury to the gut from the first hot spell, followed by a surge of damage once normal blood flow returned during the cooler weather, offers a more convincing account.

Betaine as a possible protective feed additive#

Having mapped out where the damage occurs, the review turns to betaine, a compound related to the amino acid glycine that occurs naturally in microbes, plants and animals. Once regarded mainly as a water-balancing molecule or a donor of methyl groups in metabolism, betaine is now also described as able to interact directly with heat shock proteins and assist in repairing proteins that heat has unfolded.

Several specific effects are described. Betaine build-up in gut cells is proposed to lessen osmotic strain from high-grain diets; in red blood cell membranes, betaine lowered the activity of the Na+/K+ pump by 64% and of the Ca++ pump by 73%, which could cut the sizeable energy cost of pumping ions, a cost reported to make up over 50% of the energy used by intestinal cells in cattle on concentrate rations. In one laboratory trial, betaine kept a mitochondrial protein fully functional under heat exposure at 44 degrees Celsius across a 40-minute test period during which the untreated protein failed within 15 minutes, and it sped up the repair of damaged proteins by heat shock proteins by as much as 50%.

Feeding betaine to growing pigs has also been reported to cut their maintenance energy needs by 5.5%, and in isolated rat livers put through a period of restricted and then restored blood flow, betaine lowered two markers of liver damage, lactate dehydrogenase and aspartate transaminase, by 50% and 67% respectively. The review points out that cells take up betaine in response to osmotic stress rather than heat itself, so it suggests supplementation would need to start ahead of hot weather and continue through the risk period, rather than being introduced only once a heatwave has begun.

What this means for managing heat stress in livestock#

The review positions its proposed mechanism as a different kind of lever to existing heat management advice, the sort of material presented at the RAAN conference proceedings more generally. Cutting dietary energy or the amount fed during hot spells is already used to ease heat load, but the paper notes this slows growth, adds cost in handling and storing feed, and brings its own risk of acidosis and bloat once normal feeding resumes. It argues that weather forecasting and heat index tools are not yet dependable enough to justify these trade-offs confidently, which makes approaches that build the animal's own resilience more appealing to producers than reactive feed cuts.

Betaine is put forward as a feed additive that could be used safely in animals destined for human food, unlike several other anti-endotoxin or anti-inflammatory treatments discussed in the review as unsuitable for that purpose. Its proposed value lies in potentially softening damage at several points along the injury sequence outlined earlier, from easing gut cell energy demand and osmotic strain to protecting proteins against heat damage and supporting liver recovery once blood flow is restored.

The evidence has real limits worth stating plainly. Much of the mechanistic detail rests on rodent, poultry, cell-culture and isolated-organ studies, and the review itself acknowledges that gut permeability and endotoxin levels in heat-stressed farm animals, along with any protective effect of betaine specifically against heat injury in those species, do not appear to have been directly measured. The case for betaine is best read as a well-argued hypothesis drawing threads together from several fields, not as a demonstrated outcome in commercial sheep or cattle production, in the same Livestock Library research index that holds other nutrition and physiology papers from this period.

Sources and further reading#

• Trove library search: find a library that holds the paper
• Meat & Livestock Australia: red meat industry research and marketing body

Questions#

— The Livestock Library team