How stress hammers a pig’s intestinal system

Pigs get stressed out for lots of reasons, but weaning, heat, crowding, transportation and related challenges can weaken their immune system and make them more vulnerable to disease and performance losses. This particularly holds true for gastrointestinal (GI) disorders,1 which lead to loss of appetite, slower growth, reduced feed conversion and lower meat quality.2

Largest immune organ

“The gut is the largest interface between the body and the outside world and houses the largest immune system in the body,” says Adam Moeser, DVM, PhD, Michigan State University.

The gut is very sensitive to environmental stressors, which can disrupt many gut functions critical for disease resistance and performance.  For example, stress causes loss of GI barrier function (increased gut permeability) and can trigger inflammation. Chronic stressors can weaken the gut immune system leading to increased risk for GI infections, adds Moeser, director of the university’s Gastrointestinal Stress Biology Laboratory. Further, stress-reduced feed intake hampers the digestion and absorption of nutrients, contributing to performance losses.

Moeser has co-authored studies that show that stressors such as early weaning can impede the development of the GI barrier and immune function, and make animals more susceptible to disease throughout their lives.  His studies show that pigs weaned at a later age have better gut health. In particular, one study showed that adult pigs that were previously weaned at 28 days exhibited less intestinal permeability (improved gut barrier) than those weaned at 21 days.3

“The higher the weaning age, the better for long-term resilience and gut health,” Moeser says. He acknowledges, however, that extending the weaning age could pose economic challenges for producers, and therefore, strategies other than adjusting weaning age to protect the gut are needed.

Short-term crowding and commingling of pigs can also cause stress and hurt herd performance, says Daniel Nelson, PhD, senior swine nutritionist at Zoetis.

“Physiologically, when pigs are stressed, they naturally release the stress hormones —adrenaline and cortisone,”4 he explains. “This is part of an animal’s natural fight-or-flight defenses.  But when an animal is in a pen with nowhere to go and these hormones are constantly being released because the pig is experiencing stress, they can cause adverse effects on organs of the body.

Heat stress

Heat stress can have a deleterious effect on a pig’s gut. “A finishing pig’s thermoneutral temperature is less than 70° F (21° C) — about 65° F to 67° F (18° C to 20° C), depending on the weight of the pig — which is fairly cool,” Nelson explains.5

“Anytime the environmental temperature gets above that, they have to start expending energy to try to get rid of heat. And that’s stressful.”

Pigs are not good at cooling themselves off, either. “They do not have a lot of functional sweat glands,” Moeser says.  “So, they adopt types of behavioral changes like panting, lying and reducing feed intake.”

The animals often try to adapt by reducing feed intake or drinking more water. “From a health and production standpoint, it takes a toll,” Moeser says.

Heat stress leads to a breakdown of intestinal integrity that triggers the release of endotoxins, added Jason Ross, PhD, an associate professor of animal physiology at Iowa State University. Heat stress can be particularly tough on adult pigs, such as lactating sows, Ross says.

He says lactating, heat-stressed sows often respond by eating less feed, compromising the opportunity to re-breed. “We think heat stress is contributing to seasonal infertility,” Ross says. “And every summer in North America, from the middle of July through August, we see a reduction in the pregnancy rate.”6

Reducing stress and losses

Stress has also been shown to compromise gut health, which can open the door to a host of enteric diseases that can hurt feed conversion and growth rate.

“Think of the intestinal wall as one single layer of cells that separates all the stuff passing through the intestine — feed, water, bacteria — from the blood,” Nelson says.  “What stress does, in simple terms, is punch holes in that protective layer. And when you do that, you establish routes for all the bad things that are in the intestine to pass into the blood.”7

He notes that like humans and animals, bacteria produce end-products of metabolism such as ammonia and other noxious compounds, which find their way to the intestinal cells and can damage or kill them.

If an antibiotic is present within the feed the animal is consuming, it may protect against bad bacteria that produce noxious compounds that get into the blood and have a negative effect on the host. Research by Moeser has shown pigs stressed by crowding or heat that were given the antibiotic bacitracin had a better growth rate and feed conversion than stressed pigs that did not receive the antibiotic.

For example, in a 2012 study, pigs stressed from mixing and crowding and fed bacitracin had a better average daily gain (p < 0.05) compared to pigs subjected to the same stressors but not given an antibiotic. In addition, medicated pigs subjected to stress from mixing and crowding showed improved intestinal barrier integrity, Moeser reports.8

The results were similar to earlier studies he conducted with heat-stressed pigs, which also showed better growth rate and feed conversion than heat-stressed pigs that did not receive the antibiotic bacitracin.9,10

Extra precautions needed

“The use of antimicrobials is effective,” in maintaining performance of stressed pigs, Moeser says.

“They are able to reduce [the impact of] a stressor in the environment. However, when you take a level of protection away and expose them to the same stressors, pigs likely will be more susceptible to bacterial infection.”

And lately, that’s exactly what the pork industry has been doing — taking that protection away.

For decades, pork producers added antibiotics to grower-finisher feeds to fight off enteric and respiratory disease or to optimize growth rate and feed conversion — and these medications are still being used responsibly under veterinary supervision. However, new FDA rules that took effect in January 2017 limit the use of many antibiotics to therapeutic uses only. FDA began requiring a veterinary feed directive for any medically important antibiotics used in food-animal production. This has led to a decline in the use of all in-feed antibiotics — not just those that are considered medically important.

“Using antibiotics responsibly is essential to maintain the efficacy of these medications,” Nelson says.  “Deciding not to use them at all — a path more and more producers are taking — could leave some herds’ pigs more susceptible to bacterial infections and production losses, particularly if they’re stressed.”

Finding alternatives

Some producers are experimenting with antibiotic alternatives — prebiotics, probiotics, plant extracts and essential oils, to name a few.  All have shown potential but have not shown the same consistency as FDA-regulated feed medications, which must demonstrate safety and efficacy before they’re licensed, the nutritionist says.

“As an industry, we want to keep an open mind to new ideas and technologies, but in the end, we need to weigh novelty against good science and do what’s right for the pig,” he says.

Whichever path producers choose, Moeser says producers can take steps to strengthen gut health in pigs by reducing stressors or eliminating them when possible. He urges producers to examine potential stressors such as weaning age, crowding and the mixing of animals while also striving to heighten biosecurity.

“Stressors are often additive,” he says. “If there are ways to alleviate one, two or three of the stressors, producers will see benefits there as well. Anything that happens to the gut will have a long-term impact on the pig for the rest of its life.”



1. Moeser AJ, et al. Weaning stress and gastrointestinal barrier development: Implications for lifelong gut health in pigs. College of Veterinary Medicine, Michigan State University, East Lansing, MI. Gastrointestinal Stress Biology Laboratory. Access date: April 24, 2018.
2. Effects of stress and injury on meat and by-product quality. Guidelines for Humane Handling and Transport of Livestock, Food and Agriculture Organization of the United Nations. Access date: April 24, 2018.
3. Pohl CS, et al. Early weaning stress induces chronic functional diarrhea, intestinal barrier defects, and increased mast cell activity. Neurogastroenterol Motil. 2017 Nov;29(11). doi: 10.1111/nmo.13118. Epub 2017 Jun 1. Access date: May 15, 2018.
4. Moeser AJ, et al. Stress signaling pathways activated by weaning mediate intestinal dysfunction in the pig. Am J Physiol Gastrointest Liver Physiol 2007;292:G173-G181. Access date: April 24, 2018.
5. Stewart K, Carbezon F. Fact sheet: Heat stress physiology in swine. Purdue University. Access date: April 24, 2018.
6. Ross JW, et al. Physiological mechanisms through which heat stress compromises reproduction in pigs. Molecular Reproduction & Development. Access date: April 24, 2018.
7. Moeser AJ. Stress and intestinal disease. Population Health Pathobiol. Access date: April 24, 2018.
8. Moeser AJ, et al. Dietary bacitracin improves ADG and FE in pigs in routine production environments (subjected to mixing and crowding stress). J Animal Sci. 2014;92(2). Midwestern Section Meetings.
9. Moeser AJ, et al. The effects of bacitracin on gut health and performance in heat-stressed pigs. North Carolina State University.
10. Moeser AJ, et al. Dietary bacitracin improves intestinal health in pigs subjected to heat stress. J Animal Sci. 2011;89(2).