How Food Poisoning Can Cause Symptoms That Last for Years

For most people, food poisoning ends when the digestive disruption stops. But for some, those first miserable days are only the beginning. Long after the pathogen has cleared, recurrent abdominal pain, diarrhea, and sometimes constipation can continue for months—or even years.

Doctors call this condition post-infection irritable bowel syndrome. By conservative estimates, it affects at least one in 10 people after an intestinal infection. The risk has gained new relevance amid a string of foodborne outbreaks linked to lettuce, frozen blueberries, sprouts, eggs, and jalapeños. A Cyclospora outbreak traced to iceberg lettuce alone sickened nearly 13,000 people.

Scientists have been trying to uncover its causes—and its solutions—for decades. Their research increasingly points not to a single culprit, but to lasting changes across several interconnected systems.

Post-infection IBS can follow digestive infections caused by a variety of germs, but it’s more likely to follow parasitic and bacterial illnesses than viral ones. The parasite Giardia and the bacteria Campylobacter are among the best-documented culprits, along with Salmonella, E. coli, and Shigella. Cyclospora, notably, is rarely linked to the condition.

One review estimated that the condition accounts for around 9 percent of all IBS cases, although some experts suspect it causes many more. “If I had one day of diarrhea after coming back from Mexico 10 years ago, do I remember that?” says Mark Pimentel, a gastroenterologist and researcher at Cedars-Sinai Medical Center in Los Angeles. He suspects food poisoning causes closer to 60 percent of IBS.

IBS isn’t the only digestive condition that can emerge after an infection. Chang says patients can also develop post-infection functional dyspepsia. This often-overlooked condition causes upper abdominal pain, early fullness, or uncomfortable fullness after eating—although researchers don’t yet know whether it arises through the same mechanisms as post-infection IBS.

Scientists still don’t know why one person recovers completely while another develops lasting symptoms. One set of theories focuses on changes to the gut’s microbiome. An infection blows through the gut’s resident bacterial community like a storm; normally, within days or weeks, that community rebuilds itself. In some people, it doesn’t. “It’s like it undergoes a state of shock,” says Madhusudan Grover, a gastroenterologist at the Mayo Clinic who has spent 15 years studying the aftermath of Campylobacter infections. In a small group of patients followed for a year after infection, some of the microbial residents lost during the illness never returned.

Changes to the microbial community could have a range of downstream effects. For example, Grover’s research in mice suggests that depleting the gut’s microbiome can increase the activity of proteases, protein-degrading enzymes normally kept in check by certain gut bacteria.

Other ideas center around a dysregulated defense system in the recently infected gut. Close examination of patients’ bowels shows evidence of simmering inflammation and an out-of-whack immune system: Some infection-fighting cells are overabundant, some are too scarce, and signaling molecules that govern the body’s often fiery inflammatory responses are miscalibrated. In animal studies, scientists have found evidence of an allergic response that seems localized to the gut.

Yet another set of hypotheses concerns the nerves that govern both contractions and pain sensation in the gut. In mice, gut infections can damage or kill neurons embedded in the intestinal wall, producing lasting changes in how the gut moves. Whether people experience the same neuronal losses remains unclear.

But Benjamin Levy, a gastroenterologist at the University of Chicago, sees evidence that the gut’s sensory system can become hypersensitive in his patients. Normal peristalsis, the gut’s routine muscle contractions, isn’t something most people even notice. “Patients with post-infectious IBS perceive peristalsis as pain,” he says.

Pimentel’s working theory unifies several of these ideas. He suspects that during the initial foodborne infection, the immune system creates swarms of antibodies in response to a toxin made by certain bacteria. Those antibodies may also attack the body’s own tissue—specifically, cells that coordinate the small intestine’s housekeeping functions—leading to an overgrowth of bacteria. The resulting gases and other bacterial byproducts could heighten sensitivity and make the gut lining more permeable, allowing its contents to irritate and inflame the intestine and contribute to persistent diarrhea and pain.

None of these theories fully explains post-infection IBS on its own, and scientists don’t agree on how the pieces fit together.

One notable area of disagreement relates to the brain’s involvement in post-infection IBS. Lin Chang, a gastroenterologist at UCLA who studies how the gut and brain communicate, points to evidence that people who are anxious or depressed at the time they fall ill are more likely to develop lasting gut symptoms afterward. That doesn’t mean the illness is imagined.

Rather, the brain actively processes signals from the gut and may amplify symptoms. Experiments have suggested stress measurably weakens the integrity of the gut’s lining, and stress hormones can make gut pathogens more dangerous, says Grover, who notes pathogens like Campylobacter become more virulent in the presence of the fight-or-flight hormone epinephrine.

Meanwhile, Pimentel believes that the brain is mostly an innocent bystander in post-infection IBS. He points to a study of U.S. troops deployed to Iraq and Afghanistan. In that analysis, combat exposures—including shooting a weapon, being injured, or killing someone—didn’t predict IBS, but experiencing infectious gastroenteritis while deployed did. “The problem is the gut, and it is affecting the brain, and yes, there are brain manifestations,” he says, “but I don't think the fault is the brain.”

(Why women are more likely to have IBS.)

The two views are not necessarily incompatible: An infection may trigger the condition in the gut while the brain influences how symptoms develop or persist. Determining each’s relative contribution is especially challenging because post-infection IBS is diagnosed through a characteristic pattern of symptoms rather than a single physical finding or biomarker. The syndrome likely lumps together subsets of patients with different disease mechanisms, says Grover. That makes it especially important to individualize treatment to each patient depending on the symptoms and concerns they describe.

Although treatments for post-infection IBS generally mirror those for IBS more broadly, several research groups are seeking new therapies targeting the subgroup. Grover is running a trial testing whether a fecal transplant from carefully matched donors can help patients with high gut protease activity, a signature he’s linked to Campylobacter-triggered disease.

In a randomized trial of patients with weakened gut linings, the amino acid glutamine eased symptoms and appeared to strengthen the intestinal barrier, making it less permeable. Chang points to gut-directed hypnosis, which reduces patients’ pain symptoms. Pimentel often prescribes rifaximin, an antibiotic approved for IBS with diarrhea that may alter bacterial activity in the gut. In a small retrospective study, his research group also found that declines in certain self-directed antibodies correlated with symptom improvement. However, the finding does not establish that the antibodies caused those symptoms.

Perhaps most reassuringly, time itself often helps. An eight-year study of a Canadian town sickened by contaminated water found that about half of residents with post-infection IBS no longer met the diagnostic criteria at follow-up, though others remained ill.