Research explains why stomach bugs cause a lingering loss of appetite

Anyone who has weathered a bad stomach bug knows the feeling: a loss of appetite that sets in and lingers, even after the initial illness. For the millions of people around the world who are chronically infected with parasitic worms, the same thing happens. But scientists have long puzzled over exactly why.

Now, researchers at UC San Francisco have traced the molecular pathway that connects the gut immune system to the brain during a parasitic infection, explaining how the immune system triggers a loss of appetite.

The question we wanted to answer was not just how the immune system fights parasites, but how it recruits the nervous system to change behavior. It turns out there's a very elegant molecular logic to how that happens."

David Julius, PhD, co-senior author, professor and chair of Physiology at UCSF and recipient of the 2021 Nobel Prize in Physiology or Medicine

The findings, published in Nature on March 25, reveal an unexpected communication system between two cell types, and could shed light on a range of conditions involving gut discomfort - from food intolerances to irritable bowel syndrome.

Two cells communicating

The new study focused on two rare cell types in the gut. Tuft cells detect parasites and trigger immune defenses, while enterochromaffin (EC) cells release signals that activate nerve fibers leading to the brain. EC cells are known to cause sensations like nausea, pain, and gut discomfort but whether they communicate with tuft cells was unknown.

"My lab has long been interested in how tuft cells, after they initially respond to a parasitic infection, release signals to other cell types," said co-senior author Richard Locksley, MD, a UCSF immunologist.

First author Koki Tohara, PhD, a postdoctoral researcher at UCSF, found the answer by positioning genetically engineered sensor cells directly next to tuft cells under a microscope. When tuft cells were exposed to succinate, a molecule produced by parasitic worms, the sensor cells lit up, revealing that tuft cells were releasing acetylcholine, a chemical messenger used primarily by neurons.

When acetylcholine was added to lab-grown gut tissue containing EC cells, they released serotonin. This activated vagal nerve fibers that carry signals from the gut to the brain.

"What we found is that tuft cells are doing something neurons do, but by a completely different mechanism," Tohara said. "They're using acetylcholine to communicate, but without any of the usual cellular machinery that neurons rely on to release it."

The team also discovered that tuft cells release acetylcholine in two distinct phases - explaining why people often don't develop a loss of appetite until days into an infection. In the first phase, a brief burst of acetylcholine is released. Later, after the immune system has mounted a full response, tuft cells multiply and produce a slow, sustained release of acetylcholine that is sufficient to activate EC cells.

"This explains why you feel fine at first but then start to feel sick as the infection becomes established," Julius said. "The gut is essentially waiting to confirm that the threat is real and persistent before it tells the brain to change your behavior."

Implications beyond parasites

To test whether the pathway matters beyond the lab, the researchers infected mice with a parasitic worm and tracked their food intake. Mice with normal tuft cell function ate less as the infection took hold. Mice engineered to lack acetylcholine-producing machinery in their tuft cells kept eating normally, confirming that the molecular chain drives the behavioral response. The new findings could have relevance for treating the symptoms of a parasite infection.

"Controlling the outputs of tuft cells could be a way to control some of the physiologic responses associated with these infections," Locksley said, adding that the study also could have broader implications.

Tuft cells are found throughout the body - not just in the gut, but also the airways, gallbladder, and reproductive tract - and disruptions to the newly identified pathway could contribute to conditions like irritable bowel syndrome, food intolerances, and chronic visceral pain.

The work was carried out in collaboration with Stuart Brierly, PhD, and his lab group at the University of Adelaide in Australia.