Why Do GI Symptoms Often Develop with Flu?

By Lillian So Chan

Influenza is a respiratory illness. Flu viruses multiply almost exclusively in the respiratory tract, causing symptoms such as sore throat, dry cough, runny or stuffy nose, fever, headache, tiredness, and sore muscles. 

However, some infected people, especially children, may also develop additional GI symptoms such as nausea, vomiting, and diarrhea. 

It is unclear if swallowed respiratory secretions could cause these symptoms. But there is no evidence that flu viruses replicate in the intestine in humans. Influenza virus RNA is rarely found in in the stool of infected people.

Recent studies investigating GI symptoms during flu infection are beginning to shed light on why these symptoms developed with flu.

Their findings include:
  • Flu infection can alter the gut bacteria flora through a specific immune signaling mechanism induced in the respiratory system.
  • An altered gut microbiome may result in immune system dysregulation that promotes inflammatory gut symptoms.
  • Gut injury through inflammation can result in significant impairment of the intestine’s ability to defend against harmful bacteria.
  • Primary flu virus infection can predispose the patient to secondary gut bacterial infections and GI symptoms.

Researchers suggested that flu-related changes in the gut microbiome are usually transient regardless of the infected flu virus subtypes and resolve after clearance of the flu infection.

However, after flu infection the patient may remain vulnerable to pathogenic bacterial infection before gut microbiome health is restored.

They also warned that a consistent flu-virus-sensitization of both lung and gut tissues to superbug (bacterial) infection has been observed.

References

Yildiz S, et al (2018) Microbiome 2018 6:9

Deriu E, et al (2016) PLoS Pathog 12(5): e1005572