Why does the flu cause emesis?

Several mechanisms contribute to the development of emesis during influenza infection:

1. Viral Invasion and Replication: Influenza viruses have a tropism for respiratory epithelial cells and can invade and replicate within these cells. The replication of the virus leads to cellular damage and the release of inflammatory mediators, such as cytokines and chemokines.

2. Cytokine Release: The release of cytokines, such as interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-α), is a hallmark of the inflammatory response to influenza infection. These cytokines can stimulate the area postrema (AP) and nucleus tractus solitarius (NTS), which are brain regions involved in the regulation of emesis.

3. Activation of the Vagus Nerve: The release of cytokines and other inflammatory mediators can activate vagal afferent neurons in the gut and respiratory tract. This activation sends signals to the brainstem, leading to the stimulation of the vomiting reflex.

4. Neurotransmitters and Hormones: The inflammatory response during influenza infection can also affect the levels of neurotransmitters and hormones involved in the regulation of nausea and vomiting, such as serotonin (5-HT), dopamine (DA), and gastrin. Alterations in these substances can further contribute to the development of emesis.

5. Gastrointestinal Symptoms: Influenza infection can also cause direct damage to the gastrointestinal tract, leading to nausea, vomiting, and diarrhea. This damage may be caused by the viral invasion of enterocytes or the effects of inflammatory mediators on the gut mucosa.

6. Secondary Effects: Emesis can also occur as a secondary effect of other symptoms associated with influenza, such as fever, dehydration, and electrolyte imbalances. These factors can further contribute to nausea and vomiting.

Overall, the complex interplay of viral invasion, inflammatory response, neurotransmitter changes, and gastrointestinal effects contributes to the development of emesis during influenza infection.