The gastrointestinal parasite
Entamoeba histolytica secretes serotonin, causing a sustained secretory diarrhea in some people.
[17][92] Patients infected with
E. histolytica have been found to have highly elevated serum serotonin levels, which returned to normal following resolution of the infection.
[93] E. histolytica also responds to the presence of serotonin by becoming more virulent.
[94] This means serotonin secretion not only serves to increase the spread of enteamoebas by giving the host diarrhea but also serves to coordinate their behaviour according to their population density, a phenomenon known as
quorum sensing. Outside the gut of a host, there is nothing that the entoamoebas provoke to release serotonin, hence the serotonin concentration is very low. Low serotonin signals to the entoamoebas they are outside a host and they become less virulent to conserve energy. When they enter a new host, they multiply in the gut, and become more virulent as the enterochromaffine cells get provoked by them and the serotonin concentration increases.
holy shit