This would mean trying to reduce serotonin in the cases that it is caused by high parasites is a feeble attempt - and killing parasites can cause a massive change in health. I've seen this myself. Feel free to add more studies / info on this topic:
Modulation of the immune response by helminths: a role for serotonin?
The mammalian gut is a remarkable organ: with a nervous system that rivals the spinal cord, it is the body’s largest repository of immune and endocrine cells and houses an immense and complex microbiota. Infection with helminth parasites elicits ...
www.ncbi.nlm.nih.gov