[I]nfection by the brain parasite Toxoplasma gondii, found in 10-20 per cent of the UK's population, directly affects the production of dopamine, a key chemical messenger in the brain. . . . Toxoplasmosis, which is transmitted via cat faeces (found on unwashed vegetables) and raw or undercooked infected meat, is relatively common, with 10-20% of the UK population and 22% of the US population estimated to carry the parasite as cysts. Most people with the parasite are healthy, but for those who are immune-suppressed -- and particularly for pregnant women -- there are significant health risks that can occasionally be fatal.
The parasite infects the brain by forming a cyst within its cells and produces an enzyme called tyrosine hydroxylase, which is needed to make dopamine. Dopamine's role in mood, sociability, attention, motivation and sleep patterns are well documented and schizophrenia has long been associated with dopamine, which is the target of all current schizophrenia drugs on the market.
The role of dopamine in ADHD and schizophrenia is discussed here.
I certainly admit to being absolutely shocked to discover that there is any brain parasite in human beings that can impact an important brain subsystem with observable effects is anywhere near as common as Toxoplasma gondii. It brings to mind a subplot of some of Isaac Asimov's Foundation series in which a communicable disease affecting the brain is intentionally employed to prevent humanity from making certain kinds of scientific progress.
Usually the brain is unusually free of outside agents because the brain-blood barrier does a better job of keeping these agents out than the systems that protect the rest of the body. But, these cysts somehow manage to cross that barrier.
I would very much like to know if there are other common brain parasites out there, what percentage of carriers of this parasite suffer any symptoms from it, and what kind of diagnosis and treatment regimes are out there to deal with it. Even a quite low percentage of cases that are symptomatic could conceivably account for a quite high percentage of dopamine related conditions that do not have a clear genetic basis, and if I've never heard of it, the odds that it is not widely considered as a possibility by mental health professionals is substantial. This kind of causation suggests a very different treatment regime than the default assumption in mental health circles that a condition is congenital. More here (from 2007).