A research article from the Imperial College London
where their Scientists find stronger evidence for link between cat faeces and schizophrenia
and the later ScienceDaily article
indicate a link between Toxoplasma gondii and schizophrenia. This single cell parasite also cause rats to lose fear of cats, so calling it a parasite seem to be a bit of a misnomer. It is know to cause Toxoplasmosis
which can cause problems during pregnancy for humans (presumably effect on other species unknown).
The historical myths associating witches with cats as familiars have their origins in schizophrenic hallucinations caused by toxoplasmosis.
I hope somebody with the necessary forensics skills can test this theory....