Why black cats might not be lucky for people with schizophrenia

In 1896 Scientific American published an article entitled 'Is insanity due to a microbe' which was the first time anyone had put forward the idea of an infectious cause for schizophrenia. The theory grew in popularity in the early years of the C20th but fell out of favour as the century wore on. Recent research has revived this theory linking schizophrenia with exposure to viruses such as influenza, rubella, herpes and polio while people are in the womb and polio, meningitis and encephalitis thereafter. The most research, however, has been done into a protozoa called Toxoplasma gondii which is hosted by cats and can be spread by inadvertently eating or breathing in infected material from litter trays. The organism can also be spread by eating undercooked meat from sheep, goats or other animals that have been infected by cats. Toxoplasma gondii can cause deafness, sight problems and brain damage in fetuses but show no symptoms in adults. A review of 42 studies carried out in 17 countries over five decades found that having been infected with Toxoplasma gondii increased people's risk of schizophrenia by a factor of 2.73. This is fairly modest compared to other risk factors for other diseases but it is more significant than any other genetic or environmental risk factor so far put forward for schizophrenia.

Torrey, E. Fuller ... [et al] - Antibodies to Toxoplasma gondii in patients with schizophrenia : a meta-analysis Schizophrenia Bulletin May 2007 33(3), 729-736