Parasite infection from cat shit linked to schizophrenia

Science Daily reports that researchers at Imperial College London have published a study in the Proceedings of the Royal Society B which links Toxoplasma gondii, a parasitic protozoa which as part of its life cycle is excreted within cat feces, to schizophrenia. The findings show that when rats are fed Haloperidol, an anti-psychotic, and Valproic acid, a mood stabilizer and anti-convulsant, replication of T. gondii within a host organism appears to be inhibited.

Toxoplasma gondii infects a wide range of animals including humans. Sexual reproduction takes place within the small intestine of the common house cat (Felis silvestris catus), which is then excreted by the animal. Oral contact with infected cat feces, soil, unwashed vegetables, or uncooked meat is then the usual vector of animal infection. From here the parasite begins an asexual life cycle whereby it infects cells of the host organism and divides within each until bursting the cell, thus releasing more of T. gondii within the animal to infect yet more cells. Within humans, infection is usually asymptomatic, or presents as a mild cold or flu, and has long been considered mostly benign. Occasionally those with compromised immune systems are unable to ward off the parasite, in which case the infection can progress to a more dangerous illness known as Toxoplasmosis. If untreated, toxoplasmosis can lead to encephalitis and, very rarely, even death. Estimates of human exposure to T. gondii infection range between 50% and 60% of the world population, making it highly prevalent.

Early speculation that T. gondii infection may increase the risk for schizophrenia goes back to the 1970s, when -- according to this 2001 New York Times Magazine article Pet Theory: Do Cats Cause Schizophrenia -- Psychiatrist E. Fuller Torrey first postulated the hypothesis. At the time he was a lone voice, but as research across the 1980s and early 1990s into a range of viruses and other infectious vectors began to offer tidbits of evidence in support, many in the field were swayed to the position that schizophrenia, and possibly other mental disorders, is likely caused by a combination of genetic susceptibility and environmental insults, rather than psychological factors such as childhood emotional trama. Since publication of the Human Genome Project dataset, researchers have had the tools necessary to discern at least some genetic components related to schizophrenia, which provided a critical tool for serious exploration of the hypothesis.

Interest in T. gondii heated up in 2000 when the biomedical community saw the published findings (pubmed) that otherwise seemingly normal rats infected with T. gondii were more likely to be predated upon by cats than those uninfected. The findings suggested the tantalizing possibility that a parasite could somehow change the behavior of a host organism in subtle ways to suit its own reproduction cycle. Which opened the question, could some pathogens alter behavior in other mammals, such as humans, as well? And so the notion that T. gondii might somehow change the brain to alter behavior in a complex organism turned from speculation to science. Next, researchers begin exploring the links between T. gondii infection and potential cognitive performance deficits in humans. In 2003, one research group published results showing that latent T. gondii cyst infection appeared to decrease novelty seeking behaviors and reduce psycho-motor intelligence in men (women were not part of the study) compared to a non-infected control. The researchers speculated that increased dopamine concentrations in the brains of the infected may be the cause. Then, a 2005 study appeared which linked maternal T. gondii infection during pregnancy to increased risk of schizophrenia later in the life of their offspring.

These most recent findings add further evidence to link between T. gondii and schizophrenia-like symptoms in humans. Though lead researcher Dr. Joanne Webster, of Imperial College London, doesn't assert infection with direct causation of schizophrenia in the article:

"Although we are certainly not saying that exposure to this parasite does definitely lead to the development of schizophrenia, this and previous studies do show there may be a link in a few individuals, providing new clues for how we treat toxoplasmosis and schizophrenia."

Dr. Webster next plans a set of human clinical trials using anti-T.gondii therapies in schizophrenic patients, in addition to traditional anti-psychotic medication. The outcome of these trials may offer new treatments for those suffering from this highly debilitating brain disorder.

Given the popularity of cats as pets, and the high prevalence of prior T. gondii infection throughout the human populace, such further research is obviously warranted. These findings represent just one path toward discovering other potential infectious vectors for mental illness in the future. Hopefully, with further research, science may discern the underlying brain mechanisms behind all the mental illnesses, and through that create new drugs with significantly better outcomes than current therapies offer.

cross posted at: kuro5hin.org

Thanks to: circletimessquare, Hung Fu, nostalgiphile, and trenchcoatjedi for their editorial help.

Copyright ©2006 J. Maynard Gelinas.

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