Presented at the International Congress on Schizophrenia Research

Presented at the International Congress on Schizophrenia

Research; March, 2003


Does Toxoplasma Gondii Cause Some Cases of


E. Fuller Torrey and Robert H. Yolken


Toxoplasma gondii is an intracellular parasite whose

life cycle can be completed only in cats, the definitive host.  Humans and

many other animals can also be infected and serve as intermediate hosts. 

The Toxoplasma antibody-positive rate in humans varies in different

populations from 10 to 80 percent depending on cat exposure and the eating of

undercooked meat from infected animals.  Animals infected with

Toxoplasma may show a variety of neurological and behavioral symptoms,

including changes in activity, learning, and memory.

In humans, congenital toxoplasmosis occurring early in

pregnancy is well known to cause mental retardation, seizures and other

symptoms.  Less well known are the effects of Toxoplasma from

transmission later in pregnancy or after birth.  These include changes in

personality and symptoms of psychosis, including delusions and auditory

hallucinations.  Since 1953, 16 studies of Toxoplasma gondii have

been done on individuals with schizophrenia; all except one reported a higher

percentage of antibodies in affected individuals. For example, in a recent

German study of 38 individuals with first-episode schizophrenia compared to 27

matched  controls, 42percent of the former compared to 11 percent of the

latter (p=0.007) had antibodies to Toxoplasma.  Similar increases in

Toxoplasma antibodies were found in individuals with recent onset

schizophrenia living in China.  In addition, a recent study in the United

States found that individuals with established schizophrenia who have antibodies

to Toxoplasma have increased levels of cognitive impairment compared to

age- and severity-matched individuals with schizophrenia who do not have

antibodies.  Two other studies reported that individuals with schizophrenia

have had more exposure than controls to cats in childhood.  An additional

study found that mothers who gave birth to individuals who later developed

psychosis had increased levels of IgM class antibodies to Toxoplasma at

the time of birth.

Toxoplasma is also of interest because it is known to

be neurotrophic, remain latent in the brain, selectively affect glia, and alter

neurotransmitters.  It also may interact with retroviruses, herpesviruses,

or other neurotrophic infectious agents.  Finally, many antipsychotic and

mood stabilizers inhibit Toxoplasma in cell cultures.  Studies are

underway using anti-Toxoplasma medications as adjunctive agents to treat

individuals with schizophrenia.