Presented at the International Congress on Schizophrenia
Research; March, 2003
Does Toxoplasma Gondii Cause Some Cases of
Schizophrenia?
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.