Findings from what is believed to be the largest
comparison of blood samples collected from
healthy individuals and people with schizophrenia suggest
that infection with the common Toxoplasma
gondii parasite, carried by cats and farm animals, may
increase the risk of schizophrenia.
A report on the study, conducted among U.S. military
personnel by researchers from Walter
Reed Army Institute of Research and Johns Hopkins
Children's Center, appears in the January issue
of The American Journal of Psychiatry.
Researchers found that of the 180 study subjects
diagnosed with schizophrenia, 7 percent had
been infected with toxoplasma prior to their diagnosis,
compared to 5 percent among the 532 healthy
recruits. Thus, people exposed to toxoplasma had a 24
percent higher risk of developing
schizophrenia. The difference, while seemingly small, is
important, researchers say, because the
ability to explain even a small portion of the 2 million
cases of schizophrenia in the United States may
offer clues to the disease and some possible treatments.
For example, the investigators say they plan to study
whether aggressive treatment of
toxoplasma infection with antiparasitic drugs in patients
with schizophrenia could halt the progression
of the mental disorder, characterized by paranoia,
delusions and hallucinations.
Most infections with toxoplasma occur early in life
following exposure to the parasite in cat
feces or undercooked beef or pork. Infections rarely cause
symptoms, but the parasite remains in the
body and can reactivate after lying dormant for years.
"Our findings reveal the strongest association we've
seen yet between infection with this very
common parasite and the subsequent development of
schizophrenia," said Robert Yolken, a
neurovirologist at Johns
Hopkins Children's Center, who was among those
conducting the analysis.
Previous studies have reported on the link between
schizophrenia and the presence of
toxoplasma antibodies, which are evidence of past
infection, but this is the first study to show that
infection with the parasite can precede the initial onset
of symptoms and subsequent diagnosis with
schizophrenia, Yolken said.
Because the U.S. military routinely tests its active
personnel for toxoplasma, among other
infectious agents, and stores blood samples in a central
repository, researchers were able to
determine the time line between infection and a diagnosis
"Until now, the only thing we could say is that some
people with schizophrenia also had been
infected with toxoplasma at some point, but we couldn't
tease out which came first," Yolken said.
"With our current study, we were able to show that
infection came first."
While most people infected with toxoplasma never
develop schizophrenia, the parasite may be a
trigger in those genetically predisposed to the disorder, a
classic example of how genes and
environment come together in the development of disease,
Lead investigator on the study is Col. David W.
Niebuhr, of the Walter Reed Army Institute of
Research. The study was funded by the Stanley Medical
Research Institute and the U.S. Army.
The Walter Reed Army Institute of Research conducts
research on naturally occurring
infectious diseases, combat-casualty care, operational
health hazards and medical defense against
biological and chemical weapons. It is a subordinate
laboratory of the U.S. Army Medical Research and
Materiel Command and is the Department of Defense's lead
agency for infectious disease research.