Toxoplasmosis: Some Facts Behind the Word

Toxoplasmosis is a parasitic disease caused by the protozoan Toxoplasma gondii. An estimated 25% of the world’s population carries the infection, but the most common host is the household cat.

Cats are not to blame for spreading the disease, however, as contact with raw meat is a more significant source of human infections world-wide. Animals and humans are infected by eating infected meat, or alternatively by ingesting the feces of a recently-infected cat. Toxoplasmosis is also passed from mother to fetus. Toxoplasmosis can trigger or complicate psychotic symptoms and schizophrenia in patients with such a genetic predisposition.

A new study from Johns Hopkins University provides some clues as to why toxoplasmosis can differ from person to person. The study explains that each of the three different strains sets off a unique response. These findings are published in the March issue of the journal Infection and Immunity.

According to senior investigator Robert Yolken, M.D., a neurovirologist at Johns Hopkins Children’s Center: ”We already know that toxoplasmosis can play a role in some psychiatric disorders, but up until now we didn’t know why. Working with human nerve cells, our study shows the exact alterations triggered by each strain that can eventually manifest themselves as symptoms.”

The researchers injected human nerve cells with the three most common toxoplasma strains. Cells infected with type I had the greatest impact on gene expression, altering more than 1,000 genes (including those linked to brain development and the central nervous system). Cells injected with the less virulent types II and III had low and moderate levels of gene expression (including those genes related to growth and certain hormones).

“While disease course in humans is often more unpredictable than what we see in the controlled setting of a lab, these results give us a fascinating first look into the distinct genetic cascade of reactions that each strain can unlock and may one day serve as the basis for individualized treatment of symptomatic infections,” explained lead investigator Jianchun Xiao, Ph.D., a neurovirologist at the Stanley Division of Developmental Neurovirology at Hopkins.

Most infections with toxoplasma occur early in life following exposure. Infections rarely cause symptoms, but the parasite can remain dormant in the body for years.