How Different Strains of Parasite Infection Affect Behavior Differently

Toxoplasma gondii infects approximately 25 percent of the human population. The protozoan parasite is noted for altering the behavior of infected hosts. Jianchun Xiao and colleagues of the Johns Hopkins School of Medicine find clear differences in the manipulation of host gene expression among the three clonal lineages that predominate in Europe and North America, "despite the high level of genetic similarity among them," says Xiao. Type I infection largely affects genes related to the central nervous system, while type III mostly alters genes that modulate nucleotide metabolism. Type II infection does not alter expression of a clearly defined set of genes. The research is published in the March 2011 issue of the journa Infection and Immunity.

Indeed, T. gondii can play its infected rodent hosts like a piano, converting rats' and mice's natural aversion to feline odors into an attraction, presumably to enable the parasite's sexual cycle. T. gondii can reproduce sexually only in cats. Investigations of effects on humans have found an increased risk of traffic accidents, and other reckless behavior, as well as links to hallucinations.

"Toxoplasma infections, at least for mice, are so variable in their severity and heavily dependent on which strain is doing the infecting," says Xiao. "Understanding the differential effects caused by these strains could enable predicting the outcome of infection and point out directions to be explored in future studies to eliminate transmissions or cure disease. If Toxoplasma is linked to schizophrenia, this could lead to new treatments of that disease as well."

"It is noteworthy that we found vasoactive intestinal peptide receptor 2 (VIPR2) was upregulated by all three Toxoplasma strains," says Xiao. VIPR2 "is linked to schizophrenia in some recent publications. Since the tropism of Toxoplasma for brain has been linked with specific behavioral changes and psychosis in humans, this finding will have some fundamental significance for understanding the correlation between Toxoplasma and psychosis."

Type II strains cause 70-80 percent of human cases reported in North America and Europe.

Reference: J. Xiao, L. Jones-Brando, C.C. Talbot, Jr., and R.H. Yolken, 2011. Differential effects of three canonical Toxoplasma strains on gene expression in human neuroepithelial cells. Infect. Immun. 79:1363-1373.