Humans can become infected with the protozoan Toxoplasma gondii by ingesting it during its spore-like stages, which are shed in cat faeces. The three main strains of T. gondii vary widely in terms of virulence in mice, and researchers have pinpointed a gene that may underlie this difference.

David Sibley at Washington University in St Louis, Missouri, and his co-workers generated a genetic cross between the type I strain, which is lethal to lab mice, and the milder type II. The progeny were injected into mice to assess their virulence. A genome-wide association analysis of the progeny and the parental strains revealed a locus on chromosome XII that accounted for 90% of the heightened virulence. Deleting a gene within this region, ROP5, in the type I strain completely blocked acute virulence.

The authors suggest that the ROP5 protein may regulate other proteins that control virulence in the parasite.

Proc. Natl Acad. Sci. USA doi:10.1073/pnas.1015338108 (2011)