About one-third of humanity possesses a parasite concealed in the brain. Long perceived as inactive, it has recently been discovered that this parasite has an unexpected and structured activity.
A team from the University of California, Riverside, has published results in
Nature Communications. Their work indicates that Toxoplasma gondii, which causes toxoplasmosis, exhibits a much more elaborate biological organization than previously assumed.
Our knowledge until now depicted the cysts formed by the parasite as homogeneous, dormant reservoirs. Thanks to single-cell analysis methods, researchers have identified several distinct subtypes within each cyst. Emma Wilson, a professor of biomedical sciences, compares these structures to active centers where different parasites play specific roles for survival or propagation.
These cysts develop primarily in neurons and muscle tissue, reaching up to 80 microns (approximately 0.003 inches) in diameter. They contain hundreds of parasites called bradyzoites, measuring about five microns (approximately 0.0002 inches). Consuming undercooked meat, which can harbor them, is a common route of infection in humans.
This internal diversity of the cysts plays a determining role in the persistence of the infection and the transmission of the disease. When the immune system weakens, certain subtypes can reactivate into aggressive forms, causing neurological or ocular damage.
The study overcame technical obstacles by using a mouse model close to natural infection. Mice, natural intermediate hosts, accumulate many cysts in the brain. By isolating them and analyzing the parasites individually, scientists obtained a detailed view of chronic infection in living tissues, which was previously difficult.
These discoveries now guide research towards more precise therapeutic targets. By identifying the subtypes most likely to reactivate, treatments specifically targeting cysts can be considered. This represents hope for better management of toxoplasmosis, especially in at-risk cases such as infections during pregnancy.
The life cycle of Toxoplasma gondii
This parasite follows a cycle involving several hosts. Cats, which are definitive hosts, excrete oocysts in their feces, which contaminate soil or water. Humans and other animals like mice, intermediate hosts, become infected by ingesting these oocysts or by consuming meat containing cysts.
Once inside, the parasite enters cells and multiplies rapidly as tachyzoites. It travels through the body via the bloodstream, potentially reaching various organs like the brain or muscles. This phase corresponds to acute infection, often asymptomatic in healthy individuals.
To escape the immune system, tachyzoites transform into bradyzoites and form cysts in tissues. These structures allow the parasite to persist for life, usually remaining inactive. The cycle completes when infected intermediate hosts are consumed by cats, allowing the parasite to reproduce sexually.
This knowledge of the cycle illuminates why Toxoplasma is so widespread and difficult to eradicate. Cysts ensure its transmission between species and its resistance to treatments, making it a major public health issue.