Submitted to: Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 6, 1997
Publication Date: N/A
Interpretive Summary: Infections by the protozoan parasite Toxoplasma gondii are widely prevalent in humans and animals. Humans become infected by ingesting food or water contaminated with the resistant parasite stage (oocyst) excreted in feces of infected cats and by ingesting uncooked meat containing tissue cysts (bradyzoites) of T. gondii. Although toxoplasmosis is usually asymptomatic in adults, approximately 3 to 10 percent of all AIDS patients in the U S die of toxoplasmosis. In AIDS patients clinical toxoplasmosis results from reactivation of latent infection due to rupture of tissue cysts and the conversion of bradyzoites (latent stage) to tachyzoites (active stage). Mechanisms of stage conversion are important because most drugs do not kill bradyzoites but can kill tachyzoites. A scientist at the Beltsville Agricultural Research Center has studied biological properties of tachyzoites and bradyzoites and found that digestion of tissues in acid pepsin is not reliable for distinguishing tachyzoites from bradyzoites because occasionally tachyzoites survive in acid pepsin (gastric digestion) and that infants can acquire infection from mother's milk if the mother was recently infected with T. gondii. These findings will be of interest to public health workers and parasitologists.
The effect of digestion in trypsin and acid pepsin on Toxoplasma gondii tachyzoites and bradyzoites was reevaluated because of recent use of this method to distinguish tachyzoites from bradyzoites. Toxoplasma gondii tachyzoites survived better in 0.5% trypsin solution for 1 h than in 1.0% solution and occasionally survived for 2 h in acid pepsin solution. Extracellular tachyzoites (>1000) were also infectious orally to mice and cats. Bradyzoites survived equally in trypsin and acid pepsin solutions but the digestion of brain tissue in 0.5% trypsin solution was better than in acid pepsin solution. The resistance to digestion in acid pepsin solution is not a reliable method to distinguish tachyzoites from bradyzoites.