[Inter- and intra-cephalic variations in pathogenicity in Toxoplasma gondii. Clinical and epidemiologic consequences]

Clinical and epidemiological studies, especially the ones conducted in HIV+ patients indicate that the pathogenicity of Toxoplasma gondii varies according to the strain considered. The differences observed among strains are represented by distinct genomic DNA patterns which could be experimentally evaluated by means of the RFLP (Restriction Fragment Length Polymorphism) and the RAPD (Random Amplified Polymorphic DNA). If on the one hand virulent strains present very similar genomic DNA patterns, on the other hand chronic strains are highly polymorphic. These differences may be, at least in part, due to the asexual (clonal) multiplication of virulent strains. The existence of an intraspecific pathogenicity variation and genetic heterogeneity was observed within a single strain either after attenuation (infection in Fischer or Wistar rats), or during reactivation (in immunodepressed animals). In a congenital model of toxoplasmosis, the differences are detected from one animal to another and sometimes, even from one organ to another in the same host. This finding do not seem to be related to the occurrence of mutations but rather to selective pressures, notably of immunological origin, exerted by the infected organism. A better understanding of these phenomena could result in significant therapeutic and prophylactic advances. Our first effort will be directed to the establishment of more precise diagnostic and predictive elements. The accomplishment of this step relies on the use of primers deriving from DNA sequences characteristic of virulence and which will be tested by PCR.