Unusual abundance of atypical strains associated with human ocular toxoplasmosis

To facilitate genotyping of Toxoplasma gondii in vitreous fluid of patients with severe or atypical ocular toxoplasmosis, polymerase chain reaction (P CR) restriction fragment length polymorphism (RFLP) assays were developed f or SAG3 (p43) and SAG4 (p18), 2 single-copy surface antigen genes. Together with strategies for SAG1, SAG2, and B1, multilocus RFLP analyses were perf ormed on PCR-amplified parasite DNA present in 12 clinical specimens. Most samples (8/12) were not infected by type II or type III mouse-avirulent str ains. Only 1 type III and 3 type II strains were identified, all from immun osuppressed patients. In 6 otherwise healthy adults and in 1 immunosuppress ed patient, the SAG1 allele associated with mouse virulence was amplified. Of 12 samples, 3 possessed true type I strains; 5 of 12 had new recombinant genotypes with alleles typical of type I or III strains at all loci examin ed. The unusual bias toward type I and/or recombinant genotypes bearing the SAG1 type I allele associated with mouse virulence in immunocompetent adul ts has important implications for the epidemiology and efficacious treatmen t of ocular toxoplasmosis.