Infective Toxoplasma gondii trophozoites attenuated by ultraviolet irradiation.

Both the virulence and infectivity of Toxoplasma gondii trophozoites are affected by increasing fluences of ultraviolet irradiation. All mice received unirradiated virulent trophozoites intraperitoneally died in an average of 5 days. All mice inoculated with trophozoites irradiated with 20 J m-2 died in an average of 8 days. Fluences of 35, 55, and 70 J m-2 resulted in 90, 20 and 0% deaths, respectively. Fluences up to 720 M m-2 did not increase the number of nonviable trophozoites. The minimum fluence which prevented mouse deaths (70 J m-2) also prevented trophozoite proliferation in baby hamster kidney (BHK-21) cell cultures. Minimum fluence-irradiated trophozoites attached to or entered BHK-21 cells or both at a rate similar to that for controls. BHK-21 cells fixed and stained 18 h after inoculation with unirradiated trophozoites contained both single and multiple trophozoites. In contrast, cell cultures inoculated with minimum fluence-irradiated trophozoites contained only single, nonreplicated trophozoites. No proliferation of trophozoites in these cultures was found as long as observed, 14 days after inoculation. Fluences of ultraviolet irradiation can therefore be determined which allow infection of cells but prevent trophozoite proliferation.