Competition between strains of scrapie depends on the blocking agent being infectious.

Compton White mice (Sincs7) were injected twice intraperitoneally, first with the 22A strain of scrapie agent (in brain homogenates) and then, after 105 days, with the 22C strain. Incubation periods were calculated from the time of the first injection. The experiment was designed so that, with no interaction between strains, the second strain (22C) should have produced cases about 300-350 days after the first injection, depending on the dose of 22C. This was well before the limit of 470 days set by the mean incubation period minus 3 SD of 22A alone: a limit which was used to distinguish 22C from 22A clinical cases. In fact, 22A blocked 22C as shown by (i) the lengthening of 22C incubation periods, (ii) the reduced proportion of cases due to 22C, and (iii) the reduced effective titer of 22C. The blocking efficiency of 22A was not greatly reduced by physicochemical treatments that had little or no effect on its infectivity by the intraperitoneal route. However, treatment of 22A homogenates with 6 M urea virtually eliminated infectivity and also abolished blocking ability. It is concluded that competition depends on the infectivity of the scrapie strain used for blocking.