Antibodies to ATCV-1 in Exposed Mice. Following the completion of the behavioral studies, there were a total of 47 mice from which serum could be obtained for antibody testing ∼6 mo following oral inoculation. This set included 28 of the 30 mice that had been inoculated with ATCV-1–infected C. heliozoae (15 females, 13 males; 2 males died before testing) and 19 mice inoculated with C. heliozoae alone (10 males, 9 females; 1 female died before testing). We found detectable antibodies to ATCV-1 by enzyme immunoassay (ELISA) in 10 of the 28 mice exposed toATCV-1 (5 males and 5 females) but in none of the 19 mice exposed to C. heliozoae alone (P < 0.0033, Fisher’s exact test). The presence of antibodies to ATCV-1 proteins was examined by Western blot in 12 available blood samples from mice exposed to ATCV-1 in this study. Of five tested samples that were positive by ELISA, four also reacted to multiple ATCV-1 proteins but not to C. heliozoan proteins (Fig. S9). One of the predominant proteins recognized was tentatively identified as the ATCV-1 major capsid protein. No reaction to ATCV-1 proteins occurred in Western blots with the seven samples that were seronegative with ELISAs. Sera obtained from mice exposed to C. heliozoae in the absence of ATCV-1 did not react with ATCV-1 proteins.