Natural antibodies in Microtus fortis react with antigens derived from four stages in the life-cycle of Schistosoma japonicum

The levels of antibodies which react with the cercarial antigens (CA), schistosomulum-stage antigens (SSA), adult-worm antigens (AWA) and soluble egg antigens (SEA) of Schistosoma japonicum were investigated in Microtus fortis and albino mice, using an indirect ELISA. The M. fortis studied fell into three groups: animals caught in the wild; laboratory-bred animals left unchallenged; and laboratory-bred animals that had been challenged with S. japonicum (30 cercariae/animal) 15 days previously, There were also three groups of albino mice: those without infection; those studied 15 days after challenge infection; and those investigated 42 days after infection. The antibodies detected at the highest levels in the laboratory-bred, uninfected roles and in the wild-caught animals were those reacting with SSA, followed, in descending order, by those reacting with AWA, CA and SEA. The levels of natural antibodies to SSA and AWA in these voles were significantly higher than the corresponding levels observed in the uninfected mice and even in the mice infected 15 days previously. The levels of antibodies reacting with CA, SSA, SEA and AWA in the experimentally infected M. fortis were 1.9-, 2.2-, 1.5- and 2.1-fold higher, respectively, than those in the laboratory-bred but uninfected voles. The observations indicate that even uninfected M. fortis produce antibodies which react with S. japonicum, and this presumably results in the natural resistance to infection which has been reported in these rodents.