Seroepidemiology of mycoplasmal pneumonia of swine in Japan as surveyed by an enzyme-linked immunosorbent assay.

Antibodies to Mycoplasma hyopneumoniae were surveyed by enzyme-linked immunosorbent assay (ELISA) on serum, colostrum and milk samples collected from sows and on sera of growing/finishing pigs in Japan. Only one of 196 specific-pathogen-free sows induced a low ELISA value, while 72% of 411 sows from conventional herds were seropositive. A seropositive rate in the conventional sows gradually decreased with an increase in farrowing frequency or with age. In 3267 growing/finishing pigs ranging in age from one to six months, a seropositive rate increased remarkably from the age of 4 months onwards, reaching the maximum at the age of 6 months. A survey conducted on 42 conventional farms revealed that the highest seroconversion occurred when pigs were 4 months of age. The level of maternal antibodies was proportional to that of the dam's colostral antibodies. After maternal antibodies waned, active immunity in newborn piglets from dams with high colostral antibodies appeared earlier and higher than that in piglets from dams with low colostral antibodies. In 950 slaughter pigs, there was a correlation between seropositiveness and the presence of pneumonic lesions, but the ELISA value did not correlate with the degree of the lesions. Pigs that were raised under unfavorable environmental conditions developed pneumonic lesions more frequently than pigs rearing under better conditions, regardless of their immune status. These results suggested that M. hyopneumoniae and some secondary respiratory pathogens may have been involved in the development of these pneumonias.