Actinobacillus pleuropneumoniae in Thai pig herds. Prevalence of serum antibodies and relation to performance

The aim of the study was to investigate the prevalence of Actinobacillus pleuropneumoniae infections in market weight pigs in Thailand. ELISA systems employing purified lipopolysaccharide antigens were used to detect antibodies in 549 serum samples collected from pigs of 22 herds. Relevant cut-off values were established from three herds defined seronegative. Serum antibodies were detected to all serotypes except serotype 10. Almost 60% of the samples were seropositive to at least one serotype and 45% of the pigs were seropositive to more than one serotype. Antibodies to the cross-reacting serotypes 1, 9 or 11 were found in 29% of the pigs. Other common serotypes included the cross-reacting serotypes 3, 6 or 8 (26% seropositive pigs) and serotype 5a ( also 26%). Antibodies to serotypes 2, 5b and 12 were low in prevalence (< 10%). Three herds were regarded to be seronegative and six to have a low pathogen load with respect to the prevalence of seropositive pigs. The remaining 13 herds had a high incidence of pigs with antibodies to A. pleuropneumoniae, dominated by serotypes 1-9-11 and 5a (n = 6), serotypes 3-6-8, and 5a (n = 4) or 1-9-11, 3-6-8, 5a and 4-7 (n = 3). A low pathogen load with respect to A. pleuropneumoniae, as well as small herd size and age-segregated rearing, tended to improve the performance of growers.