Characterization of Haemophilus spp. isolated from healthy swine and evaluation of cross-reactivity of complement-fixing antibodies to Haemophilus pleuropneumoniae and Haemophilus taxon "minor group".

Of 30 sows from a herd believed to be free of Haemophilus pleuropneumoniae infection, 2 had complement-fixing antibodies to H. pleuropneumoniae serotype 5. Necropsy and microbiological examination of the two sows revealed no evidence of H. pleuropneumoniae infection; however, Haemophilus taxon "minor group" and a urease-negative, indole-positive Haemophilus sp. were isolated from numerous respiratory tract sites in both sows. Isolation of these Haemophilus spp. was facilitated by serially diluting specimens in two broth media. Pigs from a closed, respiratory disease-free herd were inoculated with four strains of Haemophilus taxon "minor group" to determine whether the organism induces antibodies which cross-react with H. pleuropneumoniae in the complement fixation test. Antigenic heterogeneity among the taxon "minor group" strains was apparent; however, antibodies cross-reacting between these strains and H. pleuropneumoniae serotypes 1 through 5 and 7 were not detected.