SAFETY, STABILITY, AND EFFICACY OF NONCAPSULATED MUTANTS OF ACTINOBACILLUS-PLEUROPNEUMONIAE FOR USE IN LIVE VACCINES

Clonal, noniridescent mutants of Actinobacillus pleuropneumoniae serot ypes 1 and 5 were isolated following chemical mutagenesis with ethyl m ethanesulfonate. The absence of any detectable capsule was confirmed b y inhibition radioimmunoassay. There were no differences between the p arent and mutant strains in lipopolysaccharide or protein electrophore tic profiles or in hemolytic activity. There was no detectable reversi on to the encapsulated phenotype in vitro after passage in mice or pig s or in microporous capsules that were implanted subcutaneously in pig s for 6 weeks. The mutants were able to survive for more than 1 week i n pigs following subcutaneous inoculation, which resulted in a strong immune response to whole cells and Apx toxins I and II. Intratracheal challenge of pigs with the serotype 5 mutant at a dose 1 log greater t han the 50% lethal dose for the parent resulted in no clinical disease or lesions except in one pig that had slight pneumonia and pleuritis. Twenty-four hours after challenge, A. pleuropneumoniae could not be r ecovered from the respiratory tracts of any of the challenged pigs exc ept for the one infected pig; this isolate remained noncapsulated. Imm unization of pigs with one or both serotypes of noncapsulated mutants protected all pigs against clinical disease following intratracheal ch allenge with the virulent homologous or heterologous serotype. Nonimmu nized control pigs and pigs immunized with a commercial bacterin died or had to be euthanized within 24 h of challenge. Thus, live noncapsul ated mutants of A. pleuropneumoniae may provide safe and cost-effectiv e protection against swine pleuropneumonia. These observations support the possibility that noncapsulated mutants of other encapsulated, tox in-producing bacteria may also prove to be efficacious live-vaccine ca ndidates.