Characterization of an Actinobacillus pleuropneumoniae seeder pig challenge-exposure model

Five strains of Actinobacillus pleuropneumoniae serotype 1 were used to intranasally infect 5 groups of pigs. Using each bacterial strain, infected pigs (termed seeder pigs) were commingled for 48 hours with 5 groups of noninfected test pigs, then were removed. Seeder and test pigs were maintained in isolation and were observed for 14 days. Seeder pigs had mortality that was threefold greater than that of test pigs (24% vs 8%). Rectal temperature in excess of 40.3 C was achieved for 84% of test pigs and 88% of seeder pigs. Neither of these 2 variables was statistically different between the 2 groups of pigs. Clinical impression scores greater than or equal to 2 (on a 0 to 3 scale) were three-fold (64% vs 20%) greater for seeder than for test pigs (P < 0.05). The total number of bacterial isolations or nonrecoverable isolates was tabulated for test and seeder pigs' lungs at necropsy, irrespective of the amount of lesions. The number of A pleuropneumoniae isolations was not statistically different between test and seeder pig populations. Recovery of Pasteurella multocida or other bacteria was greater from the seeder pigs (P < 0.05), whereas the number of non-recoverable isolates was greater from test pigs than from seeder pigs (P < 0.05). Assessment of lung lesions at necropsy by either visual estimation or on a weight basis were in agreement. Fewer test pigs had lung lesions in excess of 5% of total lung volume than did seeder pigs (40% vs 84%) and, according to the odds ratio estimation, seeder pigs were 7 times more likely than test pigs to have such lesions. These results indicate a predictable, moderate intensity, natural exposure model for use in the study of Actinobacillus pleuropneumoniae-induced pneumonia. The seeder pig model appears to mimic field infection in development of clinical illness, febrile response, lung lesions, mortality, and low potential for secondary pneumonic bacterial involvement, thus providing a useful tool for preliminary evaluation of anti-infective modalities.