Identification of strain specific differences in incubation periods and distribution of lung lesions between Actinobacillus pleuropneumoniae serotype 2 and serotype 7 after experimental infection of nursery pigs

Background of this study was to determine whether differences in incubation period and distribution of lung lesions between different strains of Actinobacillus pleuropneumoniae (A, pleuropneumoniae) exist after the exclusion of influencing environmental and genetic factors. Therefore a retrospective analysis of four different experimental aerosol infection trials with the same study setup was done. A total of 120 piglets, aged 7 weeks, from the same farm and the same German Landrace breeding line, were infected either with the A. pleuropneumoniae serotype 2 strain 03656 or the A. pleuropneumoniae serotype 7 strain AP76 implementing a standardized aerosol infection model. Incubation period, based upon clinical monitoring, as well as distribution and size of lung lesions dependent on strain, infection dose (high dose: 3x10(3) cfu/liter aerosol [HD]; low dose: 1x10(2) cfu/liter aerosol [LD]) and days post infection (day 7/day 20) were assessed. Differences in incubation periods (median C3656 HD: 4.00 hours [h], LD: 8.00 h; median AP76 HD: 180.00 h, LD: 22.00 h), number of lung lesions (median C3656: 10.00; median AP76: 4.00) and size of lung lesions (median C3656: 2.39; median AP76 6.93; given as mean of marked score units) were statistically significant between the two strains. Differences were also significant for the number of lung lesions for AP76 regarding the different infection doses and necropsy days. Therefore strain specific characteristics regarding the examined attributes can be confirmed. Further studies are needed to reveal the responsible virulence factors and mechanisms.