Effects of different antimicrobial treatments on serum acute phase responses and leucocyte counts in pigs after a primary and a secondary challenge infection with Actinobacillus pleuropneumoniae.

The susceptibility to an initial challenge and a re-challenge inoculation with Actinobacillus pleuropneumoniae was analysed in pigs that were treated with antimicrobials of different efficacies following the first exposure to A pleuropneumoniae. In brief, 30 nine-week-old specific pathogen-free pigs were allocated to five groups of six. After acclimatisation, four groups were inoculated with A pleuropneumoniae serotype 2. At the onset of clinical signs, three of the groups of pigs were treated with enrofloxacin, tetracycline or penicillin. A fourth group served as the inoculated control and the fifth group as a control group that had not been inoculated. On day 28, all five groups were re-challenged with the same strain of A pleuropneumoniae serotype 2 as had been used in the first inoculation. No treatments were carried out at this time. The acute phase responses and differential leucocyte counts were monitored in detail after both inoculations. Leucocytosis and acute phase responses in the forms of serum amyloid A, pig-major acute phase protein and haptoglobin were recorded in all of the inoculated groups after the onset of clinical signs following the first inoculation. A porcine mannan-binding lectin-A response was less evident in the pigs. Acute phase responses resembling those of the first inoculation were observed in the pigs that had not previously been inoculated and in the pigs treated with enrofloxacin. Acute phase responses were not recorded in the other three groups, where the pigs had seroconverted to A pleuropneumoniae serotype 2 following the first inoculation.