Evaluation of the effectiveness of a macrolide antibiotic on reduction of respiratory pathogens in 12-day and 21-day weaned pigs

L. Kirk Clark, DVM, PhD; Ching C. Wu, DVM, PhD; William G. Van Alstine, DVM, PhD; Kay E. Knox

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Objective: To determine the effect of a new feed-grade antibiotic (tilmicosin) on Mycoplasma hyopneumoniae-challenged pigs and to determine whether the M. hyopneumoniae status of pigs weaned at 21 days, when tilmicosin is used, would be similar to the M. hyopneumoniae status of piglets weaned at 12 days. Additionally, the effect of tilmicosin on other respiratory pathogens in all treated pigs was assessed.

Methods: Fifty commercial pigs were randomly allocated to five treatment groups. Three groups were weaned when 12 days old and were either 1) challenged with M. hyopneumoniae and treated with tilmicosin (CEW treated), 2) challenged but not treated with tilmicosin (CEW untreated), or 3) left untreated and unchallenged (EW controls). Two groups were weaned when 21 days old, were not challenged with M. hyopneumoniae, and were either treated (LW treated) or not treated (LW controls) with tilmicosin. Some of the pigs in all treatment groups developed clinical signs similar to those of Haemophilus parasuis and were treated with penicillin for 3 consecutive days immediately before the two groups of the early-weaned pigs were challenged. Weight gain for each pig was measured as the difference in weight between 12 and 56 days of age. The presence of respiratory disease was measured by the number of pigs observed coughing each day and by lung lesion scores at necropsy. Pigs were euthanized when 56 days old and examined for the presence of M. hyopneumoniae, Actinobacillus pleuropneumoniae, H. parasuis, Pasteurella multocida, Streptococcus suis, and Bordetella bronchiseptica. Additionally, sera of all pigs were assessed for the presence of M. hyopneumoniae and A. pleuropneumoniae when pigs were 12 and 56 days old.

Results: Growth rates of the different groups of pigs were unaffected by tilmicosin. Tilmicosin reduced (P <.01) the coughing in the CEW treated pigs. Although the lung lesion scores of the CEW treated pigs were not significantly lower than those in the CEW untreated group (P >.05), tilmicosin did appear to delay the onset of the infection during the course of the treatment. Actinobacillus pleuropneumoniae, B. bronchiseptica, and P. multocida were not isolated from tissues of any of the pigs. Four of the seven pigs from which S. suis was isolated were from the LW control group, whereas S. suis was not isolated from any of the LW treated pigs. Haemophilus parasuis was isolated at necropsy from 26 of the pigs in this trial: 19 of 20 LW pigs and seven of 30 early-weaned pigs. Within weaning age groups, tilmicosin did not influence the rate of isolation of H. parasuis. Pigs in all five groups were seropositive to A. pleuropneumoniae when 12 days old, but titers declined during the experiment. Two of the ten LW control pigs seroconverted to M. hyopneumoniae during the experiment, whereas none of the LW treated or EW control pigs seroconverted.

Implications: Tilmicosin did not reduce lesions of mycoplasmal pneumonia when added to the feed of CEW treated pigs. Tilmicosin delayed onset of coughing and thus probably delayed the development of lung pathology while it was fed to CEW treated pigs. Tilmicosin reduced S. suis colonization and M. hyopneumoniae seroconversion in LW pigs.

Keywords: Mycoplasma hyopneumoniae, early weaning, tilmicosin

RIS citationCite as: Clark LK, Wu CC, Van Alstine WG, et al. Evaluation of the effectiveness of a macrolide antibiotic on reduction of respiratory pathogens in 12-day and 21-day weaned pigs. J Swine Health Prod 1998;6(6):257-262.

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