Effects of oral administration of tilmicosin on pulmonary inflammation in piglets experimentally infected with Actinobacillus pleuropneumoniae.

OBJECTIVES: To determine the effects of oral administration of tilmicosin in piglets experimentally infected with Actinobacillus pleuropneumoniae. ANIMALS: Forty 3-week-old specific-pathogen free piglets. PROCEDURES: Piglets were assigned to 1 of 4 groups as follows: 1) uninfected sham-treated control piglets; 2) infected untreated piglets that were intratracheally inoculated with 10(7) CFUs of A pleuropneumoniae; 3) infected treated piglets that were intratracheally inoculated with A pleuropneumoniae and received tilmicosin in feed (400 ppm [microg/g]) for 7 days prior to inoculation; or 4) infected treated piglets that were intratracheally inoculated with A pleuropneumoniae and received chlortetracycline (CTC) in feed (1100 ppm [microg/gl) for 7 days prior to inoculation. Bronchoalveolar lavage (BAL) fluid and lung tissue specimens of piglets for each group were evaluated at 3 or 24 hours after inoculation. For each time point, 4 to 6 piglets/group were studied. RESULTS: Feeding of CTC and tilmicosin decreased bacterial load in lungs of infected piglets. Tilmicosin delivered in feed, but not CTC, enhanced apoptosis in porcine BAL fluid leukocytes. This was associated with a decrease in LTB4 concentrations in BAL fluid of tilmicosin-treated piglets, compared with untreated and CTC-treated piglets, and also with a significant decrease in the number of pulmonary lesions. Tilmicosin inhibited infection-induced increases in rectal temperatures, as measured in untreated and CTC-treated piglets. Pulmonary neutrophil infiltration and prostaglandin E2 concentrations in the BAL fluid were not significantly different among groups at any time. CONCLUSIONS AND CLINICAL RELEVANCE: Oral administration of tilmicosin to infected piglets induces apoptosis in BAL fluid leukocytes and decreases BAL fluid LTB4 concentrations and inflammatory lung lesions.

Tilmicosin-induced bovine neutrophil apoptosis is cell-specific and downregulates spontaneous LTB4 synthesis without increasing Fas expression.

The pathology of bacterial pneumonia, such as seen in the bovine lung infected with Mannheimia haemolytica, is due to pathogen virulence factors and to inflammation initiated by the host. Tilmicosin is a macrolide effective in treating bacterial pneumonia and recent findings suggest that this antibiotic may provide anti-inflammatory benefits by inducing polymorphonuclear neutrophilic leukocyte (PMN) apoptosis. Using an in vitro bovine system, we examined the cell-specificity of tilmicosin, characterized the changes in spontaneous leukotriene B4 (LTB4) synthesis by PMN exposed to the macrolide, and assessed its effects on PMN Fas expression. Previous findings demonstrated that tilmicosin is able to induce PMN apoptosis. These results were confirmed in this study by the Annexin-V staining of externalized phosphatidylserine and the analysis with flow cytometry. The cell-specificity of tilmicosin was assessed by quantification of apoptosis in bovine PMN, mononuclear leukocytes, monocyte-derived macrophages, endothelial cells, epithelial cells, and fibroblasts cultured with the macrolide. The effect of tilmicosin on spontaneous LTB4 production by PMN was evaluated via an enzyme-linked immunosorbent assay. Finally, the mechanisms of tilmicosin-induced PMN apoptosis were examined by assessing the effects of tilmicosin on surface Fas expression on PMN. Tilmicosin-induced apoptosis was found to be at least partially cell-specific, as PMN were the only cell type tested to die via apoptosis in response to incubation with tilmicosin. PMN incubated with tilmicosin under conditions that induce apoptosis spontaneously produced less LTB4, but did not exhibit altered Fas expression. In conclusion, tilmicosin-induced apoptosis is specific to PMN, inhibits spontaneous LTB4 production, and occurs through a pathway independent of Fas upregulation.

Efficacy of tilmicosin in the control of experimentally induced Actinobacillus pleuropneumoniae infection in swine.

The efficacy of tilmicosin administered in the feed to control Actinobacillus pleuropneumoniae infections in pigs was evaluated through a multisite, multitrial study. For each of 6 trials, 48 pigs (stratified by weight and sex) were randomly assigned to 6 to 8 pens. Medicated feed containing tilmicosin (200 g/t) and unmedicated feed were randomly assigned at the pen level and were provided ad libitum from day -7 to trial termination (day 14). Seeder pigs (inoculated intranasally with A. pleuropneumoniae serotype 1 and showing signs of clinical disease) were introduced to each pen on day 0. Rates of death, gross lesions, and culture of A. pleuropneumoniae at necropsy, clinical scores, average daily gain in weight, and average body temperature were compared between the medicated and unmedicated pigs. Compared with the unmedicated pigs, significantly fewer (P < 0.05) pigs given tilmicosin had lesions typical of A. pleuropneumoniae or had A. pleuropneumoniae isolated from their tissues at necropsy. Together with a significant reduction (P < 0.05) in the average percentage of pneumonic lung involvement (both visually and by weight), there were reductions in the numbers of pigs with moderate and severe pneumonic lung lesions and with A. pleuropneumoniae associated mortality. With tilmicosin treatment, the average daily weight gain, daily temperature, abdominal appearance, attitude, and respiration were also significantly better (P < 0.05). The results of this study demonstrate the in vivo effectiveness of tilmicosin (200 g/t) in controlling pleuropneumonia among swine experimentally infected with A. pleuropneumoniae.