Efficacy of a chlortetracycline feed additive in reducing pneumonia and clinical signs induced by experimental Mycoplasma hyopneumoniae challenge

Abstract

Objectives: To determine the efficacy of a chlortetracycline (CTC) feed additive on pneumonia and clinical signs induced by Mycoplasma hyopneumoniae in an experimental challenge model. Methods: Three groups of pigs (12 pigs per group) were challenged with M hyopneu-moniae (Day 0). Two groups received feed containing CTC at 550 g per tonne (500 g per ton; 22 mg per kg of bodyweight) for 14 days, starting either on Day -3 (prior to challenge) or at onset of clinical signs (Day 10). Pigs were evaluated daily for clinical disease (coughing), and all were necropsied on Day 29. Percentage of lung affected by pneumonia, number of organisms isolated from lung tissue, and serum antibodies (ELISA) were measured. Results: Pigs fed CTC starting before inoculation had significantly fewer coughing days and less pneumonia than either of the other groups. Pigs in both CTC-medicated groups had significantly fewer M hyopneu-moniae organisms at necropsy than non-medicated pigs. At necropsy, 50% of control pigs were seropositive for M hyopneumoniae antibodies, while none of the CTC-treated pigs had seroconverted. Implications: Under the conditions of this study, in pigs challenged with M hyopneu-moniae, less severe clinical signs and pneumonia occur and fewer organisms may be isolated from lung tissue when treatment with CTC begins before challenge rather than after the onset of clinical signs. In addition, fewer organisms may be isolated from lung tissue of pigs treated with CTC beginning with the onset of clinical signs, compared to untreated pigs. In-feed CTC may be effective against mycoplasmal pneumonia.