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This Article Full Text Full Text (PDF) All Versions of this Article: jas.2006-396v1 85/7/1770    most recent Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Corrigan, M. E. Articles by Fox, J. T. Search for Related Content PubMed PubMed Citation Articles by Corrigan, M. E. Articles by Fox, J. T.

Effects of melengestrol acetate on the inflammatory response in heifers challenged with Mannheimia haemolytica¹

M. E. Corrigan^*, J. S. Drouillard^*,2, M. F. Spire, D. A. Mosier, J. E. Minton^*, J. J. Higgins, E. R. Loe^*, B. E. Depenbusch^* and J. T. Fox

^* Department of Animal Sciences and Industry, and Department of Diagnostic Medicine and Pathobiology, and and Department of Statistics, Kansas State University, Manhattan 66506

² Corresponding author: jdrouill{at}oznet.ksu.edu

Previous research from our laboratory has indicated that melengestrol acetate (MGA) added to the diet during the first 35 d after arrival in the feedlot improves growth rates and tends to reduce chronic respiratory disease in heifers naturally challenged with bovine respiratory disease. The current study was conducted to provide further insight into the possible immunomodulatory effects of MGA. Crossbred heifers (n = 48; 232 ± 5.5 kg of BW) were used in a randomized complete block design to determine the effects of MGA on lung pathology and markers of inflammation after Mannheimia haemolytica challenge. On d 0, cattle were blocked by BW and randomly assigned, within block, to diets (54% concentrate) that provided 0 or 0.5 mg of MGA per heifer daily for the duration of the experiment. Inoculum containing from 1.3 x 10⁹ to 1.7 x 10⁹ cfu of M. haemolytica (20 mL) was instilled at the bifurcation of the trachea on d 14. Blood samples were collected, clinical observations were made, and rectal temperatures were recorded for each animal at 0, 12, 24, 48, 72, 96, 120, and 138 h after inoculation. Heifers fed MGA had greater circulating concentrations of eosinophils and postchallenge concentrations of segmented neutrophils and white blood cells (P < 0.01) than controls, as well as elevated plasma protein, serum haptoglobin, and fibrinogen after M. haemolytica challenge (P < 0.01). Heifers fed MGA had lower plasma glucose (P < 0.01), greater plasma urea N (P = 0.02), and elevated respiratory indices (P < 0.01) compared with controls. Necropsies performed on d 6 after inoculation suggested that M. haemolytica challenge was relatively mild, because lesions were confined to a small portion of the lungs. On a 0 to 100 scale, average lung lesion scores were 3 and 1 for MGA-fed and control groups, respectively (P < 0.06). Heifers fed MGA before mild M. haemolytica challenge were more susceptible to infection, as evidenced by a greater number of heifers fed MGA exhibiting pulmonary lesions 138 h after inoculation than controls (14 out of 23 vs. 6 out of 24 for MGA and controls, respectively; P < 0.02).

Key Words: heifer • Mannheimia haemolytica • melengestrol acetate