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

¹ Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506
² Department of Diagnostic Medicine and Pathobiology, Kansas State University, Manhattan 66506
³ Department of Diagnostic Medicine and Pathobiology, Kansas State University, Manhattan 66506
⁴ Department of Statistics, Kansas State University, Manhattan 66506

^* To whom correspondence should be addressed. E-mail: jdrouill{at}oznet.ksu.edu.

	Abstract

Previous research from our laboratory indicated that melengestrol acetate (MGA) added to the diet during the first 35 d after arrival in the feedlot improved growth rates and tended to reduce chronic respiratory disease in heifers naturally challenged with bovine respiratory disease. The present study was completed to provide further insight into the possible immunomodulatory effects of MGA. Crossbred heifers (n = 48; 232 ± 5.5 kg) were used in a randomized complete block design to determine effects of MGA on lung pathology and markers of inflammation following Mannheimia haemolytica challenge. On d 0, cattle were stratified by BW and randomly assigned, within strata, to diets (54% concentrate) that provided 0 or 0.5 mg MGA per heifer daily for the duration of the experiment. Inoculum containing between 1.3 x 10⁹ and 1.7 x 10⁹ CFU M. haemolytica (20mL) were 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, and analyzed as repeated measures. Heifers fed MGA had greater circulating concentations 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 nitrogen (P = 0.02), and elevated respiratory indices (P < 0.01) compared to controls. Necropsies performed on d 6 after inoculation suggested that M. haemolytica challenge was relatively mild, as 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: heifers, Mannheimia haemolytica, melengestrol acetate