Sexual Dimorphic Innate Immune Response to a Viral-Bacterial Respiratory Disease Challenge in Beef Calves.

The potential for sexually dimorphic innate immune responses to respiratory disease was evaluated, where eight steers and seven heifers (280 ± 4 kg) were subjected to a viral-bacterial respiratory disease challenge utilizing bovine herpesvirus-1 (BoHV-1; intranasal; 1 × 10 8 PFU/nostril) and Mannheimia haemolytica (MH; intratracheal; 1.3 × 10 7 CFU/head) administered 72 h later. Body temperature was lesser in heifers than steers ( p < 0.01). There was a sex × time interaction ( p = 0.05) for white blood cells where heifers had reduced concentrations compared with steers at -72 and 0 h but greater concentrations from 36 to 60 h post-MH. Concentrations of neutrophils were lesser in heifers compared to steers from 0 to 4 h, and from 8 to 12 h ( p = 0.03). Lymphocytes were greater in heifers compared to steers at 12 h and from 36 to 60 h post-MH ( p < 0.01). The neutrophil-lymphocyte ratio was lesser in heifers compared to steers from 2 to 24 h and at 48 h post-MH ( p < 0.01). Monocytes were greater in heifers compared to steers from 24 to 60 h post-MH ( p < 0.01), while eosinophils were greater in heifers compared to steers at 48 and 60 h ( p < 0.01). Serum IL-4 was lesser in heifers compared to steers at 0 h and from 2 to 72 h post-MH challenge ( p = 0.02). Non-esterified fatty acid concentrations were lesser ( p < 0.01) in heifers compared to steers from 2 to 4 h post-MH challenge. Urea nitrogen concentrations were greater ( p < 0.01) in heifers than steers at 36 h post-MH challenge. Data from this study reveal distinct differences in the acute phase response following a respiratory disease challenge where steers produced an early response, while the response in heifers appeared to be delayed.