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

SIMPLE SUMMARY: Bovine respiratory disease is the leading cause of morbidity and mortality in cattle and is estimated to result in USD 800 to USD 900 million in losses each year. Unfortunately, few improvements have been made over the past 30 years to reduce the negative effects associated with bovine respiratory disease. Many naturally occurring factors, such as breed and temperament, have previously been observed to influence immune responses to infection. However, there is limited information on the influence of sex on the immune response to bovine respiratory disease. Thus, a study was conducted where castrated male (steer) and intact female (heifer) calves were administered a dual viral–bacterial respiratory disease challenge. In response to the challenge, heifer and steer calves produced distinct innate immune responses where steers produced a stronger early acute phase response, while heifers appeared to have a delayed response to the challenge. This suggests heifers may be more resilient to the viral–bacterial challenge when compared with steers. Therefore, the sexually dimorphic immune response should be considered when monitoring cattle for symptoms of bovine respiratory disease ABSTRACT: 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.