Effects of gut barrier dysfunction during a viral respiratory disease challenge on immune function of feedlot beef calves

Feedlot morbidity and mortality have increased in recent decades, driven in part by the prevalence of bovine respiratory disease (BRD). Research on gut barrier dysfunction (GBD) reveals similarities in predisposing factors and etiopathogenic mechanisms to BRD. This overlap suggests that GBD may serve as a predisposing factor, increasing susceptibility to BRD. To explore this connection, 15 Angus × Holstein heifers (initial body weight = 475 ± 12 kg) were used in a randomized complete block design experiment to evaluate the effects of induced changes in intestinal permeability on immune responsiveness during a viral respiratory disease challenge. Treatments were control (CT; n = 7) or GBD (n = 8), where GBD heifers underwent a protocol to increase gut permeability using aspirin (100 mg/kg of body weight every 12 hours for 4 consecutive days). Daily dry matter intake and average daily vaginal temperature (DVT) were continuously recorded. After aspirin withdrawal, all heifers were inoculated with bovine herpesvirus-1. Complete blood count, cytokines, acute-phase proteins (APP), cortisol, and intestinal morphology were evaluated. Heifer was considered the experimental unit for all analyses. The statistical model included the fixed effect of treatment and hour/day and the resultant interactions, run and heifer (treatment) were used as random effects, and hour or day was the term for all repeated statements, and heifer (treatment) was the subject. Aspirin administration increased gut permeability in GBD heifers, as evidenced by greater plasma Chromium-EDTA recovery (P = 0.02) and increased lipopolysaccharide-binding protein concentration (treatment × hour; P < 0.01) early in the disease challenge. Compared to CT, GBD heifers tended to exhibit decreased DVT (P = 0.10) and haptoglobin concentration (treatment × hour; P = 0.06) by the end of the disease challenge. No significant differences were observed in serum amyloid A, interleukin-6, tumor necrosis factor-α, interleukin-10, or cortisol concentration (P ≥ 0.20). A tendency for decreased white blood cell (P = 0.08) and lymphocyte counts (treatment × hour; P = 0.08) in GBD heifers was observed. There were no effects of treatments on intestinal morphology (P ≥ 0.16). These findings suggest that increased gut permeability influences immune responses by reducing the febrile response and decreasing the production of some APP. Greater emphasis on gut health could improve disease outcomes in BRD management.