Pathogenesis of co-infections of influenza D virus and Mannheimia haemolytica in cattle.

Bovine respiratory disease (BRD) is economically significant, and influenza D virus (IDV) is commonly identified in cattle with BRD. Mannheimia haemolytica (MHA) is an opportunistic bacterial contributor to BRD; surveillance data suggest that MHA and IDV co-infection occurs in cattle. The objective of this study was to evaluate the synergistic pathogenesis in cattle co-infected with IDV and MHA. Sixteen dairy calves were randomly assigned to four groups of four calves. The IDV + MHA + group received D/bovine/C00046 N/Mississippi/2014 (D/46 N) intranasally at 0 days post-inoculation (DPI) and Mannheimia haemolytica D153 (MHA D153) intratracheally at 5 DPI. The IDV + MHA- group received only D/46 N at 0 DPI; the IDV-MHA + group received only MHA D153 at 5 DPI; and the IDV-MHA- group received neither agent. Clinical scores were calculated twice daily. At 10 DPI, IDV + MHA+, IDV-MHA+, and IDV-MHA- calves were euthanized and evaluated for pathologic lesions. The IDV + groups seroconverted to IDV by 10 DPI. Clinical scores were higher in IDV + groups than IDV- groups on 2-5 DPI (p =  0.001). After MHA challenge on 5 DPI, clinical scores (6-10 DPI) were slightly lower in IDV+MHA+ group than IDV-MHA+ group (p <  0.05) but not significantly different between MHA+ groups and MHA- groups. The average gross pathology score was higher for IDV-MHA+ group than groups IDV-MHA- and IDV+MHA+; however, no significant differences were identified among groups. Under the conditions of this study, infection with IDV before MHA enhance neither clinical disease nor lung pathology, relative to calves infected with MHA alone.