Leukotoxin enhances nasopharyngeal colonization and shedding of Mannheimia haemolytica in bovine herpesvirus 1 infected calves.

The aim of this study was to assess the role of leukotoxin in Mannheimia haemolytica on colonization dynamics in the nasopharynx and bacterial shedding in calves concurrently infected with bovine herpesvirus 1 (BHV-1). Eight calves were assigned to two treatment groups of four and housed in isolation rooms. After acclimation, BHV-1 was administered intranasally to both groups. On the same day, calves in one group received feed containing leukotoxin (LktA) deleted mutants of the M. haemolytica serotype (ST1) and ST6 strains. Five days post-BHV-1 exposure, this group was re-exposed to LktA mutants intranasally. On day six, both groups were intranasally inoculated with wild-type ST1 and ST6 parent strains, and the animals were euthanized on day 10. Results indicate that both ST1 and ST6 can establish high-level colonization in the nasopharynx of BHV-1 infected calves. LktA mutants exhibited approximately 10-fold lower nasopharyngeal colonization compared to their wild-type counterparts. The higher shedding levels observed in wild-type strains compared to LktA mutants suggested a clear advantage in spreading the infection to other susceptible animals. These findings provide valuable insight into the nature of M. haemolytica infection in bovine hosts. They also indicate that leukotoxin expression may contribute to the competitive fitness M. haemolytica strains, potentially influencing their adaptation within the host.