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

Authors: BRIGGS, ROBERT - Retired ARS Employee; Tatum, Fred; Holthausen, David; FRANK, GLYNN - Retired ARS Employee; Casas, Eduardo; Dassanayake, Rohana

Submitted to: Research in Veterinary Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/14/2026
Publication Date: 6/16/2026
Citation: Briggs, R.E., Tatum, F.M., Holthausen, D.J., Frank, G.H., Casas, E., Dassanayake, R.P. 2026. Leukotoxin enhances nasopharyngeal colonization and shedding of Mannheimia haemolytica in bovine herpesvirus 1 infected calves. Research in Veterinary Science. 210. Article 106301. https://doi.org/10.1016/j.rvsc.2026.106301.
DOI: https://doi.org/10.1016/j.rvsc.2026.106301

Interpretive Summary: Bovine respiratory disease complex results in significant economic losses to the U.S. beef and dairy cattle industries. While respiratory viruses often make cattle vulnerable to secondary bacterial infections, we have not fully understood how these bacteria spread. The most potent virulent factor harbored by Mannheimia haemolytica is leukotoxin. Here we studied whether the leukotoxin plays any role in nasopharyngeal colonization. Our study revealed that when calves were first weakened by a virus, bovine herpesvirus 1, the bacteria, M. haemolytica, rapidly multiplied and shed in large numbers through nasal shedding. We also discovered that M. haemolytica with 'leukotoxin' were much better at the nasopharyngeal colonization and shedding than M. haemolytica without it.

Technical Abstract: Mannheimia haemolytica is a major bacterial pathogen of the bovine respiratory disease complex, with serotypes 1 (ST1) and 6 (ST6) being the most prevalent isolates. The aim of this study was to assess the role of leukotoxin in Mannheimia haemolytica colonization dynamics within nasopharynx and in 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 ST1 and ST6 strains. Five days post-BHV-1 infection, this group was re-exposed to LktA mutants intranasally. On day six, both groups were intranasally inoculated with wildtype ST1 and ST6 parent strains, and the animals were humanely 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 colonized the nasopharynx at approximately 10-fold lower level than their wildtype counterparts. The higher shedding levels observed in wildtype strains compared to LktA mutants suggested a clear advantage in spreading the infection to other susceptible animals. These findings provide valuable insight into the serotype-specific nature of M. haemolytica infection in bovine hosts and demonstrate that leukotoxin expression may offer a selective advantage in the evolution of Mannheimia.