Increased susceptibility to Pasteurella haemolytica in lambs infected with bovine respiratory syncytial virus.

Groups of conventionally reared, 6- to 8-week-old lambs were inoculated intranasally and intratracheally with either bovine respiratory syncytial virus (BRSV) or BRSV followed by Pasteurella haemolytica 6 days later or with P. haemolytica alone. Lambs infected with P. haemolytica 6 days after experimental infection with BRSV had significantly higher disease scores, greater magnitudes of fever, and higher mortality rates than those infected with BRSV alone or with P. haemolytica alone (P less than 0.05). Mononuclear cells in the peripheral blood and lung lavage obtained from the BRSV-infected lambs were more susceptible to P. haemolytica cytotoxin than those obtained from control lambs (P less than 0.05). P. haemolytica was isolated in significantly greater numbers from lambs infected with BRSV and P. haemolytica than from those infected with P. haemolytica alone (P less than 0.05).