Experimental reproduction of septicemic pasteurellosis in feedlot lambs: bacteriologic and pathologic examinations.

Septicemic pasteurellosis (SP) was induced in feedlot lambs. Twenty-eight lambs, randomly allotted into 7 groups, were given combinations of 3 treatments: (i) immunosuppression using hydrocortisone solubilized in dimethyl sulfoxide, (ii) rapid changes in feed, from 100% roughage to 90% concentrate, and (iii) oral inoculation of Pasteurella haemolytica biotype T. Feed changes and immunosuppression by hydrocortisone were needed for the production of SP. Pasteurella haemolytica inoculation was not necessary for induction of SP in all cases, indicating an endogenous source of infection. Clinical pathologic, bacteriologic, and gross and microscopic pathologic findings of induced SP were similar to those described for naturally occurring SP in lambs. Infection of lambs with P haemolytica biotype T via the gastrointestinal tract is discussed as a possible step in the pathogenesis of SP in feedlot lambs.