THE SERUM NEUTRALIZING ANTIBODY-RESPONSE IN CATTLE TO FUSOBACTERIUM-NECROPHORUM LEUKOTOXOID AND POSSIBLE PROTECTION AGAINST EXPERIMENTALLY-INDUCED HEPATIC-ABSCESSES

The serum antileukotoxin antibody response and protection against subs equent experimental challenge with Fusobacterium necrophorum were inve stigated in 30 steers vaccinated with crude F. necrophorum leukotoxoid . Culture supernatant of F. necrophorum, strain 25, containing leukoto xoid was concentrated. The steers were assigned randomly to six groups (n=5): PBS control with Stimulon adjuvant; vaccinated with concentrat ed supernatant diluted to provide 2.5, 5.0, 10.0, or 20.0 ml with the water-soluble Stimulon adjuvant; and 5.0 ml with the Ribi oil-emulsion adjuvant. The steers were injected subcutaneously on days 0 and 21. B lood samples were collected at weekly intervals to monitor serum antil eukotoxin antibody titres. On day 42, all the steers were challenged i ntraportally with F. necrophorum culture. Three weeks later (day 63), the steers were killed and necropsied for examination of their livers and assessment of protection. Steers vaccinated with crude leukotoxoid tended to have higher antileukotoxin titres than the controls, but th e difference was not significant. Also, the antibody titre did not app ear to be dose-dependent. In the control group, 3 out of 5 steers deve loped liver abscesses. The incidence of liver abscesses in steers vacc inated with Stimulon adjuvant was not dose related; however, only 8 of the 25 vaccinated steers developed abscesses. None of the steers vacc inated with the 5.0 mi dose with Ribi had any abscesses. Evidence for a relationship between antileukotoxin antibody and protection was show n by the lower titre in those steers that developed abscesses compared to those that did not. It was concluded that antileukotoxin antibody titres probably provided some degree of protection against experimenta lly induced liver abscesses, but further dose-titration studies using Ribi or possibly another more effective adjuvant will be needed to con firm this.