Isolation of Haemophilus somnus antigens and their use as vaccines for prevention of bovine thromboembolic meningoencephalitis.

Antigens extracted from Haemophilus somnus were examined for their suitability as vaccines for prevention of thromboembolic meningoencephalitis and as antigens in immunologic tests for detection of susceptible cattle. Saline extraction of whole H somnus cells produced an outer membrane complex (OMC) that contained 2 major antigens when tested against antiserum to intact cells by immunoelectrophoresis. Anion exchange chromatography was used to separate an anionic antigen (AA) from the more cationic antigen (CA). According to chemical analysis and polyacrylamide gel electrophoresis, both AA and CA were complex mixtures--probably, outer membrane fragments. Enzyme-linked immunosorbent assays were used to measure serum levels of immunoglobulin (Ig) G and IgM against AA and CA in cattle vaccinated with whole cells, OMC, CA, or AA. Protection of vaccinated cattle was assessed after IV challenge exposure to H somnus. Moderate IgG and IgM responses occurred when cattle were given 2 vaccinal doses of 0.1 mg or 1.0 mg of whole cells, OMC, or AA. Two of 10 cattle given 2 vaccinal doses (1.0 mg) of OMC died after IV challenge exposure, whereas 8 of the 10 controls died, indicating significant protection (P less than 0.05). All of the 10 cattle given 2 vaccinal doses (1.0 mg) of AA were protected from IV challenge exposure, whereas 5 controls died (P less than 0.05). Two vaccinal doses of either 0.1 mg or 1.0 mg of CA produced high IgG and IgM responses. However, 3 of 10 cattle given 2 vaccinal doses (1.0 mg) of CA died after IV challenge exposure, as did 3 of the 10 controls, indicating that CA was not protective.(ABSTRACT TRUNCATED AT 250 WORDS)