Serological diversity in Haemophilus somnus.

Serological interrelations among 46 strains of Haemophilus somnus of various geographical and pathological origins were studied by agglutination and agglutinin adsorption procedures. Sera prepared against nine representative American and Swiss isolates agglutinated all other strains to various titers. Adsorption of antisera to American strains by Swiss cultures tended to remove mainly the reactivity with Swiss antigens, whereas adsorption of such antisera to American strains in most instances abolished all agglutinating activity. Analogous observations were made when sera against Swiss cultures were adsorbed with cells of American and Swiss origin, respectively. Results of tests involving unadsorbed and adsorbed sera suggest the existence of at least three sets of antigen in H. somnus: American, Swiss, and common, which appear to exist in various combinations, accounting for four agglutination groups. These reflect to a considerable degree the geographical, although not the pathological or anatomical, origin of the respective strains.