Toxic, allergenic and immunogenic factors of Fusiformis necrophorus

Abstract

In broth cultures of Fusiformis necrophorus at 18 hours, the liquid fraction contains nearly all the toxin. The cellular fraction is separable by acetone drying and saline extraction into (i) an aqueous extract containing nearly all the precipitinogen and allergen, and (ii) extracted cells which have the surface antigens that react with agglutinin, and also probably with bactericidal antibody according to the close association between cross-protection and cross-agglutination. In the cells of a given strain the surface antigens may be present in such a form that they can induce the formation of antibody but cannot react with it. In vaccinated sheep and rabbits a dermal hypersensitivity to the allergen is of the immediate type due to circulating antibody. It apparently accounts for the accelerated inflammatory response to foot-rot in sheep vaccinated with F. necrophorus. Cross-reactions between strains indicate a considerable sharing of allergen as well as other soluble antigens. Vaccinated rabbits develop a resistance to infection that apparently depends on a humoral bactericidal mechanism and not on any enhancement of phagocytosis or neutralization of toxin. The mechanism is of a kind not inactivated by blood pigments. On the other hand, haemoglobin and haematin have a direct effect on F. necrophorus that explains the intensity with which the organism invades areas of the infected lesion where there is vascular stasis and free blood pigment. The humoral bacteriocidal mechanism destroys only 1 2 to 2 3 of a given inoculum either in vitro or in the tissues of a vaccinated rabbit; it has no influence on growth once the organism is established. Effective protection would probably require a form of vaccination which could induce antibodies able to neutralize the toxin. Vaccines made from formalin-killed broth cultures were unsuccessful in this respect.