Further observations on the weak immunogenicity of Fusobacterium necrophorum

Fusobacterium necrophorum infections: virulence factors, pathogenic mechanism and control measures

Fusobacterium necrophorum, a Gram-negative, non-spore-forming anaerobe, is a normal inhabitant of the alimentary tract of animals and humans. Two types of F. necrophorum, subspecies necrophorum (biotype A) and funduliforme (biotype B), have been recognized, which differ morphologically, biochemically, and biologically. The organism is an opportunistic pathogen that causes numerous necrotic conditions (necrobacillosis) such as bovine hepatic abscesses, ruminant foot abscesses and human oral infections. The pathogenic mechanism of F. necrophorum is complex and not well defined. Several toxins, such as leukotoxin, endotoxin, haemolysin, haemagglutinin and adhesin, have been implicated as virulence factors. Among these, leukotoxin and endotoxin are believed to be more important than other toxins in overcoming the host's defence mechanisms to establish the infection. F. necrophorum is encountered frequently in mixed infections and, therefore, synergisms between F. necrophorum and other pathogens may play an important role in infection. Several investigators have attempted to induce protective immunity against F. necrophorum using bacterins, toxoids, and other cytoplasmic components. Generally, none of the immunogens has afforded satisfactory protection against Fusobacterium infections. Because of the unavailability of suitable immunoprophylaxis, the control of F. necrophorum infection has depended mainly on the use of antimicrobial compounds.

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 subsequent experimental challenge with Fusobacterium necrophorum were investigated in 30 steers vaccinated with crude F. necrophorum leukotoxoid. Culture supernatant of F. necrophorum, strain 25, containing leukotoxoid was concentrated. The steers were assigned randomly to six groups (n = 5): PBS control with Stimulon adjuvant; vaccinated with concentrated 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. Blood samples were collected at weekly intervals to monitor serum antileukotoxin antibody titres. On day 42, all the steers were challenged intraportally 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 the difference was not significant. Also, the antibody titre did not appear to be dose-dependent. In the control group, 3 out of 5 steers developed liver abscesses. The incidence of liver abscesses in steers vaccinated with Stimulon adjuvant was not dose related; however, only 8 of the 25 vaccinated steers developed abscesses. None of the steers vaccinated with the 5.0 ml dose with Ribi had any abscesses. Evidence for a relationship between antileukotoxin antibody and protection was shown 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 experimentally induced liver abscesses, but further dose-titration studies using Ribi or possibly another more effective adjuvant will be needed to confirm this.

Purification and quantification of Fusobacterium necrophorum leukotoxin by using monoclonal antibodies

Monoclonal antibodies (Mabs) were produced to the leukotoxin of Fusobacterium necrophorum. Two mAbs (F7B10 and E12E9) partially neutralized leukotoxin activity, as determined by a tetrazolium (MTT)-dye reduction assay with bovine polymorphonuclear neutrophils as target cells. Immunoblot analysis showed that both clones reacted with antigens of 110 and 131 kilodaltons. Epitope analysis showed that the two mAbs recognized the same epitope. An affinity column containing immobilized mAb F7B10 was used to purify leukotoxin from crude toxin. Affinity chromatography of 1 ml of culture supernatant resulted in 0.67 microgram or 1350 units of leukotoxin. Leukotoxin was quantitated by a sandwich enzyme-linked immunosorbent assay using mAb F7B10 as the capture antibody and as the biotinylated indicator. The minimal detectable level was approximately 1 ng, corresponding to 2 leukotoxin units in the sample.

Pathogenicity of Fusobacterium necrophorum strains from man and animals

Necrobacillosis occurs in man and animals. The typical forms of the disease in animals are caused by Fusobacterium necrophorum biovar A; biovar B strains are much less pathogenic. In this study the pathogenicity for mice of eight human isolates of F. necrophorum was compared with that of animal biovar A and B strains. By subcutaneous inoculation seven of the human strains differed from biovar A but resembled biovar B in (1) producing, at the most, mild local lesions that rapidly healed, and (2) showing no enhancement of infectivity when suspended in sub-lethal doses of Staphylococcus aureus broth culture. The eighth human strain (A2433) resembled biovar A but differed from biovar B in (1) producing severe lesions, and (2) showing greatly enhanced infectivity in the presence of S. aureus. Nonetheless, strain A2433 differed from biovar A, both in the nature of the lesions produced and in its failure to cause severe general signs of illness and rapidly fatal infection. By intravenous inoculation one of two biovar B strains and all except one of the eight human strains produced purulent lesions, often severe, in the liver and elsewhere, but infection was not usually associated with general signs of illness. In contrast, intravenous injection of a biovar A strain gave rise to a rapidly fatal infection, with severe lesions in the liver or elsewhere. The results suggest that the term 'necrobacillosis' as used in human and veterinary medicine refers to diseases that differ in important respects.