Mannheimia (Pasteurella)
haemolytica is an important pathogen
of pneumonia in bighorn sheep (BHS).
Leukotoxin produced by M.
haemolytica is the most important virulence factor of this bacterium. It is cytolytic to all subsets of ruminant
leukocytes. Massive infiltration of polymorphonuclear leukocytes (PMNs) and
macrophages into the lungs is a characteristic feature of the pathogenesis of M. haemolytica pneumonia. Lysis and
degranulation of PMNs and macrophages is responsible for the acute inflammation
and lung tissue damage. Presence of a
number of anaerobic bacteria in pneumonic BHS lungs has also been reported. The
role of these anaerobic bacteria in BHS pneumonia is not clear. Of these, Fusobacterium
necrophorum produces a leukotoxin, similar to M. haemolytica. The long term goal of this study was to determine
whether the pathogenesis of M.
haemolytica pneumonia is enhanced by F.
necrophorum. The specific objectives
of this study were to determine the presence of F. necrophorum in archived pneumonic BHS lung tissues, and to determine
the susceptibility of BHS leukocytes to the leukotoxin of F. necrophorum. A species-specific PCR assay detected F. necrophorum in 37% of pneumonic BHS
lung tissues (n=71). Sequencing of the
PCR amplicons revealed more similarity to the low virulent F. necrophorum funduliforme
than to the high virulent F. necrophorum
necrophorum. The sequence of F. necrophorum leukotoxin was not similar
to that of M. haemolytica
leukotoxin. However, similar to M. haemolytica leukotoxin, F. necrophorum leukotoxin exhibited
cytotoxicity to BHS PMNs and peripheral blood mononuclear cells. As with the leukotoxin of M. haemolytica, susceptibility to F. necrophorum leukotoxin-induced
cytolysis of BHS PMNs was four-fold higher than that of domestic sheep
PMNs. Being an anaerobic bacterium, F. necrophorum is likely to enter the
lungs after M. haemolytica enters the
lung, initiates lung tissue damage, and creates a micro environment conducive
to the growth of anaerobic bacteria.