Vaccine Development Against Pasteurella Haemolytica Infections in Sheep

Abstract

Pasteurellosis caused by P. haemolytica is one of the most common bacterial infections of sheep, and by far the most important respiratory one, with a widespread distribution, occurring in temperate, subtropical and tropical climates (Gilmour and Gilmour, 1989). Pneumonic pasteurellosis in sheep was first described in Iceland by Dungal (1931) and the work of Biberstein (1960) and Smith (1961) in the 1960’s on serotyping and biotyping was important in defining the epidemiological aspects of the disease. In sheep 2 biotypes comprising a total of 16 serotypes are recognised, with approximately 90% of all isolates serotypable and the remaining 10% currently untypable (Fraser et al., 1982, Ball et al., 1993). The biotypes, which were differentiated on their ability to ferment either arabinose (A) or trehalose (T), are each associated with distinct clinical syndromes (Smith, 1961).The A biotypes are responsible for pneumonic pasteurellosis in sheep of all ages while the T biotypes cause a systemic disease in 6 – 10 month old lambs. Recent taxonomical studies based on DNA homologies and 16s RNA work have suggested that the T biotype strains should be placed in a new genus, P. trehalosi, (Sneath and Stevens, 1990) but for the purposes of this chapter they will still be referred to as T biotypes of P. haemolytica. A number of virulence factors which enhance the bacterium’s ability to cause disease have been associated with P.haemolytica. These include iron-regulated proteins (IRPs) (Donachie and Gilmour, 1988; Deneer and Potter,1989), leukotoxin (Kaehler et al., 1980), lipopolysaccharide, (Rimsay et al.,1981) and polysaccharide capsule (Adlam, 1989).