27       Pasteurella species, Mannheimia haemolytica and Bibersteinia trehalosi

 

Pasteurella, Bibersteinia and Mannheimia species are small, non-motile, Gram-negative rods or coccobacilli. They are oxidase-positive facultative anerobes, and most species are catalase-positive. These organisms grow best on media supplemented with blood or serum. Some species, such as Mannheimia haemolytica and Bibersteinia trehalosi, grow on MacConkey agar. In smears from infected tissues stained by the Giemsa method, pasteurellae exhibit bipolar staining. Most Pasteurella, Bibersteinia and Mannheimia species are commensals on the mucosae of the upper respiratory tracts of animals. Thus infections are frequently endogenous. Exogenous infections also occur, particularly during outbreaks of pasteurellosis in groups of animals and pathogen virulence is enhanced by animal-to-animal transmission.

Pasteurellae, Bibersteinia and Mannheimia species can be distinguished by colonial and growth characteristics and by biochemical reactions. Colonies of P. multocida are round, greyish, non-haemolytic and have a subtle characteristic odour. Colonies of M. haemolytica and B. trehalosi are haemolytic and odourless. Based on differences in their capsular polysaccharides, isolates of P. multocida are grouped into five serogroups. Twelve serotypes of M. haemolytica are recognized on the basis of extractable surface antigens. Four serotypes previously designated as T serotypes of M. haemolytica have been reclassified as B. trehalosi.

Many P. multocida infections are endogenous. The organisms may invade the tissues of immunosuppressed animals. Factors of importance in the development of disease include adhesion of the pasteurellae to the mucosa and avoidance of phagocytosis. Fimbriae may enhance mucosal attachment and the capsule, particularly in type A strains, has a major antiphagocytic role. PMT toxin, produced by serotypes A and D, is a cytotoxic protein which stimulates cell cytoskeletal rearrangements and is of importance in the pathogenesis of atrophic rhinitis in pigs. In septicaemic pasteurellosis, severe endotoxaemia and disseminated intravascular coagulation cause serious illness which can prove fatal. Four main virulence factors have been identified in strains of M. haemolytica and B. trehalosi: fimbriae, which may enhance colonization; a capsule, which enhances survival in serum; endotoxin, which can damage bovine leukocytes and endothelial cells; and leukotoxin, a pore-forming cytolysin that affects leukocyte and platelet function.

Although there are several species within the genera Pasteurella, Bibersteinia and Mannheimia, clinical infections in domestic animals are mainly attributed to P. multocida, M. haemolytica and B. trehalosi (Table 27.1). Pasteurella multocida has a wide host range, whereas M. haemolytica is largely restricted to ruminants, and B. trehalosi to sheep. The diseases associated with P. multocida infection include haemorrhagic septicaemia in ruminants, porcine atrophic rhinitis, fowl cholera and bovine pneumonic pasteurellosis. However, the main aetiological agent of bovine pneumonic pasteurellosis is M. haemolytica and this organism is also responsible for pneumonia in sheep and septicaemia in young lambs. Infection with B. trehalosi frequently results in septicaemia in older lambs.

Table 27.1 The major pathogenic Pasteurella, Bibersteinia and Mannheimia species, their principal hosts and associated diseases.

Hosts		Disease conditions
Pasteurella multocida
Type A	Cattle	Associated with bovine pneumonic pasteurellosis (shipping fever); associated with enzootic pneumonia complex of calves; mastitis (rare)
	Sheep	Pneumonia, mastitis
	Pigs	Pneumonia, atrophic rhinitis
	Poultry	Fowl cholera
	Rabbits	Snuffles
	Other animal species	Pneumonia following stress
Type B	Cattle, buffaloes	Haemorrhagic septicaemia (Asia)
Type D	Pigs	Atrophic rhinitis, pneumonia
Type E	Cattle, buffaloes	Haemorrhagic septicaemia (Africa)
Type F	Poultry, especially turkeys	Fowl cholera
Mannheimia haemolytica	Cattle	Bovine pneumonic pasteurellosis (shipping fever)
	Sheep	Septicaemia (under 3 months of age); pneumonia; gangrenous mastitis
Bibersteinia trehalosi	Sheep	Septicaemia (5–12 months of age)

Bovine pneumonic pasteurellosis (shipping fever) occurs most commonly in young animals within weeks of being subjected to severe stress such as transportation, assembly in feedlots and close confinement. The condition is associated with M. haemolytica, principally serotype A1, although recent surveys have demonstrated the increasing importance of serotype A6 in Europe and elsewhere. Several respiratory viruses including parainfluenzavirus 3, bovine herpesvirus 1 and bovine respiratory syncytial virus may predispose to bacterial invasion. Clinical signs include sudden onset of fever, depression, anorexia, tachypnoea and serous nasal discharge. In mixed infections, there is usually a marked cough and ocular discharge. At postmortem, the cranial lobes of the lungs are red, swollen and consolidated. Isolation of M. haemolytica, often in association with other pathogens, from bronchoalveolar lavage fluid or affected lung tissue is confirmatory.

Outbreaks of ovine pneumonic pasteurellosis are usually caused by M. haemolytica, a commensal of the upper respiratory tract in a proportion of healthy sheep. Predisposing factors are poorly understood and flock outbreaks usually start with sudden deaths of some sheep and acute respiratory distress in others.

Septicaemic pasteurellosis in animals between 5 and 12 months of age is usually associated with B. trehalosi infection. Bibersteinia trehalosi is found in the tonsillar tissues of carrier sheep. As with most other pasteurella infections, clinical disease may be precipitated by a range of predisposing factors including transportation.

Toxigenic strains of P. multocida type D or A cause a severe progressive form of atrophic rhinitis in pigs. These toxigenic P. multocida isolates are designated AR⁺ (atrophic rhinitis-positive) strains. Early signs, usually encountered in pigs between three and eight weeks of age, include excessive lacrimation, sneezing and, occasionally, epistaxis. As the disease progresses, a distinct lateral deviation of the snout may develop. Affected pigs are usually underweight and damage to the turbinate bones may predispose to secondary bacterial infections of the lower respiratory tract.

Vaccines have been developed for the control of many of the diseases caused by the pasteurellae and Mannheimia species. The efficacy of some vaccines remains to be determined.