Pasteurella and Mannheimia spp. infections

Historically, the term pasteurellosis has been used to signify disease caused by bacteria of the genus Pasteurella.¹⁰ In the context of respiratory disease of cattle, the term pneumonic pasteurellosis has been used conveniently and extensively to embrace disease caused by Pasteurella multocida and/or Pasteurella haemolytica without specifying which agent is of greater importance. Although the recent reclassification of bacteria in the Pasteurella haemolytica complex into five named species in the new genus Mannheimia makes continued use of the term pasteurellosis awkward, an acceptable replacement term with the same clinical implications has not emerged.^{2, 13, 34, 36}

The family Pasteurellaceae is a large group of bacteria with phenotypic and genomic similarities and currently includes the genera Pasteurella, Actinobacillus, Haemophilus, Mannheimia and Lonepinella with other less well-defined groups.^{2, 3, 40} The pasteurellas and allied bacteria are Gram negative, chemo-organotrophic, facultatively anaerobic, fermentative bacteria liked by DNA affiliation. Despite these similarities, it is a heterogeneous group.^{3, 40, 48} DNA hybridization studies, in particular, have emphasized the heterogeneity of the species within the group.^{3, 40} Consequently, the taxonomic position of many of the species within the family Pasteurellaceae is the topic of debate: a debate that has prompted nomenclatural changes and reclassification of some species.³

The genus Pasteurella comprises a number of species, of which Pasteurella multocida, P. granulomatis, P. lymphangitidis, P. caballi and P. mairi are of significance in disease processes in domestic livestock.^{10, 50, 52, 60} Although P. multocida was finally accepted as the type species only as recently as 1985, its nomenclatural origin goes back to the 1800s.^{24, 40} The causative agent of a disease in poultry (subsequently referred to as fowl cholera) first described by Rivolta in 1877 and then by Revolee in 1879, was named Micrococcus gallicidus by Burrill in 1883. The microorganism underwent several name changes until 1887, when an Italian, Count Trevisan, named the genus Pasteurella to honour Louis Pasteur’s efforts in elucidating the aetiology of fowl cholera in turkeys.⁴⁰ In spite of this signal occurrence, inconsistency in its nomenclature prevailed. Kitt is credited with the introduction of the species epithet when the microorganism was designated Bacterium (bipolare) multocidum in his Bacterienkunde II Auflage, 1893.²⁴ In 1900, the name Bacterium (bipolare) multocidum was used synonymously with Bacterium bovisepticum by Migula²⁴ and Bacillus boviseptica by Lignieres³⁹ in their texts on bacterial systematics. The latter two names referred to the causative agent of haemorrhagic septicaemia in cattle.³⁹ The genus name of Pasteurella was apparently accepted thereafter, but isolates of the microorganism were named according to their origin: P. bovicida orP. boviseptica from cattle, for example.⁴⁸ The name P. septica, mooted by Topley and Wilson in 1929, was used until 1939, after which the combination P. multocida, proposed by Rosenbusch and Merchant, gained wide acceptance. ^{24, 40, 48}

Species with phenotypic similarities were either separated from the type species (Pasteurella haemolytica in 1932,⁴⁰ [reclassified as Mannheimia haemolytica in 1999³]) or added to the genus (Pasteurella pneumotropica in 1950, Pasteurella gallinarum in 1955, Pasteurella ureae in 1962, [since reclassified as Actinobacillus ureae⁷] and Pasteurella aerogenes in 1974, among others).⁴⁰ Several Pasteurella species of clinical significance have subsequently been added to this genus. Pasteurella lymphangitidis, a cause of lymphangitis in cattle in southern India,⁵⁸ has never been isolated in South Africa.²⁶ Pasteurella caballi causes primarily respiratory and uterine infections in horses. In South Africa it has been isolated from cases of uterine infections, abortions, abscesses and other purulent infections in horses.²⁶ Pasteurella mairi has been cultured from cases of abortion in sows and septicaemia in piglets and may cause infection in other animal species.⁵⁸ In South Africa the bacterium has been isolated from aborted pig foetuses and piglet septicaemia.²⁶ The different species may be distinguished by their biochemical characteristics. ^{2, 11}

Pasteurella multocida is a species that exhibits heterogeneity in several respects.⁴⁸ The microorganism measures 0,6–2,5 × 0,2–0,4 μm and is a non-motile, non-sporogenous, Gram-negative, encapsulated, facultatively anaerobic coccobacillus or short rod. Bipolar staining is evident in Giemsa- or Wright-stained preparations of young cultures.

Based on the passive haemagglutination test, five capsular serotypes: A, B, D, E and F, and 16 somatic types of P. multocida are recognized.⁴⁶ In addition, untypable isolates are also encountered. While not the only factor involved, the polysaccharide constituent of the capsule plays a role in adherence, colonization, invasiveness and virulence, and is therefore an important determinant of pathogenicity. Encapsulated strains of P. multocida belonging to serogroups A, D and F are not readily phygocytosed in non-immunized animals.⁴⁷ Other chemical determinants of pathogenicity identified to date include:

• the lipopolysaccharide (LPS) of the outer cell membrane, which has endotoxic activity—an activity indistinguishable from that of LPS derived from other...