INTRODUCTION
The term ‘pasteurellosis’ refers to infection with bacteria of the genera Pasteurella and Mannheimia, which occurs in animals and humans. It has been also called ‘septicaemic pasteurellosis’, ‘haemorrhagic septicaemia’, or in birds ‘avian cholera’, ‘fowl cholera’ or ‘pasteurellosis of fowls or ducks’.
The classification of these pathogens is under continuous review. Infection with these bacterial species is frequently subclinical, and the bacteria can also act as opportunistic pathogens. Some species and strains of Pas- teurella and Mannheimia can also act as primary pathogens, and can occasionally cause epidemics of pneumonia or septicaemia, with significant associated mortality in domestic and, more occasionally, wild animal species. Pas- teurella spp. and Mannheimia spp. are of worldwide distribution, and most vertebrate species are susceptible to disease from one of these bacteria, which infect most mammalian families and a wide range of non-mammalian species.Pasteurella and Mannheimia are members of the order Pasteurellales, family Pasteurellaceae. They are small, pleomorphic Gram-negative rods or coccobacilli; they exhibit bipolar staining, are non-motile, facultatively anaerobic or micro- aerophilic, produce oxidase and/or alkaline phosphatase, ferment glucose without gas production and reduce nitrate to nitrite. These organisms are mucosal commensals of the oropharynx and gastrointestinal tract of healthy reptiles, birds and mammals and usually do not survive for a long time outside the host.
Pasteurella multocida is the most ubiquitous of these organisms and has been investigated since 1880 by Pasteur (after whom the name Pasteurella derives) as the causative agent of avian cholera or fowl cholera — despite the fact that, as the species name suggests (multocida — ‘killer of many’ ), it can infect many species.
Factors that allow commensal strains to become virulent are incompletely understood, and a disease outbreak could be the result of ‘activation’ of a commensal strain of the organism or the introduction of a new Pasteurella sp. strain into a given host population.