Pneumonic pasteurellosis of cattle

Pneumonic pasteurellosis of cattle

D C HODGINS AND P E SHEWEN

Introduction

Pneumonic pasteurellosis by definition refers to infection of the lung(s) with organisms of the genus Pasteurella.19 The term pneumonic pasteurellosis has served conveniently since the 1960s17, 138 as a means of speaking collectively of pneumonias caused by Pasteurella haemolytica and Pasteurella multocida without committing oneself as to the relative importance of the two agents. With the recent reclassification of P. haemolytica as Mannheimia haemolytica,7 the appropriateness of this term will need to be reconsidered.

Although these organisms can cause disease in young calves (as a component of enzootic pneumonia of beef and dairy calves5, 97, 170), pneumonic pasteurellosis is a far greater problem in recently weaned beef calves shortly after entry to feedlots (as a component of ‘shipping fever’ or undifferentiated bovine respiratory disease91, 120, 183). Economic losses to the North American feedlot industry due to respiratory disease have been estimated to be as high as one billion dollars annually,179 and losses to the beef industry in the UK have been estimated at 70 million pounds annually.128 Morbidity rates of 15 to 45 per cent and mortality rates of 1 to 5 per cent arecommonin newly received feedlot calves;94 it is estimated that 75 per cent of the morbidity79 and 50 to 60 per cent of the mortality78 are attributable to respiratory disease. The major bacterial pathogen involved in this respiratory disease is M. haemolytica; whereas P. multocida is less frequently involved. 119 In spite of continuing intensive research to develop efficacious vaccines and improved management programmes to prevent pneumonic pasteurellosis in feedlot cattle, success to date has been limited.

Aetiology

The importance of M. haemolytica and P. multocida in pneumonia of feedlot cattle was suspected in early studies because of their frequent isolation from clinical cases and necropsy specimens. Doubts arose, however, as to whether these agents were primary pathogens, due to difficulties in reproducing clinical disease and pulmonary lesions experimentally, and because bacterins did not mediate protection. The identification of numerous viral agents capable of inducing lesions in the bovine respiratory tract and/or of impairing clearance of bacteria from the lower respiratory tract, encouraged the view that infection with viral agents or perhaps Mycoplasma spp.64 was a necessary prelude to pneumonia with either species of Pasteurella. Bovine herpesvirus 1 (BHV-1) or infectious bovine rhinotracheitis virus,84 parainfluenza-3 (PI-3) virus,106 bovine respiratory syncytial virus,163 bovine virus diarrhoea virus,105, 130, 131 and more recently bovine coronavirus103, 159 have been implicated in bovine respiratory disease as predisposing to bacterial pneumonia, but viruses alone have not been considered capable of causing the fibrinous pneumonia that is characteristic of shipping fever. Challenge models employing primary infection with bovine herpesvirus or parainfluenza-3 virus followed by M. haemolytica or P. multocida have been used in studies of pathogenesis and vaccine efficacy.28,50, 80,81

The demonstration that fibrinous pneumonia could be induced in calves using logarithmic growth phase P. haemolytica alone6, 55, 56, 60, 126 if delivered to the lung (intratracheal or transthoracic administration) in sufficient numbers (>5 × 109 colony-forming units, in non-immune calves152) and the demonstration of an exotoxin (leukotoxin) specific for ruminant leukocytes30, 58, 93, 148, 149, 150 established M. haemolytica as a primary pathogen in shipping fever pneumonia. Research has focused on serotype A1 of M. haemolytica as the serotype most commonly isolated from cattle with pneumonia53, 177 in North America and South Africa,124 and the dominant serotype in the nasal passages of feedlot calves with acute respiratory disease.53

Although M. multocida has been shown to induce pneumonia when delivered to the lungs in large numbers,6 pulmonary lesions associated with this organism are usually not those of a fibrinous pneumonia;92, 138 fibrin and necrotic foci are usually absent, and suppurative lesions are more common.6 Pasteurella multocida has therefore been relegated to a secondary role in pneumonia of feedlot calves,35, 164, 183 although it may be relatively more important in disease of young dairy calves.119

Both M. haemolytica and P. multocida are Gram-negative, non-motile, non-spore-forming, fermentative, oxidase- positive, facultative anaerobic rods or coccobacilli.72, 135 Bipolar staining can be demonstrated using Wright’s stain. Both species grow best in media supplemented with serum or blood; M. haemolytica grows on MacConkey’s agar, but P. multocida does not. After 24 hours’ incubation on bovine or ovine blood agar plates, colonies of M. haemolytica are round and greyish, with a variable area of β haemolysis. Colonies ofP. multocida are generally round and greyish (some strains may be mucoid in appearance) and non-haemolytic.72, 135

Epidemiology

Epidemiological studies of pneumonic pasteurellosis in feedlot calves have been hampered by the difficulty of distinguishing, at clinical level, respiratory disease caused by Pasteurella spp. from that caused by other bacterial and viral agents. This...

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