Pasteurellosis in sheep and goats

W DONACHIE AND M QUIRIE

GENERAL INTRODUCTION: FACULTATIVELY ANAEROBIC GRAM-NEGATIVE RODS Pasteurella and Mannheimia spp. infections

Introduction

Respiratory disease is common in sheep and goats and is usually the result of a series of complicated interactions involving stress inducers, both physical and physiological, and a variety of infectious agents. The most common form of respiratory disease is pasteurellosis. In lambs and kids affected with the acute disease the signs include fever, dyspnoea, anorexia, recumbence and sudden death. If the animals survive they can become chronically affected with resultant reduced ability to thrive and survive further infections. ² Pasteurellosis can therefore have a significant impact on the productive efficacy of small stock.

Although the main clinical form of pasteurellosis in sheep and goats is pneumonia there is also a systemic form found only in sheep. The pneumonic form is caused by Mannheimia haemolytica (formerly designated Pasteurella haemolytica¹ (see below for more details), and the systemic disease by Bibersteinia trehalosi. In sheep in temperate climates Pasteurella multocida rarely causes pneumonia and little is known of the epidemiology of that infection. Pasteurellosis caused by M. haemolytica is one of the most common bacterial infections of sheep and goats, and by far the most important respiratory one, with a widespread distribution, occurring in temperate, subtropical and tropical climates.^{7, 11, 15, 21}

Aetiology

Pneumonic pasteurellosis in sheep was first described in 1931⁶ but it was not until the 1960s that serotyping and biotyping helped to define the epidemiology of the disease. In sheep, two ‘Pasteurella’ species, M. haemolytica and P. trehalosi, share a common serotyping system comprising a total of 17 serotypes,^{1, 17, 20} with approximately 90 per cent of all isolates serotypable. Each of the two species, originally biotypes of Mannheimia haemolytica, is associated with a distinct clinical syndrome. M. haemolytica (formerly biotype A) strains are responsible for pneumonic pasteurellosis in sheep and goats of all ages while P. trehalosi strains (formerly biotype T) cause a systemic disease in six to ten-month-old lambs.⁸

In 1999 the taxonomy of the family Pasteurellaceae changed in response to new information on the relatedness of strains following studies on DNA and ribosomal RNA homology. ¹ The revised taxonomy introduced a new genus, Mannheimia, to replace Pasteurella haemolytica and P. haemolytica-like strains. The prototype species of the new genus is Mannheimia haemolytica, which includes all the former P. haemolytica A serotype strains apart from A11. Strains of this latter serotype are now placed in another Mannheimia species,M. glucosida.

The three species are divided into 17 serotypes on the basis of an indirect haemagglutination test, which depends on the serotypes having specific polysaccharide capsules. Serotypes 1, 2, 5, 6, 7, 8, 9, 12, 13, 14, 16 and 17 belong to M. haemolytica, A11 is now M. glucosida and serotypes 3, 4, 10 and 15 areP. trehalosi. The taxonomy of P. trehalosi remains unchanged. The main characteristics of all the new Mannheimia spp. are shown in Table 165.1 and a more detailed list of differences between Mannheimia and Pasteurella spp. is shown in Table 165.2.

A summary of the changes affecting M. haemolytica and P. trehalosi serotyping is shown in Table 165.3.

Mannheimia haemolytica and P. trehalosi are identical in their morphology both being encapsulated, small (1–2mm× 0,3–0,6 mm), Gram-negative, aerobic coccobacilli. In carbohydrate fermentation tests it was originally thought that strains of M. haemolytica fermented arabinose (hence the former biotype A designation) but not trehalose, whereas all P. trehalosi strains ferment trehalose (hence the former biotype T designation). Colonies of M. haemolytica are small and grey with a narrow zone of haemolysis after 24 hours’ incubation. The colonies of P. trehalosi strains are darker, larger (up to 3mm in diameter) and have brownish centres.

The differentiating characteristics of M. haemolytica, P. trehalosi and P. multocida species are shown in Table 165.4.

Table 165.1 Identification of species/taxa within the genus Mannheimia (From Angen et al.¹)

(Characters common to genus: urease −, mannitol +, mannose −, trehalose −)
+ positive, − negative, d...