Pasteurellosis of Sheep and Goats: Introduction

Pasteurella and Mannheimia organisms are nonmotile, nonsporeforming, aerobic, fermentative, gram-negative coccobacilli. They are distributed worldwide, and diseases caused by them are common in sheep and goats of all ages. Mannheimia (Pasteurella) haemolytica and Pasteurella trehalosi (formerly P haemolytica biotype T) are the species most often associated with disease. These organisms are the primary agents involved in respiratory disease, septicemia, arthritis, meningitis, and mastitis and may also be important secondary invaders in respiratory diseases of ruminants. There are 12 serotypes of M haemolytica and 4 of P trehalosi currently recognized on the basis of capsular antigens. M haemolytica is most commonly associated with pneumonic pasteurellosis ( Pasteurella and Mannheimia Pneumonias), while P trehalosi mainly causes septicemia and systemic pasteurellosis (see below) in young, weaned sheep. The virulence of M haemolytica and P trehalosi is mediated by the action of several factors (including endotoxin, leukotoxin, and capsular polysaccharide) that afford the bacteria advantages over host immunity and are important in the pathogenesis of disease. Pneumonic pasteurellosis can also be caused primarily or secondarily by P multocida in sheep and goats, and outbreaks worldwide lead to high mortality and great economic loss. M haemolytica , P trehalosi , and P multocida are common commensal organisms of the tonsils and nasopharynx of healthy sheep and goats. Transition from infection to disease appears to be facilitated by various stressors, including concurrent infections; changes in climate, pasture, or feed; and other management factors.