M. haemolytica (formerly Pasteurella haemolytica Biotype A)

Of the pathogenic bacteria discussed herein, M. haemolytica is arguably the most important of the group and has traditionally been associated with the acute fulminating pleuropneumonia seen within the first weeks of cattle feeding. This Gram-negative bacterium has 12 representative capsular serotypes, but serotype 1 (S1) and S6 are the most prevalent in bovine pneumonia. In early life, calves acquire M. haemolytica through contact with their dams and other cattle. The bacterium resides in the nasopharynx and tonsils with the latter considered the major reservoir (Rice et al., Reference Rice, Carrasco-Medina, Hodgins and Shewen2007).

Virulence factors are many and include protein adhesins, capsular polysaccharide, lipopolysaccharide (LPS), iron-binding proteins, secreted enzymes and a ruminant-specific RTX toxin – leukotoxin (LKT). Specific adhesins include a glycoprotein, N-acetyl-d-glucosamine, that mediates adherence to tracheal epithelial cells and activates the oxidative burst of bovine neutrophils. Heat-modifiable outer membrane protein A (OmpA) and the surface lipoprotein 1 (Lpp1) mediate M. haemolytica binding to bronchial epithelial cells. In addition, the M. haemolytica capsule may function as an adhesin in addition to its antiphagocytic properties. Neuraminidase and sialoglycoprotease modify cell surfaces and may enhance bacterial adhesion. M. haemolytica LPS has typical endotoxic and proinflammatory properties, causes vasculitis, and complexes with LKT enhancing LKT-receptor production and augmenting LKT activity. LKT induces dose-related changes in bovine leukocytes. At high LKT concentrations, cells undergo rapid osmotic swelling, membrane pore formation and necrosis. At reduced doses, LKT can induce leukocyte apoptosis or release of proinflammatory cytokines, oxygen free radicals and cellular proteases. The sum result of these virulence factors is alveolar and vascular damage with pulmonary inflammation (Confer et al., Reference Confer, Clinkenbeard, Murphy, Donachie, Lainson and Hodgson1995; Czuprynski and Welch, Reference Czuprynski and Welch1995; Czuprynski et al., Reference Czuprynski, Leite, Sylte, Kuckleburg, Schultz, Inzana, Behling-Kelly and Corbeil2004; Rice et al., Reference Rice, Carrasco-Medina, Hodgins and Shewen2007).

M. haemolytica-induced pneumonia is characterized by acute cranioventral fibrinous to fibrinopurulent pleuropneumonia. On the cut surface, there is characteristic ‘marbling’ of the lung with lobules varying from normal, gray, or red. Whole lobules undergo hemorrhage or coagulation necrosis. Interlobular septa are usually distended with fibrin-rich edema fluid, and interlobular lymphatics may contain fibrin thrombi. Histologically, alveoli are flooded with fibrin-rich exudate, a variable neutrophil and macrophage infiltrate, and the traditional ‘oat cells’ that are flattened and streaming necrotic macrophages within the affected alveoli. Lobules undergoing coagulation necrosis appear as infarcts with thick neutrophilic infiltrates around the periphery of the necrotic lobule. Vasculitis with fibrin thrombi is frequently seen. Bronchioles often are spared or contain fibrin-rich exudate that may extend from alveoli (Caswell and Williams, Reference Caswell, Williams and Maxie2007).

P. multocida

P. multocida has been isolated from up to 40% of the cases of enzootic and shipping fever pneumonia (Welsh et al., Reference Welsh, Dye, Payton and Confer2004). This Gram-negative bacterium is acquired at an early age from the dam and is a common nasopharyngeal commensal. Serotype A:3 is the most common P. multocida isolate from bovine pneumonia, and pneumonia associated with P. multocida infections (whether in young dairy calves, in weaned and shipped beef cattle, or in calves experimentally challenged) is often difficult to discern from pneumonia associated with other bovine bacterial pathogens, i.e. M. haemolytica and H. somni. Dungworth (Reference Dungworth, Jubb, Kennedy and Palmer1985) indicated that ‘less fulminating …bronchopneumonias tend to be more often caused by P. multocida than by P. [Mannheimia] haemolytica’. In fact, isolation of more than one bacterial pathogen from pneumonic lungs in conjunction with respiratory viruses and/or Mycoplasma spp. occurs frequently (Gagea et al., Reference Gagea, Bateman, van Dreumel, McEwen, Carman, Archambault, Shanahan and Caswell2006b).

P. multocida A:3 virulence factors are less numerous than those identified in M. haemolytica. Several adhesins, a thick polysaccharide capsule and LPS are the major factors. Adhesins responsible for adherence of the bacterium to cell surfaces include type IV fimbriae, OmpA, neuraminidase and filamentous hemagglutinin (FHA); the latter is similar to FHA found in Bordetella spp. In addition, OmpA, various iron-binding proteins such as hemoglobin-binding protein A and transferrin-binding protein A bind fibronectin and other extracellular matrix proteins suggesting that they aid in invasion. P. multocida LPS induces classic signs of endotoxic shock and has biological and chemical properties and R-type LPS structure (with no polymeric O-antigen) similar to those found in many Gram-negative bacteria (Harper et al., Reference Harper, Boyce and Adler2006). P. multocida LPS is a potent stimulator of inflammatory cytokines and a predominant inciter of pulmonary inflammation. The importance of capsule as a virulence determinant in the pathogenesis of P. multocida infection is due to its antiphagocytic properties (Dabo et al., Reference Dabo, Taylor and Confer2007).

Various authors have described the P. multocida-associated gross pulmonary pathological changes differently, and those designations probably reflect (1) the age of the lesion, (2) whether or not other infectious agents were involved but not identified, and (3) descriptive preferences of the various authors. The lesion is a typical cranioventral bronchopneumonia and has been characterized simply as bronchopneumonia or as a bronchopneumonia with various descriptive modifiers including acute fibrinosuppurative, subacute to chronic fibrinopurulent, fibrinous to fibrinopurulent, suppurative and fibrino-necrotizing (Dabo et al., Reference Dabo, Taylor and Confer2007). The presence of fibrinous to fibrinopurulent pleuritis, distended interlobular septa with edema or fibrin, and/or abscesses is variable with P. multocida infection (Caswell and Williams, Reference Caswell, Williams and Maxie2007).

H. somni (formerly Haemophilus somnus)

H. somni, a Gram-negative bacterium, is associated with numerous pathological processes including pneumonia, septicemia, myocarditis, abortion, thrombotic meningoencephalomyelitis and synovitis. The bacterium is associated with enzootic or shipping fever pneumonia with epidemiological studies demonstrating marked variation in its prevalence compared to M. haemolytica or P. multocida in shipping fever pneumonia (Corbeil, Reference Corbeil2007).

H. somni are non-encapsulated, and the virulence factors include lipooligosaccharide (LOS) and various Omps – especially transferrin-binding proteins and immunoglobulin-binding proteins (IgBPs). Studies of adhesion proteins are generally lacking. Of particular interest with respect to virulence are LOS and IgBPs. LOS can mediate endothelial cell apoptosis, and through antigenic phase variation can assist the bacterium to escape the host immune response. IgBPs are surface-exposed fibrillar protein networks that bind the Fc domain of bovine IgG2 and are responsible for resistance to complement-mediated serum killing, a characteristic of pathogenic strains of H. somni (Corbeil, Reference Corbeil2007). Recently, studies demonstrated H. somni wild-type strain was cytotoxic for macrophages and inhibited phagocytosis of microspheres through disruption of the actin-filament structure. Conversely, cytotoxicity and phagocytosis inhibition were absent in an IgBPA – deficient mutant (Hoshinoo et al., Reference Hoshinoo, Sasaki, Tanaka, Corbeil and Tagawa2009). H. somni produces histamine, which in conjunction with anti-major Omp IgE, may account for early respiratory lesions (Corbeil, Reference Corbeil2007). Recently, biofilm production by H. somni within the host has been documented, and FHA proteins may be involved in that process (Sandal et al., Reference Sandal, Shao, Annadata, Apicella, Boye, Jensen, Saunders and Inzana2009).

Gross lesions of acute fulminating pneumonia due to H. somni are similar to those seen with M. haemolytica infection, i.e. a cranioventral fibrinous pleuropneumonia with hemorrhage and coagulation necrosis that can involve entire lobules. Pulmonary vasculitis is often seen, and those vessels may contain fibrin thrombi. In several studies, H. somni was isolated later in the feeding period than was M. haemolytica, and in H. somni cases, the pneumonic lesions included fibrosis, fibrous pleural adhesions and bronchiolitis obliterans (Caswell and Williams, Reference Caswell, Williams and Maxie2007).

M. bovis

This bacterium has been the subject of considerable investigation; however, its primary role in bovine bacterial pneumonia is controversial. Much of the controversy is because many healthy feedlot cattle shed M. bovis from the nasal passages with approximately 50% of cattle positive upon feedlot entry and nearly 100% positive by day 12 (Allen et al., Reference Allen, Viel, Bateman, Nagy, Røsendal and Shewen1992). In addition, M. bovis has been isolated from up to 45% of grossly and histologically normal bovine lungs (Gagea et al., Reference Gagea, Bateman, van Dreumel, McEwen, Carman, Archambault, Shanahan and Caswell2006b). The bacterium is predominately an extracellular pathogen that is present on respiratory epithelial surfaces. M. bovis antigen is present within macrophages of infected lung, and that antigen is most likely phagocytized debris (Caswell and Archambault, Reference Caswell and Archambault2007).

Virulence factors include variable surface proteins (VSPs) that function as adhesins, are responsible for phenotypic variation among M. bovis strains, and allow for evasion of host immune responses. Several other surface proteins have been demonstrated that assist in colonization. A polysaccharide toxin has been described; however, the role or even existence of that toxin remains controversial. M. bovis strains may produce hydrogen peroxide, which forms oxygen free radicals and causes host lipid peroxidation. Heat shock proteins are produced, but their role in virulence is not known. The formation of biofilm has been associated with numerous M. bovis strains, and that trait enhances immune and antimicrobial resistance and colonization (Caswell and Archambault, Reference Caswell and Archambault2007).

Transmission of M. bovis is through nasal shedding and fomites; however, direct contact with others is the most common route, because survival outside the host is limited. Lesions attributable to M. bovis are a cranioventral caseonecrotic bronchopneumonia that may have abscesses, bronchiectasis and sequestration (Gagea et al., Reference Gagea, Bateman, Shanahan, van Dreumel, McEwen, Carman, Archambault and Caswell2006a). Arthritis may accompany respiratory disease. The chronic nature of the pneumonia is apparent at the gross and microscopic levels. In addition, the highest percentages of M. bovis pneumonia are in cattle previously treated for BRD. Death often occurs after 4–6 weeks in the feedlot (Booker et al., Reference Booker, Abutarbush, Morley, Jim, Pittman, Schunicht, Perrett, Wildman, Fenton, Guichon and Janzen2008). Histologically, M. bovis antigen is present within and surrounding the caseous foci (Khodakaram-Tafti and López, Reference Khodakaram-Tafti and López2004).

A. pyogenes (formerly Actinomyces pyogenes)

A. pyogenes is a Gram-positive, rod-shaped bacterium, which is a common pathogen in bovine abscesses. In the lungs, A. pyogenes is associated with chronic abscessing pneumonia. A. pyogenes is an inhabitant of many mucosal surfaces in cattle including the nasopharynx and appears to be a secondary invader into a lung already pneumonic due to other infectious agents. Factors precipitating its proliferation and inhalation are not known; however, Catry et al. (Reference Catry, Croubels, Schwarz, Deprez, Cox, Kehrenberg, Opsomer, Decostere and Haesebrouck2008) recently demonstrated that fluoroquinolone administration to healthy cattle diminished upper respiratory P. multocida, whereas A. pyogenes became the predominant nasopharyngeal isolate. Therefore, it may survive and proliferate in cattle treated for BRD.

A. pyogenes has several important virulence factors. These include a collagen-binding protein (CbpA) that allows it to bind collagen and promote adhesion to host cells (Pietrocola et al., Reference Pietrocola, Valtulina, Rindi, Jost and Speziale2007). A cholesterol-dependent cytolysin (pyolysin) that is a pore-forming cytolysin for immune cells and is also a hemolysin has been characterized (Rudnick et al., Reference Rudnick, Jost and Billington2008). Several adhesins have been identified including two neuraminidases, which cleave sialic acids and expose cell receptors, and type 2 fimbrae. Several extracellular matrix-binding proteins that bind to collagen or fibronectin and exoenzymes (DNAse and proteases) assist in invasion of tissue and degradation of proteins and nucleic acids. In addition, A. pyogenes can evade host defenses by invasion of epithelium, intracellular survival in macrophages and formation of biofilm (Jost and Billington, Reference Jost and Billington2005).

Transmission of A. pyogenes occurs at a young age with calves acquiring the bacterium from their dams. Pulmonary lesions ascribed to A. pyogenes are primarily severe abscesses within areas of chronic bronchopneumonia or chronic pleuropneumonia (Lopez, Reference Lopez, McGavin and Zachary2007). These abscesses are typically characterized by liquefactive necrosis surrounded by a thick fibrous connective tissue band, whereas they are often larger and less caseous than M. bovis-induced lesions.

B. trehalosi (formerly Pasteurella trehalosi)

B. trehalosi was originally classified as P. haemolytica Biotype T and is primarily a sheep pathogen, especially associated with septicemia (Blackall et al., Reference Blackall, Bojesen, Christensen and Bisgaard2007). B. trehalosi has been associated with severe pneumonia in Bighorn sheep and domestic sheep (Goodwin-Ray et al., Reference Goodwin-Ray, Stevenson and Heuer2008). Most recently, B. trehalosi-associated pneumonia in young dairy calves has been described, though not documented in the refereed literature.