Bacterial pathogens associated with Bovine Respiratory Disease complex
Etiology of Bacterial Pneumonia in Bovine Respiratory Disease Complex
Mannheimia haemolytica serotype 1 is the bacterial pathogen most frequently isolated from the lungs of recently weaned feedlot cattle with bovine respiratory disease (BRD) and in dairy, beef or veal calves with enzootic pneumonia. Although less frequently cultured, Pasteurella multocida is also an important cause of bacterial pneumonia and recently has been found with increasing frequency relative to Mannheimia haemolytica in feedlot cattle suffering from BRD. Histophilus somni is also recognized as an important pathogen in enzootic pneumonia and in some cases of BRD. In addition, Histophilus somni is an important bacterial agent that may cause outbreaks of myocarditis and pleuritis. Bibersteinia trehalosi has been emerging as a major cause of cases of acute BRD in cattle.
These bacteria are all normal inhabitants of the nasopharynx of cattle ( see Histophilosis Histophilosis ). When pulmonary abscessation occurs, generally in association with chronic pneumonia, Mycoplasma bovis is frequently isolated and Trueperella pyogenes can also be found in association with pulmonary abscessation.
Under normal conditions, M haemolytica remains confined to the upper respiratory tract, in particular the tonsillar crypts, and is difficult to culture from healthy cattle. After stress or viral infection, the replication rate of M haemolytica in the upper respiratory tract increases rapidly, as does the likelihood of culturing the bacterium.
The increased bacterial growth rate in the upper respiratory tract, followed by inhalation and colonization of the lungs, may occur because of suppression of the host’s defense mechanism related to environmental stressors or viral infections. During this log phase of growth of the organism in the lungs, virulence factors such as leukotoxin are elaborated by M haemolytica. The interaction between the virulence factors of the bacteria and host defenses results in tissue damage with characteristic necrosis, thrombosis, and exudation, culminating as pneumonia.
The pathogenesis of pneumonia due to P multocida is poorly understood. This organism may opportunistically colonize lungs with chronically damaged respiratory defenses, such as occurs with enzootic calf pneumonia or existing lung lesions of feedlot cattle, and cause a purulent bronchopneumonia. H somni may also invade the lung and cause pneumonia after damage to the respiratory defenses. This organism is capable of systemic transmission from the lung to the brain, myocardium, synovium, and pleural and pericardial surfaces; often, death can occur later in the feeding period (40–60 days after arrival) from involvement of these additional organ systems.
Clinical Findings of Bacterial Pathogens Associated With Bovine Respiratory Disease
Clinical signs of bacterial pneumonia are often preceded by signs of viral infection of the respiratory tract. With the onset of bacterial pneumonia, clinical signs increase in severity and are characterized by depression and toxemia. A combination of clinical signs of depression and fever (40°–41°C [104°–106°F]), without any signs attributable to other body systems, are the classic components of a case definition for early cases of BRD. Serous to mucopurulent nasal discharge; moist cough; and a rapid, shallow respiratory rate may be noted. Auscultation of the cranioventral lung field reveals increased bronchial sounds, crackles, and wheezes. In severe cases, pleurisy may develop, characterized by an irregular breathing pattern and grunting on expiration. The animal will become unthrifty in appearance if the pneumonia becomes chronic, which is usually associated with formation of pulmonary abscesses.
Lesions on Post-Mortem Examination
M haemolytica causes a severe, acute, hemorrhagic fibrinonecrotic pneumonia. The pneumonia has a bronchopneumonic pattern. Grossly, there are extensive reddish-black to grayish-brown cranioventral regions of consolidation with gelatinous thickening of interlobular septa and fibrinous pleuritis. There are extensive thromboses, foci of lung necrosis, and limited evidence of bronchitis and bronchiolitis.
P multocida is associated with a less fulminating fibrinous to fibrinopurulent bronchopneumonia. In contrast to M haemolytica, P multocida is associated with only small amounts of fibrin exudation, some thromboses, limited lung necrosis, and suppurative bronchitis and bronchiolitis.
H somni infection of the lungs results in purulent bronchopneumonia that may be followed by septicemia and infection of multiple organs. H somni is associated with extensive fibrinous pleuritis in feedlot calves.
Pulmonary abscessation can occur as the pneumonia becomes chronic. Abscesses develop in ~3 weeks but do not become encapsulated until 4 weeks. T pyogenes is frequently cultured from these abscesses.
Unlike the preceding pathogens, M bovis is often associated with not only chronic pneumonia, but also infectious polyarthritis.
Diagnosis of Bacterial Pathogens Associated With Bovine Respiratory Disease
In feedlot scenarios, diagnosis of specific bacterial pathogens associated with bovine respiratory disease often relies on gross necropsy findings and bacterial culture, immunohistochemistry or PCR assay. Because the bacteria involved are normal inhabitants of the upper respiratory tract, the specificity of culture can be increased by collecting antemortem specimens from the lower respiratory tract by deep nasopharyngeal swab, transtracheal wash, or bronchoalveolar lavage. Lung specimens can be collected for culture at necropsy. If possible, specimens for culture should be collected from animals that have not been treated with antimicrobials to permit determination of antimicrobial sensitivity patterns. A multiplex PCR assay has been used to identify a number of bacterial agents implicated in bovine respiratory disease, including M haemolytica.
Treatment of Bacterial Infection Associated With Bovine Respiratory Disease
Early recognition of bacterial infection followed by prompt treatment with antimicrobials is essential for successful treatment of BRD. There has been substantial research evaluating automated behavioral monitoring, which utilizes feeding behavior, activity monitoring, or spatial behavior; this data may eventually provide advantages in selecting animals for early treatment. These systems are still not widely employed in commercial settings. Treatment protocols should be established so the producer has a standardized approach to identifying and treating cases. Long-acting antimicrobials such as tulathromycin, gamithromycin, tilmicosin, florfenicol, and enrofloxacin have label claims to treat BRD and are commonly used as first- or second-line treatment options in feedlot calves. Because they generally require only 1 administration, these long-acting antimicrobials allow the feedlot producer to return treated animals directly to the home pen, rather than having to maintain sick animals in a separate hospital pen. NSAIDs have been shown to be a beneficial ancillary treatment in controlling fever in cases of BRD; however, data are lacking in terms of effect on relapse and mortality outcomes.
If selection for treatment is late and pulmonary abscessation has occurred, it is difficult to achieve resolution with antimicrobials; use of a convalescent pen or culling of the animal should be considered in these cases.
Control of Bacterial Infection Associated With Bovine Respiratory Disease
General principles of control of bacterial pathogens associated with BRD are discussed under Enzootic Pneumonia of Calves Enzootic Pneumonia of Calves Enzootic pneumonia of calves is the most common respiratory disease seen in young dairy, veal, or beef calves. The disease is caused by a variety of viral and bacterial pathogens and environmental... read more . Mannheimia haemolytica vaccines have been demonstrated to be efficacious for disease prevention and may decrease morbidity in high-risk feedlot calves given one dose of vaccine on arrival by as much as 25%; however, trials have not been consistent in all risk categories of feedlot cattle. Ideally, vaccination should be done 3 weeks before transport to the feedlot and can be repeated on arrival. In dairy calves, vaccination of the dam may be of benefit by providing passive immunity to the calf.
H somni vaccines are available, and there is limited evidence that they are partially effective in control of BRD in feedlot calves even when only one dose is given on arrival. There is still a notable lack of clinical field trials evaluating the efficacy of H somni and P multocida components of vaccines. Management procedures such as preconditioning, which separates the stress of weaning from arrival at the feedlot and also includes prevaccination and adjustment to bunk feeding, can provide clear benefits in terms of respiratory disease prevention; however, it is difficult to achieve in many situations, because of the economic structure of the beef industry in many parts of North America. As a result, metaphylaxis in high-risk calves on or near arrival in the feedyard is a major control strategy in many feedyards with high risk-calves.
Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, and Mycoplasma bovis are the major bacterial etiologies of bovine respiratory disease.
Deep nasopharyngeal swabbing from early cases that have not been treated can help to differentiate which pathogens are involved and may help guide treatment protocols.
Early selection of cases and treatment with broad-spectrum antimicrobials is the primary treatment modality.
Preconditioning, minimizing mixing, vaccination for BRD bacterial pathogens, and metaphylaxis for high-risk animals are the major control methods.