Bacterial Pneumonia in Cattle with 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. Under normal conditions, M haemolytica remains confined to the upper respiratory tract (in particular, to 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 bacteria.

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 mechanisms against environmental stressors or viral infections. During this logarithmic phase of the organism's growth in the lungs, virulence factors such as leukotoxin are elaborated by M haemolytica. The interaction between bacterial virulence factors and the host's defenses results in tissue damage with characteristic necrosis, thrombosis, and exudation, culminating as pneumonia.

Although less frequently cultured than M haemolytica, Pasteurella multocida is also an important cause of bacterial pneumonia in feedlot cattle suffering from BRD.

The pathogenesis of pneumonia due to P multocida is poorly understood. This organism is also a normal inhabitant of the nasopharynx and can opportunistically colonize lungs with chronically damaged respiratory defenses (eg, with enzootic calf pneumonia or existing lung lesions of feedlot cattle) and cause purulent bronchopneumonia.

Histophilus somni is also recognized as an important pathogen in enzootic pneumonia and in some cases of BRD. It may also cause outbreaks of myocarditis and pleuritis. H somni may invade the lungs 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 occurs later in the feeding period (40–60 days after arrival) from involvement of these additional organ systems.

Bibersteinia trehalosi can be present in the respiratory tract of domestic cattle and can be an opportunistic pathogen responsible for acute cases of BRD. Often, this bacterium causes sporadic outbreaks where, in spite of aggressive antimicrobial treatment, sudden death or high mortality rates predominate.

Mycoplasma bovis is an important cause of chronic pneumonia and arthritis in feedlot cattle. Although it is often considered a secondary pathogen, it can also be the primary cause of disease. It is able to colonize the upper respiratory tract without effect on young animals; however, when cattle experience stress from weaning, transport, and relocation to feedlots, M bovis can move to the lower respiratory tract. Often, this colonization, combined with coinfection with other bacterial and viral pathogens, leads to the onset of BRD.

When pulmonary abscessation occurs, generally in association with chronic pneumonia, M bovis is frequently isolated. Trueperella pyogenes can also be present in association with pulmonary abscessation.

Clinical signs of bacterial pneumonia are often preceded by clinical signs of viral infection of the respiratory tract. With the onset of bacterial pneumonia, clinical signs increase in severity and are characterized by listlessness and toxemia. Listlessness and fever (40–41°C [104–106°F]), without any clinical signs attributable to other body systems, are the classic components in early cases of BRD. Serous to mucopurulent nasal discharge; moist cough; and a rapid, shallow respiratory rate may be noted (see BRD with clinical signs image). Auscultation of the cranioventral lung field reveals increased bronchial sounds, crackles, and wheezes.

In severe cases, pleurisy may develop, characterized by irregular breathing and grunting on expiration. The animal will develop an unthrifty appearance and pulmonary abscesses if pneumonia becomes chronic.

Bovine respiratory disease, clinical signs, steer

Image

Courtesy of Dr. Eugene Janzen.

Lesions on Postmortem Examination

M haemolytica causes severe, acute, hemorrhagic fibrinonecrotic bronchopneumonia. Grossly, there are extensive reddish-black to grayish-brown consolidated regions cranioventrally in the lungs with gelatinous thickening of interlobular septa and fibrinous pleuritis. There are extensive thromboses, necrotic pulmonary foci, and limited evidence of bronchitis and bronchiolitis. (See bacterial bronchopneumonia image.)

P multocida is associated with milder fibrinous to fibrinopurulent bronchopneumonia. In contrast to M haemolytica, P multocida is associated with only small amounts of fibrin exudation, some thromboses, limited pulmonary necrosis, and suppurative bronchitis and bronchiolitis.

H somni infection of the lungs results in purulent bronchopneumonia that can be followed by septicemia and infection of multiple organs. H somni is associated with extensive fibrinous pleuritis in feedlot calves.

M bovis is associated with multifocal caseonecrotic bronchopneumonia. It is not uncommon for affected calves to also show otitis media and arthritis or tenosynovitis.

Bacterial bronchopneumonia, lesions, cow

Image

Courtesy of Dr. John Campbell.

Pulmonary abscessation can occur as pneumonia becomes chronic. Abscesses develop in approximately 3 weeks but do not become encapsulated until 4 weeks. T pyogenes is frequently cultured from these abscesses.

• Necropsy

• Bacterial culture

• Immunohistochemical testing

• PCR assay

In feedlot scenarios, diagnosis of specific bacterial pathogens associated with BRD often relies on gross necropsy findings and bacterial culture, immunohistochemical testing, 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 species of bacteria implicated in BRD, including M haemolytica.

• Broad-spectrum antimicrobials

Early recognition of bacterial infection, followed by prompt treatment with antimicrobials, is essential for successful treatment of BRD.

Substantial research has evaluated automated behavioral monitoring, which can encompass feeding behavior, activity monitoring, and spatial behavior; data from these studies may eventually provide advantages in selecting animals for early treatment. Automated behavioral monitoring systems are not widely used in commercial settings.

Treatment protocols should be established so the producer has a standardized approach to identifying and treating cases. Long-acting antimicrobials with label claims to treat BRD are commonly used as first- or second-line treatment options in feedlot calves and include the following:

• tulathromycin (in nonlactating cattle: 2.5 mg/kg, SC in the neck, once)

• gamithromycin (in beef and nonlactating cattle: 6 mg/kg, SC, once)

• tilmicosin (10–20 mg/kg, SC, as a single dose of not more than 10 mL per site)

• florfenicol (20 mg/kg, IM in the neck, repeated in 48 hours; or 40 mg/kg, SC, as a single dose of not more than 10 mL per site, in beef and nonlactating cattle [either dosage])

• enrofloxacin (2.5–5 mg/kg, SC, not more than 20 mL per site, every 24 hours for up to 5 days; or, 7.5–12.5 mg/kg, SC, as a single dose of not more than 20 mL per site)

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 are a beneficial ancillary treatment in controlling fever in cases of BRD; however, data are lacking on the effects on relapse and mortality outcomes.

If selection for treatment is late and pulmonary abscessation has occurred, resolution with antimicrobials is difficult to achieve; using a convalescent pen or culling the animal should be considered in these cases.

General principles for controlling bacterial pathogens associated with BRD are discussed under Enzootic Pneumonia of Calves. Vaccination against M haemolytica is efficacious for disease prevention and may decrease morbidity rates by as much as 25% in high-risk feedlot calves that receive 1 dose of vaccine on arrival; however, trials have not been consistent in all risk categories of feedlot cattle. Ideally, vaccines should be administered 3 weeks before transporting calves to the feedlot and can be repeated on arrival. In dairy calves, vaccination of the dam may be beneficial by providing passive immunity to the calf.

Vaccinations against H somni are available, and limited evidence suggests they are partially effective in controlling BRD in feedlot calves even when only 1 dose is administered on arrival. There is still a notable lack of clinical field trials evaluating the efficacy of H somni and P multocida components of vaccines. Commercial vaccines against M bovis are available, including killed and newer modified live bacterial vaccines. However, with M bovis, as with H somni and P multocida, there is a lack of clinical field trials demonstrating efficacy.

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 for respiratory disease prevention. However, preconditioning is often difficult to achieve 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 feedlot is a major control strategy in many feedlots with high-risk calves.

• Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, and Mycoplasma bovis are the major bacterial etiologies of bovine respiratory disease (BRD).

• Cultures of deep nasopharyngeal swabs from early cases that have not yet been treated can help to differentiate which pathogens are involved and may help guide treatment protocols.

• Early selection of cases, followed by treatment with broad-spectrum antimicrobials, is the primary treatment modality.

• Preconditioning, minimizing mixing, vaccination against BRD bacterial pathogens, and metaphylaxis for high-risk animals are the major control methods.

• Griffin D, Chengappa MM, Kuszak J, McVey DS. Bacterial pathogens of the bovine respiratory disease complex. Vet Clin North Am Food Anim Pract. 2010;26(2):381-394.

• Maunsell FP, Woolums AR, Francoz D, et al. Mycoplasma bovis infections in cattle. J Vet Intern Med. 2011;25:772-783.