PLEUROPNEUMONIA: APPROACH TO PREVENTION, DIAGNOSIS, AND TREATMENT IN LARGE ANIMALS

摘要

Bacterial infection of the lower respiratory tract is a common disease that affects all large animals. While extension of infection into the pleural cavity and resultant accumulation of pleural effusion are commonly observed in horses, bacterial respiratory disease typically results in bronchopneumonia in ruminants. The development of bacterial pneumonia in any large animal seldom occurs without a predisposing factor. Pleuropneumonia in horses is unusual without a history of long-distance transport.Bronchopneumonia in cattle similarly occurs most frequently in beef cattle after long-distance transport ("shipping fever") or in intensively-managed dairy calves ("enzootic calf pneumonia"). Management of pneumonia in any large animal can be a challenging undertaking in both field and hospital settings. Factors that contribute to this challenge include the frequently protracted course of therapy, cost of therapy, and the development of antimicrobial resistance leading to treatment failure.There are several species-specific features of the pulmonary anatomy that the large animal practitioner should consider. Most mammalian species exhibit interalveolar pores of Kohn, a microanatomical feature that allows air to travel between alveoli, preventing rupture hyperinflated alveoli to vent air into adjoining alveoli, thus protecting the lung from volutrauma. These pores of Kohn are virtually absent in cattle, however, and over-inflated alveoli burst during extreme respiratory effort leading to pulmonary emphysema, bullae, and pneumothorax that can escape the parietal pleura and cause subcutaneous gas accumulation leading to crepitus over the withers and back. Another difference seen within mammals are the presence of pulmonary intravascular macrophages (PIMS). These unique inflammatory cells are present in equids and ruminants but not in rats, humans, or dogs. PIMS confer an increased susceptibility to the effects of the Gram-negative cell wall toxin, endotoxin. Animal species without PIMs areable to tolerate high levels of both inhaled and hematogenous lipopolysaccaride without showing significant pulmonary vascular responses, inflammation or edema.In ruminants, viral respiratory pathogens (bovine herpesvirus, bovine viral diarrhea virus, bovine respiratory syncytial virus, parainfluenza type 3 virus) probably incite colonization of the lower respiratory tract with bacteria by impairing defense mechanisms such as the mucociliary escalator and by inducing immunosuppression. These bacterial pathogens, particularly Pasteurella multocida and Mannheimia haemolytica, are normal inhabitants of the upper respiratory tract. Bovine coronavirus and Mycoplasma bovis are two pathogens whose role in the development of BRDC is not quite clear. A recent study indicated, though, that nasopharyngeal carriage of M. bovis was associated with an increased odds of clinical signs, lung consolidation, and reduced average daily gain." Chronic bronchopneumonia typically results in anaerobic bacterial colonization by Trueperella pyogenes. Because equine pleuropneumonia also results from impairment of the mucociliary escalator, normal bacterial inhabitants of the upper respiratory tract, such as Streptococcus equi ssp. zooepidemicus, are the primary agents causing disease in initial stages of pneumonia. In later stages of disease, anaerobic bacterial colonization, most commonly by Bacteroides fragilis, occurs.