Therapy for bacterial pneumonia (Proceedings)
Bacterial pneumonia encompasses a wide spectrum of disease from chronic to acute, unilobar or multilobar, and with clinical signs ranging from mild tachypnea or cough to rapidly progressive and fatal pulmonary infection. Cats are subject to bacterial pneumonia far less frequently than are dogs. It is important to understand that bacterial pneumonia is rare in otherwise healthy animals. Although there are a few primary bacterial respiratory pathogens (e.g., Bordetella bronchiseptica), most cases of pneumonia result from opportunistic infections. The animal is typically debilitated, immunosuppressed, or has a compromise to local protective mechanisms before contracting bacterial pneumonia. Reasonable efforts should be made to identify underlying predisposing factors (e.g., megaesophagus with aspiration) at the onset of treatment. These underlying conditions make treatment of bacterial pneumonia all the more challenging. Severe bacterial pneumonia results in clinical signs related to infection and clinical signs related to hypoxemia from ventilation-perfusion mismatching. Treatment must involve attempts to eradicate the causative bacterial agent plus supportive care.
Pathogens incriminated are often opportunistic, and include enteric pathogens (e.g., E. coli, Klebsiella), Pasteurella spp., coagulase-positive Staphylococci, Streptococci, and Mycoplasma spp.. Bordetella bronchiseptica is a very common cause of pneumonia in puppies. Transtracheal wash (or in cats and small dogs, transoral wash through an endotracheal tube) provides material for cytologic exam and culture prior to initiation of broad-spectrum antibiotics. Because antimicrobial treatment is often prolonged, it is important to actually identify the offending pathogen(s) and determine susceptibility. Overly broad, toxic, or expensive treatment can be avoided simply to performing a timely culture and sensitivity from the airways.
All bacterial pneumonia should be treated with antimicrobial drugs. Ideally, therapy should be based on culture and sensitivity results. Transtracheal wash can be performed with minimal or no sedation, and endotracheal wash requires only a very brief anesthetic episode. Antimicrobial drugs should be instituted before culture results become available, and then adjusted later. When considering antimicrobial choices for the respiratory tract the "blood bronchus barrier" must be considered as it effects penetration of systemic antibiotics into the airway lumen. However, severe inflammation that accompanies bacterial pneumonia may allow the penetration of drugs that otherwise would not reach adequate airway concentrations (e.g., beta lactams antibiotics). Initial spectrum of activity depends on the disease severity; the more severe the disease, the more aggressive should be the therapy. The organisms most commonly implicated in bacterial pneumonia include enteric pathogens (~50%) and anaerobes (20-25%); polymicrobial infections are common. To achieve broad spectrum coverage it is common to use combination therapy. The author typically begins with a fluoroquinolone combined with a beta-lactam, but other drugs or combinations are equally useful (e.g., imipenem-cilastin; beta lactams plus aminoglycosides or second generation cephalosporin; Ticarcillin). Parenteral administration is used as the initial delivery route for severely affected animals. Typically if the animal's condition has improved after several days an oral route of administration is adopted. Once antimicrobial susceptibilities have been determined, the antibiotic with the narrowest effective spectrum of action should be used. Duration of therapy is typically at least one week past an apparent radiographic cure, or a minimum of 3 weeks.Dogs with severe bacterial pneumonia are often hypoxemic. Ideally, PaO2 is determined via arterial blood gas, or alternatively, SpO2 is used as a rough correlate. Oxygen supplementation should be provided when PaO2 is <80 mmHg or the SpO2 is <94%. The most practical means of delivery is placement of a nasal cannula, or oxygen cages for cats or small dogs (cages use far more oxygen). Oxygen should be humidified prior to delivery to prevent drying of the airways with resultant impaired mucociliary clearance. The FIO2 should be kept at a minimum effective level since oxygen is itself toxic in high concentration over time. Ideally, a maximum of 40% FiO2 should be used. On occasion higher concentrations are required but should be used for less than 2 days if at all possible. For animals that remain markedly hypoxemic, fail to adequately eliminate CO2, or are threatened with respiratory exhaustion ventilator therapy may be the only option for continued care.