Treating Bacterial Pneumonia in Dogs and Cats

Pneumonia is a common cause of pulmonary parenchymal disease in dogs and less so in cats. Pneumonia most commonly falls into the categories of aspiration or infectious bronchopneumonia. Aspiration pneumonia is caused by inhalation of orogastric secretions and/or ingesta containing enteric bacterial organisms. Infectious bronchopneumonia (also called community-acquired pneumonia) is caused by inhalation of pathogens spread by other animals. Less frequent causes of bacterial pneumonia include infections from hematogenous spread or contaminated inhaled foreign bodies.

Many comorbidities are associated with increased risk for certain types of pneumonia (TABLE 1). Dogs frequently exposed to other dogs are at a higher risk for infectious bronchopneumonia. A definitive diagnosis of bacterial pneumonia requires cytology and/or culture.

This article explores common questions regarding the treatment and management of bacterial pneumonia and focuses on aspiration pneumonia and infectious bronchopneumonia. The authors direct readers to other sources regarding the diagnosis of pneumonia as well as treatment of nonbacterial pneumonias (e.g., viral, protozoal, fungal), which are beyond the scope of this article.^1-3

How is an antimicrobial plan developed?

Antimicrobial selection is ideally based on culture and sensitivity testing of airway samples, particularly for patients that require hospitalization. Airway sampling with culture is most indicated for patients with suspected infectious pneumonia and patients at risk for infection with multidrug-resistant organisms (e.g., those with hospital-acquired infections, recent antibiotic receipt, or chronic pulmonary disease). Samples can be collected via tracheal wash (endotracheal or transtracheal) or by bronchoalveolar lavage (which requires bronchoscopy). Endotracheal wash requires the least specialized equipment and is most feasible in a general practice setting (FIGURE 1, BOX 1). Deep oral swabs do not usually yield the same sensitivity as tracheal washes.⁴ In addition, canine- and feline-specific respiratory polymerase chain reaction panels can detect common respiratory viruses and bacteria in nasopharyngeal and nasal swab samples. A retrospective study of airway sampling and culture from dogs with pneumonia found that for 57.4% of dogs that recently received antimicrobials, bacteria with resistance to empirically selected antimicrobials were isolated.⁵

Figure 1. An endotracheal wash being performed by using a red rubber catheter, Luer lock “Christmas tree” style adapter, and a 20-mL syringe.

While culture results are pending or if airway sampling cannot be performed due to patient, practitioner, or client limitations, empiric antimicrobial therapy should be initiated. Antimicrobial selection should be based on signalment, likely route of infection, and patient risk factors. For example, puppies exposed to other dogs are at particular risk for infectious bronchopneumonia caused by Bordetella bronchiseptica. For adult dogs, common bacterial isolates include Escherichia coli, Pasteurella species, Staphylococcus species, Streptococcus species, and enteric bacteria such as Klebsiella species or Enterococcus species.^6,7 Mycoplasma species are often present as opportunistic coinfections; their relevance as a primary pathogen is unclear but should be treated if cultured.⁸ Bacterial pneumonia does not develop in all patients after aspiration; aspiration of gastric acid or food material can lead to sterile pneumonitis. This distinction is difficult to ascertain without airway sampling, and potential infection should be treated if aspiration pneumonia is diagnosed, particularly if the patient is showing clinical signs.

Given that gram-positive and gram-negative bacteria commonly cause pneumonia, broad-spectrum coverage is indicated. De-escalation to a narrower antimicrobial spectrum is based on culture and sensitivity results when available, but further escalation may be needed if the patient worsens while culture results are pending. Doxycycline is a reasonable empiric choice for infectious bronchopneumonia as both B bronchiseptica and Mycoplasma species are generally susceptible.⁹ β-Lactam antibiotics (e.g., amoxicillin, cephalexin) are not recommended for treatment of B bronchiseptica due to poor penetration into the larger airways where B bronchiseptica tends to colonize. Instead, if the dog is clinically ill (e.g., fever, systemic clinical signs) with suspected infectious bronchopneumonia or aspiration pneumonia, broader-spectrum coverage should be started (TABLE 2).¹⁰ Hospitalized patients should initially receive parenteral antibiotics, and treatment should transition to oral administration after the patient clinically improves and is eating. Penetration through the pulmonary epithelium varies by antibiotic class, but inflammation can disrupt the blood–alveolar barrier and allow some drugs that would otherwise be excluded (e.g., penicillins, cephalosporins) to reach therapeutic concentrations in the alveoli.

In human medicine, the recommended treatment of uncomplicated pneumonia is a 5- to 7-day course of antimicrobials. Longer courses are considered for nosocomial infections, for Pseudomonas aeruginosa infections, or for patients that remain clinically unstable.¹¹ Veterinary medicine has historically recommended treatment with antibiotics for several weeks. However, recent evidence has found that shorter courses of antibiotics (7 to 10 days) may be acceptable for dogs with uncomplicated pneumonia, particularly aspiration pneumonia.^12,13

What other treatments should be considered?

Patients with moderate to severe pneumonia are often hypoxemic and require supplemental oxygen, which can be provided via nasal prongs or cannulae, an oxygen hood made from an Elizabethan collar (FIGURE 2), or an oxygen cage. Some patients may require additional support, such as high-flow nasal oxygen or mechanical ventilation.

Figure 2. An oxygen hood made of plastic wrap and an Elizabethan collar. The upper 25% of the collar opening has been left open to allow for warmer, exhaled carbon dioxide to escape.

For pneumonia patients sick enough to require hospitalization, fluid therapy is usually indicated, aimed at preventing dehydration and maintaining normal perfusion parameters. The mucociliary escalator, a vital component of the respiratory immune system, relies on proper hydration for normal function. Overzealous fluid administration should be avoided as it can lead to interstitial or pulmonary edema and worsened respiratory signs and hypoxemia.¹⁴ Nebulization with 0.9% saline can be used to hydrate respiratory secretions to promote mucus clearance, although this practice is not guided by any published clinical evidence.

Use of several other adjunctive therapies is supported by varying degrees of evidence. For humans, corticosteroid use may offer a mortality benefit for those with severe community-acquired pneumonia.¹⁵ For veterinary patients, the theoretical anti-inflammatory effects of steroids must be weighed against patient contraindications and risk for immunosuppression. Early mobilization may be somewhat beneficial for humans with pneumonia, and veterinary patients should be frequently taken for walks or have recumbency changed, if tolerated.¹⁶ Because the cough reflex is an essential means of clearing respiratory pathogens, administration of cough suppressants is contraindicated. Also contraindicated are furosemide or other diuretics because they can cause dehydration of the airway surface liquid layer and decrease systemic perfusion.

How does pneumonia differ for cats?

Bacterial pneumonia is less common in cats than in dogs but is probably underdiagnosed because noninvasive diagnostic testing is often not very sensitive for the disease. Cats with histologic evidence of pneumonia may not demonstrate hematologic or radiographic changes typical of canine pneumonia.¹⁷ The clinical presentations of inflammatory airway disease and infectious bronchopneumonia in cats (e.g., wheezing) substantially overlap.¹⁸ Bronchodilators such as terbutaline, albuterol, or aminophylline may be somewhat beneficial for cats by reducing bronchoconstriction. Airway sampling (e.g., by endotracheal wash) can help differentiate between causes of pulmonary parenchymal disease.

Cats with pneumonia should be tested for feline immunodeficiency virus and feline leukemia virus because both retroviral diseases can lead to immunocompromise and a predisposition to secondary infections.³ Common bacteria associated with pneumonia in cats include Pasteurella multocida, B bronchiseptica, and Mycoplasma species.¹⁹ Cats are prone to both viral and bacterial upper respiratory infections, but secondary pneumonia is not a common sequela.

Antimicrobial dosing and adverse effects differ between dogs and cats. In cats, enrofloxacin should be used only at doses of 5 mg/kg/day due to the risk for retinal degeneration with higher dosages. Enrofloxacin can also increase the serum concentration of the bronchodilators aminophylline and theophylline and lead to systemic signs of toxicity in both species.²⁰ When given without concurrent food or water, clindamycin and doxycycline both carry a risk for esophagitis and subsequent esophageal stricture formation. That risk is greater in cats than dogs, but it is possible in any animal.

What is the prognosis for dogs and cats with pneumonia?

The prognosis for treatment of bacterial and aspiration pneumonia is generally good. Reported survival rates for dogs range from 77% to 88% with appropriate treatment, and rates for cats (according to 1 study) are 89%.^6,21-23 Dogs that experience multiple episodes of pneumonia or have previously received antimicrobials may be at higher risk for infection with multidrug-resistant bacteria. Infectious bronchopneumonia caused by B bronchiseptica often responds well to appropriate antibiotic therapy. Regardless of the cause of pneumonia, complications (e.g., pulmonary abscesses, sepsis, acute respiratory distress syndrome) carry a guarded prognosis and should be considered for patients that do not respond to treatment. Vaccination for communicable diseases and a focus on early diagnosis and appropriate treatment optimizes outcomes. B bronchiseptica vaccination should be considered for all dogs at risk for exposure.

Summary

The cornerstone of bacterial pneumonia treatment is a targeted antimicrobial plan, ideally guided by cytology and/or airway sample culture and sensitivity testing and adequate supportive care. The rise of multidrug-resistant bacteria in humans and animals emphasizes the need to avoid unnecessary antibiotic use and excessive treatment durations.²⁴ If possible, an initial broad-spectrum approach followed by culture-guided de-escalation is recommended. This along with supportive care, including supplemental oxygen (if needed), fluids, and other adjunctive therapies, provides the best chance for a full recovery. Treatment or correction of underlying disease processes that led to the development of pneumonia should reduce the likelihood of future episodes of pneumonia.