Introduction

Bacterial pneumonia remains one of the most common clinical diagnoses in dogs with respiratory signs. There have been a few recent studies that may help us to change how we manage these cases.

This topic should remind us all about the importance of antibiotic stewardship.

Definitions

  • Pneumonitis is a general term that refers to inflammation of lung tissue. While pneumonia is infection of lung tissue. Technically, pneumonia is a type of pneumonitis because the infection causes inflammation. Pneumonitis, however, is usually used by human doctors to refer to non-infectious causes of lung inflammation.
  • Bacterial pneumonia implies sepsis of the lower airway and lungs.
  • Bronchopneumonia affects the lungs in patches around the bronchi or bronchioles., whereas interstitial pneumonia involves the areas in between the alveoli.
  • Bronchiectasis a chronic condition where the walls of the bronchi are thickened from inflammation and/or infection.

Causes of Bacterial Pneumonia

  • Aspiration
    • This involves aspirating either sterile but acidic gastric contents or septic material from either gastric or oral secretions. Acid irritation promotes a local environment in which bacterial colonisation can develop and lead to pneumonia. The right middle lung lobeis the most frequently affected, although the cranial lung lobes are also commonly implicated. Because of this location focal changes can be obscured by the cardiac silhouette in a lateral view.
  • Risk Factors:
    • Anaesthesia and sedation (incidence is 0.17% in a 2014 study with over 140,000 cases)
    • Oesophageal disease
    • Laryngeal disease
    • Refractory vomiting
    • Seizures
    • Neurological disease
    • Predisposed breeds (brachycephalic breeds, golden retrievers, cocker spaniels, English springer spaniels and Irish wolfhounds)
  • Foreign Body
    • These can carry mixed bacterial and/or fungal organisms into the lung and are associated with focal or lobar pneumonias. The most common site for lodging of an airway foreign body is in the right lung.
    • Chronic pulmonary foreign bodies are associated with marked inflammation that can lead to massive airway remodelling and bronchiectasis. These are often initially responsive to antimicrobials but relapse after discontinuation of therapy.
    • Large hunting or sporting breed dogs are most commonly affected breeds. Clinical history may include an acute onset of cough and/or respiratory difficulty after exposure to an outdoor activity, although some animals present with chronic cough or relapsing pneumonia. If an airway culture reveals Actinomyces, an airway foreign body should be high on the differential list.
  • Infectious or Community-Acquired
    • This pneumonia often begins with viral colonisation and infection of the upper respiratory tract. In a subset of dogs, inflammation associated with these organisms immobilises the hosts immune defences and predisposes to infection with other respiratory pathogens. This is most common in dogs housed in boarding facilities, shelters, kennels, and pet shops and especially puppies with an incompletely developed immune system. It can also be contracted from communal water bowls.
    • Because of the combination of pathogens this syndrome is called canine infectious respiratory disease complex (used to be called kennel cough).
    • This type of pneumonia can be found in all areas of the lung and is often diffuse.
    • Associated Viruses
      • Canine adenovirus
      • Canine parainfluenza
      • Canine herpesvirus
      • Canine pneumovirus
      • Canine respiratory coronavirus
      • Canine distemper virus
    • Associated Bacteria
      • Streptococcus spp.
      • Bordetella spp.
      • Mycoplasma spp.
  • Nosocomial
    • Defined as pneumonia that develops more than 48 hours after hospital admission in the absence of any signs of infection at the time of admission. In humans this can increase a patient’s hospital stay by more than a week and mortality by three-fold. It is also 20 times more likely to occur in ventilated patients than in nonventilated patients and can occur in up to one-third of patients requiring mechanical ventilation. While the stats are not as well known in veterinary patients, similar trends are suspected.
    • The ET tube acts as a conduit to transmit pathogens into the airways. Unfortunately, hospital acquired pneumonia is much more likely to involve multi-drug resistant bacteria. Because of this, it is vital to wear gloves when handling long-term intubated or ventilated patients. This type of pneumonia usually has a similar pattern to aspiration pneumonia with the right middle lung lobe and cranial lung lobes most commonly affected
  • Immune Dysfunction
    • Both the innate and adaptive immune systems protect against the development of infectious airway disease, and a breakdown in either increases the likelihood of opportunistic infection. This type of pneumonia can be found in all areas of the lung and is often diffuse.

Clinical Signs

Theses can vary greatly depending on the cause, severity, and chronicity of disease. They can be acute or peracute in onset or can show an insidious onset, resulting in chronic illness, particularly in animals with pre-existing chronic airway disease. Early in disease, mild signs such as an intermittent, soft cough might be the only evidence of disease. As infection spreads, clinical signs worsen and often include a refractory, productive cough. As you can see, fever is not a good indicator of the presence of pneumonia.

Diagnosis

Definitive diagnosis of bacterial pneumonia needs to be confirmed by presence of septic suppurative inflammation on airway cytology, along with a positive microbiology culture. Bronchoalveolar lavage is preferred because the trachea and carina are not sterile and the sensitivity for detecting cytologic features of sepsis is greater. If foreign bodies are suspected, then a BAL sample should be obtained from both the affected airway and an additional site. It’s worth nothing that animals with mycoplasma pneumonia can have positive culture in the absence of cytologic evidence of sepsis.

When only a tracheal wash specimen can be obtained, collection of a lower airway sample is desirable to identify infecting bacteria and to determine appropriate antimicrobial therapy through susceptibility testing. Oropharyngeal swabs are not suitable substitutes for making a diagnosis of pneumonia. Typically, aerobic and Mycoplasma culture and sensitivity are requested, and, in cases with markedly purulent secretions or a history of known aspiration or foreign bodies, anaerobic cultures should also be considered.

Thoracic radiography is a major component of diagnosis and differentiating causes. However, a major limitation is that radiographic changes lag behind clinical disease – therefore unremarkable radiographs do not rule out pneumonia. Radiographic evidence of bacterial pneumonia can appear as a focal, multifocal, or diffuse alveolar pattern, although early in the disease process infiltrates can be primarily interstitial Three-view thoracic radiographs should be obtained when screening because differential aeration associated with positional atelectasis can either mask or highlight pulmonary changes.

Other possible tests that support a diagnosis of pneumonia include haematology, pulmonary function testing, further imaging like CT, or thoracic ultrasound as well as a fine needle aspirate and cytology in areas with focal lung consolidation. Bronchoscopy can be useful in diagnosis and even treatment of foreign body pneumonia and can help to guide location of BAL sampling. It is vital that the scope is sterilised before use. A blood culture can be considered if the patient is too unstable for an airway wash – although veterinary data is lacking on whether this is truly indicative of pneumonia, it is known to be ineffective in children

Treatment including New Developments

A 2021 JSAP study through the Royal Veterinary College demonstrated successful management of 14 dogs without the use of antimicrobials at all. Theoretically in aspiration clinical disease might be primarily or solely chemical pneumonitis from aspirated material making antimicrobials unnecessary. Their hospital used the criteria of a systemically well patient with recent suspicion or documentation of an aspiration event; as well as a transient disease process such as oesophageal foreign body, intoxication or resolved vomiting, even if they were displaying mild to moderate respiratory clinical signs.

Another study suggests we should consider decreasing the length of treatment. Traditionally, antimicrobials have been administered for 3 to 6 weeks, and at least 1 to 2 weeks beyond the resolution of clinical and/or radiographic signs of disease, although there is no evidence to support this practice and it is dramatically longer than the recommendations for humans with similar disease. A 2017 study with 47 dogs showed similar radiographic and clinical cures in dogs treated with a short course of antibiotic (<14 days) compared with those that received a longer duration of treatment. A slightly older study from 2014 is just reiterating the importance of sensitivity testing as resistance to empirical antimicrobials was reported in up to 26 percent of 111 cases of airway lavage samples.

Empirical Antibiotic Choices

International Society for Companion Animal Infectious Diseases recommends waiting to administer antibiotics unless there is evidence of lethargy, anorexia, and fever along with cough in an otherwise healthy dog.

Doxycycline is the first line treatment for many pneumonias, despite it being bacteriostatic. This is because Amoxicillin/clavulanic acid may not achieve adequate levels in the airways and some Bordetella isolates and all mycoplasmas are resistant. If another bacteria is deemed to be more likely to be associated with the pneumonia (for example if a foreign body is implicated) then amoxicillin clavulanate can be used as a first line treatment.

If clinical findings in dogs or cats with pneumonia suggest the existence of sepsis the Working Group recommends concurrent parenteral administration of either enrofloxacin or marbofloxacin combined with a drug with Gram-positive and anaerobic spectra until bacterial culture and antimicrobial susceptibility testing results return. Common options include ampicillin or clindamycin.

Remember, as much as we don’t want to use antibiotics when unnecessary, it is important to administer antibiotics within the first 2 hours if signs of sepsis develop.

Additional Treatment Options

  • Oxygen when needed is vital for stabilisation of patients
  • Intravenous fluids are essential to facilitate clearance of respiratory exudates.
  • Nebulisation creates liquid particles that enter the lower airways and can help to resolve consolidation
  • Severe cases may require mechanical ventilation until disease is improved with low volume, high pressure. However, prognosis for pneumonia cases requiring ventilation is poor

Treatments that can be considered if appropriate

The benefits of coupage have not been completely evaluated but can be considered in appropriate patients, especially ones that are mobile and not oxygen dependant. Coupage should not be performed in animals with regurgitation because any increase in intrathoracic pressure could exacerbate regurgitation and subsequent reaspiration. A 2018 JVECC study also showed improvement using human physiotherapy techniques.

Administration of an oral mucolytic agent such as N-acetylcysteine can be useful for animals with retention of thick respiratory secretions.

The role of antacids in management of aspiration pneumonia remains controversial. By neutralizing the pH of gastric secretions, animals with refractory vomiting or regurgitation are less likely to succumb to chemical injury related to aspiration. However, the aspirant could be more likely to contain a greater concentration of bacteria that can colonize the airways and lead to bacterial pneumonia.

Treatments that are contraindicated

Furosemide and cough suppressants as these both act to trap secretions in the airways. Remember butorphanol is a cough suppressant as well, so should ideally not be given to these patients for sedation effects.

Discharge

General prognosis of patients with pneumonia is excellent, with 77-94 percent of patients being discharged from hospital. After discharge, it is still important to track the treatment response. Serum biomarkers such as acute phase proteins are associated with inflammatory disease. Being nonspecific, these biomarkers are not clinically useful for diagnosis but might be helpful in determining treatment response, facilitating better antimicrobial stewardship by suggesting resolution of disease more rapidly than thoracic radiographs. A 2017 JVIM Study in 17 dogs used CRP normalisation to guide the duration of antibiotic treatment and found that treatment duration was significantly decreased without increasing the number of relapses. Regardless of intended duration of an antibiotic course, re-evaluation within 10-14 days is recommended, and decisions to extend treatment should be based on clinical, haematological, and radiographic findings.

Key Takeaways

  • Wear gloves when handling long-term intubated or immunocompromised patients
  • Thoracic radiography is not the best indicator of disease, severity or treatment response
  • BALs with culture are important for diagnosis and to guide appropriate antibiotic choices
  • Not all cases of pneumonia need antibiotic treatment
  • Antibiotic course lengths can be decreased
  • CRP can be used to track treatment response and guide appropriate resolution