Lower respiratory infections in dogs (Proceedings)

Infectious Tracheobronchitis (Kennel Cough)

Infectious tracheobronchitis is a contagious respiratory disease of dogs caused primarily by Bordetella bronchiseptica, associated or not with other bacterial and viral agents. It is more common in young, debilitated animals housed in crowded conditions. Dogs with uncomplicated infections have a dry hacking cough in an otherwise normal animal. Presence of pneumonia, fever or anorexia is associated with complicated infections.  Chest radiographs are usually unremarkable in the uncomplicated disease and are useful to rule-out other conditions causing loud cough. Radiographic evidences of pneumonia can be found in dogs with complicated disease. Transtracheal wash should be performed in dogs with complicated disease to obtain material for cytology, culture (including Mycoplasma cultures) and sensitivity.

Uncomplicated disease is usually resolved within 14 days without therapy. Patients should, however, be isolated to decrease spreading of the disease. Patients with systemic signs, evidence of pneumonia or that have been sick for longer than 14 days should receive antibiotics. Antibiotics should be selected based on culture and sensitivity and the ability to achieve therapeutic concentration in the bronchial tree. Good empirical choices are tetracyclines and quinolones. Support therapy with nebulization and airway humidification, rest, proper nutrition and hydration should also be introduced

Chronic bronchitis in dogs

Bacterial infections probably don't play a significant role in most cases of canine chronic bronchitis. Antibiotics are traditionally used if there is a positive culture where a single organism is recovered without an enriched medium or if the patient has fever and systemic signs. Recent studies in humans have established that inflammation in the airways and parenchyma of the lung is an important component that may be central to the pathogenesis of chronic bronchitis.  Increased airway inflammation is present in acute exacerbations and resolves with treatment. There appears to be a clear association between neutrophilic inflammation and bacterial etiology. Thus, bacterial infections are an important factor in the exacerbation of chronic bronchitis in humans. It is not known if this can be extrapolated to dogs. Dogs with bronchiectasis are known to be at higher risk for infections. One major problem in patients with bronchiectasis is the biofilm formation from Pseudomonasaeruginosa infection. The biofilm growth mode may provide organisms with survival advantages in natural environments, increasing their virulence and decreasing their sensitivity to antibiotics.

Antibiotic selection in patients with chronic bronchitis should be based in culture and sensitivity results. The ability of an antibiotic to concentrate within the airways should be considered when selecting an antibiotic. Beta-lactams are not good choices because they have poor penetration into the airways (amoxicillin + clavulanate is an exception). Fluoroquinolones, tetracylines, clindamycin, metronidazole and azithromycin have good to excellent penetration into the airways. Macrolides (e.g.; azithromicin) may be a better choice in patients with bronchiectasis because they down-regulated proinflammatory cytokines and have unconventional effects on microorganisms, including inhibiting Pseudomonas twitching motility and thus biofilm formation.

Bacterial Pneumonia

Bacterial pneumonia is the inflammation developed in response to the presence of virulent bacteria in the pulmonary parenchyma. It is usually secondary to aspiration or systemic infection (hematogenous pneumonia). Affected dogs are predominantly young males of large breeds. Clinical signs in dogs with pneumonia can change from mild signs related to infection to severe depression and evidence of systemic inflammatory response syndrome. Some dogs may have purulent nasal discharge, dyspnea, and fever. Crackels may be heard on auscultation, especially in the cranio-ventral lung fields. Cough, if present, is usually soft.

In dogs, Bordetella bronchiseptica and Streptococcus zooepidemicus are primary pathogens leading to pneumonia. In most cases, however, the bacteria are opportunistic invaders. Gram-negative aerobes: Escherichia coli, Pasteurella multocida, Klebsiella pneumoniae, and Pseudomonas aeruginosa are most commonly isolated from dogs with pneumonia. Staphylococcus spp, Streptococcus spp and Mycoplasma spp can also be isolated from dogs with bacterial pneumonia. Anaerobes are seen in patients with pulmonary abscess.

In patients with aspiration pneumonia, radiographic examination usually reveals an alveolar pattern in the cranio-ventral lung fields or in the region of the right middle lung lobe. Perioperative aspiration may have a dorsal distribution if the animal was in dorsal recumbence during surgery. Hematogenously borne infections (e.g., secondary to IV catheters) may have a caudodorsal distribution, due to increased blood flow to these lobes. Cytology obtained by transtracheal wash may show a neutrophilic inflammation with degenerate neutrophils. Bacteria can be found in less than 50% of samples. Anaerobic and aerobic cultures as well as Mycoplasma culture are thus mandatory to identify the organisms and determine their antibiotic susceptibility. Specimens should be collected prior to the initiation of antibiotic therapy, or the ability to detect infection will be severely compromised.

Therapy of the stable patient (still eats, temperature < 104, no left shift) consists of antibiotics at home for 2 weeks, nutritional support and rest. Patients should be stimulated to cough. Pain medications that do not interfere with coughing reflex (e.g. carprofen) will help by decreasing the pain associated with cough in patients with pneumonia. Growth of more than one type of bacteria may occur in more than 40% of dogs with pneumonia, therefore antibiotic selection should be based on culture and sensitivity patterns Reasonable empiric antibiotic choices before obtaining culture results include amoxicillin+clavulanic acid, cephalexin or trimethropin+sulfonamide. Antibiotic choice should be reevaluated based on culture and sensitivity or if there is no improvement in 72 hours. A stable patient that got worse should be hospitalized and rehydrated. A new antibiotic should be selected based on culture and sensitivity.

The unstable patient should be hospitalized, kept hydrated and receive nutritional support and IV antibiotic therapy. Dogs with complicated pneumonia usually have an aerobe gram negative, especially E. coli. The first choice of antibiotics is cefazoline: 15-25 mg/kg q6-8h or ampicillin: 20 – 40 mg/kg q6-8h + enrofloxacin: 2.5 mg/kg q12h.  Ampicillin + enrofloxacin can also be used in complicated pneumonia. Usually 3 to 6 weeks of antibiotics are needed, and antibiotics should be continued for at least one week beyond radiographic resolution.

Parasitic pneumonias

Most parasitic pneumonias are asymptomatic, but some patients may show, cough, dyspnea and exercise intolerance. The most common agents are nematodes. Capillaria aerophilia, Oslerus osleri and Crenosoma vulpis live in the airways, whereas Filaroides hirthi, and Andersonstrongylus milksi live in the lung parenchyma. Paragonimus kellicotti, a trematode, also infects the lung parenchyma. Oslerus osleri causes cough and induces granulomatous lesions in the region of the carina. This nodules can be visualized bronchoscopically and occasionally on chest radiographs.  Filaroides hirthi and A. milksi cause a subclinical interstitial pneumonia that can be severe in immunocompromized patients. Dogs with clinical signs tend to have diffuse broncho-interstitial pattern and alveolar infiltrates. Paragominus kellicotti infection may be associated with acute dyspnea if a cyst ruptures and pneumothorax develops. Presence of of multiloculated pneumatocysts in the radiographs is compatible with paragominiasis in dogs. Diagnosis can be achieved by fecal examination using zinc flotation (O. osleri, F. hirthi, A. milksi. C. aerophila, C. vulpis) or Baermann technique (O. osleri, C. vulpis). Fenbendazole (50 mg/kg/14 days) is effective against all lungworms. Dying worms can exacerbate the inflammatory response and worsen the clinical signs in patients with F. hirthi and A. milksi.

Viral pneumonias

Viral pneumonias are usually part of a systemic disease presentation or infectious tracheobronchitis. The epitheliotropic phase of canine distemper virus infection is accompanied by an interstitial pneumonia that is usually complicated by secondary bacterial infection. Canine adenovirus 2 and canine parainfluenza viruses associated with canine infectious tracheobronchitis may also cause mild interstitial pneumonia. Clinically apparent disease generally occurs as a result of concurrent or secondary bacterial infection. The H3N8 influenza virus can cause mild to severe hemorrhagic pneumonia in dogs.  Again, secondary bacterial infections are an important component of the disease.

Protozoal pneumonias

Protozoal pneumonias are usually part of a systemic disease, but in a few occasions, the pulmonary signs may be predominate. Toxoplasmosis can cause interstitial pneumonia, but acute necrotizing pneumonia with alveolar wall and bronchial necrosis has been identified in dogs.  Small (< 5mm) nodules can be found in the pulmonary parenchyma. The organisms can be sometimes recovered in bronchoalveolar lavages or lung aspirates, but diagnosis is based on serology. IgM correlates better with clinical disease. The drug of choice for treating toxoplasmosis in dogs is clindamycin with clinical signs improving within 48 hours after starting therapy. Neospora caninum infection also can cause non-suppurative pneumonia in dogs. It is usually subclinical with preponderance of multifocal neurologic signs and signs of polymyositis.

Fungal Pneumonias

The most common organisms causing mycotic pneumonia have a defined geographic distribution. Blastomycosis occurs in the Mississippi and Ohio River Valleys, whereas histoplasmosis is more common in the Missouri, Mississippi, and Ohio River Valleys, and coccidioidomycosis in the Southwestern USA. Cryptococcosis can occur anywhere. Diagnosis of fungal pneumonia is based on recovery and identification of the organisms. Serology may be helpful. A reliable presumptive diagnosis can be made for coccidioidomycosis based on serology. A presumptive diagnosis can also be made for blastomycosis or cryptococcosis, but not for histoplamosis. Treatment consists of long term administration of systemic antifungal drugs, often using itraconazole or fluconazole as the drug of choice.

Pneumocystis carinii can cause fungal pneumonia in immunocompromized dogs. Most case reports are in long-haired red Dachshunds. Lung biopsy is the best way to obtain a diagnosis, but is not practical in most cases. In the majority of reported cases, diagnosis was based on finding the organism on transtracheal washes. Pneumocystis shares many characteristics with protozoans, including drug susceptibility. Trimethroprim and sulfonamides, and pentamidine are the agents of choice to treat pneumocystosis.

Table 1 Disease Pulmonary involvement Radiographs Other Systems Blastomycosis 85% of cases; interstitial pneumonia, pyogranulomatous, acute or chronic, usually diffuse; Hilar lymphadenopathy Interstitial infiltrate, nodular, miliary, diffuse; Hilar lymphadenopathy Skin; lymph nodes; eyes; bones; CNS; others Coccidiomycosis Granulomatous pneumonia, usually subclinical, acute or chronic; Hilar lymphadenopathy Diffuse interstitial; localized alveolar, miliary, nodular; Hilar lymphadenopathy Bone; skin; lymph nodes; others Histoplasmosis Granulomatous pneumonia, severe fulminant, chronic; pulmonary abscess; Hilar lymphadenopathy Interstitial infiltrates, diffuse linear, nodular, patchy coalescing; Hilar lymphadenopathy Liver; intestines; liver; others Cryptococcus Usually subclinical; 50% of dogs have lung lesions at necropsy Interstitial infiltrate, nodular Nose; CNS; skin; eyes Pneumocystosis 100% of cases, immunocomprimised dogs Interstitial to alveolar infiltrate, miliary; solitary cavitary lesions Spleen; liver