Canine Infectious Respiratory Disease Complex (CIRDC)
Canine Infectious Respiratory Disease Complex (CIRDC), also colloquially known as kennel cough, is a highly contagious multifactorial disease characterized by acute or chronic inflammation of the trachea and bronchial airways. It is usually a mild, self-limiting disease but may progress to fatal bronchopneumonia in puppies or dogs infected with canine influenza virus. It is commonly seen where dogs are in close contact with each other, e.g. boarding kennels, doggie daycares or rescue centers, although CIRDC can also occur in dogs with no history of having been in such a situation. The disease can spread rapidly among susceptible dogs housed in close confinement and signs can persist for many weeks.
Etiology and Pathogenesis
Canine parainfluenza virus, canine adenovirus type 2 (CAV-2), canine influenza virus (CIV) H3N8 and H3N2, or canine distemper virus can be the primary or sole pathogen involved. More recently, canine respiratory coronavirus and canine pneumovirus have also been implicated in CIRDC. Bordetella bronchiseptica may act as a primary pathogen, especially in dogs less than 6 months of age; however, it and other bacteria (such as Streptococcus equi subsp zooepidemicus, Pseudomonas spp, Escherichia coli, and Klebsiella pneumoniae) may cause secondary infections after viral injury to the respiratory tract. Concurrent infections with several of these agents are common. The role of Mycoplasma spp has not been clearly established. Stress and extremes of ventilation, temperature, and humidity apparently increase susceptibility to, and severity of, the disease.
Clinical and Pathological Findings
The prominent clinical sign is paroxysms of harsh, dry coughing, which may be followed by retching and gagging. The cough is easily induced by gentle palpation of the larynx or trachea. Affected dogs may demonstrate few if any additional clinical signs except for partial anorexia. On auscultation, respiratory sounds may be essentially normal. In advanced cases, inspiratory crackles and expiratory wheezes are heard. Body temperature may be only slightly increased and white blood cell (WBC) counts usually remain normal. Development of more severe signs, including fever, purulent nasal discharge, depression, anorexia, and a productive cough, especially in puppies, indicates bronchopneumonia or a complicating systemic infection such as distemper or canine influenza, H3N8 or H3N2. Stress, particularly due to adverse environmental conditions and improper nutrition, may contribute to a relapse during convalescence.
During the acute and subacute inflammatory stages, the air passages are filled with frothy, serous, or mucopurulent exudate. In chronic bronchitis, they contain excessive viscid mucus. The epithelial linings are roughened and opaque, a result of diffuse fibrosis, edema, and mononuclear cell infiltration. There is hypertrophy and hyperplasia of the tracheobronchial mucous glands and goblet cells. The act of coughing is an attempt to remove the accumulations of mucus and exudate from the respiratory passages.
CIRDC should be suspected whenever the characteristic cough suddenly develops 5–10 days after exposure to other susceptible or affected dogs. Severity usually diminishes during the first 5 days, but the disease can persist for several weeks. The presumptive diagnosis of CIRDC is usually made from the history and clinical signs and by elimination of other causes of coughing. Confirmatory testing to identify the pathogen or pathogens involved employs the use of polymerase chain reaction (PCR) assays, or bacterial culture and viral isolation, which can often be performed on nasal swabs. In chronic bronchitis, chest radiographs may show an increase in linear and peribronchial markings. Bronchoscopy reveals inflamed epithelium and often mucopurulent mucus in the bronchi. In addition, the procedure allows collection of biopsy and swab samples for in vitro assay. Bronchial washing is an additional diagnostic aid that may demonstrate causative agents or significant cellular responses (eg, neutrophils, eosinophils).
Source: Merck Veterinary Manual modified