Infectious diseases affecting the respiratory tract of the dog and cat include viral, bacterial, protozoal, and fungal. Upper respiratory tract infections are more common in the cat, whereas lower respiratory tract infections are more common in the dog.
Feline Upper Respiratory Infections
Several viral and bacterial entities comprise feline upper respiratory infections. These organisms are present in saliva, nasal and ocular discharges and are mainly spread by direct contact, via fomites and by aerosol inhalation and ingestion. Infections can be more severe if there is concurrent FeLV or FIV infection. Although infections are usually limited to the upper respiratory tract they may be complicated by bronchial infection or pneumonia. Infections are more severe in kittens and elderly cats.
Feline Viral Rhinotracheitis
Feline viral rhinotracheitis is an upper respiratory infection caused by feline herpesvirus-1 and characterized by sudden onset sneezing, fever, copious mucoid nasal discharge and lacrimation. Ocular disease can be severe, with keratitis, conjunctivitis and panophthalmitis. There may also be ulcers on the tongue and necrosis of the turbinates. In kittens mortality can be very high. This virus causes about one-half of respiratory disease in cats and many cats become latent carriers. In the latter, various stresses may trigger the excretion of the virus with recurrence of clinical disease.
Feline calicivirus infection is an upper respiratory disease characterized by fever, rhinitis, conjunctivitis, palatine and/or glossal ulcerations and nasal discharge. When bronchopneumonia develops, the mortality rate may exceed 30%. Lameness occurs following infection with some strains of calicivirus. Infected cats may become carriers and the virus can be shed continuously from the pharynx and tonsils for months and sometimes years.
Feline Pneumonitis (Feline Chlamydiosis)
Feline pneumonitis is caused by the bacterium Chlamydophila felis (previously Chlamydia psittaci) and characterized mainly by a chronic follicular conjunctivitis with an ocular discharge that may become purulent. The ocular form is seen most commonly in 5-12 week old kittens. Pneumonitis is an infrequent feature of the disease.
Mycoplasma felis causes an infrequent upper respiratory infection characterized by conjunctivitis and unilateral or bilateral rhinitis. The infection can resolve spontaneously in 2-4 weeks. There is some question as to whether M. felis has a primary or secondary role. M. gatae and M. feliminutum are occasionally recovered from the respiratory tract, but they are not considered to be pathogenic.
Bordetella bronchiseptica is an important cause, either primary or secondary to respiratory viruses, of upper respiratory tract infections in cats. It has also been implicated as an infrequent cause of pneumonia. Serious outbreaks have occurred in laboratory cats and in breeding colonies with bronchopneumonia and deaths. The disease is most severe in young cats.
Canine Infectious Tracheobronchitis
Infectious tracheobronchitis is a highly contagious, non-life threatening respiratory disease of dogs characterized by paroxysms of cough that usually persist for several days or rarely for several weeks. Viral infections damage the upper respiratory mucosa and pave the way for secondary infection with bacteria such as Bordetella bronchiseptica. A variety of other bacteria can become involved as end-stage organisms in canine infectious tracheobronchitis, e.g., Staphylococci, Streptococci, Pseudomonas and coliforms. Infectious tracheobronchitis is therefore a clinical syndrome with multiple and sometimes combined aetiologies. Clinically, the syndrome is defined as being mild, upper respiratory in nature, and self limiting. Infection with multiple agents results in more severe clinical signs, but is only life threatening when a virulent agent such as canine distemper virus is involved.
Canine Parainfluenzavirus (CPIV)
CPIV is frequently isolated from dogs with an acute dry cough. CPIV only affects the surface epithelium of the upper and lower respiratory tract and does not appear to persist in the dog. CPIV produces an acute inflammatory reaction in the upper and lower respiratory tract and regional lymph nodes. Clinical signs are mild and include an acute disease with coughing, tonsillitis, and nasal discharge. With secondary bacterial infection, severe disease may develop.
Type 2 Canine Adenovirus (CAV-2)
CAV 2 is commonly isolated from the more severe cases of infectious tracheobronchitis and usually occurs in unvaccinated dogs and in pups that have lost their maternal antibody protection. CAV 2 can induce a very mild disease, or can produce a fatal bronchopneumonia. Clinical signs usually include fever and lethargy, and a dry cough of tracheal origin persisting for 10 15 days. In contrast to CPIV, CAV 2 seems to persist for long periods of time. Like CPIV, CAV 2 can cause a severe tracheobronchitis with mycoplasmas and secondary bacterial infections.
Canine distemper virus can infect epithelial tissues throughout the body, resulting in signs due to respiratory, gastro-intestinal, neurologic, or ophthalmologic involvement. Respiratory system involvement is usually identified with severe disease, and bacterial pneumonia is a common complication. Mild transient signs are often mistaken for infectious tracheobronchitis.
In contrast to newborn pups in which this virus causes generalized disease, herpes virus infection in older dogs appears to be restricted to the upper respiratory tract causing mild disease. In comparison to CPIV and CAV 2, herpesvirus is often not associated with infectious tracheobronchitis. It does not spread as rapidly from dog to dog as CPIV or CAV 2 and infected dogs remain permanent carriers.
Reovirus infection lesions are mild and confined to the lungs which show exudation of macrophages into alveolar spaces and diffuse thickening of alveolar septa. After experimental inoculation dogs develop a mucoid nasal discharge, fever and sometimes a mild cough. Reovirus is not a significant etiological agent of infectious tracheobronchitis.
Bacterial pneumonia is common in the dog and relatively uncommon in the cat. It can be a primary disease or, more frequently as a complication to other pulmonary disease processes. Dogs with bacterial pneumonias should be thoroughly investigated for underlying disease, especially if cases are unresponsive to treatment. Primary bacterial pneumonia can occur due to B. bronchiseptica and Streptococcus zooepidemicus. Secondary bacterial pneumonias can involve both gram-negative organisms (E. coli, Klebsiella, Pasteurella multocida, B. bronchiseptica, Pseudomonas aeruginosa) and gram-positive cocci (Staphylococcus and Streptococcus spp). Mycoplasmas can also play a role but are usually secondary to bacterial infections. Clinical signs usually include fever, nasal discharge, dyspnoea, congested or cyanotic mucous membranes, and abnormal lung sounds (moist crackles).
This is an uncommon condition, occurring more frequently in cats, secondary to foreign bodies, chronic lung infection, penetrating wounds, and neoplasia. Clinically these cases manifest with chronic debilitating disease, coughing, variable respiratory distress, fever and leukocytosis. Diagnosis is based on history, clinical signs, and radiographic signs of focal increased lung density and/or pleural effusion. Initial treatment involves appropriate antibiotic therapy, chest irrigation and drainage. Only after concerted medical treatment has failed should a thoracotomy with partial or total lung lobectomy be considered.
Toxoplasma gondii or Pneumocystis carinii may on rare occasions be responsible for protozoal pneumonia in immunosuppressed animals. Animals with toxoplasmosis may show acute or chronic disease and often have multiple organ involvement (lung, liver, lymph nodes, muscles, CNS, uterus, eye) whereas those with pneumocystosis usually only present with chronic pneumonia. Pneumocystosis has a high prevalence in young miniature Dachshunds. Clinical signs, radiography, and haematology are of little diagnostic value. The diagnosis is confirmed on demonstration of organisms in tracheal wash or bronchoalveolar lavage fluid or lung aspirate cytology. Faecal examinations and serum antibody titres may be useful in the diagnosis of T. gondii. If possible, the underlying immunosuppressive state should be identified and corrected (distemper, Ehrlichia canis, FIP, FeLV, FIV, Cushing's disease, exogenous immunosuppressive agents).
Mycotic pneumonia can be caused by Histoplasma capsulatum, Blastomyces dermatitides, Coccidioides immitis, Aspergillus fumigatus, or Cryptococcus neoformans. Animals may be asymptomatic or show signs of severe lower respiratory disease including a diffuse miliary interstitial pattern and hilar lymphadenopathy. Cytological identification of organisms in macrophages on tracheal wash or bronchoalveolar lavage fluid or lung and peripheral lymph node cytology is the preferred method of diagnosis. Fungal culture and serology may be used as adjuncts in the diagnosis.
Granulomatous Pulmonary Disease
There are a number of diseases that produce an inflammatory interstitial process within the lung resulting in an accumulation of inflammatory cells within the pulmonary interstitium. The pulmonary parenchyma is predominantly infiltrated with mononuclear inflammatory cells (macrophages, lymphocytes and plasma cells). Lymphomatoid granulomatosis, eosinophilic granulomatosis, SLE, FIP, mycobacterial infections (Mycobacterium tuberculosis and M. bovis), foreign body reactions, mycotic infections, neoplasia, and external allergic alveolitis have all been reported to cause granulomatous reactions within the lung. A lung biopsy is often required to make the diagnosis.
A parapneumonic effusion is the accumulation of an uninfected effusion (modified transudate to exudate) that is associated with primary pulmonary infection and inflammation. This effusion clears with antibiotic therapy for the primary pulmonary disease and does not require chest drainage.
This is the accumulation of a purulent, often foul smelling, septic exudate within the pleural space as a result of bacterial or fungal infection. The routes of infection include penetrating thoracic wounds (especially bite wounds in cats), extension from bacterial pneumonia, migrating foreign bodies, oesophageal perforation (associated with mediastinitis) and haematogenous spread. Anaerobes and Nocardia asteroides are most frequently isolated in dogs, while anaerobes and Pasteurella multocida are the most common isolates in cats.
References are available upon request.