Chapter 163 Respiratory Infections

The respiratory system is a common portal of entry for infectious agents of all varieties, with many of these infectious agents have been identified as respiratory pathogens (Table 163-1). The purpose of this chapter is to emphasize clinical aspects of common respiratory infections in dogs and cats. For details concerning specific infectious diseases, the reader is referred to Chapter 11 for respiratory virus and chlamydia infections of the cat, Chapter 12 for bordetellosis and viral tracheobronchitis of the dog, Chapter 13 for canine distemper, Chapter 20 for the systemic mycoses, and Chapter 21 for toxoplasmosis. Diagnostic procedures relevant for respiratory infections are summarized in Table 163-2 and are discussed in Chapters 158 and 159. Management of noninfective bronchopulmonary diseases and of chronic bronchitis in the dog and cat are described in Chapter 162. Management of pleural infections is described in Chapter 164.

• The nasal cavity, nasopharynx, larynx, and the trachea are normally inhabited by a variety of microorganisms. Aspirated bacteria from the upper respiratory tract intermittently populate the tracheal-bronchial tree. Thus the potential for secondary opportunistic infection is high when underlying pulmonary injury is present or when the host is immunologically compromised. In addition, the clinician can expect to culture microorganisms such as Pasteurella spp. from the upper airways when performing diagnostic studies. These results must be interpreted with care.

• Many infectious agents are primary pathogens of the respiratory epithelium, pulmonary interstitium, or pleural space. This is particularly true of upper respiratory infections caused by viral agents or bacteria like Bordetella bronchiseptica.

• The respiratory system also may be involved in multisystemic infections. This is common with the systemic mycoses, Blastomyces dermatitidis and Histoplasma capsulatum.

• Certain infectious agents, such as Bartonella spp., are emerging as potential causes of respiratory infection at multiple levels. Some of these agents are intracellular, others very difficult to culture. Diagnosis of these infections relies increasingly on molecular laboratory methods.

• It is helpful to identify the principal anatomical site(s) of infection, as many infectious agents produce a characteristic pattern of respiratory disease. This approach may help the clinician in the differential diagnosis and in directing appropriate laboratory studies. Good examples of localized respiratory disease are the viral upper respiratory infections of cats and canine infectious tracheobronchitis. Because clinicians generally speak in terms of the anatomic diagnosis—rhinitis, sinusitis, laryngitis, tracheitis, bronchitis, pneumonia, pleuritis, or mediastinitis—respiratory infections will be discussed using this classification (see Tables 163-1 and 163-2).

Table 163-1 CLASSIFICATION OF COMMON RESPIRATORY INFECTIONS BY ANATOMIC LOCATION

Tracheitis

Upper respiratory virus (see above)

Bordetella bronchiseptica [C]^‡

Osleri (Filaroides) osleri [C]

Bronchopulmonary Infections ^§

Some upper respiratory viruses (see above)

Canine influenza virus

FIV infection—can cause alveolitis in cats

Bacteria bronchopneumonia—both gram-positive and gram-negative infections

Bordetella bronchiseptica [C, F]

Gram-negative bacteria: Escherichia coli, Klebsiella spp., Pseudomonas spp., Pasteurella spp.

Gram-positive bacteria: Streptococcus spp., Staphylococcus spp.

Mycoplasma spp. [F > C]

Parasitic infections [F, C]

Aelurostrongylus abstrusus [F]

Paragonimus kellicotti [C, F]

Capillaria aerophilia [C, F]

Osleri (Filaroides) infection (F. milksi, F. hirthi) [C]

Aspergillosis (endobronchial infection)

Viral infections: feline calicivirus; canine distemper virus

Chlamydia psittaci [F]

Rickettsial infections (Ehrlichia canis; Rocky Mountain spotted fever) [C >> F]

Toxoplasma gondii [F > C]

Hematogenous bacterial infection

Nocardia spp., Actinomyces, anaerobes ^¶

Leishmania donovani [C]

Systemic mycoses: Histoplasma encapsulatum, Blastomyces dermatitidis, Coccidioides immitis, Cryptococcus neoformans [C > F]

Response to parasitic infection: migrating nematodes [C]

Dirofilaria immitis [C > F]

Lungworms and flukes [F, C]

*This list is not comprehensive; important clinical conditions are indicated with the most commonly affected species designated as [C] = canine; [F] = feline.

^† Infection commonly extends to the larynx and trachea.

^‡ Infection commonly extends to the bronchi and may cause pneumonia.

^§ Alveolar involvement may develop in some cases of interstitial pneumonia.

^¶ Pulmonary abscess, pleuropneumonia, and pyothorax may develop with these agents.

Table 163-2 DIAGNOSIS OF RESPIRATORY INFECTIONS

Nasal Cavity and Paranasal Sinuses

Signalment (age, breed, sex), vaccination status, history

Physical examination (emphasis: head, eyes, nose, oral cavity, regional lymph nodes, skin)

Auscultation

Serologic tests (aspergillosis, FeLV, FIV, and cryptococcus)

Skull, dental, and nasal radiographs

CT or MRI of the nasal cavity and paranasal sinuses

Examination of the teeth, oral cavity, oropharynx, and tonsils under anesthesia (inspection, palpation, examination with dental probes and mirrors)

Rhinoscopy (nasal cavity and retroflex rhinoscopy of the nasopharynx and posterior choanae)

Nasal culture (bacterial, fungal)

Aspiration biopsy/cytology (swab or nasal flush) of nasal exudate

Mucosal biopsy of the nasal cavity

Fine-needle aspiration biopsy/cytology of enlarged regional lymph nodes or mass lesions

Surgical exploration of the nasal cavity and paranasal sinuses for culture, biopsy, and debridement

Virology: Immunocytologic identification of viruses (e.g., immunofluorescence for canine distemper, feline herpes) and virus isolation (e.g., feline calicivirus)

Larynx

Ultrasonography of larynx

Visual examination of the larynx during light anesthesia

Radiography

Trachea

Radiography (cervical and thoracic)

Fluoroscopy

Tracheoscopy (endoscopy)

Cytologic examination of tracheal lesions (brush cytology of the tracheal mucosa)

Bronchopulmonary Diseases

History and physical examination (observation, auscultation, percussion of the thorax)

Thoracic radiography

Complete blood count

Heartworm (HW) tests (enzyme-linked immunosorbent assay [ELISA] antigen tests for dogs and cats, microfilaria tests for dogs, HW antibody test for cats)

Fecal examinations (flotation and Baermann sedimentation to detect lung parasites)

Serologic testing

(e.g., immunodiffusion or other tests for systemic mycoses, IgM ELISA for toxoplasmosis)

Arterial blood gas

Pulse oximetry

Bronchoscopy

Culture of tracheobronchial secretions

(transtracheal wash, endotracheal approach using a guarded culture swab, or aspiration through sterile tubing advanced through a sterilized endoscopic port)

Cytologic examination of the bronchi or lower airways

Transtracheal wash of tracheobronchial secretions

Endotracheal aspiration cytology

Via a catheter placed in the trachea or bronchial tree

Via an endotracheal tube (method sometimes used in cats and very small dogs)

Brush cytology of the bronchial mucosa

Bronchial aspiration cytology (selective, via a bronchoscope catheter)

Bronchoalveolar lavage with a wedged bronchoscope

Fine-needle aspiration (FNA) of the lung or a mass lesion

Lung biopsy

Pulmonary function testing

Key Point

Because many respiratory infections are secondary to another condition, consider the possibility of underlying respiratory disease or the possibility of host immunosuppression.

• The subject of infectious disease is extensive. For more detailed information on specific infections, the reader is directed to other sources such as Greene’s Infectious Diseases of the Dog and Cat.

RHINITIS AND SINUSITIS OF INFECTIOUS ETIOLOGY

Infections of the nasal cavity and paranasal sinuses are common, and clinical signs may also involve the nasopharynx. Acute rhinitis is frequently caused by viral and bacterial pathogens that directly invade the nasal mucosa. Chronic rhinitis often can be traced to another predisposing problem such as immunosuppression, foreign body, or tumor. Because the nasal cavity is not sterile, standard culture and sensitivity tests are of little value in diagnosis and management of these diseases.

Etiology

• Infections of the nasal cavity and nasal sinuses are not uncommon, especially those associated with acute, contagious viral infections in dogs and cats. Etiologic agents are summarized in Table 163-1.

History

• Cardinal signs of upper airway infection are sneezing, nasal discharge, and often a dry cough. Involvement of or discharge into the nasopharynx may cause reversed sneezing or gagging.

• Contagion is an issue with some causes of upper respiratory infections, especially with agents associated with the feline upper respiratory infections (see Chapter 11).

• Acute viral rhinitis is usually self-limiting within 1 to 2 weeks.

• Persistence of viral or bacterial rhinitis or sinusitis can occur in immunocompromised cats infected with feline leukemia virus (FeLV) or feline immunodeficiency virus (FIV).

• In addition, feline herpesvirus persists in a latent subclinical carrier form in recovered cats, and this occasionally recrudesces to result in episodic or persistent rhinitis (or conjunctivitis).

• Primary bacterial rhinitis and sinusitis are uncommon (except with Bordetella bronchiseptica); however, secondary bacterial infection can develop as a sequel to mucosal injury or obstructed nasal passages caused by upper respiratory viral infection, foreign body, periodontal disease and root abscesses, oral-nasal fistula or cleft palate, trauma (sequestrum), allergic rhinitis, lymphocytic-plasmacytic rhinitis, congenital ciliary dyskinesia (including the Kartagener syndrome), polyps, neoplasia, and fungal infection.

• B. bronchiseptica infection can lead to significant rhinitis as well as tracheobronchitis.

• Chronic bronchitis or lobar pneumonia can predispose to sinusitis, and vice versa. It is likely that a sinubronchial route of migration reinforces each infection.

• Bacterial isolates in rhinitis/sinusitis are variable and typify bacteria normally encountered in the nasopharynx, including gram-positive, gram-negative, and anaerobic organisms. Recently Bartonella spp. has been incriminated as a possible cause of nasal cavity discharge, including epistaxis, in dogs.

• Fungal infection of the nasal cavity or sinuses (see Table 163-1) can also be associated with immunosuppression; however, this immunodeficiency may be difficult to demonstrate in dogs. Aspergillus flavus, for example, is a normal inhabitant of the nasal cavity, but may invade respiratory epithelium in dogs with altered immunity or preexistent inflammation (e.g., lymphocytic-plasmacytic rhinitis, foreign body, or trauma).

• Nasal parasites, such as the nasal mite, Pneumonyssus (Pneumonyssoides) caninum, and the nasal nematode, Eucoleous, are uncommon in most localities, but are a potentially treatable cause of upper nasal/nasopharyngeal signs; this should be considered, especially in young dog with rhinitis and reversed sneezing.

Clinical Signs

• Sneezing, nasal discharge, and gagging or retching from postnasal drip are the typical signs of infectious rhinitis and sinusitis.

• Reversed sneezing (violent, inspiratory movements against a closed glottis) may occur with nasopharyngeal irritation from any source.

• Involvement of the ocular conjunctiva is not uncommon with contagious causes of rhinitis. Corneal involvement may suggest herpesvirus infection. Chorioretinitis may develop consequently to infectious canine distemper virus or cryptococcosis.

• Cough may indicate postnasal drip with pharyngeal irritation or concurrent involvement of the larynx, trachea, or bronchial tree.

• Some dogs retch and expectorate secretions that have accumulated in the pharynx.

• A serous discharge is typical of acute viral disease, whereas mucopurulent nasal exudate suggests a bacterial or fungal component.

• Other clinical findings such as fever, enlargement of the tonsils, bony swelling, regional lymphadenopathy, oral ulceration, and ocular or neurological involvement may be evident depending on the underlying causes of disease.

Diagnosis

• The differential diagnosis of upper respiratory infections is extensive (Table 163-3), including primary infectious diseases, secondary infections, and a large number of non-infective disorders. The age, vaccination status, history, and physical examination tend to focus the diagnostic considerations in most cases. The diagnostic studies chosen depend on the presumptive diagnosis and response to initial therapy (see Table 163-2).

• Primary infectious diseases, nasal mites, and foreign bodies with secondary infection are the important cause of rhinitis and sinusitis in younger dogs and cats. Older pets (older than 8 years of age) tend to be afflicted with nasal tumors complicated by secondary nasal infection.

• The poorly understood, idiopathic, lymphocytic-plasmacytic rhinitis of dogs and cats is a very common biopsy diagnosis that may be associated with allergy or infection. This disorder may allow for opportunistic secondary mucosal invasion by bacteria or fungi.

• Suppurative rhinitis is a less common biopsy diagnosis of the nasal mucosa. Primary and secondary bacterial infection may be involved; infection by Bartonella spp. should be considered.

• Infections caused by Aspergillus spp. and Penicillium spp. require histologic examination of nasal tissue, sometimes with special staining (e.g., silver stains) to detect hyphae. A positive agar gel immunodiffusion (AGID) test may also be supportive of the diagnosis. Routine culture is nonspecific because these fungi can be normal inhabitants of the nasal cavity.

Table 163-3 DIFFERENTIAL DIAGNOSIS OF UPPER AIRWAY INFECTIONS