Structural Diseases

Primary Ciliary Dyskinesia

Pathophysiology

Normal cilia are made up of nine pairs of microtubules surrounding a central pair. Inner and outer dynein arms connect the microtubules, and the enzyme dynein splits ATP to provide energy for ciliary motility. Primary ciliary dyskinesia (PCD) is a congenital or inherited disorder that is most often associated with a defect in the dynein arms (total or partial absence) resulting in defective ciliary motion. All organs that contain cilia are affected including the respiratory tract, middle ear, and reproductive tract. PCD that is accompanied by situs inversus and bronchiectasis is referred to as Kartagener’s syndrome.

History and signalment

PCD has been reported in related Bichon Frise, English pointers, Springer Spaniels, Newfoundland, Old English Sheepdogs, and other breeds. It is rarely reported in the cat. Clinical signs can be seen in puppies as early as 5 weeks of age, or animals may be older when the severity or recurrence of signs triggers evaluation. Historical complaints include chronic or recurrent sneezing, serous to mucoid nasal discharge, chronic cough, and episodes of antibiotic-responsive pneumonia. Signs of otitis media (head tilt, aural discharge, or nystagmus) or infertility may also be reported.

Physical examination

Bilateral serous to mucopurulent nasal discharge is common and dogs may display evidence of pneumonia with a moist cough on tracheal palpation, increased tracheal sensitivity, and increased bronchovesicular sounds. Severely affected animals can be cyanotic. Nonrespiratory abnormalities that may be found include hydrocephalus and otitis media.

Diagnosis

A complete blood count may be supportive of bronchopneumonia, with alterations in neutrophil numbers, and thoracic radiographs reveal alveolar infiltrates during bouts of pneumonia. Airway sampling typically reveals a septic suppurative response and infection with Mycoplasma and aerobic bacteria (Pasteurella, Streptococcus, and Staphylococcus) is common. Diagnosis of PCD requires documentation of functional and structural defects of cilia. In an intact male dog with the appropriate clinical history, lack of purposeful sperm motility is consistent with a diagnosis of PCD. Respiratory ciliary function is assessed through tracheal scintigraphy using 99-technetium-labeled macroaggregated albumin deposited at the carina. Ciliary movement of the radiolabel is followed with a gamma camera, and animals with ciliary dyskinesia have no motion detected. Mycoplasma and Bordetella infections should be appropriately treated prior to performing tracheal scintigraphy because infection with these organisms causes ciliostasis and could result in a falsepositive scintigraphic study.

Nasal or tracheal biopsies or a semen sample fixed in glutaraldehyde can be submitted for electron microscopy to identify the characteristic ultrastructural abnormalities seen in PCD; however, electron microscopy is not widely available and a properly sectioned sample is critical to assess ciliary structures. A pathologist should be consulted prior to obtaining a biopsy to insure that an adequate sample is obtained and that a proper interpretation can be provided. Dogs with PCD will have multiple defects in cilia (loss or shortening of dynein arms, loss of central pair of microtubules, triplets in place of doublets, etc.) and will have >5–20% of cilia affected. Findings must be distinguished from those of acquired or secondary ciliary abnormalities, which can be found with a variety of chronic respiratory tract diseases. These secondary defects typically affect <5% of the affected cilia, and compound cilia (multiple cilia contained within a single membranous layer) are often prominent. In human patients, PCD can occur in the absence of specific electron microscopy abnormalities and it is likely that this occurs in veterinary patients also, although it has not been specifically documented.

Treatment

Aggressive therapy for pneumonia is required and owners should be taught to recognize recurrence of disease so that treatment can be instituted immediately. Intermittent or sustained respiratory therapy with nebulization and coupage can be used to encourage clearance of respiratory secretions. Cough suppressants should not be used because of the risk for trapping infected secretions in the lower airways and promoting the development of secondary bronchiectasis.

Prognosis

Many dogs with PCD are able to survive multiple bouts of pneumonia and develop fewer episodes as they mature; however, they remain at risk for recurrent infection. If pneumonia is insufficiently treated, bronchiectasis may develop. Contact with other animals that might serve as a source of infection should be limited. Affected dogs should not be used in the breeding pool and genetic counseling is recommended.

Lung Lobe Torsion

Pathophysiology

The precise etiology of lung lobe torsion is unknown. It may be associated with atelectasis of a lobe from bronchial obstruction (by mucus impaction or a neoplasm) and twisting of the lobar bronchus on its axis. Lobar consolidation due to infection might also predispose to torsion. Alternately, it is possible that pleural effusion precedes lung lobe torsion, causing collapse of a lung lobe and an environment that allows the lobe to twist along the bronchial axis.

Typically, venous compression caused by the torsion leads to congestion and swelling of the lung lobe. This could then cause pleural effusion, which is commonly, but not always, found with lung lobe torsion. Pleural effusion is often hemorrhagic or chylous in character. The lobes most commonly affected by torsion are the right middle lung lobe (particularly in large-breed dogs) and the left cranial lung lobe (in Pugs and other small-breed dogs).

History and signalment

There is an increased incidence of lung lobe torsion in the Afghan hound and deep-chested dog breeds, although it is also seen in small-breed dogs, with Pugs affected most commonly. Torsion appears to be uncommon in the cat but it has been reported in a cat with chronic, poorly treated lower airway inflammatory disease and was likely preceded by lobar collapse associated with mucus obstruction. Clinical complaints reported with lung lobe torsion include tachypnea or difficulty breathing, lethargy, anorexia, and coughing. In chronic cases, weight loss may be noted.

Physical examination

Tachypnea is a common finding, and Heart and lung sounds can be muffled or absent focally in the region of the torsion or ventrally in the thorax due to the presence of pleural effusion. Elevated body temperature occurs in 50–60% of cases.

Diagnostic findings

Affected animals usually display a neutrophilic leukocytosis due to stress, inflammation, necrosis, or infection. Radiographs reveal pleural effusion in over 80% of cases, and thoracocentesis is indicated to alleviate respiratory distress and improve visualization of thoracic contents. Lobar opacity is a common radiographic finding (Figure 6.1), and an abnormal bronchial position can be visualized in 10–50% of dogs. Thoracic ultrasound with evaluation of pulmonary blood flow may be helpful in some cases although the most common finding appears to be hepatization of the lung lobe. A vesicular gas pattern is seen on radiographs, ultrasound, or computed tomography. Bronchoscopy can document lung lobe torsion as the cause for lobar consolidation through visualization a twisted appearance to the bronchial opening.

Figure 6.1. Dorsoventral radiograph from a 3-year-old MC Pug with torsion of the left cranial lung lobe. Note the absence of pleural effusion in this case.

Thoracic fluid analysis reveals chyle (see Chapter 7) in up to one-third of dogs and a modified transudate, hemorrhage, or exudate has been reported in the remaining cases (Neath et al. 2000). Cytology of the fluid is indicated to rule out infectious or neoplastic disease, and samples should be evaluated by aerobic and anaerobic bacterial culture. Histopathology of lung tissue usually shows pulmonary hemorrhage, inflammation, and infarction but may reveal a primary disease process responsible for the torsion such as neoplasia or bronchial obstruction.

Treatment

Lung lobectomy is required for appropriate treatment. The lobe should not be de-rotated prior to removal because of the risk for ischemic injury associated with reperfusion.

Prognosis

Over 50% of dogs will have an uncomplicated recovery from lung lobectomy; however, some dogs will experience persistent chylothorax that necessitates further intervention. Torsion of a second lung lobe can also occur.

Infectious Diseases

Thoracic radiographs obtained in the early stages of bacterial pneumonia may demonstrate mild or diffuse interstitial infiltrates. Alveolar infiltrates with air bronchograms are considered the classic radiographic finding (Figure 6.2). In severe cases, these infiltrates can coalesce to cause lobar consolidation. A lobar infiltrate can also be suggestive of foreign body pneumonia. Pleuropneumonia is uncommon in small animals unless a pleural foreign body or bite wound is the cause of pneumonia. In these cases, infection with Actinomyces can be found.

Direct airway sampling through tracheal wash, bronchoscopy with bronchoalveolar lavage, or fine-needle aspiration of the lung is indicated to confirm the etiology of pneumonia and to obtain samples for Gram stain, aerobic and anaerobic bacterial culture with antibiotic sensitivity testing, Mycoplasma culture, and cytology. Of these techniques, tracheal wash is most suited for radiographically diffuse disease, while bronchoscopy (Figure 6.3) or fine-needle lung aspiration would be preferred for focal disease (see Chapter 2). Gram staining characteristics and cytology can be useful for identifying the most likely infecting organism and initiating early antibiotic therapy.

Figure 6.2. Right lateral (a) and dorsoventral (b) radiographs from a 7-year-old MC Australian Shepherd with bronchopneumonia affecting primarily the right lung lobe. Air bronchograms are visible in both views primarily in the right lung lobe. Nodular densities likely represent metastatic lung disease.

Various bacteria can be recovered from airways of healthy cats and dogs, although only cats with pulmonary disease have had Mycoplasma isolated from lower airways to date. Therefore, documentation of pneumonia requires isolation of bacteria in conjunction with detection of septic suppurative inflammation on airway cytology (Figure 6.4). Pneumonia in the dog is usually caused by Gram-negative enteric bacteria (Table 6.1). Multiple species are isolated in almost half of all cases, and 22% of infections are complicated by the presence of anaerobic bacteria (Angus et al. 1997). Bacterial pneumonia is clinically recognized much less commonly in the cat than in the dog, and less information is available about potential causes, although it appears that Bordetella and Mycoplasma spp. are commonly involved in conjunction with Pasteurella spp (Bart et al. 2000) (Table 6.2). A more recent study also implicated Mycoplasma spp. as the most common isolate in feline pneumonia (Foster et al. 2004).

Figure 6.3. Bronchoscopy image from a dog with Mycoplasma pneumonia reveals airway hyperemia and marked mucus plugging of an isolated airway.

Treatment

While waiting to obtain results of cultures, antibiotic treatment should be initiated by using antibiotics likely to be effective against the organisms involved. The combination of a fluoroquinolone and penicillin derivative is used most commonly for initial therapy. Antibiotics are usually required for >3 weeks, and long-term treatment is based on results obtained from culture and susceptibility testing. Clinical signs, pulse oximetry, and radiographic changes are used to determine when therapy can be discontinued.

Figure 6.4. This image of airway cytology confirms bacterial pneumonia by the presence of dark blue rod-shaped bacteria within neutrophils. Note the swollen nuclei and degenerate changes in neutrophils throughout the slide. Bacterial culture identified E. coli.

Table 6.1. Bacterial organisms commonly found in dogs with lower respiratory tract infection (Angus 1997)

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