21 Parenchymal Disorders and Diseases
Dogs with pulmonary parenchymal disease may have a variety of clinical signs ranging from mild to severe. Some animals may have subtle signs despite the presence of severe disease. Owners may report a cough or increased respiratory effort. With severe disease, open-mouthed breathing, cyanosis, and dyspnea may be seen. Other signs of systemic illness may also be noted, such as weight loss, anorexia, and lethargy.
There are several causes of pulmonary disease in dogs; individual diseases will be discussed in other sections of this book. Parenchymal disease may be caused by infection, such as bacterial, viral, or fungal pneumonia. Neoplasia can also affect the pulmonary parenchyma. Tumors may be either primary or metastatic. Traumatic events may lead to pulmonary contusions. Coagulopathy or thrombocytopenia may lead to pulmonary hemorrhage. Edema, either cardiogenic or noncardiogenic, may also result in impaired pulmonary function. Noncardiogenic edema may result from many causes such as head trauma, seizures, strangulation, and inhalation of smoke. Other causes of pulmonary parenchymal disease include parasitic diseases and hypersensitivities.
The first step to diagnosis of pulmonary parenchyma disease is a thorough history and physical examination. Animals in respiratory distress may be intolerant of diagnostic procedures and the astute clinician should use their findings on history and exam to localize the disease to a specific area of the respiratory tract and institute therapy if appropriate. Thoracic radiographs, if tolerated by the dog, are extremely helpful in evaluation of the respiratory and cardiovascular systems. Thoracic radiographs should be evaluated for pulmonary masses, signs of edema, or other abnormalities. Cardiovascular structures should also be evaluated to rule out cardiogenic causes of respiratory difficulties. Baseline blood work, including complete blood cell counts, blood chemistries, and coagulation times, should be completed to evaluate for the presence of systemic illness and underlying conditions. These tests may suggest signs of infection or blood loss and may support a diagnosis. Additional evaluation of the thorax may be indicated because often radiographs may suggest disease but do not lend a diagnosis. Transtracheal wash or bronchoalveolar lavage may be beneficial. Thoracic ultrasound and transthoracic aspirate may also be helpful.
An arterial blood gas is extremely beneficial in evaluation of the dog with pulmonary disease; however, the blood gas does not differentiate from causes of disease. The blood gas will allow evaluation of ventilation and gas exchange. Elevated carbon dioxide levels suggest hypoventilation and may indicate the need for mechanical ventilation. Decreased oxygen levels indicated the need for supplemental oxygen support. Repeated blood gas analysis may also be useful. Rechecking parameters may allow for evaluation of response to therapy.
The treatment options for animals with pulmonary parenchymal disease vary and depend largely on the cause of signs. Oxygen supplementation should be instituted in animals with hypoxemia. Antibiotics or antifungal medication may be warranted in cases of pneumonia. Chemotherapy may be warranted for neoplastic disease. In some diseases, such as traumatic pulmonary contusions, treatment is largely supportive; however, advanced therapies, such as mechanical ventilation may be needed if disease is severe. Cardiogenic causes of pulmonary disease should be treated as indicated based on the underlying disease. Diuretics may be indicated to treat pulmonary edema. The reader should consult other sections of this text for more details on treatment of individual diseases.
The differentiation between cardiogenic and noncardiogenic pulmonary disease can be quite difficult at times. In animals with cardiogenic disease, a complete physical examination may reveal distended jugular veins or a hepatojugular reflex. A heart murmur may be ausculted. Thoracic radiographs may also be useful in helping to differentiate cardiogenic and noncardiogenic causes. In cardiogenic diseases enlargement of cardiovascular structures may be noted. Pulmonary edema may be seen as well. Typically, the distribution of edema in cardiogenic failure can also help differentiate the cause of respiratory disease. Infiltrates from cardiogenic causes are typically distributed in a caudodorsal pattern, although some diseases may have variable patterns. These infiltrates may also be located in the perihilar region. Infiltrates from pneumonia typically are more cranioventrally distributed. Evaluation of protein content of edema fluid may also be of benefit. Fluid from cardiogenic causes is typically lower in protein content that fluid from noncardiogenic causes. Heart rate is usually elevated with heart failure.
Initial evaluation of the dog in respiratory distress should involve some degree of oxygen supplementation. Oxygen supplementation may be provided via several routes, such as oxygen mask or hood, nasal, or intubation and mechanical ventilation. Each of these techniques has pros and cons, and the decision of which is preferred is dependent on the animal’s condition and hospital resources. Oxygen mask and hood can provide variable amounts of supplemental oxygen, whereas oxygen cages and nasal catheters supply more reliable levels. For severely dyspneic animals that do not respond to routine methods of oxygen supplementation, intubation and mechanical ventilation may be needed. The need for oxygen therapy is best assessed with clinical data such as decreased oxygen saturation noted via pulse oximetry or hypoxemia noted on a blood gas. The dog’s respiratory effort should also be considered when deciding whether oxygen supplementation is necessary. Some animals may maintain adequate oxygenation levels, but at the expense of an increase work of breathing. These animals may fatigue without supplementation and can acutely deteriorate.
Close monitoring of respiratory rates of any animal with pulmonary disease is essential. Animals should be frequently evaluated for signs of increasing respiratory rate and effort. Mucous membrane can be assessed for color; however, low oxygen saturation levels may be present before clinically detecting cyanosis. Pulse oximetry may be useful in assessing oxygen saturation. Levels lower than 92% should prompt the clinician to provide some source of oxygen supplementation. Arterial blood gas samples (as discussed previously) are also a useful indicator of oxygen status.
As discussed previously, cardiogenic causes of pulmonary infiltrates have a typical caudodorsal distribution with concentration in the perihilar areas. Abnormalities of the cardiovascular structures, such as enlargement of the cardiac silhouette, are also seen. Bacterial pneumonia typically involves a more cranioventral distribution, although appearance may vary. Fungal pneumonia and neoplastic diseases may have a nodular appearance. Noncardiogenic edema appears similar to that of cardiogenic causes; however, abnormalities of the cardiovascular structures are not seen. Traumatic pulmonary contusions and hemorrhage may have an irregular and patchy interstitial to alveolar pattern. Other atypical patterns can also be seen depending on the dog’s disease state, onset of signs, and other variables.
The primary causes of viral pneumonia in dogs are canine distemper virus (CDV), canine adenovirus type II (CAV-2), and parainfluenza virus type-II. Canine herpesvirus (CHV) can cause upper respiratory infection in adult dogs.
Parainfluenza virus type-II is the most common, followed by CAV-2 and rarely canine adenovirus type I (CAV-1). Bacterial agents may also be involved concurrently, for example Bordetella bronchiseptica, Mycoplasma, and Streptococcus spp.
They are all transmitted by inhalation of respiratory droplets or contact with contaminated fomites. In addition, CHV can be transmitted by direct contact with both respiratory and genital secretions. These viral agents can be transmitted for up to 2 weeks after infection.
Most respiratory viral infections are asymptomatic or subclinical. Mild fever, oculonasal discharge, coughing and weight loss are common clinical findings. CDV generally causes a more severe illness than the other viral infections.
Gastrointestinal signs such as vomiting and diarrhea can be seen. Neurologic signs such as myoclonus, seizures, and loss of vision have been noted. Additionally, hyperkeratosis of the footpad has been noted in chronic infections.
Supportive care is the mainstay of treatment for viral infections. This includes adequate hydration, housing the animal in a clean, warm environment, and occasionally administering bronchodilators or antitussives if there is not evidence of pneumonia. Because secondary bacterial infections are common, appropriate antibiotic therapy is often warranted. It is also important to isolate these animals and try to keep hospitalization to a minimum to prevent spread of these viruses to other animals.
Vaccination is the preventative measure against viral disease. There are vaccines against all of these viruses (except CHV) in both parenteral and intranasal routes of administration. Therefore, a good vaccination schedule, decreasing exposure of unvaccinated animals, and proper sanitation practices are all beneficial in prevention of disease and its spread.
Recently, a new canine influenza has been identified initially in the racing Greyhound populations in Florida. This virus has now spread to shelters, boarding kennels, and pet animals. Two forms of the disease have been identified. The first is a mild form in which dogs have a soft, moist cough that appears to be self-limiting and persists for several weeks. Most infected dogs have demonstrated this form. The second form is more severe in which the dogs have high fevers and display the clinical signs of pneumonia. This virus originated as an equine influenza that has infected dogs and has transformed to a new canine-specific influenza. Because this is a new virus, almost all dogs exposed become infected and about 80% show clinical signs, with most having the mild form.
Similar to the other viral diseases, serologic testing is an option ideally with both acute and convalescent samples. If the dog has died, samples of fresh lung tissue can be obtained for viral culture or polymerase chain reaction (PCR) testing.
Routine cleanliness and infectious disease precautions are important to controlling spread of disease. The influenza virus is killed by most disinfectants including quaternary ammonium compounds and diluted bleach. Animals with a suspected infection or a cough should be isolated from the other animals and caretakers should practice isolation protocols with thorough disinfection of cages and all instruments in contact with the animal.
Clinically healthy dogs normally have a low number of organisms (alpha-hemolytic streptococci, staphylococci, B. bronchiseptica, Pasteurella multocida, and Klebsiella pneumoniae) in the airways down to the first bronchial division. Microorganisms commonly cultured from the lower airways are identical to those identified from the upper airways in each individual animal. However, there is variation in what is cultured between dogs. By definition, evidence of inflammatory cells in the lower airways in addition to presence of large numbers of bacteria equate with a probable bacterial infection.
This is a highly contagious respiratory infection that is most commonly associated with a dry cough early in the pathogenesis of the disease. Owners often describe the cough as sounding like a goose honk. On physical examination, one can usually elicit a cough by palpating the trachea. B. bronchiseptica is the most commonly identified primary pathogen. It spreads through direct contact between dogs and through airborne contact leading to outbreaks in high-density situations (boarding kennels, dog shows). The incubation period is approximately 6 days and infection is typically self-limiting in adult dogs, although it can progress to pneumonia. B. bronchiseptica preferentially attaches to the cilia in the large airways in the lungs and produces a toxin that cause ciliostasis. This in turn leads to an increased the risk of secondary opportunistic infections that often result in a more severe pneumonia.
Recommended antibiotics include trimethoprim-sulfonamide, amoxicillin-clavulanate, and doxycycline which have been shown to reduce the duration of the cough. Care should be taken when using doxycycline in young dogs as it can cause permanent yellow discoloration of the enamel in unerupted teeth. This is thought to be secondary to the formation of tetracycline-calcium phosphate complexes within the enamel and dentine. Activity and exercise can lead to paroxysmal coughing and should be limited. Antitussives (i.e., hydrocodone, butorphanol, dextromethorphan) are often helpful in reducing the cough cycle, but should be limited in their use because they reduce expectoration of accumulated airway secretions and increase the likelihood of secondary pneumonia. Therefore antitussives should be used to control uncontrolled paroxysmal coughing only. Occasional coughing is expected and is helpful in the removal of airway secretions. Infected dogs should be isolated for a minimum of 3 weeks to avoid exposing other dogs to the infection. However, some dogs can shed bacteria for up to 3 months. Dogs should be routinely vaccinated every 6 months or 5 days before potential exposure in high risk environments (i.e., boarding, dog shows) if not vaccinated within the previous 6 months.
Bacteria gain access to the lower respiratory tract via the blood from extrapulmonary sources, by inhalation, and after aspiration of oropharyngeal flora. Predisposing factors include previous or concurrent viral infection, regurgitation, vomiting, neurologic abnormalities in mentation, various metabolic diseases (diabetes mellitus, hyperadrenocorticism), and immunosuppressive therapy (glucocorticoids, chemotherapy).
Colonization does not always equate with infection. Normally, bacteria are prevented from establishing residence in the lower airways by several defense mechanisms, which include filtration by the nasal turbinates, sneezing, coughing, and the mucociliary clearance system. When bacteria reach the distal airways and alveoli, the important factors related to the development of infection include (1) virulence of the bacteria, (2) adequacy of the host immune response, and (3) the number microorganisms introduced. The host immune response and effectiveness of the mucociliary clearance mechanism are probably most important in preventing bacterial colonization from proceeding to infection. Early clinical signs suggestive of infection include fever, malaise, inappetence, variable severity of dyspnea, coughing, and serous or mucopurulent nasal discharge.
P. multocida, Escherichia coli, Streptococcal spp., and less often Klebsiella spp., and Staphylococcus spp. are the organisms commonly associated with primary bacterial pneumonia. Most bacterial pneumonias occur subsequent to some predisposing cause or condition (i.e., stress, viral infection) and opportunistic bacteria are often isolated (B. bronchiseptica, Mycoplasma spp., and Pseudomonas spp.). Generally, a single organism is isolated, but mixed infections are also common with gram-negative infections predominating.