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Diagnosis and Treatment of Infectious Respiratory Disease Phil Padrid, DVM Introduction: The canine respiratory tract is frequently exposed to bacteria, viruses, parasites and fungi. Under normal circumstances the natural defenses of the respiratory tract effectively prevent these organisms from infecting the pulmonary system. Infection of the respiratory system by these agents can occur when there are overwhelming numbers of organisms, virulence of specific organisms, impaired host immunity, and/or anatomic and functional abnormalities within the pulmonary tree. It goes without saying that the best way to manage respiratory tract infections in our patients is to prevent them from occurring. All dogs in our care should be well cared for and vaccinated with the core vaccines recommended by our state and national organizations. Additionally, dogs should be vaccinated to protect them from infection against selected respiratory pathogens including canine influenza and canine bordetella, based on the best judgment of the veterinarian. Upper Respiratory Tract Infection in Dogs Upper respiratory tract infections in the dog are most commonly due to bacteria and viruses. Until recently most clinicians assumed that dogs with acute onset nasal discharge, conjunctivitis, sneeze and cough had “kennel cough” caused by Bordetella bronchiseptica. Historically, in these cases clinicians empirically prescribed antibiotic therapy, and in most cases the patient got better. It is not clear however if these patients got better because of antibiotic treatment or in spite of it. Specifically, we now know that “kennel cough” is better referred to as canine infectious respiratory disease (CIRD), because the symptoms of acute nasal discharge, sneeze and cough in dogs are more commonly the result of a complex of infectious agents including viruses rather than a single bacterial species. Clearly, Bordetella species can cause symptoms of upper respiratory infection in dogs, and bordetella species are considered the single most important causal agent for CIRD. Other bacteria that likely play a role in some cases of “kennel cough” include streptococcus equi subspecies zooepidemicus, and Mycoplasma cynos. Additionally, there are multiple viral organisms that can play a primary or synergistic role in acute respiratory tract signs in dogs including, most commonly, parainfluenza. Diagnosis and Treatment Uncomplicated Upper Respiratory Tract Infection Dogs with acute signs of cough, sneeze, nasal discharge and conjunctivitis, or some combination of these signs can sometimes be treated for the symptoms alone. In these cases an etiology is not confirmed but only assumed to be a transient viral or bacterial infection and there are no significant systemic complications Complicated Upper Respiratory Tract Infection In this category we often include patients that are depressed, inappetent, and patients with purulent nasal discharge and/or productive cough. Diagnostic tests may be recently focused on serology to confirm or deny the presence of canine influenza virus because of the emerging nature of this disease. Viral culture of canine influenza is often not helpful because viral shedding of this organism is minimal after 4-5 days of the onset of symptoms, and this is the time frame when most clinicians would begin to suspect this organism. There are no effective drugs to treat acute viral respiratory infections in dogs. The most commonly identified bacterial species in CIRD include Staph, Strep, E coli and Bordetella. Mycoplasma is often suspected but less often identified due to the unique requirements to identify Mycoplasma in a laboratory setting. In these more complicated cases we restrict cough suppressants to evening use and only if the cough is preventing sleep. Additionally, in these cases it is prudent to treat with antibiotics, and the primary antibiotic I use in these cases is a fluoroquinolone (enrofloxacin 5mg/kg po sid). The great majority of patients with upper respiratory tract infection have successful resolution of their disease within 7-10 days. In some cases the cough may linger for weeks, and this may be due in part to infection-induced erosion of mucosa where cough receptors lay. LOWER RESPIRATORY TRACT DISEASE Canine Lower airway infections in the dog may be caused by parasitic, fungal, viral or bacterial species. In practice, bacterial infection of the tracheobronchial tree and lung parenchyma is the most commonly seen clinical disease that we treat. In all cases of suspected lower respiratory tract disease in dogs, a minimum data base includes a CBC, serum biochemical profile, fecal analysis by floatation, zinc centrifugation or Baermann analysis (depending on which organisms are most likely involved) and chest radiographs. Fungal infections There are three fungal organisms that cause the majority of fungal pneumonia in dogs, including Histoplasmosis, Blastomycosis and Coccidioidomycosis. Histoplasmosis is an organism that most commonly affects dogs in the Ohio, Mississippi and Missouri river valley regions of the United States. Infection occurs by inhalation of the organism and many cases self-clear. Pneumonia with multi organ involvement is the most common clinical sequelae to cases that do not self-clear. Symptoms of respiratory tract infection are cough and dyspnea, and radiographs typically demonstrate hilar lymphadenopathy with a nodular and interstitial pattern. Specific diagnosis of histoplasmosis is based on cytologic appearance of the organism in affected tissues, including liver, lung or bone marrow, and occasionally in respiratory washings. Most treatment recommendations include itraconazole (5mg/kg PO BID) continued for 4-6 months following resolution of clinical signs. Serology and antigen detection are not currently considered accurate in the canine species for routine use in clinical practice. Blastomycosis in the dog also causes chronic cough and systemic signs of illness, and is most commonly seen in the Ohio River Valley regions. Infection may be more localized to the lung, and the interstitial and nodular radiographic pattern is not specific for this organism. Diagnosis of blastomycosis is successfully made using serology (AGID) when used in conjunction with a compatible history, physical examination and typical radiographic changes. The treatment of choice historically has been itraconazole (5mg/kg PO BID for 5 days, then SID) and is continued for at least 1 month past resolution of clinical signs. Coccidiomycosis C immitis is considered endemic in the southwestern United States (Arizona, the “Lower Sonoran Life Zone”) and is commonly referred to as “valley fever”. Serologic surveys have demonstrated that most dogs and people living in these areas have been exposed to the organism (Greene 2006). Exposure commonly results in subclinical infection and the respiratory component of this disease may cause symptoms ranging from mild intermittent cough to life threatening pneumonia. Because of the prevalence of this organism C immitis is suspected and on the differential diagnosis list for dogs in this part of the county who present with almost any unexplained lesion or symptom. Diagnosis is assumed in dogs when hilar lymphadenopathy is found on chest radiographs and is confirmed by serology (paired titers). Treatment is most successful using fluconazole (5mg/kg PO BID for cases with primarily a respiratory component). Treatment is usually continued until titers are </= 1:4 and clinical and radiographic signs have resolved. Many cases of C immitis in dogs require chronic therapy and some require lifelong treatment. Viral and Bacterial Lower Airway Disease The same viral and bacterial organisms that cause upper airway disease in dogs can progress to the lower airway to cause infectious bronchitis and pneumonia. There are no specific treatments for viral tracheobronchitis or pneumonia in dogs. Choices of antibiotics to treat suspected or confirm bacterial bronchitis or pneumonia are based on the specific organism identified, the severity and chronicity of the infection, the presence of underlying disease, and the bloodbronchoalveolar barrier (see bacterial infections in cats). Plain film radiographs are generally diagnostic for pneumonia in dogs, and most commonly demonstrate an alveolar pattern with air bronchograms, frequently with lobar consolidation. In most cases, the bacterial species are consistent with known commensal organisms. Antibiotic therapy for dogs with bronchopneumonia is based on results of culture and sensitivity results when available. If the cause of pneumonia is unknown, it is reasonable to assume that aspiration of oropharyngeal contents is a possibility, and it is also reasonable to assume a mixed infection with a population of bacteria including facultative anaerobes. For this reason, and when identification of the organism(s) is not possible, my preference is to use enrofloxacin 5m/gk PO SID and clindamycin 10 mg/kg PO BID because this combination of antibiotics can effectively treat virtually all the commonly seen bacteria reported to cause pneumonia in dogs. I repeat the plain film radiographs in 7 days and continue this treatment for a total of three weeks if clinical and radiographic changes are encouraging. I recheck the radiographs 3 weeks after the initiation of antibiotic therapy and stop treatment if the patient is back to normal and the radiographic infiltrates have been resolved. If these conditions are not met I am insistent on additional diagnostic testing to demonstrate the organism and the sensitivity pattern so that a rational treatment plan with antibiotics can be established. I have used this protocol to successfully treat more than 40 patients in the last few years with success. It is important to add that this is a protocol without published acceptance, and merely reflects my experience in a pulmonary-only specialty practice. Supportive care for canine patients with bacterial pneumonia, in addition to appropriate antibiotic therapy, includes: 1. supplemental oxygen as determined by arterial blood gas analysis, pulse oxymetry and/or clinical status 2. balanced adjunctive fluid therapy as determined by clinical status of patient 3. frequent (q2-4hr) walks as tolerated by the patient to stimulate cough 4. couppage to the affected lung segment(s) following each bout of exercise to stimulate cough 5. non-steroidal drug therapy (carprofen 1mg/kg PO BID prn) for well hydrated patients with normal hepatic and renal function. The use of non-steroidal anti-inflammatory medication to treat bacterial pneumonia in dogs is not universally recognized or accepted. However, the author has adopted this approach successfully for many years. The rationale for the use of this class of drugs is that a productive cough is a key element in resolution of bacterial pneumonia. Canine patients with bacterial pneumonia cough in a way that is classically described as “soft and moist”. This cough is not vigorous or predictably productive because of the musculoskeletal discomfort associated with the cough. Use of NSAID’s in these cases decreases the discomfort associated with coughing, allow the cough to become more vigorous, and therefore make the cough reflex more productive. Summary and Conclusions Effective strategies to manage veterinary patients with respiratory tract infections are complicated by the presence of (sometimes occult) primary non-infectious diseases that predispose to infection, the diversity of organisms that can infect the respiratory system in dogs and cats, the lack of bacteriologic studies that are allowed or available, and the costs involved in optimal care. Rational therapy must be based on the clinical presentation of the patient, the available diagnostic data, the known organisms and their sensitivity profiles, the known properties of the anti-infective drugs, and the safety of the patient. This is a daunting task when read aloud. Yet, in spite of these challenges, most patients with infections of the respiratory tract can be successfully treated and can live a long and comfortable life.