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Transcript
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.