Antibiotic therapy is obviously one of the most important modes by which bacterial infections are treated, and the lungs are
no exception.
Organisms causing bacterial pneumonia in dogs
Variable bacterial isolates have been reported in cases of bronchopneumonia in small animals. Most dogs with bacterial pneumonia
are infected with a single organism, but some may have multiple isolates. In dogs, the majority (>80%) of bacteria cultured
in pneumonia are gram negative aerobic rods such as E. coli, Pseudomonas spp, Klebsiella spp, Enterobacter spp, Pasteurella spp, and Bordetella bronchiseptica. A minority of pneumonia cases culture positive for gram positive aerobic cocci such as Enterococcus spp, Streptococcus spp, and occasionally Staphylococcus spp. The incidence of anaerobic infections in dogs with bronchopneumonia is unclear, but may be up to 20%.
Except in acute, low-grade infections, representative cultures should be obtained from the respiratory tract prior to initiation
of antibiotic therapy. Cultures may be obtained by transtracheal or endotracheal tube washes, by bronchoalveolar lavage, or
by fine needle aspiration of consolidated areas of lung. Antimicrobial therapy should be initiated immediately after obtaining
the tracheal wash for culture, and can then be fine-tuned once the result is obtained. This author has found that tracheal
cultures are usually positive and useful even if the animal has received one or two doses of antibiotics.
Obtaining cultures from the lungs
To confirm the diagnosis of bacterial pneumonia, and to help direct therapy, it is important to obtain a sample from the lungs
for cytology and culture. This can be important to help distinguish pneumonia from other causes of radiographic alveolar disease
such as hemorrhage or neoplasia. A cytologic finding of suppurative inflammation can help confirm the diagnosis and can suggest
chronicity if macrophages are found in addition to neutrophils. Cultures will subsequently confirm the presence of bacteria,
and help to direct antibiotic therapy. In order to obtain samples that are free of pharyngeal contamination, techniques that
by-pass the pharynx must be used to obtain the sample. Practical options for obtaining samples from the trachea include transtracheal
or endotracheal washes. Although bronchoalveolar lavage is another good option, it is more invasive and involves more specialized
equipment, and is not usually used as a first line diagnostic test for bacterial pneumonia.
Transtracheal aspirates
Transtracheal aspiration (TTA) can be performed in many dogs without the use of sedation, especially if the animal is debilitated.
If light sedation is required, short-acting or reversible drugs should be used, which will have little depressant effect on
respiratory function (for example, butorphanol 0.2-0.4 mg/kg IV with diazepam 0.2-0.5 mg/kg IV). At least one, and possibly
two assistants will be required for restraint. The dog should be positioned in a sitting position or in sternal recumbency,
and the head elevated. The choice of site for TTA is variable depending on the individual animal. In small dogs and in dogs
with thick neck conformation, it is easiest to enter the airway through the cricothyroid ligament. The cricothyroid ligament
can be felt as a triangular depression on the midline between the prominent thyroid cartilage and the ridge of the cricoid
cartilage of the larynx. In medium-sized to large dogs, however, it is usually best to penetrate the trachea between two tracheal
rings, on the midline, about halfway down the ventral neck. Once the site is chosen, it should be clipped and surgically scrubbed.
The area should be infiltrated with lidocaine for local anesthesia.
With the dog restrained by the assistant, the site for TTA is carefully located. The needle is inserted bevel-down in order
to minimize the risk of tearing the catheter with the sharp bevel when the catheter is inserted. The needle of the catheter
should first be advanced through the skin on the midline. The clinician then stabilizes the trachea with the thumb and forefinger
of one hand, and slowly advances the needle in a horizontal direction towards the trachea with the other. It is important
to direct the needle straight towards the midline of the trachea, since if it is approached at a tangent it will be difficult
to penetrate the lumen. When the needle contacts the trachea, it may be necessary to "walk" the needle a short distance up
or down in order to find and penetrate the ligament between two tracheal rings.
