Rational antibiotic choices for bacterial pneumonia in dogs (Proceedings)

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.

When the needle penetrates the trachea, a distinct "pop" can usually be felt. The animal will often cough or swallow, particularly when the needle is within the tracheal lumen. After penetration of the trachea, the needle is carefully held in position within the lumen, to prevent it from prematurely backing out of the airway. If the needle is advanced too far within the lumen, the dorsal wall of the trachea will prevent the catheter from passing through the needle. In some instances, the dorsal wall of the trachea may even be inadvertently penetrated. Once the needle is correctly positioned within the lumen of the trachea, it may be slowly raised from a horizontal position to a 45 degree angle, pointing bevel-down towards the carina. The catheter is then advanced through the needle as far as it will go. Most dogs will cough as the catheter irritates cough receptors of the tracheal mucosa. Once the catheter is in position, the needle can then be withdrawn a short distance from the skin, and the needle-guard secured in place. This prevents any further damage or laceration of the airway.

The TTA is then performed with 5ml or 10ml syringes of sterile saline depending on the size of the dog, using the hemostat to stabilize the end of the catheter. A syringe is placed on the catheter, and 8-9mls of saline are injected into the airway, leaving about one ml in the syringe. The clinician then attempts to aspirate as much of the fluid as possible back into the syringe. It is helpful at this time if the assistant can coupage the chest wall in order to encourage the dog to cough while the clinician is aspirating. Usually only 0.5-1 mls of fluid can be recovered from each wash. This process is repeated until flecks of mucoid or purulent material are recovered in the wash. Often the second or third attempts are more productive than the first. It is quite safe to instill 30-50 mls of saline into the lungs of all but the smallest dogs. The catheter may then be withdrawn and pressure placed over the site for a few minutes.

TTA can be hazardous in dogs with tracheal collapse, and can precipitate severe coughing and respiratory distress. TTA is a somewhat stressful procedure which should not be performed in animals in significant respiratory distress, even though these are the animals most likely to benefit from it. It can cause desaturation of hemoglobin and subsequent collapse in unstable patients. Desaturation and collapse can be accompanied by cardiac arrhythmias and hypotension. Clinicians and assistants should carefully observe the patient for cyanosis or excessive distress during the procedure, and should be prepared to discontinue and to administer oxygen by face-mask if necessary.

Other complications associated with TTA are usually few and minor. Occasionally subcutaneous emphysema can occur which is usually mild and self-limiting. Thoracic radiographs obtained after TTA may reveal self-limiting and asymptomatic pneumomediastinum. Hemoptysis may occur which is typically brief, although could be severe in animals with coagulopathy. Rarely seen but potentially serious complications include tracheal laceration, or catheter breakage with subsequent loss into the airway.

Endotracheal lavage

TTA can be difficult to perform in very small and toy breeds of dogs, and in cats, due to the small size of the airway. In such small patients, it is preferable to anesthetize and intubate, and to perform the wash through a sterile endotracheal tube. Similarly, if the patient is to be anesthetized to perform another procedure, performing an endotracheal lavage may be easy and less stressful for the patient.

To perform an endotracheal lavage (ETL, anesthesia is induced using a short-acting injectable drug. Propofol (1-4 mg/kg) is often used, although care must be taken to avoid any periods of apnea and to monitor cardiovascular function carefully when using this drug. The animal is intubated using a sterile endotracheal tube, with the operator wearing sterile gloves and taking great care to avoid contamination by touching the oral mucosa during intubation. Once the animal has been intubated, then a sterile catheter is placed down through the endotracheal tube into the airways. Ideally, the tube should reach far enough to pass the carina, although placement is blind. Catheters commonly used include red rubber urinary or suction catheters. Increments of 5 or 10 mls of sterile saline are injected into the airway through the tube, and then aspirated back out. Typically, the yield is only 0.5-1 mls per aspirate. Aspiration can be performed using suction on the syringe used to inject the saline, or using mechanical suction devices through a mucus specimen trap. After the wash, the animal should receive several large breaths with 100% oxygen prior to extubation. Careful monitoring should follow extubation, and oxygen supplementation should be provided during anesthesia recovery.

First-line antibiotic therapy for dogs with pneumonia

The initial antibiotic choice should provide broad-spectrum coverage for the most likely organisms, bearing in mind the possibility of polymicrobial infection. Cytologic results may assist in antibiotic choice, by documenting whether the bacterial organisms are gram positive or gram negative, rods or cocci. Rational antibiotic choices should initially provide broad spectrum coverage effective against both gram negative and positive organisms. Once culture and sensitivity results are available, a specific and narrow spectrum antibiotic can then be chosen for ongoing care.

The route of antibiotic administration for a pneumonia patient depends on the severity of illness. If the dog is systemically quite healthy, has no evidence of hypoxemia, is eating and drinking well and is active, then oral antimicrobials are appropriate. Good oral first line choices for a stable, normoxemic pneumonia patient could include

Amoxicillin or amoxicillin/clavulanate

• Fluoroquinolones

• Trimethoprim/sulfa

On the other hand, if the dog is anorexic, febrile or hypoxemic, then it should probably be hospitalized for administration of parenteral antibiotics, ideally intravenously. In a sick patient, it is not reasonable to rely on drug absorption from a GI tract that may have poor perfusion and low motility. When a penicillin is combined with an aminoglycoside, a synergistic effect provides excellent broad spectrum coverage in serious respiratory infections. Ticarcillin is a semi-synthetic penicillin, which when used in combination with clavulanate (Timentin®), which can be a good parenteral choice for severe pneumonia. Other new beta lactam drugs such as imipenem are also becoming available. First generation cephalosporins such as cephalexin, do not have an adequate gram negative spectrum for patients with serious pneumonia when they are used alone. They should be combined with aminoglycosides for a broader aerobic spectrum. Alternatively, second or third generation cephalosporins can be considered. Fluoroquinolones are useful because of their efficacy and excellent distribution to the cells and tissues of the lung. If concern exists about renal function, fluoroquinolones or extended spectrum beta lactam antibiotics should be used instead of aminoglycosides. Therefore, parenteral choices for a sick pneumonia patient with hypoxemia could include one of the following more aggressive combinations (bearing in mind the need for broad spectrum coverage):

• Ampicillin and a fluoroquinolone

• Ampicillin and an aminoglycoside

• Clindamycin and a third generation cephalosporin

• A potentiated penicillin such as ticarcillin/clavulanate

This author recommends that parenteral antibiotic therapy, as directed by the results of the culture and sensitivity, should be continued until the animal is no longer hypoxemic and has normal GI tract function. Once oxygenation has returned to normal and the animal is eating well, then oral antibiotics can be substituted based on the results of sensitivity testing, and the animal can be discharged from the hospital.

References available on request