Managing Bacterial Pneumonia in Adult Horses (AAEP 2011)

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Originally published on TheHorse.com

Consider how pneumonia appears in the horse, and you might picture a foal in intensive care, but in reality the condition can affect horses of all ages. While some pneumonia cases are very challenging to treat, veterinarians can rehabilitate many of these patients successfully with the proper approaches.

During a presentation at the 2011 American Association of Equine Practitioners convention, held Nov. 18-22 in San Antonio, Texas, Harold McKenzie III, DVM, MS, Dipl. ACVIM, gave attendees an in-depth look at the most effective techniques for diagnosing and managing equine bacterial pneumonia.

"Bacterial lower respiratory infections have greater clinical impact than viral infections because of the substantial risk of complications ranging from focal abscessation to pleuropneumonia," said McKenzie, an associate professor in equine medicine at the Marion duPont Scott Equine Medical Center in Leesburg, Va.

Disease Development

McKenzie explained that pneumonia arises in the airway lumen (cavity) and ranges in severity based on how much of the horse's body is involved (more on that in a moment). The disease is typically initiated by “major exposure” overwhelming and impairing the horse's defense mechanisms.

Simple cases of bacterial pneumonia involve the primary airway and rarely affect the lungs, with no abscesses or little to no consolidation (fluid-filling resulting in solidification into firm, dense masses) evident. Veterinarians consider c complicated when substantial consolidation occurs, abscesses form, or pleuropneumonia secondary to bacterial bronchopneumonia develops, characterized by infectious organisms entering the pleural space.

Diagnosis

McKenzie said veterinarians use an arsenal of diagnostic techniques to confirm bacterial pneumonia such as reviewing the animal's history and clinical signs and performing a rebreathing examination to evaluate respiratory function. Common clinical signs of bacterial pneumonia include fever, cough, nasal discharge, increased respiration rate, difficulty breathing, depression, anorexia, and pain on palpation of the thoracic wall.

Thoracic imaging is another common diagnostic tool, McKenzie said. Veterinarians often use ultrasound to detect abnormalities in the pleural space, he said, and to identify consolidated tissue and pleural effusion (swelling). The drawback with ultrasound, he noted, is that abnormalities deep within the lungs could be missed.

On the other hand, radiographs (X rays) provide a "more global assessment" of pulmonary function, he added, but these unfortunately aren't feasible in field settings and are challenging to obtain due to the size of the horse's thoracic cavity. Additionally, thoracic radiograph sensitivity and specificity can be low, he added. Specificity refers to the probability that results will be negative among patients who do not have the condition, and sensitivity indicates the probability that results will be positive when run on a group of patients with the disorder.

Using clinical pathology, McKenzie said, is important when evaluating potential bacterial pneumonia. Blood gas analysis can shed light on hypoxia (lack of oxygen) and hypercapnea (excess carbon dioxide in the blood), which could indicate diffuse lower respiratory inflammation.

"Though not usually required in the initial evaluation of patients with lower respiratory disease, airway cytology (including tracheal aspirate and bronchoalveolar lavage) is critical in the assessment of severe or persistent lower respiratory infections," McKenzie said.

These assessments provide "a clearer indication of the character of pulmonary inflammation, especially regarding the predominant type of inflammatory cells and the presence and type of bacteria."

Finally, veterinarians can use thoracocentesis (chest tap to obtain fluid for laboratory analysis) in cases of pleural fluid accumulation. "This procedure can have both diagnostic and therapeutic applications because it yields a sterile sample for cytology and culture but also allows for the removal of fluid from the pleural cavity,” said McKenzie.

Treatment

There are a host of treatment methods available for bacterial pneumonia, McKenzie explained. The majority of the medical treatments include β-lactams (such as penicillin and ceftiofur), aminoglycosides, potentiated sulfonamides, tetracyclines, fluoroquinolones, chloramphenicol, rifampin, and metronidazole. The optimal drug or combination of drugs to use is case-dependant, he said, and each medication or combination poses specific strengths and drawbacks. Veterinarians should decide the best approach for each individual case.

Other treatment or adjunct treatment options include topical antimicrobials (administered in aerosol form), bronchodilators, non-steroidal anti-inflammatory drugs, pleural drainage, and standing thoracotomy (making an incision in the chest to remove remaining fluid from the pleural cavity after pleural drainage).

Post-Treatment Management

McKenzie stressed that without proper post-treatment management and some husbandry adjustments pneumonia can recur.

"Resolution may take one to two weeks for uncomplicated cases or much longer for more complicated ones," he said.

It's especially important that the horse not return to work before receiving veterinary approval, even if he looks fresh and ready to go, McKenzie stressed. Working the horse too soon in the healing process increases the likelihood of disease recurrence.

He suggests monitoring the horse using the following recommendations for a minimum of 14 days—sometimes much longer—for any signs that the disease could be recurring:

• Take the horse’s temperature, pulse, and respiration daily;
• Evaluate his respiratory effort; 
• Watch for nasal discharge;
• Perform rebreathing examinations; 
• Repeat  imaging (ultrasonography or radiographs); and
• Run complete blood counts.

Other husbandry changes he suggested for a recovering horse included:  

• Using chopped forage or pelleted feed to reduce the amount of dust the horse inhales;
• Feeding from the floor, as opposed to using a wall feeder, to promote a more natural head position; and
• Using wood shavings instead of straw bedding to reduce the amount of dust in the horse's environment.

Take-Home Message

It’s important to understand the physiological processes behind bacterial pneumonia, along with diagnostic tools, treatment options, and husbandry practices that promote healing. Applying this knowledge and working with a veterinarian to manage cases can mean a full recovery for affected horses.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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