Pneumonia is an infection of the lungs and airways that causes difficulty in respiration and deficiency of blood oxygen; this condition is common in foals of up to six months of age. The causes of pneumonia in foals include viral infections (e.g., equine herpesvirus), bacterial infections (such as Rhodococcus equi), parasitic migration, and environmental stresses. Occasionally, a combination of these factors can give rise to the development of foal pneumonia. 

Though not the most common cause of pneumonia, the bacterial agent Rhodococcus equi is the most serious cause of pneumonia in foals one to four months old.1 This can have economic consequences due to death, prolonged treatment, detection programs, and prevention strategies. Pneumonia caused by R. equi is rare in horses older than eight months of age.

Rhodococcus equi bacteria are ubiquitous in soil and are likely to be present in any area that is frequented by horses. Although the incidence of this disease can vary from facility to facility, certain conditions—such as sandy soil, high summer temperatures and dusty barns—favor the multiplication and spread of the organism in the environment. The inhalation of dust particles containing disease-causing strains of the bacteria is the chief route of lung infection.1 Manure from infected foals is a major source of environmental contamination. 

Progress and Diagnosis

R. equi infection progresses slowly, with signs of disease being difficult to detect until lung lesions have become significantly large. When clinical signs set in, most foals will become lethargic, experience loss of appetite, run a fever, and evidence rapid breathing. Cough is an occasional sign, while pus-containing nasal discharge is less common. Some foals will also evidence inflamed joints. 

In addition to lung abscesses, intestinal and abdominal abscesses may occur. Foals with abdominal involvement often show fever, depression, loss of appetite, weight loss, colic, and diarrhea.1 The prognosis for foals with abdominal involvement of R. equi is less favorable than for those with the lung form. Equine bones are another less common site of infection. Abscesses in the liver and kidneys have also been reported.

Blood testing can reveal the abnormalities that are consistent with R. equi infection and inflammation. Lesions appearing on chest X‑rays can suggest the presence of this pathogen however, identification of the bacteria in fluid from the airways is needed for a definitive diagnosis.2

Treatment of Foal Pneumonia 

For the treatment of foal pneumonia, prompt veterinary attention and appropriate treatment is critical. The survival rate of foals with R. equi pneumonia is approximately 70% to 90% with appropriate therapy. Without therapy, or with inappropriate therapy, the death rate is about 80%.2

Combinations of antibiotics are known to be more effective in fighting R. equi infection than any single antibiotic.1 The length of antibiotic therapy typically ranges from 4 to 9 weeks. Supportive therapy should include a clean, comfortable environment and highly palatable, dust-free feeds. Depending on the severity of the infection, the veterinarian may use intravenous fluid therapy and saline mist (nebulization) to induce expectoration. Nonsteroidal anti-inflammatory drugs (NSAIDs) may be prescribed if fever is present. Treatment with oxygen may be necessary in foals experiencing severe respiratory difficulty.2

Clarithromycin’s spectrum of activity is similar to that of erythromycin, but it also has activity against a variety of bacteria that are not easily treated with other antibiotics.

Macrolides can cause foals to stop sweating. Because horses and foals are dependent on sweating to dissipate heat, when they stop sweating their body temperatures can soar in hot and humid conditions. This elevation of body temperature is termed hyperthermia, and is different from the rise in body temperature that occurs with infections resulting in fever. Thus, it is important to closely monitor foals being treated for R. equi pneumonia, especially if they are turned out in paddocks when ambient temperatures or humidity are high.2

Where to buy Clarithromycin + Rifampin

Clarithromycin + Rifampin for equines is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. 

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

1Merck Veterinary Manual.

2Yager, J. The pathogenesis of rhodococcus equi pneumonia in foals. Veterinary Microbiology, Volume 14, Issue 3, August 1987, Pages 225-232.