Rhodococcus equi is a bacterium that inhabits the soil and is often responsible for causing pneumonia in foals aged 1 to 6 months. R. equi-caused disease in adult horses, humans and other animals is rare and is often associated with an immune system deficiency. Infection with R. equi can also cause disease in other places in a horse’s body, including the eyes, bone, joints, the gastrointestinal tract and intestinal lymph nodes.¹  

R. equi is ubiquitous on horse farms, and can be detected on most horse farms globally. In most cases of infection, foals inhale the airborne bacteria into their respiratory tract or ingest contaminated soil. The infection progress is slow, and foals usually do not begin to exhibit clinical signs until the disease has become severe. Foals infected with R. equi can shed the bacteria in their feces, leading to widespread exposure on farms and potentially causing disease in foals for years afterward.²

R. equi is not the most common cause of pneumonia for foals in the one to four month-old age group, however, it has significant economic consequences because of high mortality, prolonged treatment and expensive prophylactic strategies.³

Rhodococcus equi: Background

R. equi is a gram-positive, facultative intracellular pathogen that is ubiquitous in soil.² Development of clinical disease is related to the immunocompetency of individual foals, and manure from pneumonic foals is a major source of virulent bacteria contaminating the environment. While many foals are exposed to the pathogen via the respiratory route, in other cases, a foal inadvertently ingests sputum laden with R equi, which readily replicates in their intestinal tract. “The pathogenicity of R equi is linked to its ability to survive intracellularly, which hinges on failure of phagosome-lysosomal fusion in infected macrophages and failure of functional respiratory burst upon phagocytosis of R equi.”²

Clinical signs of R equi infection are usually difficult to detect until pulmonary infection reaches a critical level, seriously compromising the foal’s constitution. The available literature holds that R. equi pneumonia should be suspected on any breeding farm where several foals, between 4-12 weeks of age appear healthy but show low grade fever and coughing.^2,4

The most common clinical sign of R. equi infections in foals is bronchopneumonia. Early clinical signs may include a slight increase in respiratory rate and a mild fever. As the disease progresses, clinical signs can include:

• Decreased appetite
• Lethargy
• Fever
• Tachypnea
• Increased effort of breathing characterized by nostril flaring and increased abdominal effort 
• Cough and bilateral nasal discharge (rare)

Treatment of R. equi Infection in Foals

Clarithromycin is a bacteriostatic, time-dependent macrolide antibiotic with broad-spectrum activity against gram-positive aerobes and some gram-negative aerobes, such as the Pasteurellaceae. It is widely distributed, including into abscesses, and can be used to treat intracellular organisms.⁴ In foals with R equi pneumonia, the combination of clarithromycin (7.5 mg/kg, PO, bid) and rifampin is superior to erythromycin-rifampin and azithromycin-rifampin.⁴ Foals treated with clarithromycin-rifampin have improved survival rates and shorter febrile periods than foals treated with erythromycin-rifampin and azithromycin-rifampin.²

Where to buy Clarithromycin

Clarithromycin is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies. 

Clarithromycin carries the potential for numerous drug reactions. Please consult your veterinarian prior to beginning any treatment regimen.

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

¹Merck Veterinary Manual.

²Von Bargen, K., et. al. Molecular and infection biology of the horse pathogen Rhodococcus equi. In: JF FEMS Microbiology Reviews, JO FEMS Microbiol Rev, YR 2009, DO 10.1111/j.1574-6976.2009.00181.

³Vengust, Modest et al. Rhodococcus equi pleuropneumonia in an adult horse. The Canadian veterinary journal = La revue veterinaire canadienne vol. 43,9 (2002): 706-8.

⁴Jacks S, Giguère S, Gronwall RR, Brown MP, Merritt KA. Disposition of oral clarithromycin in foals. J Vet Pharmacol Ther. 2002 Oct;25(5):359-62. doi: 10.1046/j.1365-2885.2002.00420.x. Erratum in: J Vet Pharmacol Ther. 2003 Feb;26(1):79. PMID: 12423226.

⁶Womble AY, Giguere S, Lee EA, Vickroy TW. Pharmacokinetics of clarithromycin and concentrations in body fluids and bronchoalveolar cells of foals. Am J Vet Res. 2006;67(10):1681-1686.

⁷Peters J, Block W, Oswald S, et al. Oral Absorption of Clarithromycin Is Nearly Abolished by Chronic Comedication of Rifampicin in Foals. Drug Metabolism and Disposition. 2011;39(9):1643-1649.