Rhodococcus equi (previously known as Corynebacterium equi) is a bacterium that lives in the soil and is the most serious cause of pneumonia in foals one to six months old.1 In rare instances, immunodeficient adult horses and humans can also become infected. Its prevalence varies by region and year, but R. equi can be detected on most horse farms globally. In terms of exposure route, foals typically 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 many years.2

R. equi is not the most common cause of pneumonia 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 Clinical disease is very rare in horses older than eight months of age.

Rhodococcus equi: Morphology

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 difficult to detect until pulmonary infection reaches a critical level, seriously compromising the foal’s constitution. “Pulmonary lesions are relatively consistent and include subacute to chronic suppurative bronchopneumonia, pulmonary abscessation, and suppurative lymphadenitis.”3 At the onset of clinical signs, most foals are lethargic, febrile, and tachypneic. Diarrhea is seen in one-third of foals with R. equi pneumonia and may be caused by colonic microabscessation. Cough may be present; purulent nasal discharge may also be present, but is less common. Breathing may reveal crackles and wheezing with asymmetric distribution. 

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 The more heavily a breeding farm is contaminated with the organism, the younger will be the age of the foals affected. Mares with foals at foot from unaffected farms have been sent for breeding to infected farms and returned with an infected foal.4

Clinical Signs of R. equi Infection

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)

Because ultrasonographic screening for early detection has become routine practice at some farms endemic for pneumonia caused by R. equi, the most frequently observed form of R. equi infection at those farms is a subclinical form in which foals develop sonographic evidence of peripheral pulmonary consolidation or abscessation without necessarily manifesting clinical signs.4 Immune-mediated polysynovitis (eg, stifle and hock effusion) is often seen in affected foals at presentation.3 Intestinal and mesenteric abscesses are the most common extrapulmonary sites of infection. Foals with abdominal involvement often present with fever, depression, anorexia, weight loss, colic, and diarrhea.3,4

Treatment of R. equi Infection in Horses

R. equi pneumonia is often not recognized until it is well advanced and, therefore, difficult to treat. Antibiotics such as erythromycin, clarithromycin, and azithromycin can be used to treat R. equi infection in horses, but they can also result in potentially serious side effects such as diarrhea and hyperthermia. Resistance of the bacteria to some macrolide antibiotics.3

Clarithromycin offers numerous advantages over erythromycin and thus, is an attractive alternative for the treatment of R. equi infections. It is also frequently used in the treatment of stomach ulcers in horses.

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 

In foals, clarithromycin is well absorbed (≈50%-60%) after intragastric administration, with peak serum concentrations occurring about 1.6 hours after administration. Elimination half-life is about 5.4 hours.6 However, when used with chronic rifampin treatment, oral bioavailability can be reduced by 90%.7

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

Where to buy Clarithromycin 

Clarithromycin is available in the U.S. through 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.

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.