Azithromycin is a macrolide antibiotic that has a broad spectrum and favorable pharmacokinetic profile. It is used to treat a variety of bacterial, rickettsial, and parasitic infections. Azithromycin has some anti-inflammatory and immunomodulatory effects however, the clinical relevance of these effects in veterinary species is unknown.1

Rhodococcus equi

R. equi is in fact the most serious cause of pneumonia in foals 1–4 mo old. It is not the most common cause of pneumonia in this age group; however, it has significant consequences because of mortality, prolonged treatment, surveillance programs for early detection, and relatively expensive prophylactic strategies.2,3 

Foals generally come in contact with R. equi through inhaling airborne bacteria into their respiratory tract or by ingesting contaminated soil. The infection progresses slowly, and foals often do not typically begin to show clinical signs until the disease has become severe. Foals infected with R. equi can shed the bacteria in their feces, leading to the pathogen spreading through a breeding farm and causing disease in the foals in subsequent years.

In foals, the clinical signs of R. equi can include fever, labored breathing, cough and depression. In some cases, it can also cause abscesses in the abdomen and elsewhere in the body, as well as joint infections.2 Many foals infected with R. equi can recover with proper treatment. 

Lawsonia intracellularis

In recent years, proliferative enteropathy (PE) caused by Lawsonia intracellularis has emerged as a significant equine pathogen.3 Early diagnosis of this disease allows prompt initiation of appropriate therapy to help decrease the severity of this disease. L. intracellularisis an obligate intracellular, gram-negative, curve-shaped bacteria. The organism typically produces lesions in the lower ileum and sometimes, the large intestine.4 Edema occurs in the serosal layer of the intestine, which results in the necrosis and mucosal thickening. The bacteria is usually seen in the apical cytoplasm of these cells. 

Affected horses are most often weanlings, 6 to 7 months of age. It is believed that the stresses of weaning and other factors that arise at several months of age (deworming, vaccinations, training, and increased population density) may contribute to the increased incidence in this age group.3,4 A decline in maternal antibodies around this age may also increase susceptibility to the organism in these young animals. 

Presumptive diagnosis of L. intracellularis can bemade based on clinical signs, hypoalbuminemia, ultrasonographic examination, and ruling out of other possible causes of enteritis and hypoproteinemia in the horse.3 This early diagnosis allows immediate initiation of treatment and may result in a better outcome for the animal. Clinical signs in horses range from mild to severe and include weight loss, rough hair coat, diarrhea, ventral edema, pot-belly appearance, and poorbody condition. The most profound pathologic finding is hypoproteinemia, specifically hypoalbuminemia.4

Based on several recent studies, azithromycin has been highly effective in mitigating L. intracellularis in horses.

Azithromycin for Horses

Azithromycin inhibits protein synthesis by penetrating bacterial cell walls and binding ribosomal subunits in susceptible bacteria. Azithromycin is a bacteriostatic antibiotic. Azithromycin can accumulate and persist in macrophages, neutrophils, and pulmonary epithelial lining fluid.1 

In foals, azithromycin is variably absorbed after oral administration, with a mean systemic bioavailability ranging from 40% to 60%. The effect of food on absorption is unclear. Azithromycin has a very high volume of distribution (11.6-18.6 L/kg). Elimination half-life is approximately 20 to 26 hours. The drug concentrates in bronchoalveolar cells and pulmonary epithelial fluid. Elimination half-life in polymorphonuclear leukocytes is approximately 2 days.1 In adult horses, intragastric administration (tablets suspended in 500 mL of water) resulted in an average oral bioavailability of 45%, with peak levels occurring ≈1 hour after administration.

Listed adverse effects for azithromycin include vomiting, reduced appetite, and diarrhea.1 As compared with erythromycin, azithromycin has significantly fewer GI adverse effects. Local reactions have occurred in patients receiving IV azithromycin.

In a preliminary study in horses, azithromycin (10 mg/kg PO once daily for 5 days) was observed to have no life-threatening adverse effects. Anhidrosis has been noted in foals but appears to be less significant than with erythromycin.1,5 Some self-limiting GI side effects (eg, decreased appetite, change in fecal consistency) were observed with azithromycin treatment.1

Where to buy Azithromycin 

Azithromycin is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. AZITHROMYCIN 200 MG/ML, ORAL SUSPENSION (500ML) is indicated for the treatment of R. equi in foals, as well as other bacterial infections in the horse.

Azithromycin carries numerous potential drug interactions. 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.

1Plumb’s Veterinary Drugs.

2Merck Veterinary Manual.

3aaep.org.

4Feary D.J., et. al. Lawsonia intracellularis proliferative enteropathy in a foal. Schweiz Arch Tierheilkd. 2007 Mar;149(3):129-33. 

5Stewart A., et. al. Effects of clarithromycin, azithromycin and rifampicin on terbutaline-induced sweating in foals. Equine Vet J. 2017;49(5):624-628.