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 in veterinary patients. Azithromycin has some anti-inflammatory and immunomodulatory effects however, the clinical relevance of these effects in veterinary species is unknown. Azithromycin has been used as a prokinetic in dogs and cats.1 

Rhodococcus equi

Azithromycin is widely used in the treatment of Rhodococcus equi infections in foals. 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 economic consequences because of mortality, prolonged treatment, surveillance programs for early detection, and relatively expensive prophylactic strategies.2 Clinical disease is rare in horses older than 8 months old. Epidemiologic data indicate that pulmonary infection probably originates within the first week of life.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

Proliferative enteropathy (PE) caused by Lawsonia intracellularis has emerged as a significant equine pathogen in recent years.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. 

The method of infection and transmission of the disease in the horse is still largely unknown, although the fecal-oral route is likely. While L. intracellularis is often seen in pigs, the main source of the disease in pigs is the mixing of chronic carriers with uninfected pigs. This seems less likely with the horse, because clinical equine cases usually occur as isolated cases or a few sporadic cases on a farm, as opposed to a wholesale herd outbreak.

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.

Where to buy Azithromycin 

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

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.

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.