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.

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.