Description

Use of macrolide antibiotics began with the discovery of erythromycin, which was isolated from soil bacteria in the early 1950's.  Newer semi-synthethic relatives of erythromycin include claithromycin and azithromycin.  Working by inhibiting the production of protein by susceptible bacteria, they may be used alone or in combination with the antibiotic, rifampin.

The macrolide antibiotics include:

• Erythromycin
• Clarithromycin
• Azithromycin
• Tylosin
• Tilmicosin
• Tiamulin

Usage

Macrolide antibiotics are commonly used in the treatment of disease caused by the bacteria Rhodococcus equi, which causes severe pneumonia in foals.  Not only does R. equi infect the respiratory tract, causing pneumonia, it also infects the gastrointestinal tract, causing abcesses, and the joints and growth plates, causing septic conditions.

The newer drug, azithromycin has better absorption qualities than the original erythromycin and is less likely to cause gastrointestinal problems. Clarithromycin is considered to be the most effective drug of the three macrolide antibiotics, but an increased incidence of diarrhea is sometimes noted with its use.

Dosage and Administration

Macrolide Antibiotics
Method	Dosage (click row for calculator)	Concentration	Period	Duration
Oral Suspension Azithromycin1	10 mg/kg	40 mg/ml	Daily	NA
Oral Azithromycin1	10 mg/kg	500 mg/tablet	Daily	NA
Oral Clarithromycin2	7.5 mg/kg	250 mg/tablet	2 times daily	NA
Oral Clarithromycin2	7.5 mg/kg	500 mg/tablet	2 times daily	NA
Oral Erythromycin	11-14 mg/lb	250 mg/tablet	2 or 3 times daily	NA
Oral Erythromycin	11-14 mg/lb	500 mg/tablet	2 or 3 times daily	NA
Notes: Federal law restricts this drug to use by or on the order of a licensed veterinarian. Extra-label use of drugs in treating animals is allowable only by licensed veterinarians within the context of a valid veterinarian-client-patient relationship, and does not include drug use in treating animals by the layman (except under the supervision of a licensed veterinarian). 1Rhodococcus equi pneumonia—Foals: Oral, 10 mg per kg of body weight every twenty-four hours for five days, followed by 10 mg per kg of body weight every forty-eight hours. Note: The above dose has been administered concurrently with 5 mg rifampin per kg of body weight every twelve hours. The doses recommended are based on a retrospective clinical study and a pharmacokinetic study of foals. 2Rhodococcus equi pneumonia—Foals: Oral, 7.5 mg per kg of body weight every twelve hours. Note: The above dose has been administered concurrently with 5 mg rifampin per kg of body weight every twelve hours. The doses recommended are based on a retrospective clinical study and a pharmacokinetic study of foals. Calculator is for educational purposes only. Follow your veterinarian's instructions regarding use of this, or any medication.

Side Effects

Mild to moderate diarrhea in foals is a common side effect.  Also, hyperthermia, with increased body temperature, has been noted.  These side effects are potentially fatal, and foals on erythromycin, azithromycin, and clarithromycin should be monitored carefully.  Exposure to heat and light should be limited.

Precautions

Erythromycin is generally not used in horses one year of age or older because of potentially fatal diarrhea occurring in these animals.  Foals on macrolide antibiotics should not be turned out during the daytime and exposure to heat should be counteracted with aggressive cooling with cool water, fans, or air conditioning.

Although rare, serious and potentially fatal diarrhea has been reported in mares suckling foals that are on erythromycin.  Extra care should be taken to prevent any exposure to the mare.  Water buckets and the foal's skin are sources of exposure.

Macrolide antibiotics are not FDA approved for use with horses.  They are commonly used with foals and are accepted practice.  They are prescription drugs and U.S. federal law restricts these drugs to use by or on the lawful written or oral order of a licensed veterinarian.

Macrolide antibiotics are not used in horses old enough for competition.

Interactions

Erythromycin is generally not used with chloramphenicol or penicillin antibiotics and should not be used with gentamicin.

Erythromycin can delay clearance of the bronchodilator, theophylline, to the point of potential toxicity.

Clarithromycin and omeprazole may increase the serum concentration of one another.

Overdose

Overdose of any macrolide antibiotic can cause severe gastrointestinal side effects.