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The combination of erythromycin and rifampin has been the treatment of choice for Rhodococcus equi infections in foals for many years. Erythromycin is a macrolide antibiotic. It works by inhibiting the production of protein by susceptible bacteria. At the usual therapeutic concentrations, erythromycin is bacteriostatic and has a relatively narrow spectrum (including R. Equi).
Rifampin is a somewhat unique antibiotic that is used in
combination with erythromycin. Rifampin penetrates abscesses and most body
tissues including bone and CNS. This property makes rifampin particularly
useful for the treatment of R. equi because one of the major problems
associated with these infections is abscess formation. Rifampin should always
be used in combination with another antibiotic because when rifampin is used
alone, the bacteria are rapidly able to develop resistance. Rifampin can be
either bactericidal or bacteriostatic depending on the specific organism and
the concentration of the drug.
Infection due to R. equi is generally thought to
be the most severe, bacterial pneumonia in foals. In addition to respiratory
disease, R. equi can cause gastrointestinal disease, and less commonly,
infections of the growth plate. R. equi infections can be both difficult
to diagnose and to treat. The disease can progress sub clinically until the
foal presents as a life-threatening emergency due to overwhelming infection
and widespread abscessation. Fatalities can occur even with prompt diagnosis
and treatment.
• The two most common side effects seen in foals due to erythromycin are mild to moderate diarrhea and hyperthermia. Occasionally the diarrhea is enough of a problem to require a change in antibiotic therapy.
• Foals on erythromycin are very sensitive to heat and possibly to bright sunlight. The mechanism of this hyperthermia is unknown but it is potentially fatal.
• Erythromycin is generally not used in horses one year of age or older. Severe, potentially fatal diarrhea can occur in these animals.
• Side effects due to rifampin are rare in the horse. In other species signs of GI pain, rashes, and increased liver enzymes have been reported especially with long term use. Rifampin should be used with caution in animals with decreased liver function.
• Rifampin can cause a harmless orange or red discoloration of urine,
or other bodily fluids.
• Most R. equi infections occur in the hot,
dry weather, making the management of hyperthermia difficult. Many veterinarians
do not turn foals on erythromycin out in the daytime, and may severely limit
their turnout time in general. Should hyperthermia occur, aggressive cooling
using water, fans or air-conditioning may be helpful.
• Although very rare, serious and potentially fatal diarrhea has been
reported in mares whose suckling foals are on erythromycin. Extra care should
be used to minimize or prevent any exposure to the mare. The water buckets
and the foal's skin are common sources of exposure.
• Erythromycin is generally not used with chloramphenicol or penicillin
antibiotics. Erythromycin can delay clearance of the bronchodilator aminophylline
to the point of potential toxicity.
• Rifampin may increase the metabolism of some drugs that are also metabolized
by the liver including chloramphenicol, corticosteroids, and anticoagulants.
None of these drugs are likely to be used with EES/rifampin in the treatment
of R. equi.
• Overdose of erythromycin can cause severe GI side effects.
• Overdose of rifampin would increase the likelihood and severity of
the previously mentioned side effects.
Dr.
Barbara Forney is a veterinary practitioner in Chester County, Pennsylvania.
She has a master's degree in animal science from the University of Delaware
and graduated from the University of Pennsylvania School of Veterinary Medicine
in 1982.
She began to develop her interest in client education and medical writing 1997. Recent publications include portions of The Pill Book Guide to Medication for Your Dog and Cat, and most recently Understanding Equine Medications published by the Bloodhorse.
Dr. Forney is an FEI veterinarian and an active member of the AAEP, AVMA, and AMWA.
You can purchase books by Dr. Forney at www.exclusivelyequine.com
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