Erythromycin In Horses
Erythromycin is an interesting drug in horses, having saved the lives of countless numbers of foals but able to readily kill an adult horse. Erythromycin is an antibiotic of the macrolide class. It is effective against many Gram-positive bacteria (e.g. Staphylococcus, Streptococcus, Rhodococcus), but not Gram-negative bacteria (e.g. E. coli, Salmonella).
There are several very good aspects of erthyromycin, including its ability to kill bacteria that have invaded cells (intracellular bacteria) and the ability to treat abscesses (many antibiotics can't do that very well at all). Erythromycin is also available in an oral form, which makes it much easier to give. It is most commonly used for the treatment of Rhodococcus equi pneumonia in foals, almost always in combination with the antibiotic rifampin, and it's a highly effective treatment.
However, the news isn't all good. Erythromycin is far from a benign drug, and should only be used in certain situations. Some of the problems associated with its use are:
Diarrhea: Any antibiotic can cause intestinal upset, ranging from mild diarrhea to fatal colitis, but erythromycin is a particularly high-risk drug. Diarrhea can occur in foals being treated with erythromycin, but this is not very common and when it occurs diarrhea is not usually very severe. However, giving oral erythromycin to an adult horse is a very high-risk proposition that can easily result in fatal colitis. In Sweden, there have been reports of fatal colitis in mares whose foals were treated with erythromycin - the hypothetical cause of this phenomenon is that mares were exposed to extremely low amounts of erythromycin from the foals' mouths or feces. Erythromycin should be avoided in adult horses at all costs.
Hyperthermia: This is a strange but rare side effect that is most often seen in foals being treated in sunny areas without access to shade. Some treated foals lose the ability to properly control their body temperature, and can develop extremely high fevers. The classic situation is an otherwise healthy foal that is responding nicely to treatment suddenly found down in the field one day with a life-threateningly high body temperature. This is uncommon, but it is a concern and treated foals should always have access to shade and be closely monitored.
Erythromycin can be a very useful drug is some situations. It's use is declining somewhat as related drugs like azithromycin are sometimes being used in its place, but it can still be a good treatment option in some cases. Because of the risks of adverse effects (and because of basic principles of antibiotic use), you should only use erythromycin under the direction of a veterinarian. Never use it in an adult horse unless you have no other choice, and realize there is a reasonable chance the horse will develop potentially fatal diarrhea.
Image: 3D model of erythromycin molecular structure (source: www.3dchem.com)
