Chloramphenicol has been used for a variety of infections in horses and is particularly useful for those caused by anaerobic bacteria. It has a wide spectrum of activity, can be given orally, and is not very expensive. One problem that has contributed to its use is a rare but serious aplastic anemia reaction in humans that come in contact with the medication. The development of aplastic anemia reported in humans, does not appear to be a significant problem for veterinary patients but suggests care must be exercised when handling this medication. This article contains information about the use, adverse effects, and dosage of chloramphenicol in horses.

Chloramphenicol is rapidly absorbed after taking orally or given by injection. It may be best absorbed when given with a meal. Chloramphenicol is widely distributed throughout the body and reaches therapeutic levels in most tissues and fluids, including the eye, joints, and CNS fluid. It also enters milk and crosses the placenta so its use in pregnant mares should be restricted. Because it is processed in the liver and urinary concentrations uncertain in the horse, chloramphenicol may not be the best choice for lower urinary tract infections.

Chloramphenicol is rapidly cleared from the blood therefore it must be given at high doses often and is best reserved for infections particularly sensitive to the drug and in locations that are difficult to get other antibiotics into. This might include CNS, intraocular, abscesses and bone infections. Chloramphenicol is considered bacteriostatic but at higher concentrations or against some very susceptible organisms it can be bactericidal. It has a wide spectrum of activity against many gram positive and negative organisms. Gram positive aerobic organisms that are generally susceptible to chloramphenicol include many streptococci, staphylococci, and anaerobic bacteria including Clostridum, Bacteroides (including B. fragilis), Fusobacterium, and Veillonella. Chloramphenicol also has activity against Nocardia, Chlamydia, Mycoplasma, Rickettsia and some gram negatives including Neissiera, Brucella, Salmonella, Shigella, and Haemophilus.