Though the drugs are similar there are other important differences between the members of the tetracycline family of antibiotics. Recently contracted tendons in very young foals have been treated with TC. This article discusses the uses, pros and cons, and dosages of this family of drugs.

Tetracycline and its close relative oxytetracycline are the drugs of choice for treating rickettsial infections in the horse including Potomac Horse Fever and Ehrlichia equi. Its poor intestinal absorption and list of adverse reactions makes its use in other infections questionable and should be reserved for those infections that better choices are not clearly available. Doxcycline, another relative to tetracycline, has been shown to be readily absorbed from the digestive tract and may be a useful antibiotic when the situation calls for it.

As a class, the tetracyclines have activity against most mycoplasma, spirochetes (including the Lyme disease organism), Chlamydia and Rickettsia. Against gram positive bacteria, the tetracyclines have activity against some strains of staphylococcus and streptococci, but resistance of these organisms is increasing. Gram positive bacteria that are usually covered by tetracyclines, include Actinomyces sp., Bacillus anthracis, Clostridium perfringens and tetani, Listeria monocytogenes and Nocardia. Among gram negative bacteria that tetracyclines usually have in vitro and in vivo activity against, include Bordetella sp., Brucella, Bartonella, Haemophilus sp., Pasturella multocida, Shigella, and Yersinia pestis. Many or most strains of E. coli, Klebsiella, Bacteroides, Enterobacter, Proteus and Pseudomonas aeruginosa are resistant to the tetracyclines. Doxycycline generally has very similar activity as other tetracyclines against susceptible organisms, but some strains of bacteria may be more susceptible to doxycycline or minocycline and additional in vitro testing may be required.