Tetracycline and the related drug doxycycline have been used in horses for a long time. Tetracycline has good activity against many common organisms infecting the lungs and is the drug of choice for serious chronic infections like Lyme disease, Ehrlichiosis (including Potomac horse fever), Babesiosis, Leptospirosis, Lawsonia and other chronic infections.
The anti-allergy, anti-inflammatory and enzyme-suppressive effects of tetracyclines should be considered when deciding on an antibiotic to treat conditions where these bonus effects would also be desirable. The day may come when tetracyclines and other immune/inflammation-modulating antibiotics are part of the treatment arsenal for conditions you would never dream of now.
In human medicine, the tetracycline family of antibiotics is known to do more than just fight infections. Autoimmune diseases like rheumatoid arthritis may show improvement with tetracycline therapy, and tetracyclines can even block the development of autoimmune arthritis.
Symptoms of chronic Lyme disease in people who have no evidence of an ongoing active infection often respond well to tetracycline anyway. Experimental studies have shown that tetracyclines can tone down inflammation and moderate lymphocyte responses like antibody formation. They can also decrease levels of circulating IgE, the immunoglobulin associated with allergic reactions.
Two studies have now shown that tetracyclines may be of benefit in treating horses with COPD, aka “heaves.” A 1997 University of Helsinki study found that fluid in the lungs of horses with COPD is high in collagenase activity. Collagenases are enzymes that attack connective tissue. Inhibition studies have shown that doxycycline can directly inhibit the activity of these enzymes.
A 2000 study from the same university also found high levels of elastinase (breaks down elastic connective tissues) in the lungs of horses with COPD. These enzymes were inhibited by another tetracycline family drug. Studies in other species have shown tetracyclines can slow the breakdown of cartilage in arthritic joints.