Basic Information

Doxycycline is a semi-synthetic tetracycline antibiotic, derived from oxytetracycline. It may be used orally (dogs, cats and horses) or intravenously (dogs and cats). Tetracycline antibiotics are broad-spectrum and bacteriostatic. Their mechanism of action is through the inhibition of protein synthesis, and the alteration of cytoplasmic membrane permeability within the susceptible organism.

Doxycycline is more lipid-soluble than other tetracycline antibiotics and may reach higher levels in some of the difficult to penetrate areas, such as synovial fluid, central nervous system, prostate, and the eyes.

Tetracycline antibiotics, including doxycycline, are effective against a broad spectrum of gram-negative and gram-positive bacteria, and wide variety of other organisms including Rickettsia, Spirochetes, Mycoplasma, Leptospira, Anaplasma, and Chlamydia. In addition to its use as an antibiotic, doxycycline is used in low doses as an anti-inflammatory and adjunct treatment for arthritis and degenerative joint-disease in dogs and horses. This use is based on the inhibition of metallo-matrix proteinases.

Doxycycline is well-absorbed after oral administration and its absorption is minimally affected by the presence of food in the stomach. Doxycycline may be used in animals with decreased kidney function because it is eliminated primarily via the GI tract.

Dogs and Cats

Doxycycline is used in dogs to treat susceptible bacterial infections and infections caused by Rickettsia, Canine ehrlichiosis (anaplasmosis), Toxoplasma, Borrelia burgdorferi (Lyme disease), Leptospirosis and Neorickettsia helminthoeca (salmon poisoning).

Doxycycline is used in cats to treat susceptible bacterial infections and infections caused by a number of other organisms including Bartonella, Hemoplasma, Chlamydia felis, Ehrlichia, Anaplasma, and Toxoplasma.

There is some interesting research regarding the use of doxycycline as a “pre-treatment” in animals with circulating microfilaria due to heartworm disease. The research indicates that the pathogenic microfilaria are generally infected with a symbiotic bacteria called Wolbachia, which is sensitive to doxycycline. Pre-treatment with doxycycline is postulated to adverse reactions to microfilaria death.

Doxycycline gel is used topically in the mouth of dogs and cats for periodontal disease. Local application in the sulcus provides higher concentrations of the drug than may be achieved with systemic antibiotics.

Horses

Doxycycline is used in horses to treat susceptible bacterial-infections and tick-borne diseases such as Ehrlichia, Anaplasma, and Borrelia burgdorferi (Lyme disease).

Side Effects

• The most-common side effects in dogs and cats are GI upset, including nausea and vomiting.
• Cats may be at increased risk for esophageal strictures after “dry” pilling with doxycycline. In order to minimize this problem, it has been suggested to dose the cat with water after pilling or use a compounded liquid product to minimize esophageal damage.

Precautions

• Tetracycline antibiotics should be avoided during pregnancy because of risk of skeletal limb abnormalities and discoloration of teeth. Doxycycline may pose less risk than other tetracycline antibiotics; although, its use should be avoided unless the benefits outweigh the potential risks.
• Intravenous doxycycline should not be used in the horse due to risk of fatal cardiac arrhythmia.

Drug Interactions

• Dosing of oral doxycycline should be separated from any oral antacids, bismuth, kaolin, or pectin-containing products by 1- 2 hours. Oral iron also interferes with the absorption of doxycycline and should be separated by three hours.
• Doxycycline and other tetracycline antibiotics are generally not used with bactericidal antibiotics such as penicillin, cephalosporin, and aminoglycosides.
• Tetracycline antibiotics may change clotting times. Animals receiving warfarin or other anticoagulant may need additional monitoring and dosage adjustment.
• Phenobarbital may reduce the half life of doxycycline.
• Overdose
• In most instances, oral overdose of doxycycline will cause pronounced GI distress. Oral antacids may be helpful to decrease gastrointestinal symptoms.