Metronidazol in Horses


Metronidazole is an antibiotic, which is commonly used to treat protozoal infections and anaerobic bacterial infections. It also has anti-inflammatory effects in the bowel. Metronidazole is bactericidal; it kills bacterial microorganisms by disrupting their DNA. It is rapidly absorbed from the GI tract, metabolised by the liver and excreted in the urine and the faeces. Because metronidazole only has activity against anaerobic bacteria, it is commonly used with other antibiotics when it is used to treat mixed bacterial infections. It is compatible with many other antibiotics including penicillin antibiotics, aminoglycosides, and some cephalosporins. Metronidazole has activity against most obligate anaerobes including Bacteroides sp. (including B. fragilis), Fusobacterium, Veillonella, Clostridium sp.(including C. difficile), peptococcus, and pep­tostreptococcus. Actinomyces is frequently resistant to metronidazole.

Metronidazole is also trichomonacidal and amebicidal in action and acts as a direct amebicide. Its mechanism of action for its anti-protozoal activity is not understood. It has therapeutic activity against Entamoeba histolytica, Trichomonas, Giardia, and Balantidium coli. It acts primarily against the trophozoite forms of Entamoeba rather than encysted forms. Cryptosporidium sp. and Eimeria leuckarti are resistant to metronidazole.


Although there are no veterinary-approved metronidazole products for horses, the drug has been used extensively in the treatment of anaerobic infections.
Metronidazole is primarily used with other antibiotics to treat mixed bacterial infections in which anaerobic bacteria are present, for example, pleuropneumonia, peritonitis, right dorsal colitis, idiopathic toxaemic colitis, hoof and abdominal abscesses. It is also used prophylactically after colic or other abdominal surgery when mixed bacterial infections are a risk. Metronidazole is generally given orally although it is also absorbed rectally. Rectal administration is occasionally used in the very sick patient when anorexia and weight loss are a problem.


Metronidazole is relatively well absorbed after oral administration. The oral bioavailability of the drug in horses averages about 80% (range 57-100%). Peak levels occur about one hour after dosing. After rectal administration the bioavailability of metronidazole is lower (range 20-40%). There are no significant differences in mean absorption time (45+/-69 for oral route vs. 66+/-18 min for rectal route) and the time to reach maximum serum concentration (65+/-36 vs. 58+/-18 min). These results indicate that rectal administration of metronidazole to horses, although inferior to oral administration in terms of bioavailability, provides an alternative route of administration when p.o. administration cannot be used. Metronidazole is rather lipophilic and is rapidly and widely distributed after absorption. It is distributed to most body tissues and fluids, including to bone, abscesses, the CNS, and seminal fluid. It is less than 20% bound to plasma proteins in humans.