Metronidazole (Flagyl®) for Horses: Pharmacology, Clinical Uses & Side Effects

Metronidazole (Flagyl®) is a commonly prescribed antibiotic in equine medicine, primarily used to treat bacterial infections in horses.

It is effective against clostridial infections, which can disrupt the delicate balance of gut bacteria and lead to severe gastrointestinal illness including colitis and diarrhea.

Metronidazole is generally well-tolerated by horses, but potential side effects include uncoordinated movement and loss of appetite. As with all antimicrobial drugs, the risk of antibiotic resistance should be taken into consideration before administering metronidazole.

To ensure safe and effective use, it is essential to follow veterinary directions and the product label carefully. Metronidazole should only be used under the supervision of a veterinarian, who can assess the need for treatment and monitor for adverse effects.

Metronidazole for Horses

Metronidazole (Flagyl®) is an antibiotic used to treat infections in horses. It is mainly prescribed for infections caused by anaerobic bacteria, but it is also effective against certain protozoa and microaerophilic bacteria. ^[1][2][3][4]

In equine medicine, metronidazole may be used to treat some forms of colitis, topically to treat periodontal disease, and deep tissue infections. It is especially useful for infections in low-oxygen environments.

The risk of antibiotic resistance must be considered before using this drug. Metronidazole should only be used when necessary and exactly as directed by a veterinarian. ^[5]

Chemical Structure

Metronidazole’s chemical name is 1-(2‑hydroxyethyl)-2‑methyl‑5‑nitroimidazole. ^[1]

The core of this molecule is imidazole, a cyclic organic compound composed of three carbon atoms and two nitrogen atoms. This ring structure forms the backbone of the drug. ^[6]

Three key side groups (substituents) are attached to the imidazole ring: ^[6][7][8][9]

• 1-(2-Hydroxyethyl): Improves solubility, helping the drug dissolve and circulate in the body
• 2-Methyl: Increases molecular stability and slightly reduces water solubility
• 5-Nitro: The primary active group that targets and disrupts microbial pathogens

Effects of Metronidazole

Metronidazole is an antimicrobial drug used to target and destroy specific types of bacteria and protozoa. ^[4][10] Metronidazole is most effective against bacteria that require an oxygen-free environment to survive (obligate anaerobes). ^[10]

There is some evidence that metronidazole is effective against certain microaerophilic bacteria (organisms that only require marginal oxygen levels to survive) such as Campylobacter fetus, but more research is needed to confirm this. ^[4][10]

Metronidazole works by penetrating the bacteria and damaging its DNA, preventing continued proliferation. ^[4][10] Metronidazole has similar effects on protozoa once the single-celled organisms ingest and metabolize the compound. ^[10]

Clinical Uses in Horses

Metronidazole is used to treat bacterial and protozoal infections in horses. It is primarily prescribed for clostridial infections, which are caused by Clostridium bacteria — anaerobic, spore-forming organisms that can produce toxins and lead to severe gastrointestinal diseases. These infections can disrupt the normal gut microbiome and result in inflammation, diarrhea, abdominal pain, and systemic illness.

“Metronidazole is an important antimicrobial agent to combat certain infectious disease processes in equine medicine, particularly for treating infections caused by anaerobic bacteria. However, careful dosing and monitoring are essential to minimize the risk of adverse effects and ensure successful outcomes.”

— Dr. Jennifer Skaggs, DVM
Equine Veterinarian

Metronidazole may also be used to treat infections caused by other anaerobic or protozoal organisms. Since metronidazole is most effective against anaerobic bacteria, it is commonly used to treat infections in deep visceral tissues within the abdominal cavity, where oxygen levels are low and anaerobic organisms thrive.

Gut Dysbiosis & Clostridial Infections

Clostridia spp. are anaerobic bacteria commonly found in soil. These species are also present in the gut microbiome of healthy horses. Under normal conditions, other microorganisms and the horse’s immune system help regulate Clostridial populations to maintain a healthy digestive system. ^[11]

Although low levels of Clostridia spp. are normal in the equine gut, disruptions in the gastrointestinal environment can lead to overgrowth and infection. When these bacteria proliferate unchecked, they produce toxins that can severely damage the digestive tract. ^[11] This imbalance in the gut microbiome is known as dysbiosis.

Several factors can trigger dysbiosis, including: ^[11]

• Sudden dietary changes
• Administration of antibiotics
• Stress
• A heavy parasite burden
• Recent deworming treatment
• Recent surgery
• Medication that results in gastric acid suppression

Clostridial gut dysbiosis

There are several species of Clostridium bacteria associated with gut dysbiosis and infection in horses. Each species affects the horse differently, with varying levels of severity, transmission risk, and susceptibility based on age and immune status.

Clostridium difficile

C. difficile is a well-known species of Clostridium and is associated with severe gastrointestinal infection and disease in many animals, including horses and humans. This bacterium can survive in the environment for long periods. C. difficile spores enter a horse’s body through contaminated soil, feces, or hospital environments. ^[11]

Once inside the gut, the spores can proliferate into active bacteria that grow and release toxins, causing diarrhea and gut inflammation. ^[11] Both foals and adult horses can be affected. The disease is more likely to occur when antibiotics or other treatments disrupt the normal gut bacteria. ^[11]

Clostridium perfringens

C. perfringens is also commonly found in soil, feces, and in the equine gut microbiome. Horses can become infected by ingesting the bacteria from contaminated feces or soil. Foals are particularly vulnerable to this species of Clostridium, and may contract the bacteria from contamination on their dam’s teats. ^[11]

While C. perfringens is normally present in healthy horses, proliferation can lead to severe symptoms of gut infection, including diarrhea and colic. These infections are usually not contagious. ^[11]

Clostridium piliforme

C. piliforme is a less common bacterium because it can only survive inside host cells. This bacterium causes Tyzzer’s disease, which mainly affects foals under 45 days old and animals with weak immune systems. Adult horses are generally resistant to C. piliforme. ^[11]

Foals contract the infection by ingesting bacteria from contaminated feces. A foal’s risk may be higher if the dam is older than six years, possibly due to lower-quality colostrum. ^[11]
Stress, poor sanitation, overcrowding, and certain medications can also increase the risk of C. piliforme and Tyzzer’s disease. ^[11]

Clostridium sordellii

C. sordellii has recently been reclassified as Paeniclostridium sordellii. This bacterium can cause severe infections in humans and animals, including horses. It is commonly found in soil and sometimes in the intestines of healthy animals. ^[11]

It is known for causing gas gangrene and symptoms similar to toxic shock or sepsis, with little or no inflammation. ^[11] In horses, P. sordellii has been linked to rare cases of severe intestinal disease, where it damages the lining of both the small and large intestines, leading to bleeding, ulcers, and tissue death. ^[11]

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Other Infections

Other equine conditions treated with metronidazole include: ^[1][10]

• Peritonitis: Inflammation of the peritoneum, the lining of the abdominal cavity
• Abdominal abscesses: Localized pockets of pus within the abdominal cavity
• Empyema: Accumulation of pus in a body cavity, most commonly the pleural space around the lungs
• Periodontitis: A serious gum infection that damages soft tissue and bone supporting the teeth, potentially leading to tooth loss.
• Infectious osteitis and arthritis: Inflammation of bone (osteitis) and joints (septic arthritis)

Since peritonitis is a potential complication of abdominal surgery, metronidazole is sometimes used prophylactically in horses that have had surgery. ^[1]

Administration

There are several methods of administering metronidazole to horses. ^[1][10] It is easily absorbed by the horse’s tissues no matter how it is administered, and tends to reach peak concentrations within one to two hours. ^[7]

Most commonly, metronidazole is administered orally. ^[1] In rarer cases, metronidazole is administered per rectum, which has the benefit of bypassing the liver. It is also absorbed more quickly using this method than the oral route, although the bioavailability of this dosing route is lower. ^[1][10]

Metronidazole may also be administered intravenously. ^[1] It is sometimes used topically to treat thrush. ^[12]

Use in Competition

Metronidazole does not appear on the FEI Clean Sport’s Prohibited Substances list. ^[13] Always check the guidelines for your sport before administering medication to your horse.

Even when not prohibited, use of any medication should be recorded and disclosed if required by your competition’s medication reporting protocols. Keep in mind that even legal medications can raise concerns during testing if not properly documented or declared.

Side Effects

Severe side effects of metronidazole are not common in horses. ^[2] When they do occur, mild reactions to oral administration may include: ^[2]

• Loss of appetite
• Excessive drooling (ptyalism)
• Incoordinated movement (ataxia)

In some cases, metronidazole can cause severe colitis in horses.

Contact your veterinarian if your horse appears to have a negative reaction to any medication. Always follow the prescription label and any instructions from the treating practitioner.

Metronidazole has been confirmed as carcinogenic in mice and rats. The relevance of this finding in horses is unknown. ^[2] However, due to this, metronidazole is not prescribed for pregnant animals. ^[10]

Frequently Asked Questions

Here are some frequently asked questions about Metronidazole in horses:

Summary

Metronidazole (Flagyl®) is an antimicrobial drug used in horses to treat infections caused by anaerobic bacteria and protozoa.

• This medication works by damaging the DNA of anaerobic bacteria and protozoa, preventing them from proliferating
• Metronidazole is primarily prescribed to treat clostridial infections in horses, including C. difficile, C. perfringens, and C. piliforme, along with other conditions like peritonitis and abdominal abscesses
• Oral administration is the most common, but it is also given rectally, intravenously, and topically
• Side effects are uncommon, but can include loss of appetite, diarrhea, and excessive drooling
• Do not administer metronidazole to pregnant animals
• Metronidazole is not on the FEI Clean Sport's prohibited substances list