While chronic diarrhea in horses is often associated with the presence of large or small strongyles (worm) infestations, giardiasis (giardia) has been reported in some cases as a cause of these symptoms. Giardia can be found in the feces of healthy horses, but is rarely recognized as a cause of diarrhea.1

Giardia (Giardia duodenalis) are anaerobic, flagellated protozoal organisms that infect the small intestine of many mammals, including horses. Recent research in several countries suggests that infection giardiasis is more common than the conventional wisdom has maintained, which could put young foals and even their human handlers at risk.2

In addition to giardia, anaerobic bacteria are significant and often overlooked bacterial pathogens of the lower respiratory tract in horses. Anaerobic bacteria are also widely believed to play a role in the development of equine endometritis.

Equine Giardia 

It is believed that approximately 0.5-35% of horses worldwide are infected with giardia. One found that between 11-14% of 398 tested foals from across Europe were found to excrete giardia cysts. Some foals shed only small numbers of cysts (50/gram of feces), but others shed up to 4 million infectious cysts/gram of feces. The study also reported that younger horses are more likely to be infected with giardia. After horses ingest giardia cysts, the parasite hatches (encysts) and gains the ability to mobilize and attach to the lining of the wall of the small intestine. Once attached, it begins acquiring nutrients and reproducing, some of which form cysts excreted in the horse’s feces.3

Since giardiasis is zoonotic, humans exposed to infected animals are also at risk of developing the infection. Giardia can cause diarrhea, ill thrift, poor hair coats, weight loss, and other common signs of intestinal parasitism in horses. Some infected foals shedding giardia cysts may not show signs of diarrhea, but they are still capable of infecting other animals, including humans. This presents the potential for a continuous cycle of transmission in a facility, especially when overcrowding is an issue.

Anaerobic Bacterial Infections 

Anaerobic bacteria (bacteria which flourish without the presence of oxygen) are often overlooked bacterial pathogens of the lower respiratory tract in horses.4 There are literally hundreds of anaerobic bacterial isolates that have been found in the pharyngeal tonsillar surface of healthy horses4; as with many bacterial infections, it is thought that immunocompromised states allow such bacteria to proliferate. 

In equine medicine, the term bronchopneumonia is often used to refer to lower respiratory tract infections, whether the infection is localized to the bronchi or the infection involves both the bronchi and the lung parenchyma.5 When there is subsequent extension of the infection from the pulmonary parenchyma to the visceral pleura and pleural space, the disease is referred to as pleuropneumonia. The spectrum of clinical signs shown by horses with bronchopneumonia is broad and reflects the severity of the disease process. Early identification of affected animals and immediate initiation of appropriate therapy are essential to prevent mortality and functional impairment of the respiratory system.4 Unfortunately, antimicrobials specifically targeting anaerobic pathogens are often not routinely employed.

Metronidazole for Equines

Metronidazole is an antibacterial and antiprotozoal agent that is available in injectable and oral forms. Its antibacterial properties are only effective against anaerobic bacteria. There are currently no veterinary-approved metronidazole products, but this drug has been used extensively in the treatment of Giardia spp in both dogs and cats. It is also used clinically in small animals for the treatment of other parasites (Trichomonas spp and Balantidium coli), as well as for treating both enteric and systemic anaerobic infections.5 It is sometimes used as a perioperative surgical prophylaxis antibiotic where anaerobes are likely to be present. In horses, metronidazole has been used clinically for the treatment of systemic and enteric (e.g., inflammatory bowel disease or IBD) anaerobic infections.

The U.S. Food and Drug Administration (FDA) has prohibited the use of metronidazole in food animals. 

Where to buy Metronidazole 

Metronidazole is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies. 

Metronidazole carries several potential drug interactions. Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

1Merck Veterinary Manual.

2ker.com.

3Kirkpatrick C.E., Skand DL. Giardiasis in a horse. J Am Vet Med Assoc. 1985 Jul 15;187(2):163-4. 

4Sweeney C.R., et. al. Anaerobic bacteria in 21 horses with pleuropneumonia. J Am Vet Med Assoc. 1985 Oct 1;187(7):721-4. 

5vetfolio.com.