Protozoan Parasites Treatment: Metronidazole (Flagyl)

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Protozoan Parasites Treatment: Metronidazole (Flagyl)

Protozoan parasites are very common in reptiles.

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Protozoan parasites are very common in reptiles and consist of unicellular organisms, which for the most part can be observed only with a microscope. This group of parasites includes the better-known amoebae, coccidia, and cryptosporidia organisms and also the lesser-known flagellates, ciliates, and parasites of the bloodstream (e.g., Plasmodium sp. and Haemosporina).  Metronidazole and the sulfa drug family are far and away the most commonly employed drugs for this group of parasites.

Metronidazole is a versatile drug that is bacteriocidal, trichomonacidal, and amoebicidal. The term cidal, when used medically, refers to something that kills rather than suppresses (static) the organisms it targets. The cidal action against susceptible bacteria appears to disrupt DNA and nucleic acid synthesis in the bacteria. Metronidazole is especially effective against anaerobes (bacteria that live in the absence of oxygen) and is considered to be one of the drugs of choice for anaerobic infections. The mode of antiprotozoan action is unknown.


Although a wide range of doses exists in the literature, four studies have given us better guidelines for dosing metronidazole (Flagyl). A study using yellow rat snakes (Kolmstetter, Cox, and Ramsay 2001) indicated that a dose of 20 mg/kg PO every forty-eight hours produced adequate serum levels of the drug to effectively control anaerobic and protozoan pathogens. In green iguanas (Kolmstetter et al. 1988), a dose of 20 mg/kg PO every twenty-four to forty-eight hours was sufficient. Bodri (2001) found that a dose of 50 mg/kg PO every forty-eight hours maintained good serum levels in corn snakes. The most recent study was performed on red rat snakes and found that an oral dose of metronidazole at 50 mg/kg every forty-eight hours was more than enough to control susceptible anaerobic bacteria and protozoa (Bodri 2006). Jacobson and Kollias (1988) reported deaths in indigo snakes at dosages above 100 mg/kg; however, dosages at 40 mg/kg have been given safely. Jarchow (J. Jarchow 1988, pers. comm.) has reported deaths in Lampropeltis zonata and Lampropeltis pyromelena at doses above 100 mg/kg.

Metronidazole is contraindicated for debilitated specimens, gravid individuals, and animals with hepatic (liver) dysfunction. Toxicity is manifested initially by lethargy, weakness, vomiting, and diarrhea. Hepatoxicity and CNS signs usually result from repeated or excessive dosages. DeNardo (D. F. Denardo 1993, pers. comm.) reported seeing CNS signs in snakes, including head tilting, star gazing, and altered locomotion, when excessive doses were given. He felt that many of these snakes had been incorrectly medicated by owners giving a “dab” of a pill that turned out to be quite an excessive dose. Doses should always be measured. Metronidazole should be used only as long as necessary to eliminate the agent being targeted. Simple overgrowth of trichomonads in a dysfunctional gut will generally require only a dose or two. Anaerobic and amoebic infections may require the administration of several doses. Although no specific data prove teratogenic (birth defects) effects in reptiles, birth defects are known to occur in mammals, so administration of this drug to gravid reptiles is not recommended.

The author prefers to use the metronidazole suspension (Flagenase 400, available in Mexico) or a custom formulation rather than the tablets. Drug formulating pharmacies can also prepare metronidazole in a variety of flavor bases and strengths. It is easier to measure and administer the liquid, and the liquid ensures precise dosage to small reptiles. Tablets can be ground up and mixed with water, but the drug is not water soluble, is bitter tasting, and can be more difficult to dose properly.

Metronidazole has also been touted by some herpetoculturists as an appetite stimulant. There is no evidence to suggest that this drug has any properties to directly stimulate appetite. It may possibly stimulate appetite indirectly by correcting an inappropriate bacterial flora, eliminating protozoan flare-ups in the GI tract, or eliminating subclinical amoebic populations (Frye 1991).

Excerpt from the book Understanding Reptile Parasites by Roger Klingenberg with permission from its publisher, Advanced Vivarium Systems.