Malachite Green in Fish: History and Why It Is Banned for Food Fish

Malachite green was an effective antifungal and antiparasitic for fish, but it is a suspected carcinogen whose residues persist, so it is banned for food fish in the US, EU and many countries.

Overview

Malachite green is a triphenylmethane (triarylmethane) dye that was historically used in fish health for its strong antifungal and antiparasitic activity. Although it is effective, it is a suspected carcinogen and mutagen, and it is banned for use in food-producing fish in the United States, the European Union and many other countries. This article describes it for reference and food-safety awareness, not as a recommended treatment for food fish.

Historical uses

Malachite green was used against Saprolegnia, the water mold that infects fish eggs and damaged tissue, against Ichthyophthirius multifiliis (freshwater Ich) and other external protozoa, and was sometimes combined with formalin. Its effectiveness is why it became widespread before its residue and carcinogenicity concerns were understood.

Why it is banned for food fish

In fish, malachite green is metabolized to its reduced form, leucomalachite green (LMG), which is lipophilic and persists in tissue far longer than the parent dye; reported figures include an LMG tissue half-life of about 10 days versus about 2.8 days for malachite green, with prolonged retention in muscle. Toxicology studies have raised carcinogenicity concerns, including an increase in lung adenomas in male rats exposed to leucomalachite green. For these reasons the United States has prohibited it for food-related use since 1983, and it is likewise banned for food use in the EU and other jurisdictions.

Residue monitoring and enforcement

Because residues of malachite green and leucomalachite green make seafood adulterated, regulators monitor for them. The FDA has detected malachite green contamination in imported seafood and refused or detained shipments on that basis. The Merck Veterinary Manual lists malachite green among compounds that should never be used in food animals for any reason, alongside chloramphenicol, nitrofurans, fluoroquinolones and quinolones, and steroid hormones.

Other cautions

Beyond the food-safety ban, malachite green is a dye that stains skin, equipment and decor, and it can be toxic to plants and to sensitive or scaleless fish. Where any non-food (ornamental) use is permitted, local law and product labeling govern it; for food fish the approved options for fungus and external parasites are drugs such as formalin and hydrogen peroxide.