Praziquantel for Fish: Flukes, Tapeworms, Dosing and Safety
How praziquantel treats monogenean flukes and tapeworms in fish, its mechanism, immersion and in-feed dosing from Merck and peer-reviewed sources, and food-fish regulatory limits.
Overview
Praziquantel is an anthelmintic drug used against flatworms, namely trematodes (flukes) and cestodes (tapeworms). In fish health it is mainly used against monogenean flukes of the skin and gills and against tapeworms. It has relatively poor aqueous solubility, which influences how it is dissolved for immersion treatments.
Mechanism of action
Praziquantel increases the permeability of the parasite cell membranes to calcium ions, which causes muscle contraction and paralysis, and it disrupts the worm's tegument (outer surface), exposing it to attack. Its molecular target is a calcium-permeable ion channel found in praziquantel-sensitive flatworms. Because it is selective for flatworms, it does not act on protozoan parasites, crustacean parasites or bacteria.
What it treats
- Skin and gill monogeneans, including Gyrodactylus spp. and gill flukes of the Dactylogyridae
- Marine capsalid monogeneans such as Neobenedenia
- Cestodes (tapeworms)
- Some trematodes (flukes)
Administration and dosing
Praziquantel is given as a prolonged immersion (bath) or in medicated feed. The Merck Veterinary Manual lists the following sourced regimens for ornamental fish.
| Route | Dose | Duration |
|---|---|---|
| Prolonged immersion bath (Merck) | 5 mg/L | Prolonged bath |
| Short-term bath (Merck) | 10 mg/L | 3 hours |
| In-feed for cestodes (Merck) | 35 to 125 mg/kg, every 24 hours | 3 days |
| In-feed, yellowtail kingfish (Merck) | 50 mg/kg per day | 7 days |
The Merck Veterinary Manual describes a 5 mg/L prolonged bath as the treatment of choice for monogenean infection in freshwater and marine ornamental fish, with prolonged baths in large marine aquaria being the most common use against capsalid monogeneans such as Neobenedenia.
Safety and tolerability
Praziquantel is generally well tolerated in fish at therapeutic doses, but because individual species and invertebrate tolerances vary, a small group should be tested before treating an entire system, and animals should be observed closely. Its low water solubility means it must be properly dissolved (for example in a small volume of solvent or premixed) so the dose is evenly distributed rather than settling out.
Regulatory status
Praziquantel is not approved for use in food fish in the United States; the Merck Veterinary Manual states there is no FDA-approved praziquantel treatment for food fish, so any food-fish use would be extralabel or investigational and governed by veterinary oversight. Legal status varies by country and applies to food fish; consult a fish-health veterinarian and current regulations before treating.