Neobenedenia and Benedenia: Marine Skin Flukes

Neobenedenia and Benedenia are marine monogenean skin flukes that infect a wide range of marine fish, causing flashing, skin irritation and cloudy eyes. Their resilient eggs and fast reproduction demand repeated treatment - praziquantel, freshwater dips and hyposalinity, plus egg control.

What they are

Neobenedenia and Benedenia are capsalid monogeneans - skin flukes of brackish and marine fish. They have low host specificity, meaning they infect many farmed and ornamental marine species, and they can cause major losses. They live on the fish's skin and eyes and reproduce with a direct life cycle.

Signs

• Flashing and rubbing the body against objects (irritation).
• Cloudy eyes or corneal ulceration when the eyes are involved.
• Skin irritation; heavy infections stress the fish and can be fatal.

Why they are hard to clear

These flukes reproduce explosively. Under standard conditions (about 24 C), a single worm can reach sexual maturity in around 10 days, lay on the order of 190 eggs per day - peaking near 500 - and can even self-fertilize and lay viable eggs without a mate. The eggs have a tough proteinaceous shell that resists most chemicals, so they survive treatments that kill the adults on the fish. Control therefore requires repeated or prolonged treatment across the egg-hatching interval, plus tank and egg hygiene.

Treatment approach

• Praziquantel at about 5 mg/L as a prolonged bath is the treatment of choice for these flukes.
• Short freshwater dips (roughly 1-5 minutes, depending on the fish's tolerance) partially remove monogeneans from marine fish.
• Hyposalinity can help - Merck notes that around 15 g/L for two days eliminated juvenile and adult Neobenedenia melleni, and 15 g/L held for five days prevented eggs from hatching.

Sources: Merck Veterinary Manual, Parasitic Diseases of Fish (www.merckvetmanual.com); PubMed Central, Reproductive strategies of Neobenedenia sp. (pmc.ncbi.nlm.nih.gov).