Jay Hemdal
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“The Eye Fluke” - Neobenedenia melleni
Jay Hemdal
Director of Animal Programs (retired)
The Toledo Zoo and Aquarium
Jay Hemdal
Director of Animal Programs (retired)
The Toledo Zoo and Aquarium
This serious disease of marine aquarium fish is caused by a type of flatworm termed a monogenean (requiring only one host to complete its life cycle). Because they do not require a secondary host, they can rapidly increase their population in marine aquariums by parasitizing the fish alone in a closed life cycle. They are often called “trematodes,” but they are actually not so closely related to digenean trematodes as was once thought – they are more properly termed Capsalid worm parasites. These relatively large (up to 8 mm), egg-laying parasites live on the skin or eyes of susceptible fish species. Neobenedenia eggs can take 14 days or longer to hatch into free-swimming larvae called oncomiracidium. Additionally, the eggs are resistant to most treatments and have sticky tendrils that attach them securely to all manner of objects in an aquarium. This makes it difficult to break the life cycle of this parasite.
There are approximately 200 Capsalid worm species, in nine subfamilies and 44–46 genera, including Benedenia, Capsala, Entobdella and Neobenedenia. Due to its broad host-specificity (known to infect more than 100 fish species from 30 families) N. melleni may actually be a complex of species (Whittington, 2004). There are probably other large Capsalid flatworms that infect aquarium fish, but they are difficult to differentiate from one another, even with a microscope.
Due to the huge numbers of fish passing through wholesale aquarium facilities each week, marine fish importers are suspected of being the primary cause of spreading this infection. The timeline of these infections is unfortunate; the importers see no losses from the low level infections their fish may have. Retail pet stores also won’t see any fish losses from Neobenedenia unless their fish remain unsold for long periods. It is the final consumer whose fish bear the brunt of this disease, seeing losses four to eight weeks after the initial infection.
Some suggest using a preventative freshwater dip as a treatment for all incoming fish. The two drawbacks to this are; 1) the dips are not 100% effective (and do not affect the fluke eggs at all) and 2) newly acquired fish often do not stand up well to the added stress of a freshwater dip when they first arrive.
The first obvious symptom of this infection is the formation of slightly cloudy eyes, caused by the transparent fluke feeding on the eye tissue and eliciting a tissue reaction. This gives this worm the common name of “eye fluke,” although the parasite feeds on other locations on the fish as well. As the infection becomes more serious, the fish will “flash” (scratch) and their coloration will become dull, their fins may become tattered, and they develop a generally “disheveled” look. Rapid breathing due to stress and possible secondary bacterial infections, and then death follow if a treatment is not begun in time.
This is an issue for captive fishes only; there are no reports of acute disease associated with infestations of Neobenedenia among fishes in the wild (Bullard et-al, 2000). Likely, this is due to a relatively low reinfection rate in the open ocean, versus the much higher rate seen when fish are held in the relatively close quarters of an aquarium – where the Neobenedenia larvae are easily able to locate a new host to attach to.
| Angelfish, Pomacanthus sp. | ++ |
| Barrimundi, Lates sp. | ++ |
| Batfish, Platax sp. | +++ |
| Butterflyfish, Chaetodon sp. | ++ |
| Cichlids, Tilapia sp. | +++ (when housed in seawater) |
| Invertebrates | 0 (but may carry eggs) |
| Jacks, Caraganidae | +++ |
| Lionfish, Pterois sp. | + |
| Lookdowns, Selene sp. | +++ |
| Pyramid butterflyfish, Hemitaurichthys sp. | +++ |
| Grouper family, Serranidae | ++ |
| Garden eel, Taenioconger sp. | + |
| Remora, Echeneis sp. | + |
| Sharks and rays, Elasmobranchs | 0 |
| Surgeonfish, Acanthurus sp. | ++ |
| Spadefish, Chaetodipterus faber | +++ |
Aquarium hosts for Neobenedenia sp. 0=not infected, + = sometimes infected,
++=commonly infected, +++=very commonly infected (From Bullard et-al 2000, and
personal obs.) Many other species can also be infected!
Other Worm Parasites of Marine Fishes
Most of the serious metazoan (multi-cellular) marine fish diseases are caused by flatworms termed monogeneans (requiring only one host to complete their life cycle). Because they do not require a secondary host, they can rapidly increase their population in marine aquariums. They are often called “trematodes,” but they are actually not so closely related to digenean trematodes as once thought.
Digeneans require multiple hosts in order to complete their life cycle, so they rarely become a problem in marine aquariums where the required secondary host species are almost never present. Digeneans either cause no visible symptoms, or the encysted state (called metacercaria) may cause white nodules under the fish’s skin on or its fins.
Dactylogyrus are tiny worms that have four eyespots and lay eggs. They are commonly found infecting the gills of fishes. Rapid breathing is the only obvious symptom of a gill fluke infection. The worms damage the gills as they feed, and secondary bacterial infections are common.
Gyrodactylus (skin flukes) are small worms with have no eyespots and are usually live bearing. Most often, they are found living on a fish’s skin or fins. These infections start off mild, with few symptoms. As worm populations increase, the fish will be seen flashing (scratching). The fins may become tattered, and the skin may develop a dull look.
Turbellarians are a group of worms related to trematodes. One group, Paravortex sp. causes very distinctive black spots on some species of fish, most notably tangs and surgeonfish. These parasites typically have eyespots.
Praziquantel is the treatment of choice for home aquarists for most of these parasites. Generally, an effective treatment is to dose Praziquantel at 2 mg/l every 5 yo 9 days for three treatments. Stubborn cases may require 4 mg/l, then, after 48 hours, change 50% of the water and re-dose with the same amount. Hyposalinity, successful with Neobenedenia, may not work for all other worm parasites, as some come from brackish water regions and are tolerant of low salinity (Hemdal 2015).
The best means to diagnose (but not treat) Neobenedenia is to give the suspect fish a five-minute freshwater dip in a clean, covered container. Tap water of the same temperature as the aquarium is the best water source for this. There is no need to worry about chlorine or the pH of the water in the dip, common practice shows that these parameters simply do not matter for short dips. Aquarists overly worry that the pH change, or the presence of chlorine will be “too stressful” to the fish receiving the dip. The obvious question is, “…but being placed into freshwater isn’t stressful?”.
Even a casual look at the bottom of the dip container afterwards will help to positively identify this disease. The worms turn whitish and fall to the bottom. Many aquarists mistake these for scales that were dislodged from the fish. However, looking at these “scales” under a dissecting microscope, or even a hand lens, will soon show them for what they are—dead Neobenedenia.
This is one disease where “partial control” isn’t possible – with even a few eggs remaining, infections can re-start. This means that total eradication of the parasite and its eggs is the only way to ensure it doesn’t return. For this reason, treatments such as garlic, copper, formalin dips or using cleaner wrasse or cleaner shrimp are simply not effective. While these treatments may limit the number of parasites, the small percentage that survive are able to begin a whole new population of Neobenedenia.
Barrett L. Christie, a public aquarium curator, has researched a variety of treatment methods and has struck upon one that is highly effective. The treatment is relatively simple; in a quarantine system, the fish are exposed to hyposalinity (low salinity) for 30 days. Exactly how low of a salinity is the variable that needs to be controlled. Some species of fish do not tolerate lower salinities, yet if the salinity is not reduced enough, the parasite population is only reduced, not eradicated. Barrett has hit upon a workable value of 17 parts per thousand, a bit less than half the salinity of normal seawater. Obviously, most invertebrates cannot be present during this sort of treatment. Sharks and some rays cannot tolerate it either. Due to inaccuracies of testing methods, it is best to target a salinity of 16 ppt, or a specific gravity of 1.012.
Assuming the fish are healthy in all other respects, you begin this treatment by lowering the salinity to the target value over 24 to 48 hours. During the low salinity treatment, water quality must be monitored closely, especially pH. Be aware that some other diseases, notably Uronema and Amyloodinium thrive at lower salinities. Luckily, another common scourge, marine ick, Cryptocaryon irritans, is also inhibited by low salinity. After 30 days, the salinity is gradually raised back to normal. It is imperative to perform this change back to normal seawater very slowly. While marine fish tolerate a drop in salinity very well, their kidneys have more difficulty adjusting as the salinity is raised. Never return fish to normal salinity faster than 72 hours, and don’t make large changes at one time.
Another alternative is a Praziquantel treatment at 4 ppm, followed by a 50% water change after 48 hours, then a second treatment 8 to 10 days later. To calculate parts per million, use the actual capacity of the aquarium (minus displacement of rocks, gravel, etc.) and multiple that by the target concentration. Divide that number by 266 to determine the grams of medication to add to the water. For example; an aquarium that holds 25 gallons of water, dosed with Praziquantel at 4 ppm would need 25 * 4 / 266 = 0.38 grams of medication. Praziquantel does not dissolve well in seawater. The best way to introduce it is to extrude it through a small brine shrimp net (while wearing gloves and a dust mask of course). This treatment works well for other species of flukes, but fails to completely clear Neobenedenia in many instances because the unhatched eggs are not affected.
At the Toledo Zoo Aquarium, we noticed that multiple Praziquantel treatments on the same system, over months to years, required higher and higher doses, combined with increased frequency of the treatments in order to maintain effectiveness. One supposition was that the target parasites were building an immunity to the drug. That seemed unlikely as genetic change in multi-cellular organisms typically takes longer to happen (as opposed to drug-resistant bacteria that can develop resistance in short order). We wondered then, what could be rendering Praziquantel so ineffective on repeat doses?
Subsequent research indicates that bacterial degradation of the Praziquantel (Thomas et-al, 2016) may be the process at work. Their study concluded that while Praziquantel is stable for over two weeks in sterile marine aquarium water, when dosed in working systems, it degrades below detectable limits in just nine days. A subsequent dose on the same system showed a reduction in Praziquantel in less than 48 hours. The presence or absence of fish in the system did not affect this rate of degradation. The natural bacterial population of the aquarium actually works to eliminate Praziquantel from the water.
Properly quarantining all new fish will tremendously reduce the problems of Neobenedenia and other fluke infestations cause aquarists. Low salinity, properly applied during quarantine will help ensure that your new fish will not carry Neobenedenia into your display aquarium.
References:
Bullard, S.A., Benz, G.W., Overstreet, R.M., Williams Jr., E.H. and Hemdal, J.F. 2000. Six new host records and an updated list of wild hosts for Neobenedenia melleni (MacCallum) (Monogenea: Capsalidae). Comparative Parasitology 67(2):190-196
Hemdal, J. F. 2015. The Salt Smart Guide to Preventing, Diagnosing, and Treating Diseases of Marine Fishes. 171 pp. Saltwater Smarts Publications. Toledo, Ohio.
Thomas, A, Dawson, M, Ellis, H., Stamper, M.A. 2016. Praziquantel degradation in marine aquarium water. PeerJ DOI 10.7717/peerj.1857
Whittington, I.D., 2004. The Capsalidae (Monogenea: Monopisthocotylea): a review of diversity, classification and phylogeny with a note about species complexes. Folia Parasitologica 51: 109–122, 2004
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