Flukes or Trematodes in Koi and Fish Ponds


FlukesMonogenetic trematodes are a separate class of parasites from ciliated protozoans. The monogenea do not have an intermediate life cycle off the fish. Those trematodes that do are called Digenetic trematodes, and use a bird or a snail as an intermediate host. These are not a considerable pathogen of Goldfish and so we are limiting our discussion to the far more common monogeneans. 
The common monogenean trematodes of Goldfish are of the genus Dactylogyrus or Gyrodactylus. Sorry about the names, I did not invent them. Their mode of reproduction [egg versus embryo] and the presence or absence of eyespots can differentiate these flukes. 
Try to remember that the gill fluke Dactylogyrus lives in the dark. It has eyespots, but still has to strain to see. 
The body fluke, Gyrodactylus, lives on the fishes’ skin and has no eyespots. What I find amusing is that the body fluke and the gill fluke hardly ever exist in homogenous populations and are distributed evenly over the body. In fact, looking back at almost a decade of light contrast microscopy and surface biopsies, I don’t recall a time when I noticed gill flukes exclusively on the gills or skin flukes restricted to the body. 
The fluke derives its nutrition from the slime coat of the fish. It does not suck blood or bite chunks of tissue from the fish, but will ingest nutritive mucus via its anterior end.  
Fish that are under attack by flukes will develop an excessive slime, isolate themselves, and clamp their fins. They may also show excessive scratching and flashing behavior. Eventually, the fish may develop sores or ulcers on their bodies as a result of the scratching behavior. Flukes are also known to carry hazardous bacterial fish pathogens on their haptens. These bacteria are deeply inoculated into the skin by the haptens. I maintain that “ulcer disease” is more commonly transmitted by flukes than by any other vector. In fact, the clinician would not be committing malpractice to assume that any fish which feature bacterial ulcerations in the body are, (or have been), parasitized by Flukes. 
Flukes can also do considerable damage to the gills of fish. With their hold-fast haptens deeply embedded in the gills of the fish, they can trigger Bacterial Gill Disease through three mechanisms. 
1) Direct trauma, chronically bleeding the fish out.
2) Indirectly causing stress, which potentiates the Bacterial Gill Disease outbreak.
3) Direct inoculation of the bacterial pathogen into the gill tissue. 
The microscopic appearance of trematodes [flukes] is extremely characteristic and it would be almost inconceivable that one would mistake a fluke for any other pathogen. They are the largest pathogen for which you would employ a microscope. They are easily witnessed at a combined power as low as 40X [forty times]. The fluke is an elongate [tubular] parasite with suction cups on one end and a gripping end which features a ring of small hooks which fan out around a pair of vicious looking hooks; the haptens. Inside the body of the fluke one will observe either a nondescript egg which is typically oval, or the observer might see a complete miniature of the mother. Indeed, in the early research done on trematodes of this nature, scientists were amazed to dissect as many as four generations of embryos from within one adult fluke. 
I maintain that flukes are the most compelling argument for owning a microscope. Let me explain. If you did not want to buy a microscope, you could use salt as a blanket treatment or panacea and you’d be “covered” in the instance of at least seven common pathogens. If you coupled that with Dimilin© or Lufenuron, there is only one parasite that would be left behind by this regimen. That sole survivor would be the Fluke. Since the flukes only have a mode of treatment which is either expensive [Praziquantel] or risky [Formalin, KmnO4] – then I would recommend the microscope to prove or disprove the necessity of these flukicide treatments. 
There are several ways in which the trematode life cycle impacts how we treat them. In the first place, the mother fluke often contains several future generations of young. Depending upon her species, she may carry a durable egg. The impact of this is that one might kill the mother with some waterborne treatment, and her body may remain on the aquarium bottom until such time as the egg or young will emerge from the remains of the mother and seek out a new host. This is the reason I recommend a repeat Fluke treatment approximately four days after the first one. 
In the second instance, we also know that this young trematode swarmer must find a viable host within a short time or it will die. This is how one can bypass the filtration system during treatments that would otherwise disable the nitrifying bacteria. Let’s work an example. 
During your first potassium treatment, it’s recommended that you would bypass your filter. Common sense might leave you wondering if there are parasites hiding in the filter, safe from the treatment. Indeed there may be. When you resume the filter, these parasites move out into the aquarium and resume the infection process. If they did not exit the filter, they would die of starvation. During your second treatment, there will be no parasites in the filter when flow is suspended. If there were, these would only be senescent adults. No larvae will have had time to re-emerge and be in your filter. Still, a third treatment (irregardless of whether you are using Formalin or Potassium permanganate) is recommended “just in case”. Understanding the life cycle of the Fluke is important in the determination of your treatment regimen. The recommendations I give you are not empirically derived. 
Additional scientific research has revealed an interesting result. It was found that a closed population of fish could not support Flukes indefinitely.  Indeed the fish seemed to develop specific surface immunity, which eliminated the flukes gradually. Only by introducing immunologically naive fish to the group did they maintain a viable, expanding population of flukes. The significance of this study is that if you “close” your Goldfish collection and resist the urge to bring in un-checked or un-quarantined fish, you might be able to avoid sustaining or introducing future fluke infestations. 
See:  Scott, M.E. & R.M. Anderson, Parasitology, 89 (Pt 1):159-94 1984 Aug.
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