Fluke resistant to treatment

Over the past few months, we have found that the traditional range of anti-fluke treatments were becoming less effective.

In fact, from evidence gathered by koi keepers with microscopes showed a sharp decline in effectiveness. They have to increase dosages by up to 5 times than normal to see these critters dyeing. Since the flukes are not eradicated the health of the fish are not improving. This is clearly not an ideal situation as the fish ends up dead. 

Slightly irritated body fluke after a five times dose of fluke treatment. [Courtesy Quinton Jones]

The most common treatments are a class of drugs called anthelmintics meant for control of internal wormlike parasites common in man and animal. Because they the work on a systemic basis they are virtually harmless to human and higher animals. Therefore, they also work for fish and kills wormlike parasites like fluke by interfering with their metabolism.

These drugs are important drugs for deworming humans, veterinarian, and farm animal husbandry. Because taken internally, dose size is small and very effective. However, for the external treatment of fish in ponds, higher dosages are needed making treatment expensive. We must treat the pond to treat the fish. So, for the fish fraternity, we are motivated to use a lower dosage due to the cost of the drugs. In the meantime, the cost of these drugs rose drastically because of the “medical” tag. Us fish-people followed by lowering the dose to a minimum. It was just a matter of time for our problematic fish parasites to develop resistance to these drugs.

This is also a worrying tendency I see developing for similar products in the animal husbandry fraternity. Farmers should be aware of the potential economic consequences of under-dosing.

Competition is the game, and it is often the manufacturers that are to blame for this under-dosing. To be relevant in the market, manufacturers tend to announce new products all the time. A new “super” version of a known effective product. These are often mixtures of known drugs and driven by the cost of effective one, the concentration is reduced of that drug and another, cheaper one added. Now the possibility of resistance is more likely if the biological control mechanisms are different for the two drugs.

There are also the possibility that what we buy is not always what we think we get. The 'dilution effect' can will eventually reach the consumer.

As in the in animal husbandry we see a constant chain of “new” drugs needed to keep our animals healthy.


To solve the above problem, we now offer Xterm as a replacement. A treatment for skin fluke (Gyrodactylus ) and gill fluke (Dactylogyrus ) and some other parasites effective by a different mechanism. Also, more affordable. We recommend you give your old favourite fluke medication a rest for awhile and hopefully things will return to normal over time.

A repeat dose of Xterm is highly recommended 3-5 days later. (Three days if water is warm and longer as it gets colder.)

It is a i any case good idea to prophylactically treat your pond at least two time a year. Before winter and before summer is a good idea. You will have much healthier pond environment.


Servaas de Kock
July 2020

Please note: Potassium permanganate dip is still effective for fluke and other parasites. But not all people can or will dip fish. It also requires skill, tools, and know-how. Treating your pond  with PPM may damage your biological filter.