Birds, Parasites and Koi Pond Health

Q: My pond has a waterfall which returns my filtered water back to the pond (2,300-gallons). As I sat in my garden enjoying my koi , I started to wonder if the wild birds are introducing pests into the pond. As they fly away after a complete preening in my pond water, I feel more and more anxious. Should I let this be or discourage it from happening?

Last April, early in the month my fish began feeding after a winter break, and they were flashing and leaping out of the water appearing to be shaking off parasites. On close inspection I cannot see any visible problems. I’m wondering now, if the problem last year was due to parasites from the birds and don’t want it to happen again now the spring is approaching and the birds are appearing. Last year I treated the pond with Malachite and formalin, but it didn’t help – do you have any ideas of what it could be and how I can prevent the problem before it happens again this year.

Thanks for providing such a detailed background to your question. I can appreciate your frustration in that your koi appear to be suffering from a parasite problem, and you are concerned to prevent it from happening again in your pond this year. Furthermore, having apparently contracted a parasite problem, your control measures did not appear to alleviate the symptoms of irritation – such as flashing, flicking and jumping.

All may not appear to be as obvious as you first thought, and a little more investigative work might unearth the real cause of your koi’s strange behaviour – putting your mind at rest. With there being two or three different areas to explore, it is difficult to know where best to start.


It is conceivable for birds to spread aquatic organisms on their feet (new waters are colonised with fish by fish eggs being transferred on bird’s feet). However, the chances of this are remote as these birds must have visited other ponds recently to have an effect. Furthermore, birds that are suffering from external avian parasites that they may remove during preening can in no way prove contagious or a risk to your koi (in the same way that we cannot be affected by fish lice).

Where koi are kept in semi-natural conditions, then fish-eating birds (such as terns, cormorants and herons) can prove to be effective vectors in the eye-fluke’s life-cycle. Droppings from infected birds contain the parasite which must find its way into an aquatic snail, which in turn must be consumed by your koi. That fish then becomes infected with eye fluke which can lead to blindness, depending on the extent of the infection. As you don’t appear to mind these birds bathing in your pond and waterfall, I assume that they are not fish-eating birds. Furthermore, I would be surprised if there are all of the necessary vectors present in your pond to complete this parasite’s life cycle. This does not mean that your fish are not suffering from a parasite infestation, but if one does exist, we can eliminate the birds from being the source of a parasite problem.

And that leads us on to the next logical step – to determine whether your fish are actually suffering from a parasitic infection. You describe in your letter that there are no visible problems. As the majority of parasites (particularly ectoparasites – those that live on the exterior of fish and cause an irritation) are microscopic, you’re not likely to see them by the naked eye. Having said that, even some of the larger parasites can be difficult to see without a close inspection, and I would recommend viewing any irritated koi closely in a net. If after closely inspecting your selected koi you do not see any evidence of larger parasites such as the virtually transparent Fish Louse or the more obvious Anchor Worm (both of which may go undetected on the underside of your koi) then you should look at eliminating microscopic parasites as the cause of your koi’s irritation. To do so you will need access to a microscope.

Taking a microscope scrape.

By simply raising the dorsal surface of a selected koi out of the water in a hand net, you will expose an area of skin for removing a sample of mucus. Gently pull a microscope slide across the skin in a head to tail direction, making contact over two to three inches of skin. Reposition the sample of mucus on to the centre of the microscope slide, add a drop of water and flatten the sample with a glass cover slip. View immediately under a x40 magnification light microscope and look for moving organisms (of various shapes and sizes) to confirm the presence of parasites on your koi. Consult a comprehensive book on disease to identify and confirm the parasites that are present.

Having identified the presence of parasites on your koi does not necessarily mean that they will cause them irritation or that your pond must be treated. The density of parasites in the field of view is a key factor for determining your next action. Even the healthiest koi, kept in some of the best and most natural environments will carry parasites, but at a naturally low level. A few parasites on a koi should be viewed as natural, with a completely ‘clean’ mucus sample proving the exception rather than the rule. I have known instances where in the pursuit of an absolutely clean sample, koi have been subjected to a continuous blitz of treatments at their highest concentration only for levels of parasitic infection to at best remain constant and at worst deteriorate further.

Through regular microscope work, will you only become sufficiently experienced to identify a level of infection that merits treatment. If your koi are showing all other signs of normal behaviour such as eating well and showing no signs of distress and yet you still find the presence of parasites on the scrape, rather than treat immediately, I would monitor the situation. However, if there is an abundance of a specific parasite on the scrape (such as Trichodina or Gyrodactylus) then this indicates that the koi’s natural balance and immune system has been upset to the benefit of the specific parasite. Treat under these circumstances, in conjunction with other exploratory work such as testing the water quality parameters with a view to identifying the stressors.

With respect to ‘safe’ levels of parasitic infection, there is a difference between keeping koi in a densely stocked clear water pond compared to a more naturally stocked, self-sustaining mud pond. Parasites will be present in either situation, but in the artificial pond, are more likely to get out of control, threatening the health of all of your koi.

You state in your letter that you treated with formalin and malachite green last year when your koi were obviously irritated, but to no effect. This suggests that one of two things may have occurred:

That parasites were causing the irritation, but did not respond to treatment. From experience, microscopic metazoan (multi-celled) parasites are difficult to control using formalin and malachite. These parasites (such as Gyrodactylus and Dactylogyrus) rarely respond to formalin and malachite pond treatments, requiring other methods of control.

The cause of your koi’s irritation was not parasite-based at all. It is well-documented that periods of poor water quality (notably the accumulation of ammonia or nitrite) can cause fish irritation, leading to them exhibiting symptoms that are similar to those shown during a parasitic infection. The accumulation of these products irritates the gills, causing a koi to scratch and jump. Furthermore such poor water quality also causes koi to hang and gasp at the surface; behaviour that could be easily confused with a parasitic problem.

Your letter states that you experienced the problem in April, the month in which your fish started feeding after the winter break. This can be the critical period for the health of your filter as it will have been through a period of inactivity through the winter, having to start to handle increased levels of waste in the pond. There is often a lag in the period between the time when koi start producing more waste after feeding and the time taken by the remnant bacterial population to respond by multiplying and utilising the new levels of waste. This lag is similar to the problems encountered during New Pond Syndrome, but less extreme, with the period of recovery being truncated to several weeks. This would tie in nicely with the changing your koi’s behaviour during the spring period, a few weeks after feeding. It may also explain why your koi’s behaviour did not improve or respond to treatment.

If you do experience similar symptoms this spring, test your water for ammonia and nitrite. If they are present, then stop feeding, carry out a partial water change and add additional aeration. For peace of mind at this time why not carry out a mucus scrape to verify the parasitic status of your koi. If as I believe, parasites were not the cause of the problem last year, then the same may be true this year. Certainly, don’t treat without verifying a parasite problem and rest assured that bathing and preening birds will not have been the cause of any parasitic problem.

Kill blanketweed and string algae.