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Step 4 - Maintaining health and preventing disease in koi ponds.

In the 4th part of the 10 steps to successful koi keeping we start to turn our focus on the koi themselves. So far we have looked indirectly at the needs of koi and how we can manage their pond environment to achieve success.

Basically, we have concluded that if you get to the environment right, the koi will inevitably be healthy. This recognises that koi are just like any other fish in that they are approximately 80 per cent water, living in an aquatic environment and are therefore by definition, a considerable part of their own environment and will be affected by changes to it.

But there is more to maintaining healthy koi and reducing the threat of disease than merely providing them with the correct environment. Unfortunately, koi keeping is presented with many other factors that can threaten the health and disease status of our koi and must be addressed by us all. Many of these factors would not necessarily affect a natural situation, but due to the artificial components that make our hobby possible, we need to be aware of the associated health implications for our koi.

Firstly we should see maintaining healthy koi and preventing disease as two different objectives. These two areas definitely impact on each other, but may prove easier for us to manage if we look at them separately. It could be argued that by simply concentrating on maintaining the health of our koi, we could still leave our koi open to attack from disease. The secret is to keep the two aspects in balance and look at maintaining healthy koi as providing and improving the beneficial aspects for our koi while preventing (or rather reducing) disease as keeping control of any negative risk factors.

1. Maintaining healthy koi.

The most effective way of maintaining a healthy koi is to provide them with an uncompromising high quality environment. This is something that we have discussed in some detail in steps one and two (providing the perfect koi environment). Time and time again, we can trace the occurrence of poor health or koi disease back to an incident or period when the quality of their environment was not as good as it should have been (perhaps a short-term peak in ammonia / nitrite, or a more chronic problem with low dissolved oxygen). It will be more useful if we can understand how and why a deterioration in the quality of their environment can lead to a health problem so we can better manage it when it happens.

Maintaining the health: disease balance. Of course, the objective of maintaining the health of our koi is preventing our koi from succumbing to disease, and in doing so we are recognising that both health and disease share an intimate relationship with each other. We need to appreciate that there is a very thin line between maintaining our koi in good health and their potential to experience disease. The single greatest factor that tips the balance in favour of disease is stress. Stress is an all embracing term for the cause of most health problems that are likely to be caused by any number of different environment and changes. If the change in pond conditions is too extreme (in either its range or duration) then the health: disease balance will be tipped in the diseases favour.

Unfortunately, when koi experience stress, a potentially lethal chain of events occurs within our koi, not necessarily causing disease, but making our koi more susceptible to the diseases efforts. Mother Natures view of disease in a koi pond is that the disease-causing organisms have just as much right to thrive and succeed in their aquatic environment as our koi do. They are there to fulfil their role and exploit their niches. However, our view is one that of course favours our koi, and we endeavour to deny natures pathogens what could be argued is rightly theirs, namely a koi that they can attack and thrive on.

Animals, including koi, battle constantly with the real and potential threat of disease. In the wild, a diseased or ill animal is the most susceptible to predation, and it is unlikely that a debilitated animal will have a second chance. In the law of the jungle, it is a case of dont get ill, or youll be the next in line for a predatory attack. Consequently, animals expend significant energy and other resources in preventing disease organisms from gaining a foothold by maintaining a range of defences. These include physical barriers between the environment and the organism and a host of other protective measures including an advanced immune system. But when our koi become distressed, through no direct fault of their own, they become more susceptible to attack from disease by their immune system becoming impaired. It seems as though Mother Nature appears to have planned stress as a way of tipping the balance in favour of her pathogens. If youre a pathogen, you could not have planned it better!

One of the first things to impact a stressed koi is the release of adrenalin, especially if it experiences a short term stressor (eg chasing with an actor). This mobilises energy allowing the koi a quick escape, but it also has a debilitating knock-on effect to a kois osmoregulatory (water/ salt balance) function. For an organism that is more than 80 per cent water in an aquatic environment, this has serious stress implications.

Another stress response that seems to play right into the hands of aquatic pathogens is an increase in mucus production under certain water quality conditions. Mucus plays an important role in a kois first line of defence, forming a barrier between itself and its environment. If water conditions deteriorate significantly, excessive mucus production can lead to a proliferation of external parasites such as Costia and Trichodina diner that feed on this nutritious secretion.

A kois ability to fight disease internally is also affected by hormonal changes that debilitate its white blood cell (disease fighting) activity. Cortisol, the hormone that is released in response to stressor has this unwelcome side effect on a kois disease fighting ability, increasing the chances yet again of a successful pathogen attack.

Rate of recovery. As soon as we become aware that our koi are under stress, we should act to remove the stressor (improve the water quality through a water change, increase aeration etc). Having intervened responsibly, we should also be aware that there is a lag between our remedial action and the complete recovery our koi. Our koi can take weeks to recover from a stress or (especially if it is a long-term background stress or) and our koi are still susceptible to health problems in this period.

2. Preventing disease.

It is not realistic for us to believe that we can prevent disease organisms from entering our pond. Neither should we feel that if a disease organism is present in our pond would it be eradicated by treatment or UVc. Our best approach is to prevent the disease organisms that will inevitably enter our pond from causing disease, and this can be achieved by managing the threat of disease at a number of different levels. Dont forget that in the wild and natural pond conditions in which koi thrive, there is a natural underlying level of disease that lives in harmony with the fish and environment. But just as koi are opportunistic feeders, so too are most of the pathogens, waiting for the opportunity to strike. As we cannot exclude these pathogenic organisms from our pond, our next best approach is to control them.

Where does disease come from?

The four different types of disease-causing organisms (virus, bacteria, parasites and fungi) can enter our pond in a number of ways. All of these organisms are experts at rapid division, multiplying to fill a new niche so our koi are potentially under threat should only a couple of virus particles, bacteria, parasites of fungi enter our pond. The most reliable route for introducing these pathogens into our pond is through koi that we introduce ourselves - even apparently healthy koi. Also, in the same way that nitrifying filter bacteria seem to appear in a new pond over time, pathogenic bacteria can also enter our ponds via airborne routes. Something that is impossible to prevent.

Quarantine or acclimatisation?

Even if you (and your dealer) are confident that your new purchase is healthy, it is highly likely (and only natural) for it to be carrying a disease organism. Ideally, all new purchases should be held in a halfway house holding facility between your healthy balanced pond and your dealers holding systems. This requires you to have a quarantine or acclimatisation facility. I prefer the term acclimatisation, because a quarantine facility suggests a recognised period of isolation beyond which a koi is confirmed as disease-free. We have seen this not to be true with respect to the threat of KHV. No period of quarantine will effectively rid a koi of the virus, but it will allow a stress-free period in which a koi is likely to avoid succumbing to its attack. Some koi dealers actively advocate a period of stress (a controlled period of high temperature) in quarantine to try to confirm the presence (or absence) of the virus.

A period of acclimatisation will allow your new koi to recover fully from importation and a potentially stressful stay in a heavily stocked dealer system. As your new koi become acclimatised, the density of any pathogens being carried will drop, reducing the risk of disease for that koi and any of those in your main pond. To be effective, an acclimatisation system must be a mature and stress-free system itself being well filtered and as large as possible.

Methods for Maintaining Health:

Effective Filtration which means

Zero ammonia/nitrite

Nitrate < 50ppm

No accumulation of DOC

Sensible Stocking Density

Regular Partial water changes remineralisation

Complete and Balanced diet

Good water circulation

Stable water conditions

Methods for Preventing Disease

Effective Quarantine Acclimatisation facility and regime

Regular koi watching for early signs of abnormal behaviour

Preventative dosing against bacteria & fungi & parasites in spring/autumn to reduce their density.

& fungi & parasites in spring/autumn to reduce their density.


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