Why Parasites Infect Koi Parasitology is a huge science in its own right but thankfully Koi-Keepers only need to appreciate some basic principles in order to protect their Koi. There are four main factors that greatly influence the level of those parasitic organisms most commonly found affecting Koi in garden ponds. The health status of the Koi themselves The water quality in which the Koi are living The stocking density of the pond How new introductions are dealt with What are parasites? A parasite is any organism that directly lives off of another and there are millions of species. Most of the common skin parasites in Koi are motile (moving) when viewed microscopically whereas fungi are often overlooked as parasites due to their static lifestyle. The success of fungi such as Saprolegnia a species commonly found infecting Koi depends on the availability of open skin lesions to invade such as wounds and ulcers. Whilst some parasites such as skin fluke take up their parasitic lifestyle at birth as this takes place directly on to the fish others have phases of their life cycle in the water rather than on the fish . Inside and out Ecto-parasites live off the host fish externally whereas endo-parasites affect the fish internally. With their unique characteristics and complex reproductive cycles endo-parasites are too diverse for generalisations. Some are capable of killing the host usually by nutrient deprivation directly from the blood thus draining body resources to a level that is harmful to health and then ultimately fatal to the host. However this is not the case with all parasites and some organisms reach a balance with the host in which both survive unharmed. Free booters Whilst it may appear parasites get the better deal with safe lodgings and a free lunch for life some actually offer a few small benefits to host fish such as keeping other more harmful organisms at bay or even utilising them as a food source. In the wild parasites are not such a threat to the well-being of fish and self-cleansing mechanisms can be adopted by the fish that cannot work in the confines of a Koi pond. Macro and Micro The macro-parasites are those than can be seen with the human eye without a microscope for example Argulus the fish the fish louse. This is spread either by the purchase of infected fish or plants and in some cases by water birds. Ducks for example often live in waters infected by lice and whilst they should not be regarded as a natural host to fish lice they can be the vector of an infection. If a duck visits a clean pond soon after leaving one infected with Argulus it is possible that some lice will also make the trip. In normal circumstances the fish louse dies on the feet or feathers of a water bird due to the defense mechanisms of the animal and the fact that fish lice do not parasitise ducks. However with little or no interval between ponds lice can be spread and such an event is largely outside the Koi-keepers control. Health Status When Koi are unwell they are unable to create an adequate immune response to defend them from secondary health problems and parasites then find fish an easy target. Koi that are recovering from disease can be afflicted for example by skin fluke or trichodina which may prove difficult to control. This is not because treatment is ineffective but because the fish are unable to aid their own recovery from within, chemicals alone are not always enough. Trichodina and skin fluke are not true fish parasites. They thrive hidden in pond debris and stricter hygiene is often more effective than chemicals. Environmental conditions If fish are frequently exposed to poor water quality parasites will gain the upper hand. Carp can be parasitised by a low level of fish lice for years and produce coping strategies via their immune system. The lice however will soon breed out of control if ammonia or nitrite levels rise unchecked or the pond is polluted by acid rain. Then secondary infection by bacteria and fungus will further weaken the fish. Stocking Density The sheer proximity of fish in overstocked ponds increases parasitism so it follows the more water each fish has to live in the less likely it becomes that a serious parasite problem will ever develop. Most Koi normally carry a few skin parasites without this being a health problem. Body contact allows exchange of mucus and parasites so ultimately all fish in overstocked ponds will have parasites. The organic load in over crowded environments peaks and troughs creating periods in which it may never be suspected that the water is unsafe. Parasites are opportunists born to recognise weakness in host fish and then attack for their own survival. Getting along together Symbiosis is observed in all branches of biology and the term is applied when a parasite depends on a host for survival without causing harm. Some internal parasites pose no risk during certain life cycles stages only becoming a threat to the fish during a specific developmental phase. Some parasitic organisms require additional hosts to carry out stages of their life cycle without which the cycle cannot be completed. Water snails for example are a common intermediate host for fish parasites. However it is not a case of any snail will do as the process requires specific teamwork in order to perpetuate each species. It is amazing just how selective parasites are when deciding on an intermediate host. The aim ensures the reproductive process produces robust future generations. Just to complicate matters fish themselves can serve as intermediate hosts for a variety of organisms and are not always parasitised directly. What a fluke Flukes are common parasites in all animals and just how many fluke species there are is unknown but they are a large and diverse family. Flukes are classified as trematodes and some species reproduce via an intermediate host. Koi keepers are familiar with the skin fluke scientifically termed Gyrodactylus and gill fluke or Dactolygyrus. There are numerous species of fluke found internally in fish and one such organism is Sanguinicola. First class travel Carp are the largest and most wide spread fish family and when parasites get a free ride on the skin, in the gills or even inside the body of Koi this ensures greater distribution. This second hand jet setting life has spread countless fish parasites to new environments allowing them to seek out new fish species as hosts. This increases the long term survival of organisms that would otherwise have had limited fish species and locations in which to live. Travel is responsible for the spread of many organisms and diseases and obviously not just amongst fish species. New Koi Quarantining all new Koi minimises the risk of parasites and serious disease being introduced to a healthy pond. In quarantine Koi can simply be observed for twelve weeks to ensure they remain healthy. If a microscope is available mucus smears can be taken if parasites are suspected due to skin redness or obvious irritation. Heat can be influential in parasitic life cycles and if applied during quarantine will minimise risks still further. Parasite control may require chemical treatment and this is often detrimental to pond and filter biology. In addition chemicals cannot specifically target any parasite and can cause side effects in Koi so ensure all chemical use is really necessary.