Fish leech (Piscicola Geometra)
As with fish lice and anchor worm, the leech is a true parasite in so much as it bites into the fish and feeds off the fluid and tissue of the koi, causing immense damage, finally leading to certain death for the koi if left unchecked. This differs to flukes which are not a true parasite. They merely latch onto the koi and feed off dead organics floating by. The same is true of Trichodina; in fact Trichodina has a mouth situated dorsally away from the fish surface, it would be impossible for Trichodina to feed on the koi's fluids and tissue.
Scientific evidence supports Piscicola Geometra being a transmission vector for SVC (spring Viraemia of carp).
Do not confuse the fish leech with mayfly or gnat lava, that hang of the pond walls and filter walls, these two are quite harmless and only about 10mm long and much darker in colour, where as piscicola geometra is deadly
While the leech is a true parasite, it differs from anchor worm and the fish louse in one major aspect: Piscicola Geometra does not need to live on the koi, it merely attaches to the koi to feed and then, once gorged, it leaves the koi and returns only to feed. They are very adept swimmers, and if you study them before you nuke them they can be seen targeting koi from across a pond and swimming quite strongly to their victim. While you may see leeches ever present on your fish once infected, I can assure you they will not always be the same ones. Its like folks jumping on a bus and jumping off again the bus always has folks on board but not always the same folks except these critters are not only along for the ride.
The leech is also oviparous and produces eggs. Like Argulus a leech has to leave the koi to perform the egg laying function in the weeds or on the pond bottom or sides. The complete life cycle can take up to 30 days., the most common route of infection is Via untreated plants introduced to the pond and birds, very rare in incoming fish
Symptoms include: extreme lethargy, paling and darkening of colour, and death! No microscopic examination is needed. The leech is pale brown/buff in colour about 1 mm in width and can measure up to 25 mm long. Above all you CAN SEE THEM! Even from 10 to 15 feet away they are that obvious.
For leech eradication is, Masoten as dosed for lice or anchor worm. Or malathion .
This treatment is not always possible. But, the fact that the leech does not reside permanently on the host presents us with alternate treatments.
You can reduce numbers drastically by hanging a small piece of bloody meat in the pond off a piece of string.
I am told they are partial to best porterhouse "rare" your wasting your time with a rasher of Smokey bacon < joke>.
This will attract the leech's for a feed, once they are present on the meat it can be removed with the leeches attached and they can be removed and destroyed and a fresh piece of meat re-hung.
The more astute of you will have noticed this possibility. If we remove the fish from the facility to another non-leech contaminated facility and we manually remove the leeches from the koi before they enter the new holding tank, we now have a vat containing koi that are no longer infected with leeches.
We can now either treat the newly emptied pond without subjecting the koi to this treatment. Or just merely leave the pond devoid of koi (empty) for about three weeks.
Alternatively we could empty the pond and clean it plus drain down the filters and allow drying out for a week even the eggs will not stand drying out. As long as the filters are not submerged in water and allowed to dry they will be fine they wont kick in straight away when you re-submerge them for a restart but they will kick in soon after and in a couple of weeks will be back up to speed
Alternatively again we could leave the pond empty but running and take the empty ponds salinity up to 3%.
Tests conducted by myself with leeches two summers ago, show the adult leech will die within 76 hours without a host and the rest will follow suit as they emerge from the egg with no host to feed upon. So thirty days should account for the life cycle of the leech at typical summer temperatures.