Gill maggots are the mature females of the parasitic crustacean Ergasilus.
Ergasilus (gill maggots) will appear as grayish black and white parasites several millimeters long infesting the gills.
Heavy infestations can cause severe damage, eroding the gill filaments and allowing secondary infections to develop.
This parasite is called a gill maggot after their preferred area of infestation and the appearance of this pest (and their maggot-like egg sacs).
This is quite a rare parasite in koi ponds but will cause an affected koi severe irritation, evidenced by flicking and flashing and the gill’s reduced ability to absorb oxygen. Infested fish are likely to be seen gasping at the water’s surface.
Ergasilus has quite a simple lifecycle with the eggs being released by the females while they are still attached to the gill, hatching in the pond and developing into further infective juveniles. Interestingly, only the female becomes parasitic.
Control is similar to any crustacean parasite and is most effectively carried out by using extremely hazardous organophosphorous insecticides.