1. Signs of infection
Small black or brown spots, up to 2 mm wide, appear on the body and fins, and sometimes on the eyes and mouth.
2. Details of infection
Spots are cysts containing larvae of intestinal trematodes, such as Fasciola pidata, which contain heavy pigments. Larval stage is also called Fasciola pigmentosa. Adults live in the intestines of waterfowl and are disturbed by eating fish that hide larvae. In the viscera of waterfowl, the membrane wrapped around each larva is dispersed, and the worm matures after a few weeks. The eggs produced by adults are discharged from the excrement of birds, and the eggs that fall into the water develop into free-swimming larvae and parasitize on water snails. A few weeks later, the parasite leaves the water snail and begins another cycle of sending suitable fish hosts. When the parasite reaches the width of about 1 mm, each worm produces a black film on its body, forming unique spots under the skin. The slight infection of black spot disease does not harm fish, because the life cycle and environmental conditions are complex, and it will not spread black spot disease from one fish to another.