also known as: black spot, black grub
Many different trematodes can cause "black spot" signs on the skin of a fish, but Neascus is one of the most common. Neascus is a digenean trematode with a complex life cycle. It requires three hosts: a snail, a fish and a fish-eating bird. Parasite metacercariae become encysted in the skin of fish, and are attacked by the fish's immune system. This leads to a buildup of pigment cells, causing the characteristic black spots.
They are the immature (metacercaria) stage of the parasite that when eaten by a bird develop into adult trematodes. They shed eggs in the bird and the bird deposits the eggs into waters where they hatch (to form miricidium) and infect snails. The organisms multiplies in the snail and the snail releases the stage of the parasite infectious for fish (cercaria). Waterborne cercaria infect fish and either mature to become adult worms or become encysted as metacercariae.
Fish normally are unaffected by Neascus unless they become heavily infected. There is no health risk associated with eating the cooked flesh of fish infected with Neascus.