also known as: flatworms, grubs, digeneans, black spot, black grub
Trematodes are unsegmented flatworms of Phylum Platyhelminthes, Class Digenea, Family Heterophyidae. There are many species of trematodes, and they can be seen commonly on the outside of fish as "black spots", they can appear also as small white, yellow or black spots in both skin, fins and meat of fish. The spots are the immature stage (metacercaria) of the parasite, which must be eaten by a bird before they develop into adult trematodes. In the bird, adult trematodes produce eggs which are shed into the environment. The eggs hatch to form a swimming miricidium stage which infects the third host, a snail. Within the snail host, the trematodes multiply and change into cercaria - which swim out of the snail to find the fish host. Depending on the trematode species, the cercaria infect fish and either mature to become adult worms or become encysted as metacercariae.
Fish normally are unaffected by grubs unless they become heavily infected. Adult worms rarely cause harm to the host and can be found in intestine, stomach, blood, gall bladder, and urinary bladder. Metacercariae are the main source of disease and can be found in skin, gills, fins, muscle and internal organs. Commonly encountered species include: Neascus sp., Apophallus brevis, Cryptocotyle lingua.
There is no human health risk with eating infected fish, as long as the fish is cooked. Humans can experience "swimmer's itch" if they swim in water with many infected snails - the trematode cercaria can cause skin irritation.