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What Makes Fish "Grubby"?

It is not uncommon to catch a freshwater fish that appears "grubby" -- a diseased fish infested with pinhead size lumps that are white to yellow or black in color. Many people wonder, is this some new disease? Is the fish safe to eat? This is not a new disease and "grubby" fish may be safely consumed by humans providing they are completely cooked, thereby killing the grubs.


Some fish have only their skin and fins affected. Others are targeted in their musculature and a few may have one or more of their internal organs involved. All of these "grubs" are dormant encysted larval flatworm parasites.


Cysts, which appear black, are a result of melanin pigment produced by the fish host and deposited around the cysts. Only certain flatworm species stimulate pigment production; otherwise their cysts appear off-white to yellow.


Flatworms are scientifically referred to as digenetic trematodes. This animal group includes numerous species of which most are parasitic - that is, they live with a host organism at the expense of the host.

Most parasitic trematodes utilize two or three hosts during their life cycle. Those which cause grubby fish usually take advantage of snails, fish and fish eating birds and mammals. The yellow grub of perch is typical. It infests the great blue heron as an adult worm and is depicted here as an example:


Life Cycle of The Yellow Grub
(Clinostomum marginatum)


grubby lifecycleThe life cycle begins as fertile eggs (2) produced by adult flukes (1) which live in the mouth of the heron (7). The eggs are shed to open water as the heron feeds. The first stage larvae, called miracidia (3), emerge from the eggs and penetrate a suitable snail host (4). Within the snail, miracidia undergo numerous cycles of asexual reproduction in various developmental phases after which second stage larvae, called cercariae (5), emerge from the snail and swim about in search of a suitable fish host, such as the yellow perch (6). Cercariae penetrate the fish and encyst to assume the third larval stage - metacercariae (6), also known as grubs. When the infested fish is consumed by the heron, the metacercariae excyst (come out of the cyst) and become adult flukes (1) in the mouth of the heron, thus completing the cycle.


Life Cycle figure adapted from: Hunter III, G.W., and Wanda S. Hunter. 1935. Further Studies On Fish and Bird Parasites. Suppl. 23rd Annual Report, NY State Conservation Department, Biological Survey. Mohawk-Hudson Watershed, 1934: 267-283, 3 pl.


Unless the parasite incidence is extreme within a given host, fish grub fluke adults and larvae usually do not appear to harm the host. The snail probably suffers the most due to the intensive reproduction of larval forms within.


In some areas of the world, adult flukes arising from fish grubs can infect humans. The majority, however, are specific for hosts other than humans. Regardless, grubby fish may be safely consumed by humans providing they are completely cooked, thereby killing the grubs.


For further information, contact:
NH Fish and Game Department, Region 2
(603) 744-5470