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Black Spot and Yellow Grub Parasites in Fish

brook trout

Brook trout with black spot disease. Photo credit: Paul Gray. Select image for larger view.


grubby fish

A yellow perch infected with yellow grubs just under the skin along the back of the fish. Photo by J. Viar.

Select image for larger view.

It is not uncommon to catch a freshwater fish that appears "grubby" – 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 (digenetic trematodes).


Cysts, which appear black, are a result of melanin pigment produced by the fish host and deposited around the cysts. This condition is known as disploptomiasis and is caused by a trematode worm in the genus Neascus. Cysts that appear off-white to yellow are caused by a trematode known as Clinostomum marginatum, also known as "yellow grub."


Digenetic trematodes include numerous species of which most are parasitic – that is, they live with a host organism at the expense of the host. These 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.


Unless the parasitic infection is extreme within a given host, fish grub fluke adults and larvae usually do not appear to seriously harm the host. The snail probably suffers the most due to the intensive reproduction of larval forms within. Occasionally, fish hosts can be harmed when vital organs are involved.


These two species of digenetic trematodes are host specific and luckily humans are not on that list. Grubby fish are safe to handle and may be safely consumed by humans providing they are completely cooked, thereby killing the grubs. We recommend that whenever you keep fish that you dispose of the unwanted parts into the trash and not back into any water body.


life cycle

Generalized life cycle of black spot and yellow grub parasites.


For more information, contact: NH Fish and Game Dept., Inland Fisheries Division, (603) 271-2501.