Yellow Spots/White Spots

Cause of disease:

Parasite (Trematoda) – Yellow Grub (Clinostomum), White Grub (Posthodiplostomum minimum)

What does it look like?

These parasites are small and cause yellow or white cysts (spots) in fish skin, muscle tissues and in most internal organs that resembles coarse salt sprinkles.

Yellow Spots/White Spots
MIDNR picture

Fish species affected:

All species of fish in Minnesota lakes and rivers are susceptible. Fish that inhabit shallow areas are most affected.

Life cycle:

Eggs are released by fish-eating birds into the water. Eggs develop into intermediate stages of the parasite in snails. These free-swimming parasites penetrate the muscles of fish and encyst, taking on the form of the yellow or white grub.  Infected fish are consumed by birds where the life cycle starts all over again. Adult worms are seen in fish-eating birds.

Where is the disease found?

Very common throughout Minnesota. Low infectivity does not appear to affect the fish’s health.

Is it safe to eat?

Yes, although unsightly, the fillet is safe to eat if it is well cooked. Always cook fish well before consuming.

How can I prevent the spread?

There are no practical means to control this parasite.

Disposal of unused portions:

Unused or uneaten portions of fish should be buried or disposed of with household waste. Fish entrails should never be discarded back into the lake.


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