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Q & A
Parasites in Fish
Over the past few years, I started finding cream colored balls in the flesh of bass I caught in ponds. Upon further inspection, I found that they are worms. This year I found the worms in bass in streams and the Susquehanna River. Is there anything that can be done to rid the bass of these worms?  Is it okay to eat the bass?

Note: We also receive inquiries about black spots on fish, comments regarding them are included in the answer.


Thank you for your inquiry and keen observation. Indeed freshwater fish may contain a variety of parasites. Without seeing the fish and parasites, a positive identification cannot be made. Two distinct parasites are typically observed by anglers in fish muscle. One is a small yellowish worm or grub, the other is a blackish spot.

Yellow grub

Your description suggests you observed encysted metacercariae larval stage of the yellow grub (Clinostomum) parasite. The encysted, yellow to yellowish white worm can be up to 1/4 inch in length. It can be found in virtually all species of North American freshwater fish.

The yellow grub is a digenetic trematode. These types of parasites require several hosts to complete their life cycles. In the case of the yellow grub, the adult parasite is found in the throats of fish eating birds, such as herons. During the feeding process, eggs produced by the adults are washed out of the bird's mouth and into the water. There they hatch, yielding a free swimming larval stage (miracidia) that will die within several hours if it does not find and infect a snail of the genus Helisoma. After further development within the snail, a free swimming cercaria leaves the snail and seeks a fish host. The cercariae burrow through the skin of the fish and encyst, where they develop into the metacercariae. These yellow grubs may live several years in the fish. If the fish is eaten by the bird host the larval metacercariae will develop into adult parasites, completing the life cycle.

Infestations by a few individuals likely cause little harm to fish, however, under certain circumstances, heavy infestations can kill fish. Yellow grubs are described as unsightly by fishermen. A related species occurring in Asia has been found to infect the upper respiratory tract of humans. Thorough cooking kills the North American yellow grub and the parasite does not alter the flavor or the infected fish; however, fish with heavy infestations are typically not eaten by anglers.

Infestation is somewhat greater for fish caught in shallow water where snails and fish eating birds are most prevalent. Fish caught from deep water typically exhibit less infestation. Like many biological phenomenon, prevalence of the grub may be greater in some years and less in others for a variety of reasons including an abundance of intermediate host mollusks and birds.

Black spots

Black spot disease is commonly observed in rock bass and other sunfish, bass, pike, perch, minnows, and other fish species. It can be identified by the presence of small black spots, usually about the size of a pin head, in the skin, the fins, the musculature, and the mouth of the fish. The black spots are caused by pigment that the fish deposits around the larval stage of a parasitic digenetic trematode, usually a Neascus spp.

The lifecycle of the "black spot" parasite is complex. The adult parasite is found in a fish eating bird, the kingfisher. The larval parasite is transferred from the infected fish to the bird during the feeding process. In the kingfisher, the larval stage develops into an adult parasite. The adult parasite in the intestine of the bird produces eggs that are eventually deposited in the water. There the eggs mature, hatch, and develop into the miracidium stage of the parasite. The miracidium infects a snail. In the snail, the miracidium develops into the cercaria life stage. The cercaria leaves the snail and actively penetrates a host fish. In the fish, the parasite becomes encysted. In about 22 days, black spots form around the cyst. This entire lifecycle takes at least 112 days to complete.

In general, the presence of the "black spot" parasite does not affect the growth or the longevity of the infected fish; however massive infections in young fish may cause fish mortality. The parasite is incapable of infecting humans and, as is the case with all fish parasites, it is destroyed by thorough cooking. When fish are heavily infected, some anglers prefer to remove the skin to improve the appearance of the cooked fish.

For further information please reference:

Numerous state Fisheries' agencies and other web sites describe parasites --- try typing "Neascus" or "Clinostomum" in search engines to learn more.

Like many biological phenomenon, prevalence of worms and parasites may be greater in some years and during some seasons for a variety of reasons, including an abundance of intermediate host mollusks and birds. Bluegills are a colonial spawner and congregations in early summer provide opportunity for infested fish to be in close proximity to one another.

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