Herring worm

A herring worm showing the characteristic coil. (Public domain)

The first wave of herring that came to feast outside of cannery row this spring were big and beautiful. The resident sea lions had their own feast eating those herring and some of the fishermen jigged up the silvery fish for bait.

When I was out on a boat trip with a few students from Anchorage and they wanted to see how to dissect and how to filet a fish, I used a couple of those bait herring to show them. One thing everyone noticed with some displeasure was the coiled up clusters of worms in the belly flaps of our subjects. Parasitic roundworms in fish are unsightly but common, and since the worms do better in warmer water temperatures, we may see more of them as the sea surface temperatures are getting higher with global climate change.

The worms we saw were one to two inches long, coiled up and almost transparent. They are a parasitic roundworm called the herring worm or seal worm. They are one of three out of hundreds of parasitic fish worms worldwide that could cause human illness if a person were to eat them alive.

The other two are the cod worm and the tapeworm, both of which also occur in our waters. The cod worm is a little bigger than the herring worm, white to brown colored and can affect a number of fish species including but not limited to cod (neither is the herring worm limited to herring). The tapeworm affects only freshwater fish and those that migrate between fresh water and the ocean, which technically includes salmon. However, only those salmon caught upstream in a river would be subject to such parasites, not the ones caught in the ocean before they enter their native streams.

Herring worms, also sometimes called seal worms, have a life cycle that requires a period spent inside a fish for the young worms. While the fish would probably rather not get eaten, that is exactly what the young worms are counting on to complete their life cycle. If their friendly host fish gets itself eaten by a seal, sea lion, porpoise or whale, the young worms mature in the mammal host and begins to produce offspring. Since the adults make their way into the hosts’ intestines, the eggs are in the right place to be pooped out right into the ocean, where they hatch into tiny larvae.

While for most plankton it is the end of their individual life when something eats them, for these parasitic round worm larvae it is the beginning. They are lucky if they get eaten by a krill, which if the worm is lucky again will later get eaten by a fish, so that the wormy cycle can continue.

Now what if the fish, and with it the young worms, gets eaten by a human? If the worms are alive and make it alive into the human intestine that is not good, either for the human nor for the worm. Being designed to survive in a mammalian host, the worms can develop in a human. Symptoms in the human would range from a tingling in the throat to sudden, severe intestinal pains. For the worms, the human is a dead-end host, unless he or she makes sure to poop directly into the ocean.

Should we all panic when we see our fish infected by worms? That would probably be an overreaction. However, I admit that if I see worms in my fish, I would take great care to remove them before adding the soy sauce and pepper. Any remaining worms are killed by cooking the fish until the flesh is flaky and no longer transparent. They are also killed by freezing at temperatures of at least minus 4 degrees Fahrenheit, which is standard procedure at fish processors but colder than most home freezers.

Care should be taken when eating fish raw, slightly cooked, or cold smoked. In commercial operations the fish filets are “candled,” which means they are inspected on a light table. Any worms can then be seen and removed. Needless to say, sushi-grade fish is inspected extra carefully and sushi chefs are trained to take extra precautions.

It has not been my intention to spoil anyone’s appetite for fresh fish. Human infestation with herring or cod worms is rather rare. However, if you dabble in becoming a homemade sushi cook and eating your fresh-caught fish raw, it is advisable to use extra caution so as not to become a host to the worms after being a host to a sushi party.

A nice piece of fresh cod cooked to perfection still makes an excellent and healthy dinner, even if that cod was host to a party of little worms while it roamed the ocean. You may conisder parasites mean, disturbing or outright revolting, but you have to admit that it is amazing how nature has come up with such an complicated life cycle involving three different hosts.

Here is a life form that thrives and prospers by taking advantage of the food web and can only survive if it gets eaten. Guten Appetit!

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