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Cod Worms Article December 2011

Originally published in Coastal Angler Boston: December 2011

On a late season tuna drift, I noticed a few decent marks passing along the bottom and I decided to drop a jig and teaser down.  It didn’t take long before we had enough cod on ice for a few dinners. 


I’ve filleted a lot of cod and I’ve seen worms in most of them, but the fillets from this day were particularly loaded with the little critters… As usual all the typical crews’ response starting getting voiced – “that’s nasty” , or “eh, just some extra protein”, and “don’t worry, they can’t survive human digestion”, and several “no way, I’m not eating that.” among others.  I used to be in the extra protein camp.  Having researched it, not anymore:


What are they?

The Cod Worm, or more correctly the Seal Worm (Pseudoterranova spp) is a parasitic nematode.  It is a roundworm that is present in an estimated 75-90% of all Atlantic Cod.  They can be brown, red, pink, yellow or white in color.  They can be found in many species of fish but are most commonly found in cod due to their living on the bottom in close proximity to grey seals.  As usual, blame it on the seals.



The Seal Worm has a 4 stage larval process.  The life cycle begins when the final infected host (usually a grey seal) releases eggs of the worm in their feces into sea water.  The eggs immediately hatch into a larval stage.  The larvae are consumed by small crustaceans, which then are preyed upon by fish.  The 3 stage of larval development has a specially formed tooth that enables penetration of the gut into the flesh of the infected fish.  This is stage that you see in the fillets of the cod.  Finally, the host fish is consumed by a marine mammal (or human) where the larvae reside in the bowel where they seek a member of the opposite sex for mating.  The life cycle is completed when the female lays eggs that exit the host’s body through its feces.


Humans are an accidental host of Seal Worms.  They easily survive your digestion process and can even complete the 4 stage of development in our bowel.  They can survive in up to 80% brine solution and can easily survive a low temperature smoking process.


The Good News (Sort of)

Fortunately, most of us are not frequent consumers of raw cod, nor should we be.  The worms can be killed by freezing at -10 degrees F or by cooking to 140 degrees (keep in mind that household freezers are typically set to 0 and cod is opaque and flaky at 130).  The commercial processors use a candling table which shines a light up through the flesh making the larvae easier to see.  This can be done at home with a pane of glass positioned over a fluorescent lamp.  The larval worms might be 1-3 cm long, but only 5mm when coiled in the fish’s flesh. Needlenose or tweezers are all you need to remove them.  Soaking the fillets in brine or lemon juice can make them easier to remove. 


Basically remove all that you can see and make sure you thoroughly cook the fillets for the ones you couldn’t.  Or, just do what restaurants do - bread and deep fry them.


Worst Case.

So let’s say you consume a quantity of infested flesh – what happens next?  Potentially nothing.  You may pass them before they take up residence.  You can even become mildly infested and be completely asymptomatic.  The 3 to 4 stage of development can take from 3 days to 3 weeks.  In this time, you may have the abdominal pain / distress and you may experience other weird symptoms like tingling in the throat or arms.  In very rare cases you can become allergic to the slime that that worms produce from their skin.  Unless you are regularly consuming undercooked or raw infected fish, the entire adult lifecycle will complete its course within a month.


Do keep in mind that parasites have not survived most of eternity by making their hosts sick.  Their entire reproductive lifecycle depends on the hosts maintaining overall and intestinal health.  This is also not a new concern, with reports and scientific studies on these worms from the 1950’s, and comprehensive studies of health effects from over 30 years ago.  You can continue to enjoy catching and eating cod.  Seal digestive systems are often loaded with these worms because they don’t fillet, inspect, freeze, and then cook their catch, but you can.

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