I have recently noticed a rather large amount of small worms in the fresh cod I am buying. I have tried to take them out as much as I can and of course have spoken to my fish supplier about them, but he assures me they are harmless. However, I don't believe my customers would be as assured. Is it okay to serve this fish, taking into account I might miss a worm or two?
Live parasitic worms are to be expected in raw fresh fish, and need to be dealt with in one of three ways:
- Cooking: If the fish is thoroughly cooked, the worms will be dead and safe to eat.
- Removal/avoidance: A skilled sashimi chef has an eye for parasites, and will either discard contaminated pieces, or remove the parasites.
- Freezing: So called "sushi grade" fish is fish that has been frozen at a temperature/duration recommended by the US FDA. That's 7 days at -20°C, or colder for shorter treatments. Note that this is much colder than domestic freezers. It is said to provide a "parasite destruction guarantee".
There may be other ways to neutralise parasites, such as Eskimo-style fermented fish - but these are unlikely to be of use to you.
There is a difference between safety and palatability, though, and your customers might reasonably expect to be served fish without visible parasites.
Parasites in fish are common. In short, your fishmonger could have done a better job of pulling them out (unless you bought them whole). They are not deemed harmful if cooked properly (see the FAO link below).
There are guidelines and standards about the number of Nematodes in a given amount of fish. Some types of fish are more susceptible than others, so you may wish to change your order from COD and Monkfish to something else.
Here is a document from FAO (Food and Agriculture Organization of United Nations). It explains how the worms get there, the safety precautions and how to avoid serving them.
The only way to reduce the numbers of parasites reaching the consumer is to inspect the fish and process them in such a way that most parasites are removed.
An excerpt on safety (for permanence):
There have been cases of human illness caused by the ingestion of live Phocanema or Anisakis larvae in countries where raw or lightly cured fish is commonly eaten. By 1980, there had been only one reported case of illness in the United Kingdom caused by larval round worms from fish; this is because in the UK fish products are normally cooked before consumption. Phocanema and Anisakis larvae are killed in 1 minute at a temperature of 60°C or over. In practice this means that cooking a fillet 3 cm thick for 10 minutes at 60°C will kill any worms present. The temperature of a cold smoking process, for example kippering, is not high enough to kill parasites, but in a commercial hot smoking process a high enough temperature is usually maintained for long enough to kill them. Freezing of fish at - 20°C for 60 hours kills all worms.
This depend entirely on how you are preparing the fish.
If you are cooking the fish to FDA's 100% safe temperature of 140F(for fish) then there should be no issue.
If you are serving the fish a bit rarer than that (120-130F), it is unlikely the worms can survive the temperature but it is still possible.
If you are serving the fish raw, then there will be issues. The parasitic worms will stay alive in your stomach for quite a while. Eventually your stomach will kill it but that can take weeks, sometimes even months. In the mean time, the host of this parasite will experience symptoms such as violent abdominal pain, nausea, and vomiting.
I pulled a curled up, dead worm out of my cod just this past weekend and still finished my meal. I read about them and they do not seem to pose a hazard to humans. They are FISH parasites and highly unlikely to survive in the human stomach for very long. And (not to be TOO gruesome) they've likely been chomped to death already in one's mouth prior to entering your acid-filled stomach. Anyway, 5 of us ate the cod and nobody had any sort of adverse reaction (and the fish was delicious).