Sashimi, thin slices of raw fish, is becoming a popular method of preparing trout. Originally from Japan, sashimi is commonly served molded over vinegared fingers of rice and called sushi. If you prepare your own sushi you should be aware of health risks caused by the presence of parasites in raw fish, even though incidences of parasitic infection are rare.
Parasitic worms are killed by thorough cooking, or adequate freezing. Only the ingestion of raw, lightly cured, or insufficiently cooked infected fish can transfer the live worms to humans. Many authorities recommend using only seafish for sushi because the primary hosts for the parasites are marine animals such as seals, dolphins and whales.
Most of these parasites cannot adapt to human hosts, and, if an infected fish is eaten, the parasites may be simply digested with no ill effects. Freshwater fish parasites often rely on mammals with digestive tracts more closely resembling ours, and can easily switch host.
The parasitic worm infests a number of different hosts during its life cycle: mature worms reproduce in mammals, the parasite eggs pass with the faeces into water, and hatch into larvae. Then, small water dwelling invertebrates such as crustaceans or snails swallow the larvae. Fish become infected when they eat the invertebrate host and finally, mammalian predators eat fish, and the whole cycle starts again.
Humans join this cycle by eating raw fish, inadvertently ingesting the contaminated flesh. There are three types of parasitic worms that can infect humans. i. Roundworms (Nematodes); ii. Tapeworms (Cestodes); iii. Liver flukes (Trematodes). Symptoms can include abdominal problems and fever, and may resemble appendicitis or intestinal obstruction, jaundice, fatigue, diarrhea, weakness, numbness of the extremities, and a feeling of hunger.
Adequate freezing and/or cooking eliminates infection by the parasites.
In commercial freezing, a temperature of -40 °F kills any parasite in 15 hours. In a home freezer, at 0° to 10°F, it can take up to five days to kill all the parasites, especially in large fish.
Fish is also safe to eat when it reaches an internal temperature of 145°F for five minutes. Thus traditional cooking methods and microwaving will kill any potential parasites providing the fish temperature reaches 145°F for five minutes.
Similarly, hot smoking of fish, provides an effective method of eliminating parasites provided the fish is smoked at 150° to 200°F for four to six hours. On the other hand, cold-smoked products may not be safe to eat unless they've been properly frozen first. Unlike hot smoking, cold smoking does not use heat and the fish doesn't reach the temperature required to kill the parasites. Likewise, ceviche, or raw fish marinated in lemon or limejuice, may contain parasites unless it has been properly frozen before marinating.
If you do choose to eat raw fish, a process called candling reduces the risk of infection by parasites. Candling means holding thin slice of filleted flesh in front of a light so that any parasites can be seen and then removed. The parasite is a tightly coiled, clear worm, 1-4mm in diameter, that imbeds itself in the flesh. Candling also reveals any pinbones left in a product intended to be boneless. In New Zealand good sanitation reduces the risk, unlike some tropical countries where raw fish should never be consumed.
Consumers should be aware of these risks, and take care when preparing sashimi for sushi. [Other risks include mercury from geothermal sources]