Who lives in seafood?
In cooking, fish and other aquatic inhabitants are very respected. True, mostly in a well, sincerely, and diligently prepared form. Except for planning in the Far North, their heat treatment is provided by shock freezing at a very low temperature, to which freezers of household refrigerators are very, very far away.
But for quite some time now, Asian cuisine has been in fashion, offering to enjoy the authentic taste of raw or minimally processed seafood and fish. How dangerous is it to eat such foods? Who can be accidentally eaten with sashimi, and what it threatens?
Helminthiases of aquatic inhabitants
In 2020, American scientists from the University of Washington published an article in which they presented the results of their analysis, which follows that over the past 40 years, the risk of contracting helminthiasis when eating raw seafood and fish has increased by 283 times. This may be due both to the increased fashion for poorly processed seafood and to the increase in the prevalence of helminthiases in aquatic inhabitants.
Naturally, the question arises: if everything is so bad, why does this “fashion” not disappear either in Japan, Korea, or in other countries where the diet traditionally includes raw seafood and fish? Perhaps the Japanese have developed some defense mechanism against parasites? The northern peoples acquired a certain resistance to cadaveric poison, for many generations eating kopalchen – meat aged for several months in a swamp, from which any European would die in agony within a few hours even if he survived the process of eating this extremely unappetizing dish for our noses and taste buds.
But no. Asians have a natural defense only against certain bacteria, which can also be infected from raw fish, but they suffer from helminths in the same way as everyone else. And even more often: according to statistics, it is in Japan that the largest number of cases of anisakiasis is recorded.
What can you pick up from insufficiently salted, frozen, or fried fish?
The “range” includes only three components: nematodes, flatworms, flukes, and tapeworms. But at the same time, as they say, “with good coverage,”: nematodes, for example, are found in the inhabitants of almost all seas and oceans of all climatic zones. Tapeworms live mainly in cold-water species of freshwater fish, and flatworms, or flukes, prefer freshwater inhabitants of warm waters.
At the same time, a person is not the main “host” for any of the helminths, which does not prevent those, after an accidental “settlement” in the human body, from feeling relatively well there, with rare exceptions.
According to statistics, this is a frequently diagnosed helminthiasis from aquatic animals that a person becomes infected with. The worm is also known as a wide tapeworm, capable of growing in the intestines up to 12 m in length and living there for decades. Sometimes the parasite is not alone but with relatives – there are cases when more than 100 individuals were found in a patient’s intestines. In this case, even the development of intestinal obstruction is possible. And so usually, the patient complains of appetite disorders, increased salivation, belching, nausea, weakness, irritability, rumbling in the abdomen, and unstable stools.
The source of infection can be insufficiently processed or poorly salted pike, pike perch, burbot, pink salmon, and chum salmon, and most fish caught in the rivers and lakes of Siberia, the Far East, the northern regions of the European part of our country, in the reservoirs of the Volga basin, etc. take into account that the drying of lightly salted fish does not eliminate parasites.
Symptoms of opisthorchiasis may appear 2-3 weeks after the infected fish is eaten. And a person often does not even remember this episode and does not associate a meal with a deterioration in well-being in the form of fever (sometimes up to 40 ° C), nausea, bitterness in the mouth, pain in the right hypochondrium, and weakness. And if the disease proceeds in an erased form, much more time can pass before going to the doctor.
The causative agent of this disease is the Siberian fluke, which belongs to trematodes, or flatworms. This is a small parasite – up to 12 mm in length, which prefers the liver, bile ducts, and gallbladder for residence. The main hosts of this parasite are terrestrial mammals, including cats (so, by the way, you should not feed raw river fish to domestic purrs). Intermediate hosts are freshwater mollusks and fish of the carp family: crucian carp, bream, roach, asp, ide, carp, etc.
Approximately the same symptoms in clonorchiasis are caused by the Chinese fluke, which affects fish that live in fresh water and rivers of the Far East, China, Korea, and Vietnam.
Although some people believe that there are no worms in the “redfish” of salmon breeds, this belief is erroneous: it happens, and how. And nano psychosis caused by small trematodes is an example of this. Intermediate parasite development stages pass in the bodies of chum salmon, whitefish, taimen, and Siberian grayling, as well as kunja, lenok, and other fish. The final hosts are mainly terrestrial predatory mammals and humans in case of accidental infection.
The disease is manifested by diarrhea and pain throughout the abdomen.
The disease is associated with infection by anisakid larvae, a representative of the nematode family. As definitive hosts, anisakids prefer marine mammals – dolphins, seals, whales, large predatory fish, and fish-eating birds. The larvae can live in fish, mollusks, crustaceans, etc. Anisakids are roundworms with a body length of up to 5-7 cm, usually coiled up, and feel great both in the abdominal cavity of fish and in muscle tissue, where they are much harder to spot.
According to available data, in some years, the contamination of commercial marine fish with antacids can reach 100% and affect representatives of herring, cod, salmon, etc.
It is especially unpleasant that anisakiasis is difficult to diagnose. According to some experts, many doctors need a sufficient level of awareness of this disease’s symptoms, which applies to doctors from a wide variety of countries and regions.
For example, this is the case in Australia, where, according to official statistics, the incidence of anisakiasis is relatively low.
According to available data, fish and seafood contamination with anisakids has increased significantly in recent years. Australia is a region where the seafood is eaten in significant quantities. Moreover, most of the seafood is sold in the markets and does not go through strict control. However, the country has a relatively low number of cases of diagnosed anisakiasis. We assumed that the incidence could be higher, and such low rates are largely due to insufficiently accurate diagnosis and a low level of awareness of both patients and doctors.
Our survey results partly confirmed that. However, since only 11% of all invited experts took part in the survey, and this group was very heterogeneous in many respects, it is too early to conclude. However, there is reason to believe that helminthiases associated with eating contaminated seafood are not diagnosed in many cases due to certain difficulties in detecting infection and insufficient awareness of people about the high risk of infection.
The Portuguese authors also note the insufficiently high level of awareness of doctors regarding the possible manifestations of anisakiasis. In particular, this applies to allergic reactions provoked by these helminths. This article was published in 2022.
In her work, Polish researcher Jolanta Morozinska-Gogol, published in 2019, reports that even a fairly good heat treatment of fish contaminated with anisakids does not eliminate the risk of developing allergic reactions when eating such fish. And this is because some of the anisakid allergens are thermostable and resistant to pepsin treatment.
Other manifestations of the disease depend on where the parasites are located. If it is the stomach, there may be epigastric pain, vomiting, and fever. Ingestion of larvae from the stomach into the esophagus can cause a sore throat and cough. The disease’s intestinal form can give very mild symptoms and a picture of an “acute abdomen.” In the most severe cases, the integrity of the intestinal wall may be violated, followed by peritonitis.
How to avoid infection?
First, refrain from eating raw and poorly processed fish, squid, octopus, and shellfish. And from some of the illusions associated with the concepts of what “well-processed seafood” is. And also with the conviction that parasites are necessarily visible to the naked eye.
Here is what the CPS recommends.
To avoid infection with opisthorchiasis, it is necessary either to boil or fry the fish in oil for at least 15-20 minutes or to freeze it at a temperature of -40 ° for at least 7 hours. Household refrigerators cannot provide such freezing.
As for salting fish, you should forget about lightly salted instant fish: small fish should be salted for at least 14 days, and large fish for at least 40 days, while it is recommended to use 2 kg of salt for every 10 kg of fish.
In the same way, you can get rid of anisakid larvae. The difference lies in resistance to low temperatures: parasites not related to trematodes can die at not-so-low temperatures. For example, it may be sufficient to keep seafood at -20°C daily or at -35°C for 15 hours. However, not all home refrigerators can produce such a temperature; therefore, first, you need to ensure the freezer’s capabilities.
In Europe, it is believed that trematodes can be killed by freezing to -10°C for five days or to -28°C for 32 hours. Regarding exposure to high temperatures, heat treatment at 70°C for 30 minutes is considered sufficient even for the most resistant species of trematodes.