Unfortunately, raw fish ingested in dishes such as sushi and ceviche can transmit parasitic diseases and other infectious diseases caused by bacteria. It is important to mention that lightly salted, smoked, or pickled fish are considered as raw fish and can also transmit these infectious agents. In fact, parasitic infections are considered as the most important health risk when eating dishes containing raw fish.
There are three main types of parasites susceptible to be acquired from raw fish, namely the nematodes Anisakis simplex and Pseudoterranova decipiens responsible for the disease called anisakiasis, as well as the fish tapeworm Diphyllobothrium responsible for the disease called diphyllobothriasis and the Chinese liver fluke Clonorchis sinensis. However, cases of infection with the parasites Anisakis simplex and Pseudoterranova decipiens are extremely rare and mainly observed in Japan where raw fish ingestion in sushi is widespread. Infections with Diphyllobothrium and Clonorchis sinensis are more commonly observed, even though the latter one is almost exclusively observed in Asia, but has also been related to the ingestion of imported fishes from Asia in other countries. It is interesting to note that these infections are now observed more frequently in North America and Europe since sushi became a more popular dish.
Infections with parasites occurring when eating raw fish are easily preventable. First of all, larvae and adult worms can be visualised by the naked eyes before preparing meal with raw fish. Contaminated fish must then be discarded and not be part of raw fish dishes, such as sushi or ceviche. It is interesting to note that visual inspection of raw fish is mandatory in restaurant in some country, such as France. However, the safest mean in order to kill the parasites present within the fish before doing the sushi or the ceviche is to freeze it. As some parasites or larvae appear to be longer to kill than others, it is recommended to freeze the fish below -20°C for at least 7 days in industrial freezer. It is noteworthy that home freezer (that are usually kept at -18°C) might not be powerful enough and that the temperature might not be enough stable to efficiently kill parasites. Some producer also used quick ultra-low freezing at temperature as low as -60°C in order to prevent the deterioration of the fish, such as discoloration of the flesh. It is then recommended to buy fresh fish that have been frozen industrially if you want to do sushi or ceviche at home. As such, many countries, such as the United States and the European Union, have now regulations that forbid the use of unfrozen raw fish in restaurants and transformed raw fish products in grocery store.
Finally, eating raw food, such as raw fish, present certain health risks, such as the risk of being infected by parasites and bacteria. People more prone to infections, such as pregnant women and people with weakened immune system, should avoid eating raw fish as a mean of precaution.
Source: Centers for Disease Control and Prevention.