Fish-borne Parasitic Zoonoses and Allergens

Article Citation: Al-Awadhi, M. A. (2020, November 8). Fish-borne Parasitic Zoonoses and Allergens. Kuwaiti Journal of Medical Parasitology.

Fish-borne parasitic zoonoses are parasitic infections which are transmitted from fish to humans when infected raw or undercooked fish is consumed. Cuisines which consist of raw/undercooked fish including sushi and sashimi have been increasing in popularity worldwide, and consequently, the incidence of fish-borne zoonoses is also on the rise (Lopes et al., 2020). Furthermore, several parasitic species in fish have recently been reported in European countries, posing a potential public health concern (Caffara et al., 2020).

Numerous species of fish-borne parasites exist which may be classified into 3 major groups: i) parasitic Nematodes (roundworms), ii) Cestodes (parasitic tapeworms), and iii) Trematodes (parasitic flatworms, also known as flukes). Respectively, the most common parasitic diseases associated with the consumption of raw/undercooked fish are anisakiasis (nematodes), diphyllobothriasis (tapeworms) and clonorchiasis (flukes).

Anisakiasis is caused largely by 3 parasite species, i.e., Anisakis simplex, Contracaecum osculatum and Pseudoterranova decipiens. The symptoms of infection include mild fever, abdominal pain, vomiting, diarrhea, bloody stool and bowel obstruction (Shimamura et al., 2016). A recent study from Egypt had reported 40 hospitalized patients due to infection with Capillaria philippinensis (a fish-borne parasitic nematode) caused by consuming raw/undercooked freshwater fish (El-Dib & Ali, 2020). Cases of anisakiasis were also reported in Spain due to the consumption of raw hake fish, which required endoscopic removal of several larval nematodes from the stomach of patients (Roca-Geronès et al., 2020). A recent study from Turkey showed that 98% of Atlantic mackerel fish were infected with Anisakis spp. larvae (Simsek et al., 2020). In Slovakia, various parasite species were detected in 25% of the examined European perch fish from the Danube river, including the parasitic nematode Eustrongylides sp. and the trematode Clinostomum complanatum, both of which can cause illness in humans (Juhásová et al., 2020).

Diphyllobothriasis is caused by several Diphyllobothrium spp., most commonly being D. latum. Acute symptoms are similar to those of anisakiasis. However, chronic symptoms may present as headaches, megaloblastic anemia (enlarged red blood cells with low cell count), weight loss, dyspnea (shortness of breath), optic neuritis (painful inflammation of the optic nerve) and skin allergy (Scholz et al., 2009). Most of the chronic symptoms are caused by the parasite competing for vitamin B12 uptake in the small intestine (Durrani et al., 2020).

Diphyllobothriasis is commonly reported in East Asia, but has recently been reported in a French patient who had no travel history to Asia but consumed undercooked salmon, indicating a potential intercontinental spread of parasitic infections due to increasing global demand for seafood (Greigert et al., 2020). In addition, a study carried out in Lake Iseo, northern Italy, reported that the prevalence of D. latum in the European perch fish reached up to 10% (Menconi et al., 2020).

Clonorchiasis (also known as the Chinese liver fluke) is caused by Clonorchis sinensis. Although acute infection is commonly asymptomatic, some patients experience right upper quadrant abdominal pain, fever, fatigue, nausea, vomiting and weight loss. The chronic stage, however, is more severe due to a heavier parasitic load in the biliary tract. The symptoms of chronic clonorchiasis include indigestion, diarrhea, cholecystitis (inflammation of the gall bladder), cholangiohepatitis (inflammation of the liver and bile ducts) and cholangiocarcinoma (bile duct cancer) (Locke & Richardson, 2020). Clonorchiasis is prevalent in China, Korea, and Vietnam, where 15-20 million people are estimated to be infected (Na et al., 2020). A recent study in northern Vietnam’s rural communities had reported that the C. sinensis infection was prevalent in 40.4% of the population, and was significantly associated with raw fish consumption, low education levels and having unhygienic toilets (Nguyen et al., 2020). In addition to C. sinensis, around 30 different species of trematode parasites, including Pygidiopsis macrostomum and Ascocotyle pindoramensis, currently infect about 7 million people due to raw/undercooked fish consumption (Santos and Borges, 2020). A recent study in Myanmar found that 46% of nearly 700 examined fish were infected with carcinogenic (cancer-causing) trematodes in which the prevalence reached up to 83% of a particular fish species (Cyclocheilichthys repasson) (Phyo Myint et al., 2020). In Lake Endine in northern Italy, researchers published the first report on the prevalence of Clinostomum complanatum, a trematode responsible for Halzoun syndrome (allergic pharyngitis) in humans. The parasite was found in about 1 in 5 examined European perch fish (used in traditional cuisine), emphasizing the possible health risk for consumers (Menconi et al., 2020).

Although parasites are killed in processed fish (e.g. frozen, smoked or marinated), more than 28 different parasite allergens (proteins) can potentially cause IgE-mediated hypersensitivity (allergic reaction) in sensitive individuals (Kochanowski et al., 2020a). Parasite allergens have been shown to persist in smoked mackerel, herring, cod and hake, and in marinated herring, canned cod liver and mackerel, and surimi sticks (Kochanowski et al., 2020b). Therefore, adequate frying, baking, grilling or boiling of fish meat is necessary to kill the parasites and to denature (alter the shape and function) allergens, rendering them inactive.

In conclusion, it is of utmost importance that consumers of raw/undercooked fish delicacies, including sushi, sashimi and smoked salmon lovers, are aware of the health risks associated with new trends in seafood cuisine, which include potentially fatal cancer-causing parasitic infections and anaphylactic shock caused by parasite allergens.


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