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A review was conducted to identify the most common causative agents of anisakidosis, the methods used for identification of the causative agents, and to summarize the sources of infection, and patients’ demographics. A total of 762 cases (409 articles, inclusive of all languages) were found between 1965 and 2022. The age range was 7 months to 85 years old. Out of the 34 countries, Japan, Spain, and South Korea stood out with the highest number of published human cases of anisakidosis, respectively. This raises the question: Why are there few to no reports of anisakidosis cases in other countries, such as Indonesia and Vietnam, where seafood consumption is notably high? Other than the gastrointestinal tract, parasites were frequently found in internal organs such as liver, spleen, pancreas, lung, hiatal and epigastric hernia, and tonsils. There are also reports of the worm being excreted through the nose, rectum, and mouth. Symptoms included sore throat, tumor, bleeding, gastric/epigastric/abdominal/substernal/lower back/testicular pain, nausea, anorexia, vomiting, diarrhea, constipation, intestinal obstruction, intussusception, blood in feces, hematochezia, anemia, and respiratory arrest. These appeared either immediately or up to 2 months after consuming raw/undercooked seafood and lasting up to 10 years. Anisakidosis commonly mimicked symptoms of cancer, pancreatitis, type I/II Kounis syndrome, intussusception, Crohn’s disease, ovarian cysts, intestinal endometriosis, epigastralgia, gastritis, gastroesophageal reflux disease, hernia, intestinal obstruction, peritonitis, and appendicitis. In these cases, it was only after surgery that it was found these symptoms/conditions were caused by anisakids. A range of not only mainly marine but also freshwater fish/shellfish were reported as source of infection. There were several reports of infection with >1 nematode (up to >200), more than one species of anisakids in the same patient, and the presence of L4/adult nematodes. The severity of symptoms did not relate to the number of parasites. The number of anisakidosis cases is grossly underestimated globally. Using erroneous taxonomic terms, assumptions, and identifying the parasite as Anisakis (based solely on the Y-shaped lateral cord in crossed section of the parasite) are still common. The Y-shaped lateral cord is not unique to Anisakis spp. Acquiring a history of ingesting raw/undercooked fish/seafood can be a clue to the diagnosis of the condition. This review emphasizes the following key points: insufficient awareness of fish parasites among medical professionals, seafood handlers, and policy makers; limited availability of effective diagnostic methodologies; and inadequate clinical information for optimizing the management of anisakidosis in numerous regions worldwide.

Anisakidosis, human infection with nematodes of the family Anisakidae, is caused most commonly by Anisakis simplex and Pseudoterranova decipiens. Acquired by the consumption of raw or undercooked marine fish or squid, anisakidosis occurs where such dietary customs are practiced, including Japan, coastal regions of Europe, and the United States. Severe epigastric pain, resulting from larval invasion of the gastric mucosa, characterizes gastric anisakidosis; other syndromes are intestinal and ectopic. Allergic anisakidosis is a frequent cause of foodborne allergies in areas with heavy fish consumption or occupational exposure. Diagnosis and treatment of gastric disease is usually made by a compatible dietary history and visualization and removal of the larva(e) on endoscopy; serologic testing for anti—A. simplex immunoglobulin E can aid in the diagnosis of intestinal, ectopic and allergic disease. Intestinal and/or ectopic cases may require surgical removal; albendazole has been used occasionally. Preventive measures include adequately freezing or cooking fish. The ocean is a wilderness reaching round the globe, wilder than a Bengal jungle, and fuller of monsters. —Henry David Thoreau, Cape Cod [1, p 188]

Northeast Arctic cod, saithe and haddock are among the most important fisheries resources in Europe, largely shipped to various continental markets. The present study aimed to map the presence and distribution of larvae of parasitic nematodes in the Anisakidae family which are of socioeconomic and public health concern. Fishes were sourced from commercial catches during winter or spring in the southern Barents Sea. Samples of fish were inspected for nematodes using the UV-press method while anisakid species identification relied on sequencing of the mtDNA cox2 gene. Anisakis simplex (s.s.) was the most prevalent and abundant anisakid recorded, occurring at high infection levels in the viscera and flesh of cod and saithe, while being less abundant in haddock. Contracaecum osculatum (s.l.) larvae, not found in the fish flesh, showed moderate-to-high prevalence in saithe, haddock and cod, respectively. Most Pseudoterranova spp. larvae occurred at low-to-moderate prevalence, and low abundance, in the viscera (Pseudoterranova bulbosa) and flesh (Pseudoterranova decipiens (s.s.) and Pseudoterranova krabbei) of cod, only 2 P. decipiens (s.s.) appeared in the flesh of saithe. Body length was the single most important host-related factor to predict overall abundance of anisakid larvae in the fish species. The spatial distribution of Anisakis larvae in the fish flesh showed much higher abundances in the belly flaps than in the dorsal fillet parts. Trimming of the flesh by removing the belly flaps would reduce larval presence in the fillets of these gadid fish species by 86–91%.

Clinicians can be forgiven for thinking of anisakiasis as a rare condition low in the differential diagnosis of abdominal pain. Gastrointestinal anisakiasis is a zoonotic parasitic disease caused by consumption of raw or undercooked seafood infected with nematodes of the genus Anisakis. Even though the reported cases indicate that this is a rare disease, the true incidence of the disease could be potentially higher than what is reported in the literature as cases can go undiagnosed. Diagnosis and treatment of gastric anisakiasis are made by a compatible dietary history, direct visualization, and removal of the larvae via gastroscopy. Serologic testing and imaging studies are useful in the diagnosis of intestinal anisakiasis and conservative management should be considered. This disease may mimic other diseases and lead to unnecessary surgery. This emphasizes the importance of suspecting gastrointestinal anisakiasis by history taking and by other diagnostic modalities.

Fish in wild and cultured populations may be infected with numerous types of pathogens but the host responses vary dependent on both host and parasite species. The present study demonstrates how an experimental infection with endoparasitic nematode larvae ( Anisakis simplex ) induces cellular and humoral immune responses in rainbow trout Oncorhynchus mykiss . The nematode larvae invaded the peritoneal cavity of the fish following oral administration and became encapsulated by a range of host cells including macrophages, neutrophils, mast cells, fibroblasts, and lymphocytes. The main part (92.7%) of the recovered larvae was located in the body cavity and 51.3% along the pyloric caeca with only few in or on the stomach, liver, spleen, swim bladder, and musculature. The cellular reaction was documented by transmission electron microscopy (TEM) and histochemistry. Real time quantitative PCR (qPCR) showed that a series of immune-relevant genes in the host spleen became regulated by the infection. Thus, A. simplex induced downregulation of immune-genes (encoding IgD and lysozyme) and upregulation of the gene encoding the immune-regulating cytokine IL-10. Nematode molecules influencing the antiparasitic host reactions are discussed.

Anisakiasis is a foodborne zoonosis caused by ingesting raw seafood containing anisakid larvae. Its global incidence has increased with rising seafood consumption, highlighting the need for effective prevention methods. While freezing at −20 °C for 24 h is a standard preventive measure, it compromises seafood quality. Pulsed power (PP) technology, delivering high-voltage microsecond pulses, has emerged as a promising non-thermal alternative. Here, we evaluated PP inactivation efficacy using agar penetration and rabbit infection tests. Larvae treated with 10 pulses of 13 kV for 10 µs failed to penetrate agar (0/40), whereas 35/40 untreated larvae penetrated agar ( p  = 0.0202). In the rabbit model (four animals/group), PP-treated larvae caused no gastric wall penetration, and only three nonviable larvae were observed in the gastric lumen. Conversely, untreated larvae invaded the gastric mucosa (mean: ~ 21/50), lumen (~ 7.8/50), and peritoneal cavity (~ 2.3/50). Gastric wall penetration was significantly lower in the PP group than in the untreated group ( p  = 0.0211), with a relative risk of 0.006. These results objectively demonstrate complete inactivation of anisakid larvae after PP treatment. PP represents a promising alternative to freezing and holds potential for broader application, provided that compact devices are developed for practical use in seafood processing.

The increasing presence of Anisakis spp. in fish is having significant implications for public health due to a rise in cases of anisakiasis. Given this situation, there is a critical need to develop new strategies to fight this parasite. Satureja montana L., commonly known as savory, is a plant recognized in folk medicine for its therapeutic activity, such as being antispasmodic and digestive, among other properties. The aim of this study was to assess the nematicide activity against A. simplex larvae of the essential oil from two varieties of S. montana (subsp. montana (SMM) and variegata (SMV)). The essential oils were obtained via hydro-distillation of the flowering aerial parts. In vitro assays demonstrated the complete inactivation of anisakis larvae after 24 h when exposed to both essential oils, along with a significant reduction in their penetration capacity. Moreover, both essential oils showed an inhibitory effect on acetylcholinesterase (AChE). No differences between the subspecies were observed in any of the assays. Hence, the nematicidal activity of essential oils could be attributed to their capacity to inhibit AChE. These findings suggest the potential of S. montana essential oil for therapeutic and food industry applications.

Marine nematodes of the genus Anisakis are common parasites of a wide range of aquatic organisms. Public interest is primarily based on their importance as zoonotic agents of the human Anisakiasis, a severe infection of the gastro-intestinal tract as result of consuming live larvae in insufficiently cooked fish dishes. The diverse nature of external impacts unequally influencing larval and adult stages of marine endohelminth parasites requires the consideration of both abiotic and biotic factors. Whereas abiotic factors are generally more relevant for early life stages and might also be linked to intermediate hosts, definitive hosts are indispensable for a parasite’s reproduction. In order to better understand the uneven occurrence of parasites in fish species, we here use the maximum entropy approach (Maxent) to model the habitat suitability for nine Anisakis species accounting for abiotic parameters as well as biotic data (definitive hosts). The modelled habitat suitability reflects the observed distribution quite well for all Anisakis species, however, in some cases, habitat suitability exceeded the known geographical distribution, suggesting a wider distribution than presently recorded. We suggest that integrative modelling combining abiotic and biotic parameters is a valid approach for habitat suitability assessments of Anisakis , and potentially other marine parasite species.

Anisakid nematodes are widespread marine parasites with complex life cycles involving invertebrates and fish as intermediate or transport hosts, and marine mammals as definitive hosts. Despite their ecological importance, and the zoonotic potential associated with the larval stages found in fish, recent data on anisakid species diversity in pinnipeds from Norwegian waters remain scarce. In this study, we investigated anisakid infections in two juvenile harbour seals ( Phoca vitulina ) stranded along the southern coast of Norway. Gastrointestinal nematodes were collected, morphologically classified to the genus level, and subsequently identified to species level through molecular analyses of mitochondrial (mtDNA cox2 ) and nuclear (rDNA ITS) markers. Five anisakid species were identified: Contracaecum osculatum sp. A (reported here for the first time in harbour seals), C. osculatum (sensu stricto), Phocanema decipiens (s.s.), P. krabbei , and Anisakis simplex (s.s.). The latter species was found in unexpectedly high abundance and in fully developed adult stages in one of the seals. Notably, these adult A. simplex (s.s.) exhibited large body size, in contrast with previous studies reporting either absence or minimal presence of adults in harbour seals. The underlying mechanisms promoting growth and reproductive development of A. simplex (s.s.) in this host species remain unclear, but may involve a combination of host-specific physiological traits, environmental factors, and parasite phenotypic plasticity. Gross pathological examination revealed multiple gastric and intestinal ulcers in the same seal, including seven crateriform lesions consistent with ulcerative gastritis and enteritis, associated with nematode attachment and feeding. These findings expand the current knowledge on anisakid diversity in P. vitulina and provide novel evidence of its role as a definitive host for A. simplex (s.s.) in Norwegian coastal waters. Furthermore, the results suggest that competitive interactions among anisakid species, combined with ecological and physiological host factors, may facilitate the development and maturation of A. simplex (s.s.) in harbour seals. Further studies are warranted to assess the frequency and health implications of such infections in wild pinniped populations.

Parasitic nematodes are prevalent in fish populations. The parasites are pathogenic but depress host responses, which limit clearance of the pathogens from the invasion sites. We hypothesized that one of several control strategies, which could augment protection, is immunization of the fish host with parasite antigens prior to live pathogen exposure. We used rainbow trout as a host model and third stage larvae (L3) (Nematoda, Ascaridoidea, Anisakidae) as pathogen model. We used a total of 120 fish and immunized 40 of the fish with a homogenate (adjuvanted) of parasite larvae (i.p. injection), 40 fish received adjuvant only and 40 PBS. Following 38 days (d) half of the fish in each group were exposed to infection with live worms (oral administration), and after an additional 25 d the infection success was evaluated together with antibody responses in the different groups. Injection of L3 antigens induced a series of adaptive and innate host responses. ELISA and Western blot analyses indicated specific IgM reactions in immunized trout against worm antigens with molecular weights (MW) of approximately 39, 103 and 119 kilodalton (kDa). Fish immunized and subsequently infected with live larvae reacted to those three and six additional antigens with MW approximately 61, 73, 84, 152, 186 and 277 kDa. The immunized fish showed a significantly lower worm burden following exposure to live parasite larvae (when compared to naïve fish), but no full protection was achieved. Expression analyses of both adaptive and innate immune genes in fish showed a general down-regulation following infection. Prior immunization with L3 homogenate induced a strong antibody response, but the protection was incomplete. It was noteworthy that an infection period (25 d) with live parasites merely induced an insignificant antibody response. It may be explained by the immunosuppressive compounds released by live worm larvae. With the aim of increasing the protective response, we suggest in future immunization experiments to target immunosuppressive worm antigens by immunizing the host organisms with excretory/secretory (ES) proteins and extracellular particles from L3.