Anisakis Nematodes in Fish and Shellfish- from infection to allergies.

Anisakidosis is a zoonotic parasitosis induced by members of the family Anisakidae. The anisakid genera includes Anisakis, Pseudoterranova, Hysterothylacium and Contracaecum. The final definitive hosts of these nematodes are marine mammals with a complex life cycle. These nematode parasites use different crustaceans and fish species as intermediate or paratenic hosts and humans are accidental hosts. Human anisakiasis, the infections caused by members of the genus Anisakis, occurs, when seafoods, particularly fish, contaminated with the infective stage (third stage larvae [L3]) of this parasite, are consumed. Pseudoterranovosis, on the other hand is induced by members of the genus Pseudoterranova. These two genera of anisakids have been implicated in human disease globally. There is a rise in reports of gastro-intestinal infections accompanied by allergic reactions caused by Anisakis simplex and Anisakis pegreffii. This review provides an update on current knowledge on Anisakis as a food-borne parasite with special focus on the increasingly reported diversity of fish and crustacean hosts, allergens and immunological cross-reactivity with invertebrate proteins rendering this parasite a significant public health issue.

Introduction

The World Health Organization (WHO) (World Health Organization, 2012) estimates approximately 56 million cases of parasite infections associated with the consumption of fish products. Among the parasites implicated, are the group of anisakids, known to be capable of inducing severe pathologies in humans and widely distributed geographically across all continents (Pampiglione et al., 2002).

Anisakids are parasitic nematodes belonging to the Phylum Nemathelmintes, Class Nematoda, Ascarida order, suborder Ascaridina, superfamily Ascaridoidea, Anisakidae family and subfamily Anisakinae. This family includes the genera Anisakis, Pseudoterranova, Contracaecum and Hysterothylacium (Smith and Wootten, 1978). However, three species of the genus Anisakis are reported to be the causative agent of infections in humans: A. simplex sensu stricto, Anisakis pegreffii and Anisakis physeteris (Mattiucci and Nascetti, 2008). For the purpose of this review, the focus will be on A. simplex and A. pegreffii.

Anisakiasis is the zoonotic disease triggered by the third stage larvae of nematodes, Anisakis (Nieuwenhuizen and Lopata, 2013). This parasite habitually parasitises adult marine mammals. Intermediate and/or paratenic hosts of the larvae are crustaceans, cephalopods and fish (Nieuwenhuizen and Lopata, 2013). Humans are however accidentally infected when hosts are ingested either as raw or inadequately cooked or treated fish/shellfish meals. Hence, the infection has been directly linked to eating habits (Pampiglione et al., 2002). Infected dishes of raw fish such as sushi and sashimi commonly found in Japan's national dishes as well as the culinary tradition of consumption of marinated or raw fish in European countries such as Italy are a significant source of the infection (Ivanovic et al., 2015; Yorimitsu et al., 2013). Globalization of such cuisine improved public health diagnosis (better diagnostic tool development) and greater awareness of anisakiasis infection has resulted in increase in the frequency of reported anisakiasis in most continents (Ivanovic et al., 2015).

Symptoms of acute anisakiasis include severe abdominal pain, nausea, and vomiting. Some of these symptoms closely mimic peptic ulcer, appendicitis, or peritonitis with the most concerning presentation being allergic sensitisation, which is usually serious and range from urticaria to anaphylactic shock (Villazanakretzer et al., 2016). The L3 larvae of the genus Anisakis have been found in several economically important fish species. However, the prevalence of Anisakis in the fish host has been reported to vary between different geographical fishing grounds and seasons in the same fish host species. Serraca and colleagues (2014) attributed this to the fact that some invertebrate hosts for Anisakis may not be part of the habitual diet of some fish hosts in certain fishing grounds in some seasons as they may prefer other invertebrates as foods (Serracca et al., 2014). It is therefore possible for a fish host in one region to be heavily contaminated with Anisakis depending on the types of crustaceans eaten; and in another region, such a fish host may be free of Anisakis, as it may prefer other invertebrates as food than those harbouring Anisakis (Serracca et al., 2014). Hence, the composition of the zooplankton communities in fishing regions is a significant contributing factor to Anisakis parasitosis (Serracca et al., 2014). Widening our knowledge on the fish and crustacean hosts used by this parasite to propagate its life cycle, would be significant in reducing the risk for human anisakiasis via consumption of potentially infective seafoods, which are a food safety risk.

Anisakis biology and life cycle

The definitive host of anisakid, where the adult stage of the parasite is found, are cetaceans, (marine mammals such as dolphins and seals). Eggs of the parasite are passed with faeces of definitive host into the marine environment where they hatch and develop into the 2nd stage larvae of the parasite (Baird et al., 2014; Buchmann and Mehrdana, 2016). The larvae that develop from the eggs are eaten by the intermediate hosts, crustaceans (copepods, decapods, isopods, amphipods, euphausiids) and molluscs (Baird et al., 2014). It is well established that one of the most important first intermediate hosts in the Anisakis life cycle are the Euphasiids (Krill) (Smith and Wootten, 1978). Infected krill eaten by fish or squid become a source of the infective 3rd stage larvae of Anisakis for the paratenic hosts (fish or squid). The Anisakis larvae encyst on the intestines and other visceral organs of these hosts. The parasite does not develop further and remains at the third stage of larvae development (L3 stage) in these hosts. The life-cycle comes to a completion when infected fish/squid are eaten by marine mammals such as whales, seals and dolphins. In these definitive hosts, the larvae grow to the L4 stage and subsequently to the adult stage (Fig. 1). The nematode feeds, grows, mates and then releases eggs via the host faeces into the sea water to continue its life cycle (Pozio, 2013). As is common with parasites with complex life cycle, the morphology of Anisakis varies with the different stages and infected hosts. In fish, the L3 stage larvae displays a coiled shape, which when uncoiled, is about 2 cm long. Humans become accidental hosts when undercooked or raw fish and cephalopods, contaminated with the parasite are consumed.

Fig. 1

Anisakid Life Cycle. Location of the L3 larvae may be gastric, intestinal or ectopic.

Molecular identification and classification of anisakids

Classification of anisakid nematodes was previously based on some of their morphological features, which include presence, or absence of mucron at the tail tip, the length of the ventriculus position and extent of the cecum. This led to the classification of Anisakis into two types: Anisakis type 1 and II by Berland (1961). However, due to lack of precise details for species identification using morphological features, molecular tools such as sequencing of the internal transcribed spacer (ITS) region of ribosomal DNA, mitochondrial gene cytochrome oxidase subunit 2 (Cox2) and allozyme analysis, polymerase chain reaction coupled with restriction fragment length polymorphism; have been employed in species-specific identification (Iglesias et al., 2008; Shamsi et al., 2009; Umehara et al., 2006). The cox2 gene is shown to be highly polymorphic in Anisakis spp. and is therefore particularly useful for identifying the parental species involved in the production of hybrids. It provides valuable information in the recognition of sibling species of Anisakis larvae, as independent genetic lineages (Abollo et al., 2003; Mattiucci et al., 2009; Valentini et al., 2006). It has also been established that it provides additional genetic characters useful for molecular epidemiological approach to the study of anisakiasis in human (Audicana and Kennedy, 2008).

Several studies have also shown that the first and second internal transcribed spacers (ITS-1 and ITS-2) of nuclear ribosomal DNA (rRNA) provide appropriate genetic markers for anisakid species identification irrespective of their stage of development (Jabbar et al., 2012, 2013; Shamsi et al., 2011). PCR-coupled mutation scanning of the ITS-1 and/or ITS-2, integrated with targeted sequencing (Gasser et al., 2006) and phylogenetic analysis also furnishes a powerful approach for investigating the genetic composition of populations of anisakids (Jabbar et al., 2012, 2013).

Different genera of anisakids have been described using molecular tools and classified into one of the two families: Anisakidae or Raphidascarididae (Kalay et al., 2009). Within the genus Anisakis, ten species and four distinct clades have been described (Mattiucci et al., 2014, 2018). Anisakis species identified as Clade 1 include A. berlandi, A. pegreffii, and A. simplex (s.s.) and are usually referred to as the A. simplex (s.l.) complex. A. simplex and A. pegreffii are inferred to be sister taxa (Mattiucci et al., 2014). Anisakis spp in clade 2 are also considered as sister taxa and have been identified as A. ziphidarum and A. nascetti (Mattiucci et al., 2014). Clade 3 Anisakis have been identified as A. physeteris, A. paggiae and A. brevispiculata. Anisakis sp 2, a newly identified taxon, has been found to be closely related to A. physeteris, clustering closely with the clade 3 group of Anisakis and implying that Anisakis spp 2 might represent an undescribed sibling species belonging to this complex. Furthermore, A. typica, another species of Anisakis, is reported not to cluster with any of the previously identified clades. However, it was recently identified to belong to a new clade, clade four (Mattiucci et al., 2018). A recently identified Anisakis spp, Anisakis sp 1 was genetically identified in Malaysian Nemipterus japonicus using mitochondria DNA (mtDNA) and allozyme analysis. Though it is stated that its precise position from a phylogenetic point of view is yet to be resolved, it is however found to be genetically distinct from all previously known species of Anisakis, but closely related to A. typica from central Atlantic waters (Mattiucci et al., 2018). A. simplex and A. pegreffii have been reported as the most important zoonotic species of the genera Anisakis (Nieuwenhuizen and Lopata, 2013).

Epidemiology

The first case of anisakid disease was described in 1876 by Leuckhart (Leuckart, 1876); the disease was however only widely recognized in the 1960s, when epidemics of anisakidosis occurred in the Netherlands with the consumption of lightly salted herring (Van Thiel et al., 1960; Van Thiel, 1962). The larva was identified as A. simplex 3rd-stage larva. Since then, many cases of this zoonotic infection have been described in other countries such as Japan where consumption of raw fish is customary (Arizono et al., 2012; Suzuki et al., 2010). Over 20,000 cases of anisakiasis had been reported worldwide prior to 2010 (EFSA-BIOHAZ, 2010), with the highest prevalence (over 90%) from Japan (Baird et al., 2014). Infected sushi and sashimi, which are the national dishes of raw fish are the main source of human infection, with an annual report of 2000–3000 cases of anisakiasis in Japan alone (Yorimitsu et al., 2013). The worldwide adoption of different cuisine, growth of better tools for diagnosis and greater knowledge on Anisakis and its infection, has resulted in significantly increased reporting of anisakiasis. Other countries in which cases of anisakiasis have been reported include Korea (Sohn et al., 2015), China (Qin et al., 2013), Malaysia (Amir et al., 2016), Taiwan (Li et al., 2015), the United Kingdom (Audicana and Kennedy, 2008), Spain (Herrador et al., 2018)., Italy (Mattiucci et al., 2018; Guardone et al., 2018), France (Audicana and Kennedy, 2008), Germany (Audicana and Kennedy, 2008), Denmark (Andreassen and Jorring, 1970), Norway (Jacobsen and Berland, 1969), Croatia (Mladineo et al., 2016), the United States of America (Kojima et al., 2013), Southern America (Borges et al., 2012; Eiras et al., 2018), Egypt (Audicana and Kennedy, 2008), South Africa (Audicana and Kennedy, 2008), and Australia (Shamsi and Butcher, 2011), implying anisakiasis occurrence in all continents of the globe excluding Antarctica. Recent data shown by Orphanet (Orphanet, 2016) as reported by Guardone and colleagues in a recent retrospective epidemiological study of human anisakiasis (Guardone et al., 2018), estimated a global incidence of 0.32/100,000.

A strong tradition of eating raw or undercooked fish in the form of traditional recipes such as ‘ceviche’ in South America (Eiras et al., 2018), marinated anchovies in Spain (Herrador et al., 2018), raw fish prepared according to traditional Japanese dishes, such as ‘sushi’ and ‘sashimi’ and an exponential increase in the number of restaurants serving ‘sushi’ and ‘sashimi’ globally, are risk factors for the occurrence of anisakiasis (Eiras et al., 2018). According to Shamsi and Sheorey (2018), consumption of these raw or undercooked seafoods increase the risk of infection when viable parasites are present. In a recent study of anisakiasis cases reported between 2000 and 2017 in the European union, a total of 236 cases were reported with the highest incidence in Spain, followed by Italy (Serrano-Moliner et al., 2018). Spain was identified as the country with the second highest incidence worldwide (Herrador et al., 2018). Marinated anchovies in vinegar, a dish popular in the Spanish community, was implicated as the main food vehicle of infection (Herrador et al., 2018). In Australia, it has been estimated that 1259 tonnes of raw finfish are consumed annually with 115.6 million servings per annum used for sushi and sashimi fish meal preparation (Sumner et al., 2015). As reports from certain parts of the world such as Australia on anisakiasis human infections are unexpectedly low, it is suggested that among factors implicated is the possible absence of clinical symptoms or presence of non-specific symptoms, resulting in underdiagnosis of anisakiasis (Shamsi and Sheorey, 2018). This constitutes a serious public health problem. This risk of infection could be minimised by following the recommended standard of freezing fish (FDA/CFSAN, 2001); however, it is reported that freezing fish in some cultures, is believed to affect the taste of some of these fish meals, hence there is a reluctance to freeze fish (Iwata et al., 2015). Avoiding the consumption of raw or undercooked fish can minimise anisakiasis infection. Fish that has undergone inspection for anisakis larvae have been encouraged to be used for meal preparation in addition to freezing the fish prior to consumption (Serrano-Moliner et al., 2018).

Anisakiasis

Humans are an accidental host of the parasite, Anisakis. Infection occurs upon consumption of raw or under-processed marine fish and crustaceans contaminated with the third stage larvae (Ivanović et al., 2017). Two main mechanisms are reported to be responsible for anisakiasis: allergic reactions and direct tissue damage as a result of the penetration of the infective larvae into the target organ site (Caramello et al., 2003; Choi et al., 2009). The survival time of Anisakis in humans is very short and they usually become expelled or destroyed in few days or weeks (Audicana and Kennedy, 2008). However, within a few hours of ingesting this parasite through contaminated fish, the worm burrows into the human intestinal wall resulting in an acute and transient infection with symptoms such as abdominal pain, vomiting, nausea and/or diarrhea (Audicana and Kennedy, 2008). Invasion of the gut wall by the parasite sometimes results in development of eosinophilic granuloma or perforation, which causes direct tissue damage (Choi et al., 2009).

The part of the digestive tract in which Anisakis larvae is lodged, after consumption of raw or lightly cooked fish infected with L3 larvae and the type of Anisakis spp ingested, determines largely the clinical manifestation of anisakiasis observed. Penetration of the gastric mucous results in inflammation which gives rise to some of the symptoms (Valls et al., 2003). Anisakis can cause gastrointestinal infection, which may be classified as acute, chronic, ectopic or allergic reactions (Bucci et al., 2013). In gastric anisakiasis, a predilection for penetration into the greater curvature of the stomach body has been suggested, with the larva embedding itself into the walls of the stomach (Kakizoe et al., 1995; Shimamura et al., 2016). Usually, a prominent gastric mucosal oedema around the area of penetration is observed endoscopically in the gastric mucosa (Kakizoe et al., 1995; Shimamura et al., 2016). Typical clinical presentation is usually reported to be acute severe epigastric pain, few hours after infected fish has been consumed, with symptoms developing within 12 h (Lee et al., 2009; Takabayashi et al., 2014). For intestinal anisakiasis, non-specific clinical characteristics such as nausea, vomiting or diarrhoea, are reported to develop within 5 days after consumption of infected fish. It is known that it takes longer time for anisakiasis symptoms to manifest in intestinal infection than in gastric anisakiasis (Takabayashi et al., 2014). Most cases of anisakiasis have previously been reported to be gastric anisakiasis and was suggested to represent about 95% of the disease burden, with intestinal anisakiasis accounting for the remaining burden of anisakiasis (Kojima et al., 2013). However, it has been recognised that in intestinal anisakiasis, patients are frequently misdiagnosed with other diseases such as inflammatory bowel disease, bowel obstruction, ulcer, acute appendicitis, diverticulitis, ileitis or cholecystitis (Miura et al., 2010) and this might have resulted in the low record of intestinal anisakiasis. In a recent report by Guardone and colleagues, it was shown that in Italy there was a similar frequency for localisation of gastric and intestinal anisakiasis with only a slight increase for gastric lesions (Guardone et al., 2018). In another retrospective study in Japan, results were similar to Guardone's study with 47% of patients presented having gastric anisakiasis and 53% presented with small intestinal anisakiasis (Takabayashi et al., 2014). Recent cases of Anisakis infection have shown that contaminated raw fish may harbour several Anisakis larvae which may attach to multiple sites on the digestive system (Mizumura et al., 2018), with the implication that a serving of raw fish containing more than one Anisakis larvae results in intestinal anisakiasis after gastric anisakiasis has occurred (Mizumura et al., 2018). We speculate that in such anisakiasis infection with multiple larvae, some of the larvae may fail to embed in the walls of the stomach and after being moved by peristaltic motion, migrate into the intestine where they are able to cause intestinal anisakiasis.

Clinical manifestation of anisakiasis are not confined only to gastrointestinal symptoms but has been reported to be associated with allergy reactions in some individuals (Guardone et al., 2018; Ivanović et al., 2017; Shimamura et al., 2016). Symptoms of allergic reactions range from urticaria and angioedema to life-threatening anaphylactic shock, usually associated with gastrointestinal symptoms (Choi et al., 2009). Allergic reactions may occur after the primary infection with Anisakis and exposure to allergenic proteins in the food.

Anisakis as food allergen

The first allergic reaction from consumption of Anisakis contaminated fish was reported in Japan by Kasuya et al. (1990). The allergenic potency of Anisakis antigens despite the active penetration of the larvae into the gastro-intestinal tract was first pointed out in their report (Kasuya et al., 1990). Thereafter, another case of anaphylaxis allergic reaction attributed to anisakiasis was reported in a 52-year-old woman in Spain (Audicana et al., 1995). Shortly after this incidence in Spain, 28 new cases of Anisakis implicated allergic reaction were reported (Fernandez de Corres et al., 1996). Symptoms described for these anisakiasis cases varied and included urticaria/angioedema, facial angioedema, gastrointestinal symptoms (vomiting, diarrhea and abdominal pain), respiratory symptoms, anaphylactic shock and respiratory arrest (Fernandez de Corres et al., 1996). The fish host implicated included Merluccius merluccius (hake), Engraulis encrasicholis (anchovies) and Gadus morhua (cod). Importantly, some of the allergenic proteins of Anisakis have been found to be thermostable as well as pepsin resistant (Nieuwenhuizen and Lopata, 2013a). Not surprising, it has been documented that the ingestion of dead parasites or fragmented parts of the parasite in contaminated fish might result in allergic symptoms after consumption. Furthermore, occupational allergies in aquaculture and fishery workers, cooks as well as fishmongers have been reported, on inhalation of or contact with A. simplex allergens (Nieuwenhuizen et al., 2006; Nieuwenhuizen and Lopata, 2013, 2014). It is however, generally accepted that a viable larva associated with active infections is needed to induce allergic symptoms in most cases (Ivanović et al., 2017). A. simplex has been recognised by the WHO/IUIS nomenclature committee as the parasite with the largest number of known allergens (Fitzsimmons et al., 2014) and it has been proposed that more allergens for these parasite nematodes are yet to be discovered (Baird et al., 2016). Table 1 lists the WHO/IUIS registered Anisakis allergenic proteins.

Table 1

WHO/IUIS registered Anisakis allergenic proteins.

Name	Protein Name	Molecular Weight (kDa)	Organism	References
Ani s 1	Ani s 1	21	A. simplex	Moneo et al. (2000)
Ani s 2	Paramyosin	100	A. simplex	Perez-Perez et al. (2000)
Ani s 3	Tropomyosin	33	A. simplex	Asturias et al. (2000)
Ani s 4	Cystatin	9	A. simplex	(Moneo et al., 2005; Rodriguez-Mahillo et al., 2007)
Ani s 5	SXP/RAL-2	15	A. simplex	Kobayashi et al. (2007)
Ani s 6	Serine protease inhibitor	7	A. simplex	Kobayashi et al. (2007)
Ani s 7	Ua3 recognized allergen	139	A. simplex	Rodriguez et al. (2008)
Ani s 8	SXP/RAL-2	16	A. simplex	Kobayashi et al. (2007)
Ani s 9	SXP/RAL-2	15	A. simplex	Rodriguez-Perez et al. (2008)
Ani s 10	Not given	23	A. simplex	Caballero et al. (2011)
Ani s 11	Not given	30	A. simplex	Kobayashi et al. (2011)
Ani s 12	Not given	33	A. simplex	Kobayashi et al. (2011)
Ani s 13	Haemoglobin	37	A. simplex	Gonzalez-Fernandez et al. (2015)
Ani s 14	New major allergen	23.5	A. simplex	Kobayashi et al. (2015)

Note: Allergens accepted in the International Union of Immunological Societies (IUIS) Database.

A considerable problem in diagnosing and interpreting allergic reactions is the presence of cross-reactivity of Anisakis allergens with antigens from other nematodes as well as related invertebrates including insects, crustacean and mollusc. Some allergens of Anisakis, including tropomyosin and paramyosin (native and recombinant products) have demonstrated strong cross-reactivity to homologous proteins in other invertebrates, including crustaceans and mites (Nieuwenhuizen and Lopata, 2013a). Table 2 lists the identified and reported allergens in Anisakis, which are immunologically cross-reactive with allergenic proteins from other invertebrates. However additional allergenic proteins in Anisakis pegreffii have been identified, which have cross-reactivity with proteins of shellfish (Asnoussi et al., 2017).

Table 2

Anisakis allergens confirmed for cross-reactivity.

#	Anisakis (IUIS-accepted) allergen	Cross-reactivity with other Invertebrates	Invertebrates Common name	References
1	Ani s 2 (Paramyosin)	Blomia tropicalis (Blo t 11)Dermatophagoides pteronyssinus (Der p 11)Dermatophagoides Farina (Der f 11)	Dust mite	Guarneri et al. (2007)
2	Ani s 3 (Tropomyosin)	Periplaneta americana (Per a 7)	Cockroach	Guarneri et al. (2007)
		Dermatophagoides farina (Der f 10)Dermatophagoides pteronyssinus (Der p 10).Lepidoglyphus destructor (Lep d 10)	Dust mite
		Charybdis feriatus (Cha f 1).Homarus americanus (Hom a 1).Panulirus stimpsoni (Pan s 1)Metapenaeus ensis (Met e 1)	Crustaceans
		Mimachlamys nobilisHeliotis diversicolorHelix aspersa (Hel as 1).Perna viridisCrassostrea gigasTurbo cornutus (Tur c 1)	Mollusc
		Chironomus kiiensis (Ch k 10)	Midge
		Lepisma saccharina	Insect
3	Ani s 9 (SXP-RAL-2 family)	Hymenoptera spp	Insects (Wasps)	Rodriguez-Perez et al. (2014)

Increasing numbers of fish and crustacean hosts for anisakis spp

Almost three decades ago, the most common fish and shell fish sources of A. simplex were the spotted chub mackerel (Scomber japanicus) and Japanese flying squid (Todarodes pacificus) (Nagasawa and Moravec, 1995). However, just over a decade later, Abollo and colleagues concluded that most species of cephalopods and fish can potentially harbour these marine parasitic nematodes as 200 fish and 25 cephalopods species have been identified as hosts for Anisakis spp. (Abollo et al., 2001). Anisakis larvae are found in diverse species of fish, crustacean and cephalopods of commercial importance as detailed below (Vidaček et al., 2009).

Distribution in fish hosts

In Western Europe, herring (Clupea harengus) was identified as the main fish species mostly infected and involved in food-borne anisakiasis (Audicana et al., 2002), although cases with other fish species that were insufficiently cooked were also reported, these include hake (Merluccius merluccius), anchovy (Engraulis encrasicolus), mackerel (Scomber scombrus) and cod (Gadus morhua) in countries such as the Netherlands (Rodríguez et al., 2018). Most cases in Spain were related to the consumption of pickled anchovies and raw sardines (Sardina pilchardus) (Audicana et al., 2002; Guardone et al., 2018). Marinated anchovies are also traditional recipes in Italy and Croatia where lime may also be used to marinate the anchovies (Cipriani et al., 2018; Mladineo et al., 2016).

In Italy, most cases of human anisakidosis appear to be caused by Anisakis pegreffii with the L3 larvae stage of the parasite found mostly in marinated anchovies (Guardone et al., 2018). This species is the most relevant species for the Italian fishery representing 25–35% of the national fishery production (Guardone et al., 2018). In France, Salmon (Salmo salar) was demonstrated to be the most frequent food source associated with human anisakiasis followed by anchovies (Yera et al., 2018). In addition, European pilchards (Sardina pilchardus) are also reported to be frequently consumed fresh in western Mediterranean countries such as Portugal, Spain, Italy or France (Molina-Fernandez et al., 2015). Anamnesis of acute cases of anisakiasis in Spain were associated with consumption of marinated pilchards (Molina-Fernandez et al., 2015). Furthermore, in Spain, among the allergic patients with anaphylaxis a few were linked to the ingestion of pilchards that were either cooked or canned and A. pegreffii was found to be the predominant Anisakis species in pilchards from the Mediterranean basin (Buselic et al., 2018).

In Australia, Anisakis pegreffii L3 larvae has been identified in Tiger flathead fish (Neoplatycephalus richardsoni), Eastern school whiting (Sillago flindersi) and White trevally (Pseudocaranx dentex) (Asnoussi et al., 2017); but has not been associated with anisakiasis occurrence. This, has been attributed to be likely due to the high frequency of self-limiting characteristic of the infection, missed diagnosis and under-reporting since symptoms of anisakiasis are usually non-specific (EFSA-BIOHAZ, 2010), as well as probable low-level knowledge of seafood parasitology among medical experts (Shamsi and Sheorey, 2018) or low occurrence of Anisakis migration into the musculatures of infected fish. In China, the white-spotted conger (Conger myriaster) was recently reported to have a high prevalence of Anisakis pegreffii (Chen et al., 2018). Atlantic cod fish (Gadus morhua) have been associated with numerous penetrating A. simplex larvae (Levsen and Berland, 2012), while Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) returning to rivers in Scotland, England and Wales have also been documented to be infected with a high number of A. simplex larvae (Buchmann and Mehrdana, 2016). In Korea, 81.3% of human anisakiasis were due to Anisakis type I larvae (Sohn et al., 2015); and etiologic agents of human anisakiasis in Korea were recently confirmed to be A. pegreffii larvae by molecular methods (Lim et al., 2015). The fish commonly reported to be implicated in these cases was the common Conger (Conger myriaster), followed by Croaker (Pseudosciaena spp.) and yellowtail fish (Seriola spp.) (Sohn et al., 2015). Table 3 summarizes our current knowledge on different fish hosts in different continents where Anisakis species, (A. pegreffii and A. simplex), have been identified.

Table 3

List of Fish in which Anisakis has been found.

Continent	Fish Species	Anisakis found	Reference
AFRICA
Algeria	Merluccius merluccius (European Hake)	A. pegreffii, A. simplex	Farjallah et al. (2008a)
	Trachurus trachurus (Atlantic horse mackerel)Boops boops (Seabream fish)	A. simplex	Ichalal et al. (2015)
Egypt	Merluccius merluccius (European Hake)	A. simplex	Abou-Rahma et al. (2016)
Libya	Merluccius merluccius (European Hake)Scorpaena porcus (Scorpion fish)	A. pegreffiiA. pegreffii	(Farjallah et al., 2008a);Eissa et al. (2015)
Mauritania	Merluccius merluccius (European Hake)	A. pegreffii, A. simplex	Farjallah et al. (2008b)
Morocco	Scomber japonicus (Chub Mackerel)	A. pegreffii, A. simplex	Abattouy et al. (2011)
	Trachurus trachurus (Horse mackerel)	A. pegreffii	Abattouy et al. (2014)
	Merluccius merluccius (European Hake)	A. pegreffii, A. simplex	Farjallah et al. (2008b)
Tunisia	Merluccius merluccius (European Hake), Scomber japonicus (Chub Mackerel)	A. pegreffii	Farjallah et al. (2008a)
ASIA
China	Conger myriaster (Brevoort)	A. pegreffii, A. simplex	Chen et al. (2018)
	Scomberomorus niphonius (Japanese Spanish mackerel)Gadus macrocephalus (Pacific cod)Trichiurus lepturus (Largehead hairtail)	A. simplexA. pegreffiiA. pegreffii	Kuhn et al. (2013)
	Pseudorhombus cinnamoneus (Cinnamon flounder)	A. pegreffii	Li et al. (2017)
Indonesia	Auxis rochei (Bullet tuna)	A. pegreffii	Palm et al. (2017)
Iran	Saurida tumbil (greater lizardfish)Nemipterus japonicus (Japanese thread fin bream)Tylosurus crocodilus crocodiles (crocodile longtom)Carangoides armatus (longfin trevally)Thannus tonggol (Long tail tuna fish)Scomberomorus commerson (Narrow-barred Spanish Mackerel)	Anisakis spp	Dadar et al. (2016)
Japan	Scomber japonicus (Chub Mackerel)	A. simplexA. pegreffii	(Suzuki et al., 2010);Toyoda and Tanaka (2016)
	Trachurus trachurus (Horse mackerel)Oncorhynchus spp (Salmon)	Anisakis spp	Fujikawa et al. (2018)
Korea	Paralichthys olivaceus (olive flounder)Sebastes schlegeli (Korean rockfish)	A. pegreffii	Sohn et al. (2014)
	Oncorhynchus spp (Salmon)Gadus macrocephalus (Cod)	A. simplex
	Sepiida spp (cuttlefish)	Anisakis spp	Choi et al. (2017)
	Conger myriaster (Conger)	A. pegreffii	Sohn et al. (2015)
	Pseudoscianea spp (Croaker)
	Seriola spp (Yellowtail fish)
Taiwanese coast	Trichiurus lepturus (Cutlass fish)	A. simplex	Shih (2004)
	Scomber australasicus (Spotted Mackerel)	A. pegreffii, A. simplex	Chen and Shih (2015)
AUSTRALIA AND OCEANIA
Antarctica	Electrona Antarctica (Lantern fishes)	A. Pegreffii, A. simplex c	Klimpel et al. (2010)
South Australia	Sillago flindersi (Eastern School Whiting)	A. pegreffii	Jabbar et al. (2012)
Western Australia	Pseudocaranx dentex (White trevally)	A. pegreffii	Jabbar et al. (2013)
Victoria (Melbourne)	Neoplatycephalus richardsoni (Flat head Tiger Fish)	A. pegreffii	Asnoussi et al. (2017)
EUROPE
Adriatic Sea	Engraulis encrasicolus (Anchovies)	A. pegreffii	Cipriani et al. (2016)
Austria	Clupea harengus (herring)	Anisakis spp	Kapral et al. (2009)
Belgium	Pollachius pollachius (Pollock)Gadus morhua (Cod),Pollachius virens (Saithe),Merlangius merlangus (Whiting)	A. simplex	Piccolo et al. (1999)
	Micromesistius australis (Southern blue whiting)
	Macruronus magellanicus (Hakes)
German coast	Clupea harengus (Atlantic herring)	A. simplex	Kuhn et al. (2013)
Italy	Merluccius gayi (Hake Merluza)	A. pegreffii	(Ciprianni et al., 2015; Mattiucci et al., 2013)
	Engraulis encrasicolus (raw Anchovies)	Anisakis spp	(Bucci et al., 2013; Fumarola et al., 2009)
	Merluccius merluccius (European hake)Scomber japonicus (Chub Mackerel)	A. pegreffiiA. pegreffii	Kuhn et al. (2013)
Norway	Micrelaps muelleri (Bristle-mouth fishes)Pollachius virens -Pollock (White fish)	A. simplex	Klimpel et al. (2004)
Portugal (Madeira)	Aphanopus carbo (Black scabbardfish)	A. simplex	Kuhn et al. (2013)
Spain	Merluccius gayi (Hake Merluza)	A. pegreffiiA. simplex	Cipriani et al. (2015).
	Trachurus trachurus (Horse mackerel)	A. simplex	Adroher et al. (1996).
	Micromesistius poutassou (Blue whiting fish)	A. simplex	Bucci et al. (2013)
	Engraulis encrasicolus (Anchovies)	A. simplex	(Repiso et al., 2003)
United Kingdom (UK)	Oncorhynchus spp (Salmon) and pickled fish	Anisakis spp	Lucas et al. (1985)
NORTH AMERICA
Canada	Salvelinus alpinus (Arctic char)	Anisakis spp	Bhat and Cleland (2010)
	Oncorhynchus spp (Salmon)	A. simplex	(Madi et al., 2013); (Vaughan et al., 2015)
California (USA)	Sebastes sp (Red snapper)Oncorhynchus spp (Salmon)Anoplopoma fimbria (Sable fish)	Anisakis type 1A. simplex	Kliks (1983)(Kuhn et al., 2013)
Alaska	Oncorhynchus keta (Chum salmon)Oncorhynchus nerka (red salmon)	A. simplex	Kuhn et al. (2013)
Hawaii	Cololabis saira (Mackerel pike)	A. simplex	Kuhn et al. (2013)
SOUTH AMERICA
Ecuador	Merluccius gayi (Hake Merluza)	A. pegreffii	Castellanos et al. (2018)
Chile	Merluccius gayi (Chilean hake)Macrouronus magellanicus (tail-hake) Genypterus chilensis (redconger-eel) Paralichthys microps (flat fish),Trachurus murphyi (Chilean mackerel)	A. simplex	Mercado et al. (2001)
	Merluccius australis (Southern Hake).	Anisakis spp	Chavez et al. (2012)
	Micromesistius australis (Southern blue whiting)
	Macruronus magellanicus (Hakes)

Distribution in invertebrate hosts

Anisakis species have a complex life cycle which involves several hosts starting with the formation of the first-stage larvae (L1) in the eggs and subsequent hatching into the second stage (L2) free-swimming larva which in turn is ingested by crustaceans such as krill (euphausiids) and squid (Cephalopods). The intensity of larval nematodes is suggested to be low in the crustacean which are the first intermediate hosts. However, larger fishes are mostly infected only via the already infected crustaceans or small fishes (Mouritsen et al., 2010). The most important intermediate crustacean hosts for A. simplex seem to be the euphausiids for most fish hosts (Hojgaard, 1999). However, there are regions where euphausiids seem to have no significance for successful transmission of A. simplex such as in the Norwegian Deep (Klimpel et al., 2004). It has been suggested that water temperature may contribute to anisakids distribution (Mattiucci and Nascetti, 2006). This may in turn affect the distribution of invertebrate host available for Anisakis in any location. A. simplex sensu stricto have been identified as a colder water species while A. typica appear to be distributed in warmer temperate and tropical waters (Mattiucci and Nascetti, 2006). Zhao and colleagues suggested that this may prevent the euphausiids, which are the most important first intermediate hosts of A. simplex, from being distributed in certain regions (Zhao et al., 2016). In Antarctica, krill has been identified as an intermediate host for both A. simplex c and A. pegreffii (Klimpel et al., 2010). A. simplex has also been found in krill in Mexico (Gomez-Gutierrez et al., 2010). Other intermediate host reported for Anisakis include squid, octopus-molluscs and one-eyed crayfish. Squid have been identified in Korea to be an intermediate host for Anisakis type 1 (Sohn et al., 2015), with most species of Anisakis type 1 in Korea identified recently as A. pegreffii by molecular methods (Lim et al., 2015). In Spain A. simplex has been found to use krill, squid and octopus as invertebrate intermediate hosts (Abollo et al., 1998; Gregori et al., 2015). Klimpel et al. (2004) have identified the one-eyed crayfish (Paraeuchaeta norvegica) as an intermediate host for A. simplex. The discovery of the presence of Anisakis in different mesozooplankton organisms indicates that transmission route for Anisakis is wide (Gregori et al., 2015; Klimpel et al., 2004).

Prevention of anisakiasis

Anisakids are known to survive and be resistant to different treatment conditions such as freezing, microwaving, heating, salting, as well as use of anthelmintic drugs and condiments (Brutti et al., 2010; Tejada et al., 2015). Storage, after-harvest handling and fish preparation are the focus of preventive measures. Migration of larvae into the muscle might be prevented by immediate evisceration of fish after being caught (Chen et al., 2018). However, since this immediate cleaning may most likely be performed at sea, resulting in removed contaminated viscera being thrown back into the sea and eaten by other fish, the prevalence of infection may be heightened through this practice (McClelland et al., 1990).

The recommended and common method to kill the larvae in fish before consumption is the application of high or low temperatures (Chen et al., 2014). The US Food and Drug Administration (FDA) recommended that fish be cooked to a temperature of at least 63 °C–74 °C before consumption (Beldsoe and Oria, 2001). Larvae of Anisakis species have been observed to be alive if frozen for a short period at −20 °C. The FDA has recommended that fish be kept frozen at −20 °C for at least 168 h or blast-frozen at −35 °C for at least 15 h (Beldsoe and Oria, 2001).

Food safety consideration

Parasitological surveys for Anisakis larvae in consumed seafoods and particularly fish host is crucial to ensure food safety. The parasite burden in the edible flesh of the fish need to be evaluated as this constitutes the main threat to public health (Cipriani et al., 2016). In addition, since the possibility of intra-vitam migration of larvae to the fish flesh has been documented (Cipriani et al., 2016), the storage temperature after fish capture becomes crucial in the post-mortem motility of Anisakis larvae into the flesh of fish. This emphasizes the importance of the regulations which specify freezing of fish products at −20 °C for 24 h, or at −18 °C for 96 h in domestic freezers before utilisation in food preparation, to prevent human anisakiasis. It is however important to consider that allergenic proteins from Anisakis may still be present in food products after removal of intact worms (Audicana et al., 1997).

Conclusion

In conclusion, the existence of Anisakis parasites in a variety of mesozooplankton organisms suggests, according to Gregori and colleagues (Gregori et al., 2015) that the transmission routes of A. simplex and A. pegreffii are much broader than previously known. This indicates that Anisakis species may not be specific to any intermediate host and may use any of the named intermediate hosts to move from one habitat to another depending on the seasonal and ecological availability of such intermediate hosts. This may result in Anisakis species expanding their pathways to find their definitive host. The implication of this is an increase in occurrence of anisakiasis across the globe with a potential increase in anisakiasis and allergic reactions accompanied by cross-reactivity to homologous proteins in other invertebrates. More attention needs to be given to this parasite to develop better risk management system of the cascading reactions that may follow infection and sensitisation caused by consumption of Anisakis contaminated raw or inadequately treated fish and/shellfish.