What is anisakis?
The genus Anisakis includes some species of parasites that usually inhabit the digestive system of certain fish, molluscs and marine mammals.
But that's not all: in addition to anisakiasis, these nematodes can be the protagonists of allergies. After taking raw food infected with Anisakis, the body of some people produces type E immunoglobulins (IgE); this results in an even severe allergic reaction (eg anaphylaxis).
- What is the biological cycle of anisakis?
- What risk does the raw fish consumer take?
- How can the risk of Anisakis infections be reduced?
Description and biological cycle
DESCRIPTION
Anisakis is a nematode belonging to the Anisakidae family, composed of 5 genera, of which 4 cause diseases in humans and other animals:
- Anisakis, including Anisakis simplex and Anisakis physeteris
- Pseudoterranova
- Contracaecum
- Phocascaris
Among these, the nematodes belonging to the genus Anisakis are probably the most frequent parasites transmitted by fish products to humans.
Anisakis has all the typical characteristics of nematodes: the body is cylindrical and vermiform, with a circular section, which is very different from that of flatworms (flat worms).
The Anisakis are large parasites, visible to the naked eye, often curled up on themselves: these nematodes, with a whitish or pinkish color, measure from 1 to 3 cm in length. These are pseudocelomed parasites, therefore constituted by a false celoma, the cavity interposed between the alimentary canal and the body wall.
The parasite wall consists of three basic sections:
- Multi-layered cuticle, formed from collagen. To sneak in the host's stomach, the cuticle is essential to the parasite: this, in fact, acts as a shield for acidic gastric juices.
- Intermediate epidermal layer
- Longitudinal muscle layer
The Anisakis are so-called "proctodea" organisms, that is they have a mouth for food intake and anus for faeces.
BIOLOGICAL CYCLE
Similar to the vast majority of parasites, the Anisakis have a complex life cycle. There are many intermediate guests, and the man - in this specific case - is not the definitive host as one might believe. The biological cycle takes place in the marine environment passing through various stages of development. To simplify, we report a list of intermediate hosts of the parasite:
- Anisakis live in the stomach of marine mammals (whales, seals, dolphins); the non-fertilized eggs of Anisakis are therefore released into the sea via the faeces → first stage
- Once in the water, the Anisakis eggs ripen (they are fertilized); subsequently, they exit the egg and spread (larvae L2) → second stage
- The larvae are ingested by an intermediate host (eg planktonic crustacean): in this phase, the L2 larvae mature in L3 → third stage
- After eating the Anisakis, the crustacean is in turn ingested by a second intermediate host (fish, squid, etc.). When the host dies, the larvae of Anisakis migrate into the liver, gonads, mesentery and, above all, muscle tissues. Through predation, the larvae pass from fish to fish.
Herring, cod, mackerel, anchovies, monkfish and sardines are the most contaminated fish by Anisakis
At this point, the life cycle can be completed or interrupted:
- Marine mammals feed on fish, cuttlefish or squid with L3 larvae → L3 larvae become adult worms, produce eggs and the cycle resumes
- The man, eating raw fish or malcotti, embodies the accidental guest of the Anisakis: having reached the human intestine, the parasite, unable to complete its life cycle, dies.
Let us not forget, however, that anisakis infections in humans can create gastrointestinal disorders (abdominal pain, diarrhea, vomiting), up to perforations of the stomach or intestine.
Anisakis infections
Anisakiasis are emerging zoonoses, due to the rapid spread of Anisakis nematodes: the intake of raw or undercooked fish contaminated with Anisakis larvae can trigger a gastrointestinal infection. The species most involved in anisakiasis is Anisakis simplex.
Symptoms
To learn more: Symptoms Anisakiasis
After a few hours of ingesting these contaminated fish, the victim complains of violent abdominal pain, nausea and vomiting. In some cases, the Anisakis larvae reach the intestinal level: in similar situations, after a couple of weeks after infection, the body can respond with a severe eosinophilic granulomatous response, causing symptoms that mimic Crohn's syndrome.
Fortunately, in the majority of diagnosed cases, the cure for the removal of the parasite is rather simple.
Complications
Anisakis infection tends to self-purify in a short time: the victim, using therapeutic aids, completely heals in a few days.
Only rarely can Anisakis cause serious obstructions in the small intestine: in this case, the patient will undergo surgery. Some patients may avoid surgery: albendazole therapy may be sufficient.
Treatment
Information on the therapies available for treating anisakiasis is available in the article "Anisakis - Drugs and Treatment of Anisakiasis".
Prevention
The implementation of simple precautions allows to consume the fish in total safety: the heat treatment (freezing / cooking), for a sufficiently long time, is essential to prevent anisakiasis.
The FDA always recommends freezing shellfish and fish for consumption at (at least) -35 ° C for 15 hours, or at -20 ° C for 7 days. Even cooking (the heart of the product must reach 60 ° C for at least one minute) guarantees the removal and killing of the parasite from the fish.
Neither salting nor marinating or smoking are effective preparatory methods for killing Anisakis from fish.
Raw food fanatics should be assured that they have been immediately eviscerated after fishing before consuming freshly killed fish. Clearly, all the fish destined for consumption should be carefully inspected "with the naked eye" for the detection of Anisakis parasites: remember, in fact, that these nematodes reach considerable dimensions which make them visible to the naked eye.
Allergies from Anisakis
For some years now, anisakis has been recognized as a possible carrier of allergies.
Particularly sensitive individuals may develop allergy by simply manipulating infected fish. Anisakis allergies can also be contracted simply by inhaling airborne allergens when processing infected fish products.
In sensitive patients, not even adequate heat treatment guarantees immunity from these allergies. Parasitic larvae such as Anisakis and other related species (eg Pseudoterranova spp., Hysterothylacium aduncum ) can release biochemical substances (antigens) in fish tissues that are extremely resistant to freezing and cooking. The hypersensitive or allergic subject, in contact with these substances, develops allergic reactions of varying severity: the most frequent allergies are hives, angioedema and anaphylactic reactions, sometimes accompanied by gastrointestinal symptoms. Less frequent are asthmatic attacks, contact dermatitis and conjunctivitis after inhalation / intake of fish contaminated with Anisakis larvae.
Exposure to antigenic proteins of Anisakis larvae → † ↑ IgE → IgE mediated reactions
The prick test and the search for specific antibodies against the larvae of these parasites are useful diagnostic tests to ascertain the suspicion of allergy from Anisakis. Hypersensitivity to these parasites is assessed by the rapid increase in IgE levels in the days immediately following consumption of Anisakis contaminated fish.
