What are Parasites?
Parasites are small animals that require one or more host animals in order to complete their life-cycle. Generally they cannot survive outside of their host. Their presence may or may not cause health effects in the host animal.
Which parasites can humans catch from fish?
There are three types of fish parasites of public health importance; roundworms (nematodes), flatworms or flukes (trematodes) and tapeworms (cestodes).
From a human infection perspective, the most common roundworms are from the family of Anisakidae and include Anisakis spp., Pseudoterranova spp., Phocascaris spp., and Contracaecum spp. The most common flatworms or flukes involved in human infection are liver fluke worms belonging to the family Opisthorchiidae and some species of intestinal fluke worms belonging to the Heterophyidae and Echinostomatidae families. Human infections caused by fish tapeworm are most commonly caused by the genus Diphylloborothrium.
Are fresh water and salt water fish involved?
Both fresh water and salt water fish are a potential source of human infection with parasites. Fish roundworms are associated with salt water fish from all areas of the seas, whereas fish tapeworm, tends to come from fresh water fish in cold waters. Fish flatworms or flukes, on the other hand, are normally associated with fresh water fish in temperate and warm waters.
How do fish get infected?
Fish get infected with parasites when they feed on intermediate hosts. The definitive host for fish roundworms are marine mammals like seals and dolphins and birds that feed on fish like cormorants and seagulls. These hosts shed eggs in their faeces which hatch into larvae in the water. The larvae are eaten by small crustaceans like prawn, crabs and shrimp and in turn these are eaten by squid and fish. The larvae then bore through the intestinal wall and enter the muscle tissue where they are encapsulated and can survive for long periods. Fish can also harbour live worms that penetrate the gut and these are often visible in the flesh.
In contrast, the definitive host for flatworms or flukes are land mammals like foxes, dogs and cats. Eggs are again excreted with fecaes and enter fresh water courses where they are ingested by fresh water snails. Inside the snails the eggs develop into cercariae larva. These are released into the water and actively penetrate the fish beneath the scales where they become cysts in the fish muscle and can survive for long periods.
With fish tapeworms, fish eating mammals and birds infected with the tapeworm, shed eggs which find their way into water and hatch into motile embryos. These are ingested by tiny crustaceans called copipods where they develop into first stage larvae. Fresh water fish eat the copipods and the larvae enter the flesh through the wall of the digestive tract where they develop into a second stage larvae that remain inactive for several years.
How can humans get infected with fish parasites?
Infection in humans is associated with the consumption of fish containing live parasites. Humans are not the ‘intended’ host of the parasites as we are not an aquatic mammal. Hence, in this respect humans are considered an accidental host.
What are the human health effects of infection with fish parasites?
Fish roundworms cause a condition in humans called anisakiasis. According to Centres for Disease Control in the USA, “symptoms of this infection are abdominal pain, nausea, vomiting, abdominal distention, diarrhoea, blood and mucus in stool, and mild fever. Allergic reactions with rash and itching, and infrequently, anaphylaxis, can also occur.”
Fish flatworms or flukes cause a condition in humans called trematodosis. According to the World Health Organisation “early and light infections often pass unnoticed, as they are asymptomatic or only scarcely symptomatic. Conversely, if the worm load is high, general malaise is common and severe pain can occur, especially in the abdominal region. Chronic infections are invariably associated with severe morbidity [illness]. Symptoms are mainly organ-specific and reflect the final location of the adult worms in the body. In clonorchiasis and opisthorchiasis [two conditions associated with fish trematodes], the adult worms lodge in the smaller bile ducts of the liver, causing inflammation and fibrosis of the adjacent tissues.”
Fish tapeworms cause a condition in humans called diphyllobothriasis . According to the Centres for Disease Control in the USA it “can be a long-lasting infection (decades). Most infections are asymptomatic. Manifestations may include abdominal discomfort, diarrhoea, vomiting, and weight loss. Vitamin B12 deficiency with pernicious anaemia may occur. Massive infections may result in intestinal obstruction.”
Can fish parasites cause allergenic reactions in humans?
The European Food Safety Authority considers that the A. simplex species of roundworm is the only fishery product parasite causing a clinical allergic response. The allergic response can be due to ingestion of live or more rarely, dead roundworms of this species. It is not a common cause of allergy although under-diagnosis cannot be ruled out. (For further details on this allergy see http://www.efsa.europa.eu/it/scdocs/doc/1543.pdf )
How is human health protected from infected fish?
Fish parasites are killed by freezing and heating treatments. For fish parasites other than flatworms or flukes (trematodes), freezing treatments must be at a temperature of -20oC for not less than 24 hours or -35oC for at least 15 hours in all parts of the fish. Heating treatments need to be >60oC for at least 1 minute.
Flatworm or fluke larvae of Opisthorchis spp. and Clonorchis spp. are killed by freezing at -10°C for 5 days. Whereas 3-4 days may be necessary to kill the larvae of Clonorchis sinensis if frozen at -20°C and 32 hours to kill the larvae of Opisthorchis felinus at -28°C. The European Food Safety Authority refers in its Opinion on Parasites in Fishery Products to a temperature of 70°C for 30 min for killing the Clonorchis and Opisthorchis larvae. (see http://www.efsa.europa.eu/it/scdocs/doc/1543.pdf)
What checks are carried out on fish before it is placed on the market?
Point D of Chapter V of Section VIII of Annex III to Regulation (EC) No 853/2004 requires that FBOs ensure that fishery products have been subjected to a visual examination for the purpose of detecting visible parasites before being placed on the market. They must not place fishery products that are obviously contaminated with parasites on the market for human consumption.
In addition Article 14 of Regulation (EC) No. 178/2002 on the general principles of food law requires food placed on the market to be safe. This is a general requirement requiring FBOs to conduct a risk assessment. Therefore fresh water fish at risk of flatworm or fluke (trematode) infection would need to receive a more severe freezing treatment than listed in Regulation (EC) No. 853/2004 as they are more resistant than other parasites.
Is there any legislation requiring anti-parasitic treatment of fish placed on the European market?
Point D1 of Chapter III of Section VIII of Annex III to Regulation (EC) No 853/2004 contains a requirement that certain fishery products shall undergo a treatment sufficient to kill viable parasites that may represent a health hazard to the consumer. The details in this legislation were amended by Commission Regulation (EU) No. 1276/2011 as follows:
1. Food business operators placing on the market the following fishery products derived from finfish or cephalopod molluscs:
(a) fishery products intended to be consumed raw; or
(b) marinated, salted and any other treated fishery products, if the treatment is insufficient to kill the viable parasite;
must ensure that the raw material or finished product undergo a freezing treatment in order to kill viable parasites that may be a risk to the health of the consumer.
2. For parasites other than trematodes the freezing treatment must consist of lowering the temperature in all parts of the product to at least:
(a) – 20 °C for not less than 24 hours; or
(b) – 35 °C for not less than 15 hours.
Are there any exemptions to the above requirements to treat for viable parasites in certain fishery products?
Yes there are exemptions to the above requirement to treat for viable parasites. Point D3 of Chapter III of Section VIII of Annex III to Regulation (EC) No 853/2004 contains details of the exemptions. The details in this legislation were amended by Commission Regulation (EU) No. 1276/2011 as follows:
Food business operators need not carry out the freezing treatment set out in point 1 for fishery products:
(a) that have undergone, or are intended to undergo before consumption a heat treatment that kills the viable parasite. In the case of parasites other than trematodes the product is heated to a core temperature of 60 °C or more for at least one minute;
(b) that have been preserved as frozen fishery products for a sufficiently long period to kill the viable parasites;
(c) from wild catches, provided that:
- there are epidemiological data available indicating that the fishing grounds of origin do not present a health hazard with regard to the presence of parasites; and
- the competent authority so authorises;
(d) derived from fish farming, cultured from embryos and have been fed exclusively on a
diet that cannot contain viable parasites that present a health hazard, and one of the
following requirements is complied with:
- have been exclusively reared in an environment that is free from viable parasites; or
- the food business operator verifies through procedures, approved by thecompetent authority, that the fishery products do not represent a health hazard with regard to the presence of viable parasites. ”
The European Commission have indicated that in the case of wild catches, the food business operator (FBO) must demonstrate that there are epidemiological data available indicating that the fishing grounds of origin do not present a health hazard with regard to the presence of parasites. Furthermore, the absence of freezing treatment of those products must be authorised by the competent authority.
EFSA in its Scientific Opinion on risk assessment of parasites in fishery products, mentioned above, concluded that "all wild caught seawater and freshwater fish must be considered at risk of containing any viable parasites of human health concern if these products are to be eaten raw or almost raw. For wild-catch fish, no sea fishing grounds can be considered free of Anisakis simplex".
So, to date, requirements are not met to allow the FBO to avoid the freezing treatment required as regards wild caught fish.
However, the situation of fishery products derived from fish farming is different. It is important to note that to be included in this category the fishery products must originate from cultured embryos and have been fed exclusively on a diet that cannot contain viable parasites that present a health hazard.
In this case the freezing treatment can be avoided if one of the following conditions is applied:
- have been exclusively reared in an environment that is free from viable parasites, for example a freezing exemption automatically applies to farmed Atlantic Salmon reared in Ireland
- the food business operator verifies through procedures approved by the competent authority, that the fishery products do not represent a health hazard with regard to the presence of viable parasites
In condition (1), the food business operator harvesting the farmed fish must ensure that documentation accompanies (physically or electronically) each batch of farmed fish, assuring that the correct information as regard the exemption from the freezing treatment is available before the fishery products are placed on the market. This information can be included in the commercial documents or in any other information accompanying the fishery products.
Under Condition (1), a freezing exemption automatically applies to farmed Atlantic Salmon reared in Ireland and, when placed on the market only in Ireland, the label /documentation indicating farmed Atlantic Salmon reared in Ireland is considered to meet the freezing exemption information requirements.
In condition (2), the documentation, always under the responsibility of the food business operator harvesting the farmed fish, must accompany each batch of farmed fish, as described before, demonstrating in addition that the verification procedures received the approval of the Competent Authority. Also, in that case the correct information as regard the exemption from the freezing treatment must be available before the fishery products are placed on the market.
Freezing Exemption and potential labelling implications
Currently, a freezing exemption automatically applies to farmed Atlantic Salmon (reared in Ireland in floating cages or onshore tanks, that have been cultured from embryos fed exclusively on a diet that cannot contain viable parasites that present a health hazard; and, exclusively reared in an environment that is free from viable parasites (Condition (1) above). If such fish are subsequently frozen and thawed, this fact must be included on any labelling i.e. the term “defrosted” must accompany the name of the food.
My food business serves fish that is not cooked thoroughly– what precautions should I take?
All FBOs must have procedures or a set of procedures based on the principles of the hazard analysis and critical control point system (HACCP). FBOs serving undercooked (<60oC/1min) fish, raw fish, cold smoked wild fish (not farmed fish) or uncooked marinated/salted fish should source their fish from a reputable supplier that can provide assurance that the fish has been subjected to a freezing or heating or marinating/salting process that is capable of killing fish parasites.This assurance is provided in a document issued by the supplier. If the FBO is sourcing product that is exempt from the freezing treatment then a record of the exemption declaration must accompany the batch..
Point D 4 of Regulation (EC) No. 853/2004, as amended by Regulation (EC) No.1276/2011, reads as follows:
" 4. (a) When placing on the market, except when supplied to the final consumer, fishery products referred to in point 1 must be accompanied by a document issued by the food business operator performing the freezing treatment, stating the type of freezing treatment that the products have undergone.
(b) Before placing on the market fishery products referred to in points 3(c) and 3(d) which have not undergone the freezing treatment or which are not intended to undergo before consumption a treatment that kills viable parasites that present a health hazard, a food business operator must ensure that the fishery products originate from a fishing ground or fish farming which complies with the specific conditions referred to in one of those points. This provision may be met by information in the commercial document or by any other information accompanying the fishery products. ”
Further information on the exemption declaration is available in the exemption question above.
Further information on fish parasites is available on the European Commission website:
Guidance on viable parasites in fishery products that may represent a risk to the health of the consumer
Last reviewed: 9/2/2021