Causing agent

Diphyllobothrium latum is a ribbon tapeworm belonging to the Cestode class, to the Diphyllobothriidea order and to the Diphyllobothriidae family. It can reach ten meters in length (Fig. 1) and can live several years. It is responsible of a parasite infection called the diphyllobothriosis.

Life cycle

The biological cycle (Fig. 2) of the parasite includes a definitive host: Man (and other fish-eating mammals) and at least two intermediate hosts: a planktonic crustacean and one or several freshwater fish. The eggs are excreted in freshwater with the feces of the definitive host. In favorable environment conditions, after 8 to 12 days of maturation, the eggs hatch and release a ciliated embryo that is ingested by a microscopical crustacean belonging to the Cyclops or Eudiaptomus genus. The embryo transforms into a procercoid larvae in the general cavity. Once ingested by a carnivorous fish, this larvae turns into a second type of larvae, called plerocercoid (of few millimeters in length, Fig. 3). The plercocercoid encysts into the fish musculature or viscera. Man and other fish-eating mammals contaminate themselves by ingesting flesh of freshwater fish that is raw or insufficiently cooked. Once in the intestine of the definitive host, the plerocercoid larvae grows of several centimeters per day and the first eggs are excreted in the feces approximately one month following infestation. Several species of this parasite are pathogenic to human beings but D. latum is the sole species to be contracted from metropolitan freshwater fish. Nevertheless, cases of diphyllobothriosis caused by D. nihonkaiense (species from the Pacific) have been notified in consumers of salmons (Onchorynchus sp.) imported from the Pacific ocean (Canada).

Prevalence

The diphyllobothriosis is still encountered in Western Europe. It is decreasing in the historical areas of endemicity: the Scandinavian countries. On the contrary, it seems to emerge in French- and Italian-speaking areas of the subalpine lakes where a professional fishing activity often exists. Since 1987, more than 200 cases have been reported around the lakes of Geneva, Morat, Bienne, Majeur, Côme, Iseo and Garde. The lake of Geneva seems particularly endemic since 48 cases of contamination have been identified on the Swiss and French catchment basins in 2001 and 2002. Between 2002 and 2007, 44 cases have been identified in medical analysis laboratories in the Haute-Savoie region. The food involved is the raw flesh (marinated fillets, carpaccio, etc.) or raw eggs of freshwater fish: perch (Perca fluviatilis), pike (Esox lucius), char (Salvelinus alpinus), burbot (Lota lota), etc. Four to 10 % of the perch fillets consumed on the boarders of the lake of Geneva harbor the parasite. The Coregonidae (Féras) and probably, the European salmonids of the Salmo genus are refractory to D. latum. The Canadian salmonids of the Onchorynchus genus can harbor larvae of D. nihonkaiense.

Clinical signs

The parasitism becomes apparent with abdominal pain and excretion of proglottids resembling those of the beef or pork tapeworm. Rare cases of anemia due to lack of B12 vitamin have been described in case of prolonged infestation in undernourished populations.

Prevention

The prevention is based on sufficient cooking of the fish (65°C) or freezing at  - 20°C during 8 to 72 hours depending on the fish thickness. As far as general hygiene practice is concern, the treatment of sewage water in a modern water treatment plant should interrupt the transmission cycle.

References

Dupouy-Camet J,Haidar M, Espinat L, Dei-Cas E, YéraH, Ben MostafaA, Guillard J, Aliouat-Denis CM. 2014. Prévalence de l’infestation par Diphyllobothrium latum de différents poissons des lac Léman, du Bourget et d’Annecy et évaluation de l’incidence des cas humains auprès des laboratoires d’analyse médicale de la région (2011-2013). Bulletin épidémiologique de l’ANSES. Santé animale et alimentation. Sous presse.

Dupouy-Camet J & Peduzzi R. 2004. Current situation of human diphyllobothriasis in Europe. Euro Surveillance, 5:31-35.

Nicoulaud J, Yéra H & Dupouy-Camet J. 2005. Prévalence de l'infestation par Diphyllobothrium latum, L., 1758 chez les perches (Perca fluviatilis) du lac Léman. Parasite, 12(4):362-4.

Yéra H, Estran C, Delaunay P, Gari-Toussaint M, Dupouy-Camet J, Marty P. 2006. Putative Diphyllobothrium nihonkaiense acquired from a Pacific salmon(Oncorhynchus keta) eaten in France; genomic identification and case report. Parasitol Int, 55(1):45-9.

Von Bonsdorff B. 1977. Diphyllobothriasis in man. Editions Academic Press, Londres.

Wicht B, Peduzzi R & Dupouy-Camet J. 2010. Diphyllobothriose. In: Actualités permanentes en bactériologie clinique, vol. IX, Editions ESKA, Paris.

Useful links

CDC

FAO/OMS

Report Swiss Television on the risk of eating raw fish (in French)

ANSES  form on Diphyllobothrium

What ?

What you should know about fish parasites.

Where ?

At the Pasteur Institute of Lille in the Buttiaux amphitheatre.

When ?

on Tuesday, the 20^th of January 2015, from 5.00 pm until 7.00 pm.

Who ? Some of the Fish-Parasites partners

Dr. Mélanie Gay, responsible for research, ANSES (National Agency of Food Security, Environment and Work) laboratory of food security, Boulogne-sur-Mer.

Dr. Gabriela Certad, Researcher, Biology and Diversity of Emerging Eukaryotic Pathogens Eucaryotes Emergents (BDEEP), Center for Infection & Immunity of Lille (CIIL), Pasteur Institute of Lille.

Dr. Cécile-Marie Aliouat-Denis, Researcher-Lecturer, Faculty of Pharmacy, University of Lille 2, Biology and Diversity of Emerging Eukaryotic Pathogens Eucaryotes Emergents (BDEEP), Center for Infection & Immunity of Lille (CIIL), Pasteur Institute of Lille.

How ?

You need to subscribe. It's free ! Follow this link: Pasteur Institute

Summary of the conference

Numerous parasites can occur in or on fishing or aquaculture products. Amongst these, some are responsible of zoonoses that impact on human health. A research program (Fish-Parasites, ANR-10-ALIA-004) targeted a better understanding of the risk linked with the parasites of fish in France. During this conference, we will focus on two groups of parasites that were studied in this program: on one hand, the microscopical parasites belonging to the genus Cryptosporidium, that may cause diarrhea in fragile subjects. No data were available on the presence of these parasites in fish in France. Cryptosporidium has been more frequently detected in freshwater fish than in marine fish. On the other hand, we will also talk about the Anisakidae which are macroscopical nematodes or roundworms, that occur in many fish species with variable intensities. These parasites may induce either light digestive pathologies folowing the ingestion of insufficienltly cooked or raw fish either more rarely allergies. While the consumption of raw fish is trendy, some advices to prevent the consumer against the risk will be given at the end of the conference.

Mechanical means of prevention

•    The fish evisceration is recommended before cooking or freezing. Evisceration should be done as soon as possible after fishing. This procedure diminishes the risk of larvae migration. Indeed, larvae that may be present in the fish digestive tract can migrate in the fillets after the death of the fish. But the evisceration does not completely prevent the parasite risk because larvae can also be present into the fillets before catching the fish.

•    The trimming of the fish fillets: parts of the fillets surrounding the visceral cavity (= the flanks) are removed. The trimming eliminates the most highly infected parts of the fillets that are close to the visceral cavity. Thus, the parasite risk is reduced but not completely eliminated.

•    The meticulous observation of the filet, by trans-illuminating the fillet on a candling table for example, allows the manual elimination of visible larvae. This technique is time-consuming and not always efficient according to the species (dark flesh) or to the thickness of the filet. When preparing raw fish in carpaccio or sushi/sashimi, the cook should finely slice the fillet, be meticulous and remove any larvae eventually present in the flesh. Freezing is required (see below) prior to the consumption of a raw fish fillet.

The temperature as a prevention mean

•    To maintain the cold chain: it reduces the mobility of the parasite larvae and thus diminishes the risk of larvae migration towards the consumed parts of the fish.

•    The freezing is efficient to eliminate the infectious risk at certain temperature/time pairs. The efficiency depends on the thickness of the piece to be frozen, its weigh, the species and the fat contents. Official standards are available. The fish must be frozen at -20°C in all parts of the product during at least 7 days (domestic freezer).

•    Cooking allows the efficient killing of the parasite larvae. A temperature of 60°C must be reached in the center of the product during at least one minute (70°C for microwave cooking). For a 3 cm-thick filet, cooking should be 10 minutes long.

Other prevention methods

The salting, marinating and cold smoking methods are overall inefficient to eliminate the infectious risk. Of note, some efficient salting methods exist for herrings (kippers): immersion in saturated salted solution for at least 16 days at 15°C.

Irradiation or pressure methods are inefficient because efficient doses are too strong and would alter the product.

Pasteur Institute of Lille, the 26th of May 2014

Dowload the program and abstracts of the conference below.