The preferred hosts of the parasite Echinococcus multilocularis are rodents and foxes, but when this parasite accidentally infects humans, it is responsible for thealveolar echinococcosis. The occurrence of this disease is quite rare, about forty new cases per year in France, but its consequences are serious, requiring daily medication for life. The first human cases of the disease described in the 19th century were confined to the Alpine regions from France, Germany, Switzerland and Austria. But for the past thirty years, human cases and/or infected foxes have been reported in other regions of France, as well as in European countries such as the Netherlands, Estonia, Poland, Serbia or Croatia. Foxes infected at the intestinal level with wormsEchinococcus multilocularis excrete parasite eggs in their droppings. These eggs can be ingested by rodents when they eat contaminated plants. The parasitic cycle continues when a fox in turn consumes infected rodents, which have developed the larval form of the parasite. Humans can become infected accidentally, for example by eating fruit picked near the ground or raw vegetables or by touching the coat of infected dogs or cats after eating rodents.
Foxes responsible for transporting the parasite
A previous study had shown that the spread of the parasite was achieved by the progressive migration of infested foxes from the historic European focus of the Alps to bordering countries: the genetic profiles of the parasite are common but with less diversity in terms of away from the Alps. This decline in diversity is explained by the fact that each animal only brings with it the strains it carries. As alveolar echinococcosis is also very present in Asia, where it is caused by strains ofEchinococcus multilocularis distinct from those in Europe, the question was whether geographic expansion of the parasite on the whole European continent was carried out only from the alpine historic hearth or also from the Asian hearth. The recent identification of Asian strains in Poland has reinforced this question. To answer this question, scientists from different European countries collected samples of the parasites. The French reference laboratory on Echinococcus spp.for which ANSES is responsible, studied small portions of DNA worms sampled, whose variable size makes it possible to distinguish the different strains of the parasite.
A presence in Europe almost exclusively linked to the Alpine focus
The study was published in the journal Infection, Genetics and Evolution. It shows that the populations of the parasite observed in Belgium, Luxembourg and the Netherlands are similar. The exclusive presence of strains from the historic focus testifies to the spread of the parasite from the Alps. These strains are mostly common with those observed in Denmark and Sweden. This suggests an expansion of the worm to Scandinavia via Benelux and then northern Germany.
Of the 528 samples from the nine Eastern European countries, all were of European origin, except for one, taken from the European part of Russia. A spatio-temporal scenario of the expansion could be established: the parasite spread from the Alps through the Czech Republic, Slovakia and Hungary, then reached the southern part of Poland, before spreading in management of the Baltic countries and Ukraine. ” Given the slowness of the phenomenon, the expansion took place several decades, says Gérald Umhang, head of the national reference laboratory for Echinococcus spp., and we can think that the parasite is also present in countries bordering those where its presence is proven, even if no case has yet been identified. Understanding how the worm moved therefore allows to anticipate the possibility of cases of alveolar echinococcosis in places where the disease is not known historically. » However, the climatic conditions impact the survival of the eggs in the environment and limit the expansion of the worm: areas such as the south of France and Spain are a priori too dry to allow the survival of the eggs and no case n has also been detected there.
A blend on the border between Europe and Asia
« We have seen that the presence of Asian strains in Europe is very punctual. The two strains mix in the Asian part of Turkey, Poland and the European part of Russia, but the Asian strain has not been identified beyond that. says Gérald Umhang. The scientists hope to be able to continue their study by replicating their work for Asia. ” NOTWe assume that the Asian strains spread in the same way as the European ones but this is only a guess and we do not know the original focus of this strain, although we suspect it is probably in China , where more than 90% of alveolar echinococcosis cases occur worldwide. »