20/09/2021 4 min

How a parasitic worm travelled across Europe

By analysing the DNA of Echinococcus multilocularis parasitic tapeworms, an international group of scientists led by ANSES has determined how foxes carried this worm, responsible for alveolar echinococcosis, from the Alps to Northern and Eastern Europe.

Rodents and foxes are the preferred hosts of the Echinococcus multilocularis parasite, but when it accidentally infects humans, it causes alveolar echinococcosis. This disease occurs only rarely, with about 40 new cases a year in France, but it has serious consequences, requiring daily medication for life. The first human cases of the disease were described in the 19th century and were confined to the Alpine regions of France, Germany, Switzerland and Austria. However, in the last 30 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 and Croatia. Foxes infected intestinally with Echinococcus multilocularis worms shed the parasite's eggs in their faeces. These eggs can then be ingested by rodents when they feed on contaminated plants. The parasite cycle continues when a fox consumes an infected rodent in which the larval form of the parasite has developed. Humans can be infected accidentally, for example by eating fruit picked close to the ground or raw vegetables, or by touching the fur of infected dogs or cats after they have eaten rodents. 

A parasite carried by foxes

An earlier study showed that the parasite had spread through the progressive migration of infested foxes from the historical European endemic area in the Alps to neighbouring countries, since the genetic profiles of the parasite shared common features but with lower diversity as it moved away from the Alps. This decrease in diversity can be explained by the fact that each animal only brings with it the strains it carries. As alveolar echinococcosis is also widespread in Asia, where it is caused by strains of Echinococcus multilocularis distinct from those in Europe, it was important to determine whether the geographical spread of the parasite across the European continent had occurred solely from the historical Alpine area or from the Asian one as well. The recent identification of Asian strains in Poland has made this issue even more relevant. To provide some answers, scientists from different European countries collected parasite samples. The French reference laboratory for Echinococcus spp., for which ANSES is responsible, studied small portions of DNA from the worms collected, the difference in size of the DNA made it possible to differentiate between strains.

European presence of the disease almost exclusively originating in the Alpine endemic area

The study published in the journal Infection, Genetics and Evolution shows that the parasite populations observed in Belgium, Luxembourg and the Netherlands are similar. Since almost all of the strains can be traced back to the historical origin in the Alps, this confirms that the parasite did indeed spread from there. These strains are mostly all the same as those observed in Denmark and Sweden, suggesting that the disease travelled to Scandinavia via the Benelux countries and northern Germany. 
Of the 528 samples from the nine Eastern European countries, all were of European origin, with the exception of one sample from the European part of Russia. The scientists were then able to draw up a spatio-temporal scenario of this expansion: the parasite spread from the Alps through the Czech Republic, Slovakia and Hungary, then reached the southern part of Poland before spreading towards the Baltic countries and Ukraine. "Given the slow pace of the phenomenon, the expansion occurred several decades ago," believes Gérald Umhang, head of the national reference laboratory for Echinococcus spp., "and we can assume that the parasite is also present in countries bordering those where it is known to occur, even if no cases have yet been identified. Understanding how the worm got there therefore helps us predict the possibility of cases of alveolar echinococcosis in places where this disease was not historically known." However, climate conditions can affect the survival of eggs in the environment and limit the spread of the worm; areas such as the south of France and Spain are in principle too dry to promote survival of the eggs, and no cases have been detected there.

A mix on the border between Europe and Asia

"We have seen that cases of Asian strains in Europe are very sporadic. However, the two strains have mixed in the Asian part of Turkey, in Poland and in the European part of Russia, but the Asian strain has not been identified beyond there," says Gérald Umhang. Scientists hope to continue their study by replicating their work for Asia. "We assume that the Asian strains have spread in the same way as the European ones, but this is only a hypothesis and we do not know the origin of this strain, although we suspect that it is probably in China, where more than 90% of the world's cases of alveolar echinococcosis occur."

What is alveolar echinococcosis and how can it be prevented? 

Alveolar echinococcosis is a parasitic disease caused by Echinococcus multilocularis that can be passed from animals to humans. Most infections do not result in the development of disease, but if they do, it destroys the liver and can spread to other organs. It is fatal if left untreated, and the drugs available only stop the parasite's development and cannot eliminate it. Immunocompromised people are particularly at risk.

A few simple measures can prevent infection:

  • thoroughly wash fruit and vegetables picked close to the ground;
  • wash hands after contact with soil and after handling foxes, dogs or cats, which may carry eggs of the parasite on their fur;
  • fence off vegetable gardens to prevent foxes from entering and contaminating the soil and plants with their faeces;
  • for dog or cat owners, deworm the animals regularly if there is a risk that they may eat rodents.