2023-09-26 19:08:02
Certain vectors of infectious diseases are in the news. This is the case of the tiger mosquito, whose bite can cause dengue fever, or ticks, which can transmit Lyme disease in particular.
But a wide range of so-called “parasitic” animals are much less publicized, even though they are widespread and, too, potentially devastating to human and animal health.
Who, for example, has ever heard of parasitic flatworms called trematodes? However, they are responsible for many diseases linked to water and food…
Some of them, liver flukes, are the cause of a disease which can have serious consequences: fasciolosis (also sometimes called fasciolasis or hepatic distomatosis).
France is not spared, but until now the number of annual human cases remained very low. However, the arrival of new snails of tropical origin in our country worries some specialists.
What is fasciolosis?
Among the diseases caused by trematode flatworms, fasciolosis is the one with the widest global distribution.
It takes its name from the fact that it is caused by worms belonging to the genus Fasciola (notably the species Fasciola hepatica), commonly called “large liver flukes” because, at the end of a complex process, they migrate and then develop in the liver of the mammalian hosts that they infect.
Many domestic and wild mammals can fall victim to the liver fluke, and humans themselves are a potential host of this parasite. It is generally contaminated by ingesting raw aquatic plants which carry encysted larvae of the parasitic worm.
In France, “wild” watercress is the food most frequently involved in the transmission of liver fluke. Other wild lettuces, such as dandelion or lamb’s lettuce, may also be involved. Exceptionally, cultivated watercress can also be contaminated in the event of flooding.
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In human fasciolosis, there is an acute phase and a chronic phase. The first is characterized by fever, nausea, severe abdominal pain, and swelling of the liver. In the second, epigastric pain, nausea, fat intolerance, inflammation and obstruction of the bile ducts, as well as liver damage are manifested. The patient suffers from jaundice and anemia. If left untreated, symptoms can last for several years.
Where is the disease rampant?
Fasciolosis is found on all continents, with the exception of Antarctica. It occurs in tropical or temperate environments, even at very northern latitudes, in the plains as well as at high altitudes. In addition, the human prevalence of fascioliasis varies greatly.
According to some estimates, around 50 million people are affected by this disease and almost 180 million live in regions at high risk of infection.
In France and Europe, there is no monitoring system allowing us to get an idea of the number of people infected each year. According to figures from the National Health Security Agency dating from 2016, the number of cases remains very low in the country (on average, 5 people were hospitalized each year between 2008 and 2013).
A re-emerging parasite
The low number of human contaminations recorded in France should not mask the overall situation: on a global scale, we are indeed seeing an increase in the transmission of fasciolosis. Invasions of limnaea, the aquatic snails in which the worm develops before infecting a mammal, are one of the factors explaining this increase.
Antonio Alejandro Vazquez Perera, Provided by the author
Indeed, although limnaea are not capable of traveling long distances on their own (their “active” dispersal is limited), the fact remains that a “passive” migration of these molluscs takes place. dfor millions of years: they are transported by currents or by various animals, notably birds or certain mammals.
Furthermore, more recently, human activities have changed the situation: they have taken over, favoring the dispersal throughout the world of certain limnae considered to be major hosts. Thus, due to international trade, and in particular that of plants used in aquariums, several tropical species have been introduced very far from their native areas.
In the context of globalization, this situation poses the risk of a drastic increase in the species of limnae present in certain territories, particularly in Europe.
Antonio Alejandro Vazquez Perera, Provided by the author
Exotic species arrive in Europe
In Europe, the local mollusk species that plays a major role in fasciolosis is the lesser amphibious limnaea (Galba truncatula), a species well adapted to temperate and cold environments. But three particularly dangerous species, introduced recently, have already been detected on European territory.
The oldest, Pseudosuccinea columella, is native to North America. Spotted for the first time in 2004 on the banks of the Lot, it was also found very recently in several stations on the Canal du Midi and in Corsica.
Two other exotic limnae species, one native to Central and South America as well as the Caribbean (Galba of Cuba), the other from Asia and Oceania (Orientogalba viridis) have also extended the list of European molluscs. Both have been sampled in the Ebro Delta region, Spain, since 2010. The Asian limnea, in particular, has become very well established in this region, where stable populations were detected in 2020.
Read more: Podcast: All parasitized?
In their native ranges, these exotic species are able to maintain stable populations during the warmer months of the year. In a context of climate change and a trend toward warming temperatures, these species of limnae introduced into Europe might begin to fully establish themselves and play a role in the epidemiology of liver flukes.
This new situation poses the risk of uninterrupted transmission of fasciolosis in southern Europe. This constitutes a latent threat that must be taken seriously.
Farm animals, main players in the epidemiology of fasciolosis
The relationship between breeding activities and liver fluke circulation is very close. Domestic ruminants (sheep and cattle) play the role of the main definitive hosts of the parasite, and ensure its dissemination through their excrement.
In France, in 80 departments, the prevalence of liver flukes in cattle herds exceeds 60%. Cher, Pyrénées-Orientales and Lot even display 100% prevalence. In total, it is estimated that 86% of farms are affected.
Grazing in risky areas characterized by the presence of molluscs, such as streams, irrigation canals or flood-prone areas, are determining factors in the expansion potential of these worms. The movements of domestic animals (cattle, sheep, and goats) must also be taken into account during trade. Finally, wild fauna (wild boar, coypu, etc.) can also play a role in the dispersal of parasites between habitats not connected to each other, from a high prevalence outbreak linked to livestock activities.
In practice, better identification of molluscs can help organize the grazing of herds: depending on the presence of limnaea on the sites, it may be wise to prohibit either the free movement of animals in the breeding areas, or their movement towards watering areas.
No vaccine or preventive treatment once morest fasciolosis
Let us end by emphasizing that there is currently no vaccine or preventive treatment for fasciolosis.
The World Health Organization, however, recommends a drug for the management of acute and chronic phases, triclabendazole. But this is sometimes administered when the parasite has already damaged the body.
In the case of herd treatment, although there are several alternatives, triclabendazole continues to be the most effective in eliminating the youngest parasites. On the other hand, cases of resistance are increasingly observed.
In order to control this disease, it therefore appears essential to implement, in the field, monitoring actions for limnaea, these aquatic snails which serve as intermediaries in fasciolosis. And more specifically, potentially invasive exotic species.
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