Montpellier pioneers parasitic ecology

To better manage the effects of parasites on human health, we need to understand how they fit into their ecosystems. This is the aim of parasite ecology, a discipline born of the work of Louis Euzet at the University of Montpellier.
It's a very strange living organism. It uses another living organism, called a host, as its habitat and source of energy. Parasites can be responsible for serious human diseases such as malaria, bilharzia, dengue fever and chikungunya. To control these diseases, doctors can't do without the contribution of researchers in parasite ecology. Their work: " understand how parasites circulate in ecosystems, decipher their relationships with their hosts and the parameters that drive their evolution "explains Catherine Moulia, a teacher-researcher at Montpellier Institute of Evolutionary Sciences (ISEM).
While this discipline is now practiced by researchers all over the world, it owes a great deal to the work of Professor Louis Euzet, who headed the Laboratory of Comparative Parasitology at Montpellier 2 University from 1969 to 1990. The father of the "Montpellier school of parasitology", he and his pupil Claude Combes were among the precursors of parasite ecology, one of whose aims was to understand the evolutionary aspects of host-parasite interactions. " Our teams in Montpellier, ISEM, MIVEGEC... are the heirs of this Euzet-Combes school. "says Catherine Moulia.

When the parasite takes control

Students in Louis Euzet's laboratory were among the first to demonstrate that parasites could modify their host's behavior to increase their chances of being transmitted to another host. "This is what we call the phenomenon of favorization ", explains Alain Lambert, a retired teacher-researcher who spent his career at UM2 with Louis Euzet. One example is the small liver fluke. During its life cycle, the fluke first parasitizes an ant. To continue its cycle and become an adult, it must then parasitize a sheep.
Problem: how do you get from one to the other, given that sheep aren't particularly fond of ants? The liver fluke has come up with a fascinating answer: it modifies the ant's behavior, so that instead of hanging around the anthill with its fellow ants, it perches at the top of a blade of grass to increase its chances of being eaten by a sheep grazing there. " The parasite takes control of its host's brain ", Alain Lambert sums up. Combes has shown that a whole part of the food chain is controlled by parasites," explains Laurent Gavotte, research professor at ISEM. This is a key factor in the management of populations and ecosystems ".

Cutting transmission between host and parasite

And when it comes to combating human diseases, the most detailed possible knowledge of the parasite's lifestyle and interactions with its host is vital. Researchers have thus succeeded in limiting the transmission of bilharzia, a tropical disease that has long plagued Guadeloupe. "It's not a question of eradicating the schistosome, the parasite responsible for this disease, but of implementing sanitary measures to prevent people from coming into contact with the waters where these parasites live," explains Catherine Moulia. " This cuts off transmission between the host and the parasite. In the same way, malaria, which was rife in France - and in the Hérault region in particular - was eradicated after the war. How was this achieved? "By sanitizing breeding grounds and generalizing the use of quinine," explains Alain Lambert.
But the disappearance of a parasite is not without consequences for its host. "There are areas in Africa where both intestinal worms and a serious neurological form of malaria are rife. We found that children who had received treatment to eliminate these worms were much more likely to catch neuromalaria, which is much more dangerous," explains Laurent Gavotte. It is therefore vital to understand the cross-interactions between parasites in order to provide the best possible care for the resulting pathologies.

Finding environmental solutions

" The contribution of non-medical scientists is crucial to understanding these interactions," stresses Catherine Moulia. These researchers are increasingly called upon by doctors, who need a more global view of the situation. " We need a multifactorial approach, which means understanding each system and each environmental context," says Alain Lambert. " And to take account of changing global dynamics," adds Laurent Gavotte. This was one of Louis Euzet's fundamental contributions: to go beyond the medical framework and take a broader view. Today, researchers in parasitic ecology are focused on finding sustainable environmental solutions. We never totally eradicate a parasite," explains Catherine Moulia, "but we try to maintain it at a tolerable level to limit the impact, particularly economic, of the diseases associated with it. A never-ending task in a constantly changing environment. Parasite ecology and its daughter epidemiology have a bright future ahead of them...