**Schistosome Cercarial Emission as a Tool to Face Rising Challenges: Parasite Hybridizations, Reservoir Hosts and Climate Change for Disease Control Using the One Health Paradigm**

**Hélène Moné and Gabriel Mouahid** Domitia, CNRS, IFREMER, Univ Montpellier, Perpignan, France

Schistosomiasis is a human-neglected tropical parasitic disease that affects impoverished communities and

causes devastating health, social, and economic consequences to more than one billion people. It is due to a flatworm, the schistosome, which lives in the venous vessels of vertebrates. Human schistosomes belong to the genus Schistosoma, which has more than 20 species infecting domestic and wild animals. The life cycle includes two obligatory hosts: a homeothermic vertebrate where male and female worms reproduce and a freshwater snail where clonal multiplication occurs. Transmission between the two hosts is due to freeswimming larvae, miracidia for the vertebrate-snail transmission, and cercariae for the snail-vertebrate one. Such an introduction is commonly used in papers dealing with this disease. However, schistosomiasis control is challenged by three major rising phenomena, which will imply a substantial changing approach, that of One Health. We conducted field and experimental research on the cercarial emergence trait in high and low-endemic areas. The cercarial emission from the infected-snail host occurs each day (24 hours) during a specific period coinciding with the aquatic activity of the target vertebrate host in order to maximize transmission to the host. The cercarial emergence patterns associated with the molecular biology approach showed that natural hybridization between Schistosoma species is much higher than expected in both endemic areas and areas where the disease emerges. Zoonotic and human schistosome hybrids play a major role in schistosome transmission, placing schistosomiasis in the zoonotic diseases, even the species that, until now, were considered specific to humans. Climate change enhances the geographical range of the hybrid species from tropical areas to northern temperate areas (Europe). Hybridization between schistosomes gave rise to hybrid forms capable of simultaneously having several chronobiological profiles, allowing them to infect several hosts whose behavior with respect to water is different. We can no longer consider each species of schistosome as a separate entity. Hybrid forms lead us to treat schistosomiasis as a zoonotic parasitic disease involving humans, animals, and changes in the environment, the framework of the One Health approach.

Hélène Moné and Gabriel Mouahid

Domitia, CNRS, IFREMER, Univ Montpellier, Perpignan, France

#### Introduction

Schistosomiasis is a human neglected tropical parasitic disease affecting more than one billion people. It is caused by a flatworm, the schistosome, which lives in the venous vessels of vertebrates. Transmission to the vertebrate host is due to free-swimming larvae, the cercariae, which emerge from the snail host each day during a specific period coinciding with the aquatic activity of the target vertebrate host.

#### Methodology

The cercarial emergence pattern was used as a tool to identify hybridization and the implications of reservoir hosts in the field.

#### Results

Hybridization between schistosomes gives rise to hybrid forms capable of having several simultaneous chronobiological profiles, allowing them to infect several hosts whose behaviors with respect to water are different. (a.) Schistosoma bovis early morning profile, adapted to cattle (yellow); (b.) schistosoma haematobium midday and afternoon profile, adapted to humans (blue); (c.), (d.) and (e.) hybrid combination profiles simultaneously adapted to bovines, humans and/or nocturnal rodents (purple).

#### Conclusions

Schistosomiasis control is challenged by three major rising phenomena which will imply a huge changing approach, that of One Health. We can no longer consider each species of schistosome as a separate entity. Hybrid forms lead us to treat schistosomiasis as a zoonotic parasitic disease involving humans, animals and changes in the environment.

*Freshwater snail collection in the field.*

*Filtration, staining and counting of cercariae.*

Schistosome Cercarial Emission as a Tool to Face Rising Challenges: Parasite Hybridizations, Reservoir Hosts and Climate Change for Disease Control Using the One Health Paradigm
