Protozoa such as Cryptosporidium spp, Giardia duodenalis and Toxoplasma gondii are identified as public health priorities because of possible waterborne transmission to humans. Protozoa are currently detected by immunofluorescence (IF), except T. gondii, on samples from filtered water. Limitations have been identified for both the IF detection method (requires antibodies, high-level expertise, etc.) and the matrix (water). The use of the water matrix to monitor the contamination level of water bodies gives variable results that depend on physico-chemical and meteorological parameters that are particularly important in the present context of global climate change. This could represent a limitation when applying a monitoring approach based on this matrix. Bioaccumulation measurements of these protozoa in invertebrate species, mainly bivalve molluscs, could represent an alternative tool to evaluate water contamination and avoid the limitations due to the water matrix. They would also break away from current approaches to protozoa research. Advantages would be the integrative (temporal) character of pathogen loads in invertebrates and the representativeness of pathogen measurements according to site (attached organisms). For us to have tools applicable on a large spatial scale (freshwater-seawater continuum), two bivalve mollusc species will be considered: a coastal one (the blue mussel, Mytilus edulis), and a continental one (the zebra mussel, Dreissena polymorpha). Using the capacity of our indicator species to accumulate parasites as a tool requires overcoming various scientific challenges by answering questions centred around three main issues: 1- It is necessary to develop and validate a sensitive procedure for simultaneous protozoa extraction and detection, and strict quantification of protozoa (oo)cysts in a complex matrix such as bivalve molluscs. The consortium proposes to develop innovative, rapid and sensitive molecular approaches, with special focus on the crucial step of (oo)cyst extraction/purification from molluscs. 2- It will be necessary to determine whether the parasite loads measured in zebra mussel and blue mussel tissues are correlated with the levels of contamination. To that end, the consortium proposes to define the kinetics of protozoa bioaccumulation by bivalves during single or combined exposures under laboratory conditions. 3- The physiology of mussels, in link with environmental and endogenous parameters, could modify their capacity to accumulate pathogens. The MOBIDIC project proposes to investigate the potential effect of environmental parameters (temperature, food availability, chemical stress) on pathogen bioaccumulation. In addition, in light of the lack of data in the literature, the project proposes to investigate the potential effects of (oo)cysts on mussel’s health, particularly on immune responses. This project proposes to provide new knowledge about interactions between protozoa (Cryptosporidium spp., Giardia duodenalis., and T. gondii) and mussels, with the aim to define a reliable indicator of the biological quality of water bodies to preserve the quality of water resources and protect human health. The MOBIDIC project will also provide interesting data in ecotoxicology in a multi-stress context, i.e. cumulative/interacting effects between biological and chemical contaminants, to better estimate the risk related to global climate change. To achieve the different objectives, a multidisciplinary consortium will associate environmental sciences (ecotoxicology and ecophysiology) and the fields of health (parasitology) on the one hand, and academic, institutional and private partners to combine basic and applied research and transfer skills to stakeholders, on the other hand. To facilitate skill transfer, the consortium applied for the HYDREOS cluster label. Members of this cluster will participate to the advisory board of the project.