Estimer le risque associé aux maladies vectorielles dans un contexte d’émergence : cas de la maladie de Lyme et du virus du Nil occidental au Québec

In recent decades, vector-borne diseases, diseases transmitted by bloodsucking arthropods, have been emerging or re-emerging around the world. The management of vector-borne diseases is mainly based on the reduction of contacts between the human and the vector, which requires knowledge of the spatiotemporal distribution of vector populations, i.e. the entomological risk. For emerging vector-borne diseases, the estimation of the entomological risk may be the only way to estimate the risk to humans since the circulation of the pathogen and the detection of human cases may be weak. Based on the emergence of Lyme disease and West Nile Virus (WNV) in Québec, the aim of this research was to understand the representativeness of the entomological risk, estimated by surveillance or modeling, in relation to the vector distribution and the risk to humans, according to the ecology of the vectors of interest and in a context of emergence. In the first part of the thesis, I explored the spatial and temporal distribution of entomological risk of Lyme disease as measured by entomological surveillance of its tick vector. The first study showed a significant increase in tick populations in terms of presence and abundance between 2007 and 2014 in southern Québec. These results support long-term monitoring to regularly reassess entomological risk and identify new risk areas. The second study looked at the spatial distribution of tick populations at different spatial scales. The spatial heterogeneity highlighted at each scale implies a local spatial variation of the risk associated with the ecology of ticks. This result asks question about the use and interpretation of information on entomological risk, particularly in areas where the presence of ticks could not be confirmed by surveillance; and about the representation of the risk and the choice of the geographic scale used. Understanding spatial variation in risk can be an asset for local communities, park managers or citizens by allowing interpretation of the information provided on a wider scale by public health authorities and adaptation to the local situation. The third study looked at the relationship between the measured entomological risk and the risk to humans estimated by the number of human cases. Our results suggest that the quality of the estimate of the risk to humans based on the entomological risk depends on the chosen entomological indicator. In our study, passive entomological surveillance was a better predictor of the number of human cases in a municipality than active entomological surveillance. We were thus able to develop an indicator, the number of people who submitted a tick in passive surveillance, to discriminate municipalities with at least three reported human cases. This indicator was included in the official definition of risk levels in Québec in 2017. Despite the success of this indicator, the relationship between entomological risk and risk to humans remains biased because of the spatial and temporal heterogeneity of vector populations, the mobility of vectors, the distribution of the human population and human behavior. In the second part of the thesis, I examined the representativeness of the entomological risk estimated from the modeling of vector populations, using the example of WNV. The resulting models provide a daily estimate of the abundance of Culex pipiens-restuans and Aedes vexans during the summer depending on weather conditions, which made it possible to predict the periods of highest WNV transmission risk due to higher vector density. However, environmental factors, such as the habitat, as well as the use of larvicide treatments, drive local variation in vector population abundance. These factors may nevertheless be taken into account in models to adjust the estimated mean value. This study also highlighted the importance of vector ecology which requires adaptation of the models to each vector species of interest. This thesis has questioned the effect of vector ecology and emergent context on the interpretation and the use of entomological risk. This will provide knowledge to the various actors in public health to optimize the use of the tools at their disposal for the management of emerging vector-borne diseases.

Keyphrases: Québec, vector disease, Lyme disease, West Nile virus, emergence, ecology, vector, entomological risk, representativeness, public health