Geospatial risk analysis of mosquito-borne disease vectors in the Netherlands

The availability of data on distribution and density of mosquito vectors of disease is needed to understand the risk of mosquito-borne diseases. In case of an outbreak of a newly introduced mosquito-borne pathogen of medical or veterinary importance, such information is required in order to decide on a contingency and eventual control plan. In the recent decade, several exotic mosquito species have become established in European countries and they have rapidly expanded their distribution. The rationale for the present study was the increasingly frequent reporting of invasive mosquito species (IMS) in the Netherlands, some of which are known vectors of infectious diseases, as well as the lack of detailed knowledge on the spatio-temporal distribution of the indigenous mosquito fauna. The aim was therefore to develop methodologies for acquiring accurate information on the actual and potential distribution of indigenous and exotic mosquito species in the Netherlands, and to evaluate the surveillance and control methodologies applied after IMS findings.

To establish a baseline for the spatio-temporal distribution of the indigenous mosquito fauna present in the Netherlands, a survey was conducted at the start of the study. Cross-sectional mosquito field surveys were carried out over a period of four years (2010-2013). These surveys provided occurrence maps for 26 indigenous species. One invasive mosquito species, Aedes japonicus, was discovered using this strategy. Furthermore, data on seasonality of the species, biodiversity and habitat preferences were also provided. Using the collected data on occurrence and abundance, a special study using random forest models was done to investigate the potential spatial distribution and population density of Anopheles plumbeus, a native nuisance mosquito species. I found a high environmental suitability and abundance of this species in the south-eastern provinces, mostly associated with abandoned pig farm buildings, and reports of biting nuisance. The identification of pathways for introduction of IMS was investigated in a next step taking into account the current knowledge and expert opinion. This was done in order to decide on the surveillance strategies needed to reduce the risk of future IMS introductions and/or potential outbreaks of mosquito-borne diseases. Four main routes for IMS introduction and dispersal were identified: the trade in used tires, the import of Lucky bamboo plants from Asia, the passive transport of IMS in vehicles (traffic by road, airplanes, and sea), and the natural dispersal of IMS. The results of the risk-based surveillance of IMS revealed yearly introductions of Ae. albopictus since 2010 at used tire companies and Lucky bamboo greenhouses, sporadic findings of Ae. japonicus associated with used tire trade, the first aircraft associated import of Ae. aegypti in Europe, and the first associated Ae. japonicus import with Lucky bamboo plants from elsewhere in the world. The control of these IMS, implemented after detection, has proven effective to avoid proliferation at these locations and their surroundings. Due to the yearly findings of Ae. albopictus, the potential risk of establishment of this invasive species was further investigated using habitat suitability models. Results show that the current average climatic conditions limit the overwintering of eggs of Ae. albopictus and their survival as adults in many inland areas of the country. However, due to the expected increase of the temperatures in the next decades as a result of climate change, these parts of the Netherlands will offer climatic conditions suitable for sustain populations of this species.

The results presented in this thesis show that nationwide surveillance of mosquitoes is pivotal to gain detailed information on the spatio-temporal distribution and abundance of mosquito species, which is useful to study the habitat suitability of vector species. Furthermore, this thesis highlights the main pathways for introduction and dispersal of IMS, designed a risk-based surveillance of IMS, and evaluated the surveillance and control measures applied in the Netherlands against IMS introductions. The work presented provides essential insights for identifying locations at risk of vector-borne disease transmission, and for designing targeted control of newly introduced IMS in the Netherlands, which is expected in the future.