Effectiveness and economic assessment of routine larviciding for prevention of chikungunya and dengue in temperate urban settings in Europe

In the last decades, several European countries where arboviral infections are not endemic have faced outbreaks of diseases such as chikungunya and dengue, initially introduced by infectious travellers from tropical endemic areas and then spread locally via mosquito bites. To keep in check the epidemiological risk, interventions targeted to control vector abundance can be implemented by local authorities. We assessed the epidemiological effectiveness and economic costs and benefits of routine larviciding in European towns with temperate climate, using a mathematical model of Aedes albopictus populations and viral transmission, calibrated on entomological surveillance data collected from ten municipalities in Northern Italy during 2014 and 2015.We found that routine larviciding of public catch basins can limit both the risk of autochthonous transmission and the size of potential epidemics. Ideal larvicide interventions should be timed in such a way to cover the month of July. Optimally timed larviciding can reduce locally transmitted cases of chikungunya by 20% - 33% for a single application (dengue: 18–22%) and up to 43% - 65% if treatment is repeated four times throughout the season (dengue: 31–51%). In larger municipalities (>35,000 inhabitants), the cost of comprehensive larviciding over the whole urban area overcomes potential health benefits related to preventing cases of disease, suggesting the adoption of more localized interventions. Small/medium sized towns with high mosquito abundance will likely have a positive cost-benefit balance. Involvement of private citizens in routine larviciding activities further reduces transmission risks but with disproportionate costs of intervention. International travels and the incidence of mosquito-borne diseases are increasing worldwide, exposing a growing number of European citizens to higher risks of potential outbreaks. Results from this study may support the planning and timing of interventions aimed to reduce the probability of autochthonous transmission as well as the nuisance for local populations living in temperate areas of Europe.Author summary: Larvicides are a key tool to prevent the growth of mosquito populations and decrease both the risks of outbreaks of mosquito-borne diseases and the nuisance deriving from bites. In order to assist municipalities from temperate areas in Europe in effectively planning vector control programs, we modelled the effect of larviciding in public areas on populations of Aedes albopictus using mosquito collection data from 10 municipalities in Northern Italy, over two years with very different temperature conditions. We then evaluated the resulting probabilities of potential outbreaks of chikungunya and dengue and their expected final sizes, and we compared the intervention costs to the economic and health benefits due to the avoidance of clinical cases. By assessing several different intervention strategies, we found that the optimal timing should be centred on the month of July, corresponding to the period of maximal growth of the mosquito population. Municipality-wide interventions are likely to be cost-effective in small-to-medium towns (below 35,000 inhabitants) even where mosquito infestation is moderate, whereas for larger cities a neighbourhood-based intervention should be considered. The involvement of citizens to apply larvicides within private premises resulted effective but generally too costly.