Aedes albopictus (Diptera: Culicidae) population size survey in the 2007 chikungunya outbreak area in Italy. II: Estimating epidemic thresholds.

Our study compared different estimates of adult mosquito abundance (Pupal Demographic Survey [PDS], Human Landing Collection [HLC], Number of Bites declared by Citizens during interviews [NBC]) to the mean number of eggs laid in ovitraps. We then calculated a disease risk threshold in terms of number of eggs per ovitrap above which an arbovirus epidemic may occur. The study was conducted during the summers of 2007 and 2008 in the Emilia-Romagna region of Italy where a chikungunya epidemic occurred in 2007. Ovitrap monitoring lasted from May to September, while adult sampling by means of PDS, HLC, and NBC was repeated three times each summer. Based on calculated rate of increase of the disease (R(0)) and the number of bites per human per day measured during the outbreak, we estimated that only 10.1% of the females transmitted the chikungunya virus in the principal focus. Under our conditions, we demonstrated that a positive correlation can be found between the females' density estimated by means of PDS, HLC, and NBC and the mean number of eggs in the ovitraps. We tested our hypothesis during the 2007 secondary outbreak of CHIKV in Cervia, and found that R(0) calculated based on the number of biting females estimated from the egg density was comparable to the R(0) calculated based on the progression of the human cases. The identification of an epidemic threshold based on the mean egg density may define the high risk areas and focus control programs.