Danilo O. Carvalho; Andrew R. McKemey; Luiza Garziera; Renaud Lacroix; Christl A. Donnelly; Luke Alphey; Aldo Malavasi; Margareth L. Capurro

“ Sterile males do not immediately reduce the target population; rather they lead to mortality in the next generation. Therefore, there is a delay of approximately one generation between the release of sterile males and any consequent effect on population size. ”

“ Ovitraps provide only an indirect measure of the adult population density, reduction of which is the key target for vector control purposes. Though we consider that changes in ovitrap metrics provide good indications of changes in adult density within an area, they provide a poor guide to absolute number [28] . To provide an independent measure of the adult population we used mark-release-recapture methods to estimate the standing crop of wild Ae. aegypti adults in the trial area [32] , exploiting the periodic releases of OX513A males. Adult collection was initially based on aspiration surveys ”

“ Abstract The increasing burden of dengue, and the relative failure of traditional vector control programs highlight the need to develop new control methods. SIT using self-limiting genetic technology is one such promising method. A self-limiting strain of Aedes aegypti, OX513A, has already reached the stage of field evaluation. Sustained releases of OX513A Ae. aegypti males led to 80% suppression of a target wild Ae. aegypti population in the Cayman Islands in 2010. Here we describe sustained series of field releases of OX513A Ae. aegypti males in a suburb of Juazeiro, Bahia, Brazil. ”

“ By comparing the ratio of OX513A:wild males with the fluorescent:non-fluorescent ratio in hatched eggs (‘fluorescence ratio’) we were able to estimate the mating competitiveness of the released OX513A males. We define field mating competitiveness (C) as the relationship between the numerical density of wild-type (W) and OX513A (O) males and the relative mating success, such that C = PW/ (O (1-P) ) where P is proportion of sterile mating (= proportion fluorescent larvae) [9] , [27] . ”

“ Assuming wild populations remained constant, likely release numbers required to achieve a predetermined ratio of released to wild population males in order to achieve suppression could then be estimated from the release rates and corresponding mating competitiveness estimate observed in the rangefinder phase. ”

“ The target release rate sufficient to achieve suppression should be proportional to the initial wild population. This may be assessed in terms of the ratio of release to wild males. Alternatively the mating fraction achieved represented a more direct impact of a given release rate without need to account for potential differences in mating competitiveness. Results from this and previous [9] studies support the use of target release rate sufficient to achieve >0.5 mating fraction in large operational programs. ”

“ It is primarily transmitted by one mosquito species, Aedes aegypti. We released transgenic ‘sterile’ male mosquitoes in Itaberaba, a suburb of Juazeiro, a Brazilian city. Sustained release of these males, whose offspring typically die before adulthood as a consequence of the transgenic modification, strongly suppressed the target wild population—by 80–95% according to different measures. ”

“ Conventional sterile insect based programs generally achieve greater efficiency by initiating releases in conjunction with reduced mosquito population, either by utilising naturally occurring seasonality or temporary knockdown with alternate control such as insecticide application. ”

“ It is a well-documented feature of SIT programs that laboratory caged study estimates of mating competitiveness are not representative of results in the field, where competitiveness values are much lower [26] . This is has also been true for OX513A as observed mating competitiveness in the field are lower than results from laboratory [21] , [36] , and larger semi-field mating cage studies [37] that have consistently shown OX513A males to be highly competitive achieving equal mating success to non-transformed males counterparts. ”

“ Optimising and scaling up production of OX513A
We next developed rearing and distribution systems in Brazil, building on Oxitec’s prior experience from field release in other countries [9] , [21] , [29] . This involved significant modification and optimisation of methods to make them appropriate for the Brazilian environment, including local sourcing of material [25] . Adult male production correspondingly increased from approximately 30,000 per week during the rangefinder to 540,000 per week in early 2012 [25] . ”

“ Weekly numbers of adult males released per hectare and percentage of larvae recovered from ovitraps in treated areas with the OX513A transgene as detected by fluorescence. (C) Relative ovitrap index. Relative Ovitrap Index (Treated/Control) in treated areas A and B. The horizontal line represents an equal value of ovitrap index in treated areas and control area. (D) Adult density in treated area and Ovitrap index in control area. Estimated adult wild population of Ae. aegypti per hectares (error bars = 95% CI) in treated area and ovitrap index in control area. ”

“ However this situation is itself transient as death of OX513A heterozygous offspring will in time start to have an impact on the number of wild mosquitoes, depending on the numbers of OX513A males released and their mating competitiveness in relation to wild males. Females additionally have to blood feed and find oviposition sites. ”

“ The mating competitiveness of the released males (0.031; 95% CI: 0.025-0.036) was similar to that estimated in the Cayman trials (0.059; 95% CI: 0.011 – 0.210) , indicating that environmental and target-strain differences had little impact on the mating success of the OX513A males. We conclude that sustained release of OX513A males may be an effective and widely useful method for suppression of the key dengue vector Ae. ”

“ The community engagement protocol was approved by the Institutional Review Board in Human Research (Comissão de Ética em Pesquisa com Seres Humanos do Instituto de Ciências Biomédicas/USP) and Comissão Nacional de Ética em Pesquisa—CONEP, protocol #1115. ”

“ Sterile Insect Technique 2005. p. 177–208 .1 2 3 Mayer DG, Atzeni MG, Stuart MA, Anaman KA, Butler DG. Mating competitiveness of irradiated flies for screwworm fly eradication campaigns. Preventive Veterinary Medicine. 1998;36 (1) :1–9 . pmid:96776231 2 3 Focks D, Brenner R, Hayes J, E D. Transmission thresholds for dengue in terms of Aedes aegypti pupae per person with discussion of their utility in source reduction efforts. ”

“ Late-acting dominant lethal genetic systems and mosquito control. BMC Biol. 2007;5:11. pmid:17374148↑ Alphey N, Alphey L, Bonsall MB. A Model Framework to Estimate Impact and Cost of Genetics-Based Sterile Insect Methods for Dengue Vector Control. PLoS ONE. 2011;6 (10) :e25384. doi: 10.1371/journal.pone.0025384 pmid:219986541 2 3 4 5 6 Harris AF, Nimmo D, McKemey AR, Kelly N, Scaife S, Donnelly CA, et al. Field performance of engineered male mosquitoes. Nat Biotech. ”