Increasing Effectiveness of Genetically Modifying Mosquito Populations: Risk Assessment of Releasing Blood-Fed Females.

Releasing mosquito refractory to pathogens has been proposed as a means of controlling mosquito-borne diseases. A recent modeling study demonstrated that instead of the conventional male-only releases, adding blood-fed females to the release population could significantly increase the program's efficiency, hastening the decrease in disease transmission competence of the target mosquito population and reducing the duration and costs of the release program. However, releasing female mosquitoes presents a short-term risk of increased disease transmission. To quantify this risk, we constructed a Ross-MacDonald model and an individual-based stochastic model to estimate the increase in disease transmission contributed by the released blood-fed females, using the mosquito Aedes aegypti and the dengue virus as a model system. Under baseline parameter values informed by empirical data, our stochastic models predicted a 1.1-5.5% increase in dengue transmission during the initial release, depending on the resistance level of released mosquitoes and release size. The basic reproductive number (R0) increased by 0.45-3.62%. The stochastic simulations were then extended to 10 releases to evaluate the long-term effect. The overall reduction of disease transmission was much greater than the number of potential infections directly contributed by the released females. Releasing blood-fed females with males could also outperform conventional male-only releases when the release strain is sufficiently resistant, and the release size is relatively small. Overall, these results suggested that the long-term benefit of releasing blood-fed females often outweighs the short-term risk.

Variation in Wolbachia effects on Aedes mosquitoes as a determinant of invasiveness and vectorial capacity.

Wolbachia has been introduced into Aedes aegypti mosquitoes to control the spread of arboviruses, such as dengue, chikungunya and Zika. Studies showed that certain Wolbachia strains (such as wMel) reduce replication of dengue viruses in the laboratory, prompting the release of mosquitoes carrying the bacterium into the field, where vectorial capacity can be realistically assessed in relation to native non-carriers. Here we apply a new analysis to two published datasets, and show that wMel increases the mean and the variance in Ae. aegypti susceptibility to dengue infection when introgressed into Brazil and Vietnam genetic backgrounds. In the absence of other processes, higher mean susceptibility should lead to enhanced viral transmission. The increase in variance, however, widens the basis for selection imposed by unexplored natural forces, retaining the potential for reducing transmission overall.

Dengue burden in India: recent trends and importance of climatic parameters.

For the past ten years, the number of dengue cases has gradually increased in India. Dengue is driven by complex interactions among host, vector and virus that are influenced by climatic factors. In the present study, we focused on the extrinsic incubation period (EIP) and its variability in different climatic zones of India. The EIP was calculated by using daily and monthly mean temperatures for the states of Punjab, Haryana, Gujarat, Rajasthan and Kerala. Among the studied states, a faster/low EIP in Kerala (8-15 days at 30.8 and 23.4 °C) and a generally slower/high EIP in Punjab (5.6-96.5 days at 35 and 0 °C) were simulated with daily temperatures. EIPs were calculated for different seasons, and Kerala showed the lowest EIP during the monsoon period. In addition, a significant association between dengue cases and precipitation was also observed. The results suggest that temperature is important in virus development in different climatic regions and may be useful in understanding spatio-temporal variations in dengue risk. Climate-based disease forecasting models in India should be refined and tailored for different climatic zones, instead of use of a standard model.

Comparing vector­host and SIR models for dengue transmission.

Various simple mathematical models have been used to investigate dengue transmission. Some of these models explicitly model the mosquito population, while others model the mosquitoes implicitly in the transmission term. We study the impact of modeling assumptions on the dynamics of dengue in Thailand by fitting dengue hemorrhagic fever (DHF) data to simple vector­host and SIR models using Bayesian Markov chain Monte Carlo estimation. The parameter estimates obtained for both models were consistent with previous studies. Most importantly, model selection found that the SIR model was substantially better than the vector­host model for the DHF data from Thailand. Therefore, explicitly incorporating the mosquito population may not be necessary in modeling dengue transmission for some populations.

Assessment of dengue risk in relief workers in Puerto Rico after Hurricane Georges, 1998.

Health risk assessment is important in the safe deployment of workers to tropical areas. We monitored dengue incidence in 204 of 222 North American relief workers visiting Puerto Rico after Hurricane Georges and during a dengue epidemic in 1998. We recorded information regarding participants' living conditions and any illness they experienced from arrival to 2 weeks after their departure. Virus isolation, polymerase chain reaction, and serological tests for anti-dengue immunoglobulin (Ig) M and IgG antibodies were used to diagnose dengue infection by means of departure and follow-up serum specimens. Among respondents, 82% (164 of 199) reported mosquito bites, 97% (156 of 161) reported having insect repellent available, and 41% (79 of 195) reported using repellent every day. Twelve participants reported a mild denguelike illness. No participants had laboratory evidence of dengue infection after 1.8 person-years of assessable exposure to areas with dengue transmission (upper 95% confidence limit of 1.67 cases per person-year). The risk of acquiring dengue among relief workers in this study appears low, possibly as a result of protective factors. Travelers to dengue-endemic areas should continue to be advised to protect themselves against mosquito bites.

Aedes aegypti in Tahiti and Moorea (French Polynesia): isoenzyme differentiation in the mosquito population according to human population density.

Genetic differences at five polymorphic isoenzyme loci were analyzed by starch gel electrophoresis for 28 Aedes aegypti samples. Considerable (i.e., high Fst values) and significant (i.e., P values >10(-4)) geographic differences were found. Differences in Ae. aegypti genetic structure were related to human population densities and to particularities in mosquito ecotopes in both Tahiti and Moorea islands. In highly urbanized areas (i.e., the Papeete agglomeration), mosquitoes were highly structured. Recurrent extinction events consecutive to insecticidal treatments during dengue outbreaks tend to differentiate mosquito populations. In less populated zones (i.e., the east coast of Moorea and Tahiti), differences in ecotope characteristics could explain the lack of differentiation among mosquitoes from rural environments such as the east coast of Tahiti where natural breeding sites predominate. When the lowest populated zones such as Tahiti Iti and the west coast of Moorea are compared, mosquito are less differentiated in Moorea. These results will be discussed in relation to the recent findings of variation in mosquito infection rates for dengue-2 virus.