The time it takes for malaria parasites to develop within a mosquito, and become transmissible, is known as the extrinsic incubation period, or EIP. EIP is a key parameter influencing transmission intensity as it combines with mosquito mortality rate and competence to determine the number of mosquitoes that ultimately become infectious. In spite of its epidemiological significance, data on EIP are scant. Current approaches to estimate EIP are largely based on temperature-dependent models developed from data collected on parasite development within a single mosquito species in the 1930s. These models assume that the only factor affecting EIP is mean environmental temperature. Here, we review evidence to suggest that in addition to mean temperature, EIP is likely influenced by genetic diversity of the vector, diversity of the parasite, and variation in a range of biotic and abiotic factors that affect mosquito condition. We further demonstrate that the classic approach of measuring EIP as the time at which mosquitoes first become infectious likely misrepresents EIP for a mosquito population. We argue for a better understanding of EIP to improve models of transmission, refine predictions of the possible impacts of climate change, and determine the potential evolutionary responses of malaria parasites to current and future mosquito control tools.Electronic supplementary materialThe online version of this article (10.1186/s13071-018-2761-4) contains supplementary material, which is available to authorized users.
The dengue, Zika and chikungunya viruses are transmitted by the mosquito Aedes aegypti and pose a substantial threat to global public health. Current vaccines and mosquito control strategies have limited efficacy, so novel interventions are needed 1,2. Wolbachia are bacteria that inhabit insect cells and have been found to reduce viral infection-a phenotype that is referred to as viral 'blocking' 3. Although not naturally found in A. aegypti 4 , Wolbachia were stably introduced into this mosquito in 2011 4,5 and were shown to reduce the transmission potential of dengue, Zika and chikungunya 6,7. Subsequent field trials showed Wolbachia's ability to spread through A. aegypti populations and reduce the local incidence of dengue fever 8. Despite these successes, the evolutionary stability of viral blocking is unknown. Here, we utilized artificial selection to reveal genetic variation in the mosquito that affects Wolbachia-mediated dengue blocking. We found that mosquitoes exhibiting weaker blocking also have reduced fitness, suggesting the potential for natural selection to maintain blocking. We also identified A. aegypti genes that affect blocking strength, shedding light on a possible mechanism for the trait. These results will inform the use of Reprints and permissions information is available at www.nature.com/reprints.
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