Abstract
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. Despite such epidemiological importance, the estimation of EIP has been limited to a small number of vector-parasite combinations, and sources of variability in EIP remain largely unknown. Besides temperature and parasite species, evidence that EIP can be influenced by other genetic and environmental factors, and possibly be associated with other key transmission parameters such as mosquito longevity, is limited. The reason for this gap of knowledge probably lies in the difficulty of measuring EIP in the laboratory.The objective of our study was to use the Anopheles/Plasmodium system to investigate the genetic and environmental determinants of fEIP of Plasmodium falciparum. Several species of Anopheles (Anopheles arabiensis, Anopheles coluzzii, Anopheles gambiae, and Anopheles stephensi) were experimentally infected with either natural isolates of P. falciparum for the first three species or with parasite cultures for the last species. The mosquitoes were maintained under standard insectary conditions (12 h: 12 h light: dark, 27 ± 2 °C, 70 ± 5% relative humidity). In addition to the classical method of salivary gland dissection, a non-destructive approach based on the detection of parasites in mosquito saliva deposited on cotton soaked with a 10% glucose solution was developed and used to estimate the EIP. Our results showed that EIP varied with parasite genotype and how it varied for each parasite genotype depended on the mosquito species. It also varied with the parasite load in the head-thoraxes of females such that females with many sporozoïtes in the salivary glands produced short EIP. In addition, a positive relationship was observed between EIP and longevity of mosquitoes placed individually in the tubes with short EIP that were associated with short life spans. Finally, EIP did not appear to vary with mosquito age or parasite density in blood. These results provide important information for assessing how genetic and environmental influences on parasite transmission traits may shape malaria dynamics and evolution in response to malaria control.