At the last VectorBiTE meeting at Imperial College, the EIP working group sketched out a plan for a review paper on how to define and improve measurements of the extrinsic incubation period (EIP) for malaria parasites. Our review is finally complete and has been published in Parasites & Vectors. The article is available here.
Don’t have time to read the manuscript? Here are a few of the key points we made in the paper:
(1) Malaria’s EIP has been inconsistently measured and reported in the past. Many studies report EIP as the time until the first observation of sporozoites in the salivary glands following an infectious feed. However, this ignores variation in development time that is likely present in a mosquito population and may not be representative of the EIP we want to capture in models of transmission if this earliest time of development is not the time when infectious mosquitoes are also feeding. See our Figure 1 for variation in EIP documented from the literature compared to the Detinova model that is used to predict EIPs based on temperature.
(2) Many factors are likely to affect EIP but most studies only consider temperature. The classic model to predict malaria’s EIP (the Detinova model) uses mean temperature to predict EIP. We evaluated evidence from the literature that fluctuating temperatures, parasite genetics, vector genetics, and other environmental factors such as mosquito diet, may also impact EIP.
(3) The best way to report EIP may be using the median EIP value observed from a mosquito population, rather than the time until first observed sporozoites. We run through an example of how using EIP estimated as the time until first observation vs estimated as the median EIP changes predictions of a proxy for potential transmission intensity (the number of alive and infectious mosquitoes). Future studies that make use of non-destructive sampling techniques to look at sporozoites in the salivary glands of individuals over time will hopefully make studies on EIP easier to do and better capture variation in EIP between individual mosquitoes.