A New Test of a Theory about Old Mosquitoes

Abstract

In vector control, it is widely accepted that killing adult mosquitoes would sharply reduce the proportion of old mosquitoes and cause the greatest changes to malaria transmission. The principle is based on a mathematical model of the sporozoite rate (the proportion of infective mosquitoes) that emphasized changes in mosquito age. Killing adult mosquitoes also reduces mosquito population densities, which are directly proportional to human biting rates (the number of bites, per person, per day). Eect sizes of vector control can be compared using sporozoite rates and human biting rates, which are commonly measured. We argue that human biting rates convey more use- ful information for planning, monitoring and evaluating vector control, and operational research should focus on understanding mosquito ecology.

Keywords: dynamics and control; malaria; mosquito ecology; vector control.

Figure 1:

Effects of IRS on the human biting rate and sporozoite rate. During a longitudinal study in Nagongera, Tororo, Uganda, the government implemented an IRS spraying program (the vertical orange line marks the beginning of the first round). The sporozoite rate (shown as smoothed lines in grey) changes seasonally, and it declined sharply after control. a) The average sporozoite rate changed from 1.9% to 0.6%, a factor of approximately 3.1 (blue horizontal lines show the means before and after control). b) Mean mosquito counts declined from 33.9 per day before control to 2.6 per day after control, corresponding to a change in the human biting rate by a factor of 12.8 (blue horizontal lines show the means before and after control). Measured crudely, the effect size on the human biting rate was roughly 4.1 times as large as the effect size on the sporozoite rate.