Malaria remains a dreaded human infection with around 300 million clinical cases and 3 million deaths reported every year. Controlling this disease requires an understanding of its transmission and this involves a study of its basic reproductive number, R0.
The basic reproduction number is the average number of secondary cases infected by a typical single infected case, in a population with no immunity to the disease and in the absence of interventions to control the infection. It is often denoted R0. Generally, the larger the value of R0, the harder it is to control the epidemic.
In a new study published in PLoS Biology, David Smith and colleagues demonstrate that estimates of R0 range from around one to over 3,000, providing much higher estimates than previously thought, with serious implications for the control of the disease.
The author provides 121 estimates of R0 for Plasmodium falciparum malaria in African populations. In addition to the higher estimates of R0, they also show that in small human populations, R0 approximates transmission when counting infections from mosquito to mosquito, but overestimates it from human to human.
Previous studies showed that transmission is amplified if some humans are bitten more than others. The authors confirm that such heterogeneous biting amplifies transmission counting from mosquito to mosquito, but it can also dampen transmission counting from human to human. Humans who are bitten most both infect a large number of mosquitoes and absorb many infectious bites.
What does this mean for control? When R0 is in the thousands, eliminating malaria may seem impossible. If transmission from the humans who are bitten the most can be targeted, however, local elimination can still be within reach.