With declining malaria transmission, reliable quantification of ongoing transmission is vital and becomes increasingly difficult using classical entomological measures. The molecular force of blood-stage infection (molFOB), a measure of exposure to new blood-stage infections, has been demonstrated to be a suitable proxy for measuring transmission but is yet to be applied to directly assess the impact of intensified control measures on P. falciparum and P. vivax transmission. Using three consecutive longitudinal child cohorts, we investigated the impact of improved control measures on the changing patterns of molFOB, incidence of clinical illness and the cumulative prevalence of infections and to gain insight into how these metrics perform for the two main species, P. falciparum and P. vivax.
The cohorts included children 1-5 years of age and were conducted before (2006), during (2009) and after scale up of control interventions. Incidence analyses were conducted using the number of clinical episodes and molecularly determined new blood-stage infections in each interval between active case detection (ACD) time-points when the child was considered at risk. Prevalence analyses were conducted using longitudinal binary outcomes measuring the presence of infection which was measured at each ACD time-point and diagnosed by microscopy and PCR.
After scale-up an immediate reduction in P. falciparum molFOB and incidence of clinical episodes, as well as P. vivax clinical episodes was observed. There was a delayed impact on P. vivax molFOB. After 5 years of intensified control, there was a marked reduction in the incidence of new infections by 73% for P. falciparum and 87% for P. vivax, and the incidence of clinical episodes by 90% and 88%, respectively.
We demonstrate the utility of molFOB in measuring the impact of control interventions on transmission for the first time, observing marked differences for P. falciparum and P. vivax.