Hello everyone, it is no news that malaria is a devastating disease that needs to be controlled. However, malaria epidemiology and control are affected by the intensity of transmission and other factors such as the percentage of the population that is infected, total mortality due to malaria, the rate at which the infection spreads through the population, or the average number an infected person contacts a susceptible person (R0). Estimating these parameters is therefore imperative in evaluating and interpreting the effectiveness of the control interventions in different places and times.
The basic reproductive number - R0, provides a transmission intensity index and establishes threshold criteria. It accounts for the expected number of vertebrate hosts or vectors that would be infected through one complete generation of the parasite by a single infected mosquito or a single infected human (Shown below). If R0 < 1, the number of infected people declines but if R0 > 1, then that number increases. Additionally, if control interventions curb transmission by a factor that exceeds R0, then the elimination of parasites will occur. However, the downside to using R0 to determine the transmission intensity is that it is based on an idealized scenario. For instance, it assumes that the population is infinite and is susceptible to the disease. What does this mean in terms of malaria transmission? Well, first of all, human populations are finite, secondly, not everyone receives the same number of infected bites. Because some receive more bites and are more likely to become infected, they will infect more mosquitoes and amplify transmission. Therefore, R0 is underestimated in infinite human populations.
Most of the equations that estimate R0 for malaria assume that each infective mosquito bites a different host. However, the reality is that malaria transmission is local, and therefore, some infectious bites will land on a previously infected host. That is why Smith et al. (2007) used a finite population with heterogeneous biting to determine R0. They used two indices: the entomological inoculation rate (EIR) defined as the average number of infectious bites a person receives per year and the parasite ratio (PR) which is the prevalence of malaria infection in humans. Because of the many factors that affect malaria transmission such as the ecology and biting behavior of mosquitoes, R0 varies from place to place. Smith et al. (2007) estimated R0 to be around one and to over 3 000. As mentioned earlier, any value of R0 equal to or above one will sustain the transmission, but this also means that our vector control efforts of malaria elimination are insufficient or failing. So, are you the next victim?
Sources
Routledge, I., Chevéz, J.E.R., Cucunubá, Z.M., Rodriguez, M.G., Guinovart, C., Gustafson, K.B., Schneider, K., Walker, P.G., Ghani, A.C. and Bhatt, S., 2018. Estimating spatiotemporally varying malaria reproduction numbers in a near elimination setting. Nature communications, 9(1), pp.1-8.
Shaw, W.R., Holmdahl, I.E., Itoe, M.A., Werling, K., Marquette, M., Paton, D.G., Singh, N., Buckee, C.O., Childs, L.M. and Catteruccia, F., 2020. Current estimates of malaria basic reproduction number underestimate parasite transmission efficiency due to multiple blood feeding.
Smith, D.L., McKenzie, F.E., Snow, R.W. and Hay, S.I., 2007. Revisiting the basic reproductive number for malaria and its implications for malaria control. PLoS biology, 5(3), p.e42.
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