Temperature and population density determine reservoir regions of seasonal persistence in highland malaria

Abstract

A better understanding of malaria persistence in highly seasonal environments such as highlands and desert fringes requires identifying the factors behind the spatial reservoir of the pathogen in the low season. In these 'unstable' malaria regions, such reservoirs play a critical role by allowing persistence during the low transmission season and therefore, between seasonal outbreaks. In the highlands of East Africa, the most populated epidemic regions in Africa, temperature is expected to be intimately connected to where in space the disease is able to persist because of pronounced altitudinal gradients. Here, we explore other environmental and demographic factors that may contribute to malaria's highland reservoir. We use an extensive spatio-temporal dataset of confirmed monthly Plasmodium falciparum cases from 1995 to 2005 that finely resolves space in an Ethiopian highland. With a Bayesian approach for parameter estimation and a generalized linear mixed model that includes a spatially structured random effect, we demonstrate that population density is important to disease persistence during the low transmission season. This population effect is not accounted for in typical models for the transmission dynamics of the disease, but is consistent in part with a more complex functional form of the force of infection proposed by theory for vector-borne infections, only during the low season as we discuss. As malaria risk usually decreases in more urban environments with increased human densities, the opposite counterintuitive finding identifies novel control targets during the low transmission season in African highlands.

The Lancet Countdown: tracking progress on health and climate change

“…32 The distribution and impacts of infectious diseases are already being altered by the various dimensions of climate change observed so far, and are projected to worsen for many infectious diseases in the future. [33][34][35] Given the existing information about climate-sensitive infectious diseases, we will fi rst derive a shortlist of relevant diseases or disease groups to road-test the indicator and then expand the list to include other relevant infectious diseases, following wider input from-and consultation with-infectious disease experts. Examples from three key groups will be tracked: food-borne diseases, vector-borne diseases, and parasitic diseases or zoonotic diseases.…”

Is there a disease‐free halo at species range limits? The codistribution of anther‐smut disease and its host species

“…Understanding the ecological drivers of species range limits is a fundamental aspect of ecology, and one increasingly relevant in the face of global change (Gaston, ; Sexton, McIntyre, Angert, & Rice, ). For pathogenic organisms, elucidating the drivers of range limits is critical to human health (Bozick & Real, ; Siraj et al., ), food security (Chakraborty & Newton, ) and for predicting the response to environmental change (Altizer, Ostfeld, Johnson, Kutz, & Harvell, ). Large‐scale studies of the codistribution of hosts and pathogens that span ecological gradients are critical to this endeavour.…”

“…Large‐scale studies of the codistribution of hosts and pathogens that span ecological gradients are critical to this endeavour. Studies focused on human pathogens (Lafferty, ; Siraj et al., ) have usually been in areas predetermined by political boundaries and only some include explicit ecological gradients (Balls et al., ; Moyes et al., ). Moreover, in human populations pathogen distributions have perforce been affected by disease control measures.…”

“…2009; Siraj et al, 2015) have usually been in areas predetermined by political boundaries and only some include explicit ecological gradients (Balls et al, 2004;Moyes et al, 2014). Moreover, in human populations pathogen distributions have perforce been affected by disease control measures.…”

Climate forcing and infectious disease transmission in urban landscapes: integrating demographic and socioeconomic heterogeneity

“…Characterizing the effects of demographic factors, especially human population density on transmission intensity and disease persistence, is essential to formulate better spatiotemporal models of transmission. Most temporal models for the population dynamics of vector‐transmitted diseases are extensions of the well‐known Ross–Macdonald equations, which assume frequency‐dependent transmission . Because the force of infection depends on the ratio between the number of vectors and that of the hosts, host population size dilutes transmission intensity, with no explicit distinction between the effect of density and that of population size.…”