Habitat Hydrology and Geomorphology Control the Distribution of Malaria Vector Larvae in Rural Africa

Larval source management is a promising component of integrated malaria control and elimination. This requires development of a framework to target productive locations through process-based understanding of habitat hydrology and geomorphology. We conducted the first catchment scale study of fine resolution spatial and temporal variation in Anopheles habitat and productivity in relation to rainfall, hydrology and geomorphology for a high malaria transmission area of Tanzania. Monthly aggregates of rainfall, river stage and water table were not significantly related to the abundance of vector larvae. However, these metrics showed strong explanatory power to predict mosquito larval abundances after stratification by water body type, with a clear seasonal trend for each, defined on the basis of its geomorphological setting and origin. Hydrological and geomorphological processes governing the availability and productivity of Anopheles breeding habitat need to be understood at the local scale for which larval source management is implemented in order to effectively target larval source interventions. Mapping and monitoring these processes is a well-established practice providing a tractable way forward for developing important malaria management tools

Plots of <i>An</i>. <i>arabiensis</i> estimates per water body type.

<p>(A) Bootstrap prediction estimates of late-stage <i>An</i>. <i>arabiensis</i> larvae per dip and (B) area-weighted abundance estimate of late-stage <i>An</i>. <i>arabiensis</i> larvae for each water body type. Area-weighted abundances and their 95% confidence intervals were calculated by multiplying estimated habitat size by the number of late-stage <i>An</i>. <i>arabiensis</i> larvae per dip estimated by bootstrapping a mixture distribution generated from GEE estimates of number of late-stage anophelines and the probability of finding <i>An</i>. <i>arabiensis</i> in the PCR samples. The hydrometric data is added for reference including hourly areal average rainfall, river stage recorded in the middle of the study site catchment and water table depth recorded towards the south of the study area.</p

Examples from each water body type.

<p>The water body types were classified according to their geomorphological and hydrological characteristics. (A) Topographic convergence: saturated areas driven by topographic convergence of subsurface moisture; (B) Floodplain basins: depressions within floodplains of active river channels with well-developed levees; (C) Palaeochannels: associated with relict palaeochannel systems; (D) River channels: pools located in perennial or seasonally active river channels; and (E) Spring-fed pools.</p

Contrasts in bootstrap estimated number of late-stage <i>An</i>. <i>arabiensis</i> larvae per dip using Method of Variance Estimates Recovery [67].

<p>Black = significant difference (95% confidence), grey = no significant difference, blank = not available (due to absence of larvae in one or both habitat types). T = topographic convergence, F = floodplain basin, P = palaeochannel, R = river channel and S = spring-fed pond.</p

Graphs showing (A) areal averaged rainfall from eight gauges distributed throughout the study area summarised as daily totals; (B) mean hourly river stage height with bank level reference line for the middle and lower gauges; and (C) mean hourly water table depth below the surface.

<p>Graphs showing (A) areal averaged rainfall from eight gauges distributed throughout the study area summarised as daily totals; (B) mean hourly river stage height with bank level reference line for the middle and lower gauges; and (C) mean hourly water table depth below the surface.</p