Table 1 Summary of studies on persistence mechanisms of malaria mosquitoes in sub-Saharan Africa
Method, persistence mechanism being tested & reference | Strengths | Weaknesses | Key assumptions | Comments |
|---|---|---|---|---|
Mark Release Recapture (MRR) to determine whether malaria mosquitoes survive the dry season by aestivation [4, 10, 63, 80] | Informs population size, survival rate and movement May be the only method that can provide unequivocal proof for aestivation and migration | Affected by the dry season with few or no mosquitoes Without sequencing of recaptured mosquitoes, results are not confirmatory (aestivation/local refugia) Low recapture rate, thus affecting the accuracy of the method Does not reveal where mosquito shelters are and how they cope with the dry season | The marked mosquitoes become re-integrated into the rest of the population Marking mosquitoes does not adversely affect them Mortality of marked mosquitoes caused specifically by their recapture is ignored Mortality rate is constant throughout | The aestivation process is difficult to reproduce Some females break their aestivation more readily than others |
Lab studies to determine whether malaria mosquitoes survive the dry season by aestivation [81] | In this study, the maximum lifespan of Anopheles mosquitoes was over 100 days representing maximum longevity compared to standard insectary conditions by 2.2–3.5-fold | Laboratory colonies lose genetic diversity in a few generations Laboratory conditions do not recapitulate all of the possible cues present in the field The lack of unambiguous markers of aestivation in Anophelines made it difficult to clearly confirm whether it really happened Demonstrating aestivation in its entirety in the lab is still a challenge [81] | This study used somewhat exaggerated climatic conditions to induce longevity with reduced temperature and photoperiod | Using exaggerated photoperiods beyond what happens in Mali is likely to have pushed An. gambiae to have similar longevity to that of An. coluzzii, something studies carried out to date have not reported |
Field collections to confirm whether mosquitoes survive by aestivation or as local refugia [8, 14, 28, 31, 52] | No specific functional approach [82] Areas with determinants of high mosquito density are established to show sources of dry-season populations | Vector density too low during dry season Distinguishing between absences that are a result of poor sampling and those which are legitimate is a challenge | Changes occur in mosquito physiology and behaviour in dry season Ovaries undergo one gonotrophic cycle in dry season and develop slowly | In dry season, females occupy hidden habitats Low temperatures and relative humidity induce a state of arrested development |
Time series analysis (Field collections) to confirm whether mosquitoes survive by aestivation or migration [7] | More reliable results as mosquitoes are collected over a relatively long period of time | Not clear whether not collecting mosquitoes during dry season is a weakness of the sample collection method or because of hidden shelters | When the mosquito recapture rate was less than 3%, the effect of removing them from the subsequent density instead of releasing them was negligible | Climate is one of the Selective pressures responsible for the ecological divergence between An. coluzzii and An. gambiae species The An. arabiensis collected during the dry season Could be representative of backcrossed hybrids between An. coluzzii and An. arabiensis |
Aerial sampling of mosquitoes at 40-290 m above ground level to confirm whether mosquitoes undergo long distance migration [29] | Results disprove previous studies that malaria mosquito dispersal doesn’t exceed 5 km [63, 64] | There is need to separate the role of Odyssean malaria from windborne migrants Protocol optimisation is time consuming, takes close to 12 months | Mosquitoes ascend by their own flight but are also passively carried by wind atltitude Mosquitoes fly in a layer between 50 and 250 m above ground level (and probably higher) Mosquito flights started at or after 18:00 and ended by 06:00 the following morning LDM-based migrants remain viable /reproductively fit | The likelihood of capturing Anopheles species increased with altitude Malaria mosquitoes migrated over tens to hundreds of kilometres in a single night Females outnumbered the males collected (4:1) |
Semi-field study (SFS) to test whether malaria mosquitoes survive the dry season by aestivation or migration [83] | SFS bridge the conceptual and methodological gaps between laboratory and field experiments Lifecycle completion is feasible inside the SFS | Laboratory colonies do not represent the wild type as they lose genetic diversity in a few generations A few larvae are sampled to avoid population depletion The hidden mosquito shelters used give a biased representation of the natural environment | Aestivation and migration are the main mechanisms that explain variation in population dynamics | Study results showed that An. coluzzii and An. arabiensis aestivate while An gambiae could adopt a different dry season survival strategy such as LDM Host feeding preferences could be involved in causing species variation of the SFS |
The indirect approach: Using genetic data | The method used is sensitive to bottlenecks of population size (robust) Ne depends on both population density and patterns of movement Additional inference based on inter-annual and inter-seasonal changes in private alleles and other measures of pop genetic constitution may be key to identify continuation of breeding vs. migration | Reliable estimates of Ne are difficult to obtain for natural populations Violation of the assumptions considered could result in larger Ne values More information is required to assess the effect of constraints on Ne estimates Ne is not meaningful if we don’t know the geographical area it represents and the population structure model these species follow | Random mating between individuals, discrete generations, a sex ratio of one, negligible selection, migration and mutation | Large populations are maintained throughout the dry season Large populations could be maintained by individuals hidden with respect to sampling Large populations could be maintained by extensive movement of adults |