Ecological and genetic determinants of malaria vectors feeding and resting behaviours

Maliti, Deodatus Vincent (2015) Ecological and genetic determinants of malaria vectors feeding and resting behaviours. PhD thesis, University of Glasgow.

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Ecological and genetic factors play a key role in determining the behaviour of mosquito vectors, which in turn influences malaria transmission and epidemiology. Malaria vector control strategies such as long lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) rely on the timing and location of mosquito vector feeding behaviour, and their choice of resting habitat (inside houses versus outside). In some settings where these control measures have been intensively used, the major malaria vectors have been reported to change their behaviour to bite more often outdoors and/or earlier in the evening before people are protected by LLINs, and to increasingly rest outdoors. Such shifts in vector behaviour may jeopardize the effectiveness of LLIN and IRS strategies. The potential for such changes to undermine malaria control can only be understood by: (1) developing better, standardized sampling tools for the surveillance of mosquito behaviours, and (2) identifying the environmental and genetic factors that contribute to these behavioural changes as is required to predict how quickly they can occur and spread.
This study developed and evaluated a range of novel tools to sample host seeking and resting African malaria vectors. These tools were used to characterize a range of epidemiologically relevant malaria vector behaviours within an endemic area of southern Tanzania, and investigate the role of potential ecological and genetic determinants of behavioural variation. Firstly, a novel mosquito electrocuting trap (MET) was developed and evaluated relative to a commercially available insect electrocuting trap (CA-EG) and the gold standard human landing catch (HLC) technique for measuring the abundance and host seeking behaviour of Anopheles gambiae s.l. and An. funestus s.l. A Latin Square experiment was conducted in a rural setting in the Kilombero Valley of Tanzania where the sampling performance of MET was promising, especially in outdoor sampling where it achieved >58% sampling performance relative to the HLC. In contrast, the CA-EG had poor performance relative to both the MET and HLC and was considered unlikely to be a viable sampling method. This study showed that electrocuting traps can be developed and used as alternative, realistic and exposure-free sampling tools to the HLC technique.

Secondly, a series of new lightweight, portable and standardized sampling traps were developed and compared relative to one another to identify which traps are optimal for measuring African malaria vector resting behaviour. Two existing resting traps, the Resting Bucket (RBU) and Resting Box (RBO) were used along with two modified versions of the RBU designed to test the influence of specific design features to mosquito catchability: a modified entry resting bucket (MERBU) and a sticky resting bucket (SRBU). The performance of all traps for sampling indoor and outdoor resting An. gambiae s.l. and An. funestus s.l. were evaluated relative to one another, and the back-pack aspirator method (BPA, indoor collections only). Mosquito vector densities in all resting traps were relatively low (<3 per night), but were consistently higher in the RBU and RBO. The SRBU had significantly poorer performance outside than inside, which gave rise to highly biased estimates of exophilic behaviour. The MERBU trap performed consistently poorly inside and outdoors. Based on their relative and consistent sampling performance, the RBO and RBU are recommended as the best choice for wider scale surveillance of vector resting behaviour and its response to control measures.

Thirdly, a candidate gene approach was used to test if variation in the host seeking behaviour of An. arabiensis is associated with genetic polymorphisms in their circadian rhythm genes. Single nucleotide polymorphisms (SNPs) from 34 loci across 8 circadian genes in An. arabiensis were identified and analyzed for association with the timing (“early=7pm-10pm vs “late” = 4am-7am) and location (indoors vs outdoors) of their host seeking. No associations were found between the host seeking phenotypes and SNP polymorphisms in An. arabiensis. However, a strong genetic population structure was detected within An. arabiensis from the study area, which was correlated with polymorphisms in the Timeless gene (irrespective of the feeding phenotypes or geographical location). The cause of this structuring remains unknown, and further studies to investigate the potential mechanism and epidemiological implications are recommended. Although no association was found in this study, the role of genetics in determining malaria host seeking behaviour cannot be discounted. Other approaches such as transcriptomics and whole genome sequences are recommended in future studies.
In combination, results from this study give insights into the optimality of different sampling tools for reliable, ethically conducive monitoring of malaria vector behaviour. Furthermore, they provide baseline assessment of the contribution of some mosquito genetic and environmental factors to mosquito vector behaviour. It is hoped that the most promising sampling tools developed here can be improved and integrated into malaria vector surveillance programmes to obtain reliable information on vector behaviour and how vectors respond to environmental change and wide-scale use of malaria control measures.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Malaria, mosquito traps, population genetics, mosquito ecology, mosquito behaviour, mosquito sampling methods, Anopheles gambiae, Anopheles arabiensis, Anopheles funestus, phenotype-genotype association, single nucleotide polymorphisms, behavioural genetics, entomology
Subjects: Q Science > QL Zoology
Colleges/Schools: College of Medical Veterinary and Life Sciences > Institute of Biodiversity Animal Health and Comparative Medicine
Supervisor's Name: Ferguson, Dr. Heather
Date of Award: 2015
Depositing User: Dr Deodatus Maliti
Unique ID: glathesis:2015-6933
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Dec 2015 14:27
Last Modified: 06 Dec 2018 10:55

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