Trends in Parasitology

Trends in Parasitology

Volume 36, Issue 1, January 2020, Pages 39-51
Journal home page for Trends in Parasitology

Review
Mosquito Age Grading and Vector-Control Programmes

https://doi.org/10.1016/j.pt.2019.10.011Get rights and content

Highlights

  • New guidelines on evaluating vector-control tools highlight the need for entomological-based measures that predict epidemiological outcomes.

  • Mosquito survivorship is the key variable in epidemiological models of vector-borne disease, but existing morphological measures of age are technically demanding, subjective, and have little utility for arbovirus vectors.

  • Emerging techniques in spectroscopy and existing transcriptional approaches may overcome many of the problems of conventional morphological techniques but their true utility has yet to be tested.

  • Future studies on age grading must focus on the ability of the new technologies to estimate the age of wild-caught adults and provide convincing field validations, replicated across field sites. This will require blinded trials against existing correlates of age (e.g., parity, infection, or the recapture rates of released mosquitoes).

An ability to characterize the age of mosquito populations could provide cost-effective and compelling entomological evidence for the potential epidemiological impacts of vector control. The average age of a mosquito population is the most important determinant of vectorial capacity and the likelihood of disease transmission. Yet, despite decades of research, defining the age of a wild-caught mosquito remains a challenging, impractical, and unreliable process. Emerging chemometric and existing transcriptional approaches may overcome many of the limitations of current morphological techniques, but their utility in terms of field-based monitoring programmes remains largely untested. Herein, we review the potential advantages and disadvantages of new and existing age-grading tools in an operational context.

Section snippets

New Regulatory Requirements and the Importance of Ageing Mosquito Populations

The increasing incidence of mosquito-borne diseases, such as dengue, chikungunya, and Zika, the failure of vaccine development programmes, and stalled efforts to reduce the global malaria burden have underscored the continuing requirement for new, effective vector-control tools and accelerated research and development efforts on this topic. New interventions include the release of Wolbachia-infected mosquitoes to block virus transmission [1] or sterilize vectors [2], genetic manipulations that

Recognizing a Need for a Reliable Age-Grading Method

Classifying a mosquito’s chronological age by examining qualitative or quantitative changes in its reproductive morphology is one of the oldest and most widely acknowledged methods of age grading. Its adoption was driven by its apparent technical simplicity: dissection tools, a steady pair of hands, and a light microscope were all that was required. The simplest of these microscopy techniques, the ovary tracheation method, distinguishes whether a female mosquito is nulliparous (has not laid

Transition to the Biochemical Age

The identification of age-dependent changes to particular biochemical signals in mosquitoes marked a move from subjective, dissection-based approaches to the adoption of contemporary analytical methods. Two classes of biochemicals, shown to vary with age, include the pteridines (pigments common in the wings and eyes of insects) and cuticular hydrocarbons, common in the mosquito integument.

Although initial studies isolated individual pteridines using high-pressure liquid chromatography (HPLC),

Genes and Proteins

Transcriptional profiling provided the next potential advance in mosquito age grading [23,51]. This measures gene expression using reverse transcriptase quantitative PCR (RT-qPCR), and it may be used to explore correlations between the transcription products of multiple genes and age-dependent expression patterns. Age-responsive genes identified in the model insect Drosophila melanogaster were used to find related homologues in Ae. aegypti mosquitoes whose expression subsequently proved to be

Chemometrics: NIRS and MIRS

In the last decade, one of the most widely reported innovations for differentiating a variety of mosquito traits (age, species, and infection) has been the use of NIRS and MIRS. NIRS measures the absorption of organic compounds within a sample using an electromagnetic spectrum in the near-infrared region (350–2500 nm) [28], and subsequent chemometric analysis may detect compositional differences between samples according to the near-infrared energy absorbed [27]. The derived spectra are

Areas for Consideration

A major challenge for contemporary analytical methods of mosquito age prediction is to develop models that are robust to the variance induced by field environments. Currently only a handful of studies have attempted to validate emerging chemometric [64] and molecular [22,32,50,55] approaches against wild-caught adult mosquitoes. Most proof-of-principle studies restrict themselves to validating models against established laboratory colonies or field-sourced larvae reared to adults [28,61,62].

Concluding Remarks

Calls for the rigorous evaluation of vector-control programmes have become louder in response to emerging pandemics of arboviral diseases (dengue, chikungunya, Zika), the continued commitment to malaria elimination across diverse habitats, and the strategic priorities of the new WHO Global Vector Control Response [10]. Unfortunately, the tools that we have for characterizing the entomological drivers of transmission are limited and, although the need for standard measures of the components of

Acknowledgements

G.J.D., O.T.W.O., and T.S.C. were supported by USAID grant AID-OAA-F-16-00094. T.S.C. received additional support from the UK Medical Research Council (MRC) grant MR/P01111X/1.

Glossary

Age grading
a method of determining the age structure of a target population by analysing morphological, biochemical, or genetic factors that are age-related (chronologically or physiologically).
Chemometrics
the use of mathematical and statistical methods to design optimal models for the interpretation of chemical data. It is the basis for interpreting hydrocarbon signatures, gene expression, or NIRS/MIRS outputs for the purposes of mosquito age grading.
Daily survival probability
the probability

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