Surveillance on mosquitoes is important in determining factors related to disease transmission and can help to prioritize areas and seasons for vector control. It can also be used to monitor the impact of interventions and allow detection of increases in vector density.
Larval and pupal surveys can be conducted by trained field technicians, who inspect sources of standing water for the presence of immature mosquito. Standard dipping techniques should be used to sample the water, and if they are, the stage of larval development and the number of larvae per dip can be recorded[1]. Typically 3 – 5 dips are made per site.
Depending on the purpose of the survey, the technician may then be required to use control measures, such as emptying the water source or treating it with biological or chemical control, or take no action if the survey is being done as part of monitoring the effects of an intervention.
A set of indices has been developed for surveillance of Aedes, with index thresholds being associated with risk of disease transmission[2].
- House index (HI): percentage of houses infested with larvae and/or pupae.
- Container index (CI): percentage of water-holding containers infested with larvae or pupae.
- Breteau index (BI): number of positive containers per 100 houses inspected.
- Pupa index (PI): number of pupae per 100 houses inspected.
Whilst these indices can have value for monitoring the effects of interventions and changes over time, caution should be taken extrapolating larval densities and disease transmission. A systematic review found little evidence of quantifiable associations between vector indices and dengue transmission that could reliably be used for outbreak prediction[3,4], and it has been argued that indices correlate poorly with abundance of adult mosquitoes.
References
- Vector Disease Control International. Larval Mosquito Surveillance and Control Methods. 2015 September. Available at:
http://www.vdci.net/blog/larval-mosquito-surveillance-and-control-methods - World Health Organisation and TDR. Dengue guidelines for diagnosis, treatment, prevention and control. 2009 April.
https://apps.who.int/iris/bitstream/handle/10665/44188/9789241547871_eng.pdf - Bowman LR, Runge-Ranzinger S, McCall PJ. Assessing the relationship between vector indices and dengue transmission: a systematic review of the evidence. PLoS neglected tropical diseases. 2014 May 8;8(5):e2848.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4014441/ - Focks, DA & UNDP/World Bank/WHO Special Programme for Research and Training in Tropical Diseases. A review of entomological sampling methods and indicators for dengue vectors. World Health Organisation. 2004.
https://apps.who.int/iris/bitstream/handle/10665/68575/TDR_IDE_DEN_03.1.pdf?sequence=1&isAllowed=y