Validation of a method for the dry preservation and rehydration of Anopheles gambiae sensu lato for parity analysis to assess the impact of vector control measures in the field

Abstract

Background: As the control of malaria remains heavily dependent on vector management interventions, it is important to understand the impact of these on mosquito populations. Age-grading is a valuable tool for this; however, logistical challenges in remote, resource-poor areas make current methodologies difficult to incorporate into clinical trials and routine surveillance. Our aim was to validate a methodology that could be easily implemented in such settings. Using dried mosquito specimens instead of freshly killed ones, we validated the commonly used ovarian tracheation technique for assessing population age structure.

Methods: Laboratory-reared Anopheles coluzzii mosquitoes with known parity status were dry preserved in silica gel for up to 12 weeks and rehydrated prior to parity assessment. The results were compared to parity results for freshly killed mosquitoes from the same colony. Preserved, field-caught Anopheles gambiae sensu lato (s.l.) from Guinea-Bissau were assessed by three different assessors blinded to each other's scores. An overall index of agreement was calculated using inter-rater reliability of all assessor pairings. The impact of preservation time was investigated using a one-way ANOVA to look for differences in assessor agreement over three time periods.

Results: The parity status was correctly identified for 90% of dry preserved and rehydrated insectary-reared An. coluzzii and for 98% of freshly killed insectary-reared An. coluzzii. The inter-rater reliability was highest (0.94) for freshly killed An. coluzzii. The results for all time points showed excellent strength of agreement between assessors. For field-caught An. gambiae s.l., the overall index of agreement between all three assessors was 0.86 (95% confidence interval 0.78-0.93), indicating almost perfect agreement. There was no significant difference between assessor agreement between time frames.

Conclusions: Dry preserving and rehydrating Anopheles mosquitoes provides an alternative to using freshly killed mosquitoes to assess the efficacy of a control intervention in remote settings where it is logistically difficult to dissect fresh specimens. This method also provides the flexibility required for parity assessment to be done on larger scales over bigger areas.

Keywords: Age-grading; Anopheles; Guinea-Bissau; Malaria; Mosquito; Parity; Vector control.