Evaluating preservation methods for identifying Anopheles gambiae s.s. and Anopheles arabiensis complex mosquitoes species using near infra-red spectroscopy

Authors: MAYAGAYA, VALERIANA - Ifakara Health Institute; NTAMATUNGIRO, ALEX - Ifakara Health Institute; MOORE, SARAH - Ifakara Health Institute; WIRTZ, ROBERT - University Of Basel; Dowell, Floyd; MAIA, MARTA - Ifakara Health Institute

Submitted to: Parasites & Vectors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/14/2015
Publication Date: 1/27/2015
Publication URL: http://handle.nal.usda.gov/10113/60917
Citation: Mayagaya, V.S., Ntamatungiro, A.J., Moore, S.J., Wirtz, R.A., Dowell, F.E., Maia, M.F. 2015. Evaluating preservation methods for identifying Anopheles gambiae s.s. and Anopheles arabiensis complex mosquitoes species using near infra-red spectroscopy. Parasites & Vectors. 8(60):1-6. DOI: http://dx.doi.org/10.1186/s13071-015-0661-4.

Interpretive Summary: Determining age and species of preserved mosquitoes that transmit malaria is important to evaluate mosquito control methods. Near infra-red spectroscopy (NIRS) has been successfully used on fresh and RNAlater® preserved mosquitoes to determine species and age. However, no cheaper or more convenient preservation methods have been tested. The aim of this study was to compare different preservation methods that are cheaper and more field-friendly over a long period of storage time. Mosquitoes of each species were placed in sample tubes and preserved using one of the following preservation methods: (i) refrigeration at 4ºC, (ii) freezing at -20ºC, (iii) drying over a silica-gel desiccant, (iv) inclusion in RNAlater® at room temperature, and (v) inclusion in RNAlater® at 4ºC. Mosquitoes were preserved for up to 50 weeks before they were scanned. Refrigeration at 4°C was the only preservation method that up to 32 weeks of storage did not result in predicted values that were significantly less than that obtained from fresh insects. However, refrigerated samples did not give meaningful results at 50 weeks. Desiccation over silica gel was the best preservation method when storing up to 50 weeks.

Technical Abstract: Near infra-red spectroscopy (NIRS) has been successfully used on fresh and RNAlater® preserved Anopheles gambiae complex to identify sibling species and age. No preservation methods other than RNAlater® have been tested to preserve mosquitoes for species identification using NIRS. However, RNAlater® is not the most operational preservative for field settings, since it is expensive, requires basic laboratory conditions for storage and is not widely available in sub-Saharan Africa. The aim of this study was to compare different preservation methods that are cheaper and more field-friendly over a long period of storage time. In this study we describe the use of NIRS to identify sibling species of preserved An. gambiae s. s. and An. arabiensis. Mosquitoes of each species were placed in sample tubes and preserved using one of the following preservation methods: (i) refrigeration at 4ºC, (ii) freezing at -20ºC, (iii) drying over a silica-gel desiccant, (iv) inclusion in RNAlater® at room temperature, and (v) inclusion in RNAlater® at 4ºC. Mosquitoes were preserved for 1, 4, 10, 32 and 50 weeks before they were scanned. Refrigeration at 4°C was the only preservation method that up to 32 weeks of storage did not result in predicted values that were significantly less than that obtained from fresh insects. However, refrigerated samples did not give meaningful results at 50 weeks. Desiccation over silica gel was the best preservation method when storing up to 50 weeks, with partial least squares regression cross-validation of >80% . Prediction data values were analysed using a generalized linear model. NIRS can be used to identify species of desiccated Anopheles gambiae s.s. and Anopheles arabiensis for up to 50 weeks storage duration with more than 80% accuracy.