Preserved mosquitoes create a buzz for NIR spectroscopy

As can be imagined, determining what species an individual mosquito belongs to is quite a challenge. This is especially the case for members of the Anopheles gambiae complex, which comprises seven physically-identical species of mosquito. What makes this challenge of more than mere academic interest is that these seven species include the main transmitters of the malaria parasite in sub-Saharan Africa, particularly An. gambiae sensu stricto and Anopheles arabiensis.

Although these seven species are physically identical, they exhibit different behaviours, which is why they are classed as different species. For example, An gambiae sensu stricto prefers to suck blood from humans, whereas its sister species A. quadriannulatus prefers to suck blood from animals. Because of this difference in behaviour, different control measures for preventing the spread of malaria are required for each species, hence the importance of knowing which species are present in a certain region.

Recently, scientists have discovered that NIR spectroscopy offers a simple and relatively accurate way to determine the species of collected mosquitos, based on characteristic variations in biochemical components such as water, proteins, lipids and carbohydrates (see NIR News, 2014, 25, 4). Up to now, though, scientists have only shown that NIR spectroscopy works on either fresh mosquitoes or those preserved using an aqueous, nontoxic tissue storage reagent known as RNAlater.

Such preservation is usually required prior to transporting the mosquito samples back to the laboratory for storage and subsequent testing. However, many other simpler and cheaper preservation methods are available, such as freezing and drying with a desiccant, which are easier to perform out in the field.

Now, in a paper in Parasites & Vectors, an international team of researchers led by Valeriana Mayagaya at the Ifakara Health Institute in Tanzania has shown that NIR spectroscopy works just as well with these simpler preservation methods. Using various techniques, including freezing at -20°C, drying with a silica-gel desiccant and RNAlater, Mayagaya and his team preserved samples of both An. gambiae sensu stricto and An. arabiensis, which they stored for up to 50 weeks. For all three of these preservation techniques, NIR spectroscopy could distinguish between the two species with an accuracy above 80%, but after 50 weeks storage the average accuracy was actually higher for freezing and drying than for RNAlater.

Drying with a silica-gel desiccant is perhaps the easiest preservation technique to conduct in sub-Saharan Africa, where the electricity supply required for freezing is often erratic. So, conclude the researchers, the discovery that it works perfectly well with NIR spectroscopy should help this new way of determining mosquito species become a central component of malaria control efforts.

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