Thank you for the opportunity to reply to the letter by Arya regarding our study recently published in the American Journal of Tropical Medicine and Hygiene.1 Dr. Arya has raised some interesting points about storage of specimens for RNA extractions, particularly the use of commercially available RNA and DNA preservatives (RNAlater® and GenoFix, respectively). Indeed, when the study was in its infancy, these or similar RNA preservatives were considered. However, there are a number of difficulties associated with using such chemicals. 1) Large volumes (perhaps liters) of the preservatives would need to be used to store the thousands of mosquitoes than can be collected in the mosquito trap at any time. 2) The cost of such preservatives (currently AUS $310 for 500 mL of RNAlater®) and the large volumes required to store large numbers of mosquitoes would make their use very expensive. 3) Mosquitoes stored in liquid are likely to be difficult to identify to species. 4) The use of such chemicals in mosquito traps that are maintained and handled by lay personnel without an appropriate scientific background is unlikely to meet Occupational Health and Safety Regulations. 5) The preserving chemicals may make transport of field-collected mosquitoes via the postal system difficult. 6) Mosquitoes are held for one week in a mosquito trap that is not sealed from the external environment, and as such are exposed to high temperature and humidity (not room temperature), and any liquid would not be protected from evaporation. Furthermore, the use of alcohol-based preservatives would not be recommended in a mosquito trap that combusts propane for generation of power and carbon dioxide (e.g., MosquitoMagnet™; American Biophysics Corporation, East Greenwich, RI) that is currently being tested in northern Queensland and northern Western Australia).
For these reasons, safer and more economically viable methods were chosen and tested for their efficacy at preserving mosquito specimens and in particular, virus RNA. Thymol is a commonly used insect preservative that is readily available and relatively inexpensive. Commercially available RNA preservatives certainly would be useful in many instances. However, it is difficult to imagine that they would be suitable for our purposes, given the expense and logistical difficulties outlined above.
Johansen CA, Hall RA, van den Hurk AF, Ritchie SA, Mackenzie JS, 2002. Detection and stability of Japanese encephalitis virus RNA and virus viability in dead infected mosquitoes under different storage conditions. Am J Trop Med Hyg 67 :656–661.