I still remember the day ten years ago in a workshop in Sudan on the establishment of the Sterile Insect Technique (SIT) project for the control of An. arabiensis in northern Sudan. The discussion was mainly about the isolation of the area, in the middle of which old Prof. Osman Abdelnour, chief entomologist of Sudan who passed away two weeks ago, raised his hand and hardly pulled his body from the chair and asked: and what about passive transportation?...
Getting back to my very busy life I still remember that very simple direct question and wonder whether its significance has been overlooked. It is not only the absence of scientific research determining the impact on Sterile Insect Technique Studies, but also more generally as a factor that might affect our understanding of the dynamics of mosquito populations in any part of the world.
With respect to Sudan the sole published paper about the isolation of northern Sudan mentioned the expectation of passive transportation of An. arabiensis mosquitoes into the control area as improbable. However, field observations from Sudan show that luggage cabinets in big buses are places that are frequently found to accommodate mosquitoes, in addition to areas underneath seats inside “air conditioned” buses. The journey by bus between Khartoum in central Sudan and Khariema in northern Sudan through the desert takes around 4.5 hours. Only one train a week travels from Khartoum to Wadi Halfa at the Egyptian border. Despite the trip taking 12 hours Egyptian authorities spray it with insecticide when the train reaches Wadi Halfa to limit the possibility that passive transportation disrupts their successful An. gambiae control programme. This leads me to wonder how the numerous short bus trips might compare to 12 hours by train in terms of the risk of passive transportation.
Another obvious potential passive transportation route are airplanes. Luckily, in northern state they are not favoured as it is very costly in comparison to buses and small vehicles. However, the image of an air hostess on certain plane routes taking two bottles of insecticide fluid and spraying the inside of a plane is one familiar to many travelers around the world. This activity has always made me wonder about the scientific basis of such precautions (and what insects are being targeted).
It has been stated that “SIT site[s] should be an island or an area on the main land as isolated as possible so that reinfestation does not occur or is so slight that it does not distort the evaluation of the experiment”1. Conceptually it seems that passive dispersion may be more of an issue when the objective is local stable eradication (where releases may stop after success) and might be less important where the objective is long term suppression (based on continuous long-term releases of sterile males). However despite my sustained interest in passive transportation I consider that there is a lack of suitable scientific studies to determine if we underestimate or overestimate its role in SIT projects? In addition I wonder what is the evidence of the relative effectiveness of risk reduction techniques such as the spraying of planes and trains?
Personally I think that more scientific studies should be carried out to increase our understanding of passive transportation of mosquitoes to better anticipate its impact on control efforts. Primarily to identify factors which have the biggest impact on the frequency of passive transportation e.g. seasonal climatic conditions, socioeconomic factors or the design of vehicles? Once there is this type of knowledge we will be in a much better position to assess the value (if any) of current control techniques or if it is necessary to develop new ones.
Planes, trains or vehicles? This is a plea for more scientific input on passive transport.
1. Knipling E F, Laven H, Craig GB, Pal R, Smith CN, Brown AWA: genetic control of insects of public health importance. Bull World Health Organ 1968, 38: 421-438.
The views expressed in this article are the authors' alone and not intended to reflect those of their organizations.
Rasha Azrag is a medical entomologist working in the department of Zoology/ University of Khartoum, Sudan and used to teach basic entomology courses to undergraduate students and molecular entomology to master students in the Medical Entomology and Vector Control program. She has experience from working in different vector control programmes, from basic classic control methods to the use of genetic methods.
Guy Reeves is a research scientist at the Max Planck Institute for Evolutionary Biology in Plön, Germany. Part of his research involves the exploration of genetic methods to control vectored diseases.
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