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Establishment of a large semi-field system for experimental study of African malaria vector ecology and control in Tanzania.

Ferguson HM, Ng'habi KR, Walder T, Kadungula D, Moore SJ, Lyimo I, Russell TL, Urassa H, Mshinda H, Killeen GF, Knols BG - Malar. J. (2008)

Bottom Line: The development of Semi-Field Systems (SFS) has been proposed as the best mechanism for bridging this gap.The SFS at Ifakara was completed and ready for use in under two years.Preliminary observations indicate that realistic and repeatable observations of anopheline behaviour are obtainable within the SFS, and that habitat and climatic features representative of field conditions can be simulated within it.

View Article: PubMed Central - HTML - PubMed

Affiliation: Division of Infection and Immunity, University of Glasgow, G128TA Glasgow, UK. H.Ferguson@bio.gla.ac.uk

ABSTRACT

Background: Medical entomologists increasingly recognize that the ability to make inferences between laboratory experiments of vector biology and epidemiological trends observed in the field is hindered by a conceptual and methodological gap occurring between these approaches which prevents hypothesis-driven empirical research from being conducted on relatively large and environmentally realistic scales. The development of Semi-Field Systems (SFS) has been proposed as the best mechanism for bridging this gap. Semi-field systems are defined as enclosed environments, ideally situated within the natural ecosystem of a target disease vector and exposed to ambient environmental conditions, in which all features necessary for its life cycle completion are present. Although the value of SFS as a research tool for malaria vector biology is gaining recognition, only a few such facilities exist worldwide and are relatively small in size (< 100 m2).

Methods: The establishment of a 625 m2 state-of-the-art SFS for large-scale experimentation on anopheline mosquito ecology and control within a rural area of southern Tanzania, where malaria transmission intensities are amongst the highest ever recorded, is described.

Results: A greenhouse frame with walls of mosquito netting and a polyethylene roof was mounted on a raised concrete platform at the Ifakara Health Institute. The interior of the SFS was divided into four separate work areas that have been set up for a variety of research activities including mass-rearing for African malaria vectors under natural conditions, high throughput evaluation of novel mosquito control and trapping techniques, short-term assays of host-seeking behaviour and olfaction, and longer-term experimental investigation of anopheline population dynamics and gene flow within a contained environment that simulates a local village domestic setting.

Conclusion: The SFS at Ifakara was completed and ready for use in under two years. Preliminary observations indicate that realistic and repeatable observations of anopheline behaviour are obtainable within the SFS, and that habitat and climatic features representative of field conditions can be simulated within it. As work begins in the SFS in Ifakara and others around the world, the major opportunities and challenges to the successful application of this tool for malaria vector research and control are discussed.

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Average hourly temperatures at ground-level within the central section of the SFS and a nearby site outside of the SFS (3 m away) from May 9 – 14th 2008.
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Figure 5: Average hourly temperatures at ground-level within the central section of the SFS and a nearby site outside of the SFS (3 m away) from May 9 – 14th 2008.

Mentions: A primary aim was to create climatic conditions within the SFS representative of the natural environment within the Kilombero region. Initial consultations with the greenhouse manufacturers indicated that the netting walls would allow temperatures inside the SFS to equilibrate with those outside. However, hot temperatures substantially higher than what is generally deemed acceptable for An. gambiae and An. arabiensis survival and reproduction (e.g. > 30°C for several hours each day) were soon observed within the SFS. These periods of high temperature, however, were similar to those of ambient conditions nearby but outside of the SFS where temperatures at ground level exceed 40°C up to 8 hours each day during the hot rainy season (Figure 5). Thus, although temperatures within the SFS were above the threshold for adult mosquito survival for periods of the day, they did not in general differ in mean or variability from those experienced in the nearby environment (e.g May 9–14th 2008: mean temperature inside SFS: 34.24°C ± 10.64°C SD, mean temperature outside the SFS: 34.33°C ± 11.20 SD). For mosquitoes to survive periods of excessively high temperatures both in nature and within the SFS, environmental refugia of substantially lower and less variable temperatures such as houses must be available [65]. The simple shaded refugia that were constructed within several areas of the SFS successfully reduced temperatures to within the acceptable range for adult and larval survival (Figure 6). The average temperature within the mud-walled house in the central SFS section was 3.5°C lower than within exposed areas of the SFS, and was substantially less variable (Table 2). Notably, temperatures inside the mud house did not exceed 35°C which is a critical threshold above which An. arabiensis in the laboratory begin to exhibit avoidance behaviour [66]. At 29.20°C (± 3.29°C), the average temperature in our artificial larval habitat was also within the natural range observed in An. gambiae s.l. aquatic habitats in east Africa, and did not exceed the upper tolerable limit of 40°C [67] (Table 2, Figure 6). The construction of a simple thatched roof over the insectary section of the SFS reduced temperatures by approximately 4°C in comparison to exposed areas of the SFS (Table 2, Figure 6), and considerably reduced the maximum temperature from 51.91°C to 34.69°C. Thus the climatic conditions within the SFS successfully represented the range of temperature extremes experienced in nearby field conditions, while providing realistic environmental refugia with temperatures appropriate for mosquito growth, survival and reproduction.


Establishment of a large semi-field system for experimental study of African malaria vector ecology and control in Tanzania.

Ferguson HM, Ng'habi KR, Walder T, Kadungula D, Moore SJ, Lyimo I, Russell TL, Urassa H, Mshinda H, Killeen GF, Knols BG - Malar. J. (2008)

Average hourly temperatures at ground-level within the central section of the SFS and a nearby site outside of the SFS (3 m away) from May 9 – 14th 2008.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2543042&req=5

Figure 5: Average hourly temperatures at ground-level within the central section of the SFS and a nearby site outside of the SFS (3 m away) from May 9 – 14th 2008.
Mentions: A primary aim was to create climatic conditions within the SFS representative of the natural environment within the Kilombero region. Initial consultations with the greenhouse manufacturers indicated that the netting walls would allow temperatures inside the SFS to equilibrate with those outside. However, hot temperatures substantially higher than what is generally deemed acceptable for An. gambiae and An. arabiensis survival and reproduction (e.g. > 30°C for several hours each day) were soon observed within the SFS. These periods of high temperature, however, were similar to those of ambient conditions nearby but outside of the SFS where temperatures at ground level exceed 40°C up to 8 hours each day during the hot rainy season (Figure 5). Thus, although temperatures within the SFS were above the threshold for adult mosquito survival for periods of the day, they did not in general differ in mean or variability from those experienced in the nearby environment (e.g May 9–14th 2008: mean temperature inside SFS: 34.24°C ± 10.64°C SD, mean temperature outside the SFS: 34.33°C ± 11.20 SD). For mosquitoes to survive periods of excessively high temperatures both in nature and within the SFS, environmental refugia of substantially lower and less variable temperatures such as houses must be available [65]. The simple shaded refugia that were constructed within several areas of the SFS successfully reduced temperatures to within the acceptable range for adult and larval survival (Figure 6). The average temperature within the mud-walled house in the central SFS section was 3.5°C lower than within exposed areas of the SFS, and was substantially less variable (Table 2). Notably, temperatures inside the mud house did not exceed 35°C which is a critical threshold above which An. arabiensis in the laboratory begin to exhibit avoidance behaviour [66]. At 29.20°C (± 3.29°C), the average temperature in our artificial larval habitat was also within the natural range observed in An. gambiae s.l. aquatic habitats in east Africa, and did not exceed the upper tolerable limit of 40°C [67] (Table 2, Figure 6). The construction of a simple thatched roof over the insectary section of the SFS reduced temperatures by approximately 4°C in comparison to exposed areas of the SFS (Table 2, Figure 6), and considerably reduced the maximum temperature from 51.91°C to 34.69°C. Thus the climatic conditions within the SFS successfully represented the range of temperature extremes experienced in nearby field conditions, while providing realistic environmental refugia with temperatures appropriate for mosquito growth, survival and reproduction.

Bottom Line: The development of Semi-Field Systems (SFS) has been proposed as the best mechanism for bridging this gap.The SFS at Ifakara was completed and ready for use in under two years.Preliminary observations indicate that realistic and repeatable observations of anopheline behaviour are obtainable within the SFS, and that habitat and climatic features representative of field conditions can be simulated within it.

View Article: PubMed Central - HTML - PubMed

Affiliation: Division of Infection and Immunity, University of Glasgow, G128TA Glasgow, UK. H.Ferguson@bio.gla.ac.uk

ABSTRACT

Background: Medical entomologists increasingly recognize that the ability to make inferences between laboratory experiments of vector biology and epidemiological trends observed in the field is hindered by a conceptual and methodological gap occurring between these approaches which prevents hypothesis-driven empirical research from being conducted on relatively large and environmentally realistic scales. The development of Semi-Field Systems (SFS) has been proposed as the best mechanism for bridging this gap. Semi-field systems are defined as enclosed environments, ideally situated within the natural ecosystem of a target disease vector and exposed to ambient environmental conditions, in which all features necessary for its life cycle completion are present. Although the value of SFS as a research tool for malaria vector biology is gaining recognition, only a few such facilities exist worldwide and are relatively small in size (< 100 m2).

Methods: The establishment of a 625 m2 state-of-the-art SFS for large-scale experimentation on anopheline mosquito ecology and control within a rural area of southern Tanzania, where malaria transmission intensities are amongst the highest ever recorded, is described.

Results: A greenhouse frame with walls of mosquito netting and a polyethylene roof was mounted on a raised concrete platform at the Ifakara Health Institute. The interior of the SFS was divided into four separate work areas that have been set up for a variety of research activities including mass-rearing for African malaria vectors under natural conditions, high throughput evaluation of novel mosquito control and trapping techniques, short-term assays of host-seeking behaviour and olfaction, and longer-term experimental investigation of anopheline population dynamics and gene flow within a contained environment that simulates a local village domestic setting.

Conclusion: The SFS at Ifakara was completed and ready for use in under two years. Preliminary observations indicate that realistic and repeatable observations of anopheline behaviour are obtainable within the SFS, and that habitat and climatic features representative of field conditions can be simulated within it. As work begins in the SFS in Ifakara and others around the world, the major opportunities and challenges to the successful application of this tool for malaria vector research and control are discussed.

Show MeSH
Related in: MedlinePlus