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Haiti (Les Cayes)
- Rapid
emergance of insecticide resistance.
Purpose
- To demonstrate
combined use of bioassay and biochemical assay techniques.
- To show
how these methods may be used to document rise and spread of a resistance
focus.
- To illustrate
how a malaria mosquito control cycle can select for higher levels of
insecticide resistance.
Background
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Figure
1 shows map of study area.
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In early
1985, sample Anopheles albimanus mosquitoes from the Les Cayes
area on the southern peninsula of Haiti showed evidence of an elevated
esterase resistance mechanism. A previous baseline run in the area in
1981 had shown complete susceptibility in the mosquito populations. Intensive
investigations of the area were done using bioassay and biochemical techniques
in May and October, 1985. These periods of sampling coincided with the
beginning and end of an intensive control cycle in which fenitrothion
was sprayed onto the walls of huts.
Data
| Baseline
data collected in 1981 showed a completely susceptible mosquito population.
Note the data are normally distributed and no individuals showed esterase
levels above the threshold absorbance of 0.9 when read at 550 nanometers.
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Les
Cays 1981
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| In
May 1985, you can see that the former normal distribution has become
skewed to higher absorbance values. A small percentage of the population
shows absorbance values above the threshold of 0.9. This shows a small
subset of the population is less succeptible. |
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May
1985
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| In
October 1985, following anintensive malaria spray campaign, resistance
levels rose to more than 50%. This is a clear indication disease control
measures can sometimes lead to amplification of resitance in vectors
such as mosquitoes. |
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October
1985
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| Figure
3, shows the same results expressed in bioassays of mosquitoes. These
bioassay data are consistent with the biochemical data presented above.
Both bioassay and biochemical data that should be mutually supportive
before you reach conclusions on resistance levels. |
Figure
3 |
Conclusions
- Both
bioassay and biochemical assay methods gave a consistent measurement
of insecticide resistance levels.
- The resistance
was associated with elevated esterase levels.
- The fenitrothion
spray cycle significantly increased the level of resistance to this
insecticide in Anopheles albimanus mosquitoes from the Les Cayes
region of Haiti.
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