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Background

Dengue fever is a major public health issue in Cambodia, with over 185,000 cases estimated annually. Transmission occurs primarily through the Aedis aegypti vector, which favours environments where water storage is abundant and solid waste disposal is deficient. This vector can be targeted through insecticide treatments. This study characterised the insecticide resistance status of Aedes aegypti across urban and rural locations, evaluating its susceptibility to temephos, permethrin and deltamethrin in accordance with World Health Organisation instructions.

Methods

Four geographical areas in Cambodia were selected for field sample mosquito collections (Phnom Penh, Kampong Cham, Battambang and Siem Reap), with two urban villages and two rural villages selected as collection points in each. Twenty-five households were randomly selected within each village and all containers were inspected for larvae and pupae. In accordance with WHO instructions, late third instar larvae of F1 generation were used for determining the resistance of mosquito larvae to temephos. For both larvae and adult assays, a USDA (United States Department of Agriculture) reference susceptible strain was used as positive and negative control with water and ethanol in plastic beakers.

Results

Larval Bioassays

The highest LC50 and LC90 values were obtained with Battambang urban populations (LC50 = 0.125 ± 0.004 mg/L and LC90 = 0.221 ± 0.008 mg/L) and Kampong Cham. In Phnom Penh and Siem Reap, the LC50 and LC90 were lowest with LC50 values ranging between 0.012 mg/L (Siem Reap rural) and 0.020 mg/L (Phnom Penh rural).

Adult Bioessays

Results showed a very high level of resistance to permethrin regardless of province or rural/urban classification. The
average mortality to permethrin at the WHO diagnostic dose is 2.22% ± 0.02% for all the populations. While all populations showed resistance to permethrin, six of the eight populations showed no mortality to permethrin at all. Seven of the 8 field populations had a percentage less than 90% of mortality due to deltamethrin, meaning that these populations are resistant. The average mortality of Ae aegypti populations from Phnom Penh and Siem Reap provinces ranged between 4.0% and 8.3% only.

Conclusions

Our results and those of neighboring countries are alarming. From a regional point of view, it seems essential to rapidly change control methods and replace temephos with another larvicide that remains to be determined. Finally, and perhaps most worrying, it seems that in the event of an epidemic the adulticides used in the Southeast Asia region are no longer effective. We must quickly find an alternative.

Published in Asia Pacific Journal of Public Health

Citation: Boyer, Sébastien, Sergio Lopes, Didot Prasetyo, John Hustedt, Ay Sao Sarady, Dyna Doum, Sony Yean et al.

Country: Cambodia