Evaluation of the Time Course on the Effectiveness of WHO Standard Pyrethroid and Carbamate Impregnated Test Papers against Anopheles stephensi, the Main Malaria Vector in Iran.

Background: Anopheles stephensi is a major vector of malaria in some parts of the world. A standard method for determining resistance in adult mosquito populations is the bioassay test recommended by the world health organization (WHO). The papers used in this method have an expiry date. This study aimed to determine the effectiveness of outdated susceptibility test papers for use in insecticide resistance monitoring programs.

Methods: Beech and Bandar Abbas strains of An. stephensi were reared in the insectary. Permethrin 0.75%, Deltamethrin 0.05%, and Bendiocarb 0.1% impregnated test papers prepared by Universiti Sains Malaysia were used. Probit analysis was used to analyze the results and prepare time-mortality regression lines of LT₅₀ and LT₉₀.

Results: There was a difference in the mortality of both tested strains of An. stephensi was exposed to all tested insecticides. Both expired and not expired Permethrin and Deltamethrin papers induced 100% mortality at the diagnostic time (60min), but their insecticidal properties were reduced gradually in serial times. The highest efficacy of test papers was in the first trimester after the expiry date and decreased over time.

Conclusion: At the diagnostic time of 60 minutes, the mortality rate of both dated and expired papers was 100% in the pyrethroid insecticides, even three years after expiry dates, if stored in the package provided by the producer, in a refrigerator. This value was reduced to less than 100% in the expired papers of Bendiocarb comparing the dated papers that induced 100% mortality.