In 2006 Zanzibar Malaria Control Programme re-introduced the Indoor Residual Spraying in all households except in Stone town and increase coverage with Long Lasting Insecticidal Nets to pregnant women and under-fives. IRS and LLINs were treated with the same class of pyrethroid insecticide. Baseline information on insecticide resistance on malaria vectors in Zanzibar was conducted in 2008 whereby pyrethroids and other classes of insecticides were found effective. Due to a predictable increase in insecticide selection pressure on the vector as a result of intensive use of LLINs and application of IRS, insecticide resistance monitoring program has been conducted for the past ten years to investigate the susceptibility status of local malaria vectors so that control efforts are not compromised.
Based on the WHO guidelines, insecticide resistance monitoring has to be done at least after every 12 months. Two years after the baseline monitoring, pyrethroid resistance was detected in Pemba islands in 2010 and spread to Unguja in subsequent years. Molecular analysis on tested mosquitoes has shown that resistance is more increased in Anopheles arabiensis which is also currently the main malaria vector in Zanzibar. In the subsequent insecticide resistance monitoring assays, the results have shown the persistence of pyrethroid resistance across Zanzibar, however, the intensity assays on pyrethroids revealed that resistance in Pemba is much higher than Unguja. However, malaria vectors in Zanzibar are still susceptible to Bendiocarb and pirimiphos-methyl.
Following the detection of pyrethroid resistance in 2010, an insecticide resistance management plan was developed for IRS and rotation of insecticides with different modes of action was proposed and adopted by the Zanzibar Malaria Elimination Program. Pyrethroids were replaced by Bendiocarb in 2011 and Actellic 300CS replaced Bendiocarb from 2014 to date. This IRM strategy has allowed ZAMEP to continue its progress towards the goal of malaria elimination in Zanzibar.