Evolution of insecticide resistance and its mechanisms in Anopheles stephensi in the WHO Eastern Mediterranean Region

Background: While Iran is on the path to eliminating malaria, the disease with 4.9 million estimated cases and 9300

estimated deaths in 2018 remains a serious health problem in the World Health Organization (WHO) Eastern Mediterranean

Region. Anopheles stephensi is the main malaria vector in Iran and its range extends from Iraq to western China.

Recently, the vector invaded new territories in Sri Lanka and countries in the Horn of Africa. Insecticide resistance in

An. stephensi is a potential issue in controlling the spread of this vector.

Methods: Data were collated from national and international databases, including PubMed, Google Scholar, Scopus,

ScienceDirect, SID, and IranMedex using appropriate search terms.

Results: Indoor residual spaying (IRS) with DDT was piloted in Iran in 1945 and subsequently used in the malaria

eradication programme. Resistance to DDT in An. stephensi was detected in Iran, Iraq, Pakistan, and Saudi Arabia in

the late 1960s. Malathion was used for malaria control in Iran in 1967, then propoxur in 1978, followed by pirimiphosmethyl

from 1992 to 1994. The pyrethroid insecticide lambda-cyhalothrin was used from 1994 to 2003 followed

by deltamethrin IRS and long-lasting insecticidal nets (LLINs). Some of these insecticides with the same sequence

were used in other malaria-endemic countries of the region. Pyrethroid resistance was detected in An. stephensi in

Afghanistan in 2010, in 2011 in India and in 2012 in Iran. The newly invaded population of An. stephensi in Ethiopia

was resistant to insecticides of all four major insecticide classes. Different mechanisms of insecticide resistance, including

metabolic and insecticide target site insensitivity, have been developed in An. stephensi. Resistance to DDT was

initially glutathione S-transferase based. Target site knockdown resistance was later selected by pyrethroids. Esterases

and altered acetylcholinesterase are the underlying cause of organophosphate resistance and cytochrome p450s

were involved in pyrethroid metabolic resistance.

Conclusions: Anopheles stephensi is a major malaria vector in Iran and many countries in the region and beyond.

The species is leading in terms of development of insecticide resistance as well as developing a variety of resistance

mechanisms. Knowledge of the evolution of insecticide resistance and their underlying mechanisms, in particular,

are important to Iran, considering the final steps the country is taking towards malaria elimination, but also to other

countries in the region for their battle against malaria. This systematic review may also be of value to countries and

territories newly invaded by this species, especially in the Horn of Africa, where the malaria situation is already dire.