Resistant status of Culex pipiens complex species to different imagicides in Tehran, Iran

Sara Rahimi; Hassan Vatandoost; Mohammad Reza Abai; Ahmad Raeisi; Ahmad Ali Hanafi-Bojd; Fatemeh Rafi

doi:10.4103/0972-9062.308800

RESEARCH ARTICLE

Year : 2020 | Volume : 57 | Issue : 1 | Page : 47-51

Resistant status of Culex pipiens complex species to different imagicides in Tehran, Iran

Sara Rahimi¹, Hassan Vatandoost², Mohammad Reza Abai¹, Ahmad Raeisi³, Ahmad Ali Hanafi-Bojd¹, Fatemeh Rafi¹
¹ Department of Medical Entomology and Vector Control, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran
² Department of Medical Entomology and Vector Control, School of Public Health; Department of Environmental Chemical Pollutants and Pesticides, Institute for Environmental Research, Tehran University of Medical Sciences, Tehran, Iran
³ National Program Manager for Malaria Control, Center for Communicable Diseases, Ministry of Health and Medical Education, Tehran, Iran

Date of Submission	13-Apr-2018
Date of Acceptance	01-Aug-2018
Date of Web Publication	05-Feb-2021

Correspondence Address:
Prof. Hassan Vatandoost
Department of Medical Entomology and Vector Control, School of Public Health, Tehran University of Medical Sciences, Tehran
Iran

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/0972-9062.308800

Abstract

Background & objectives: Insecticides are the most important means of controlling pests in Iran especially for Culex pipiens complex species. The rational use of insecticides largely depends on understanding the susceptibility levels of these species. The study was designed to determine the susceptibility levels of Cx. pipiens complex (field and insectary strains) to various insecticides in the city of Tehran.
Methods: The mortality rates of the field strain of Cx. pipiens complex after different exposure times to DDT (4%), bendiocarb (0.1%), propoxur (0.1%), malathion (5%), fenitrothion (1.0%), permethrin (0.75%), deltamethrin (0.05%), lambda-cyhalothrin (0.05%), etofenprox (0.5%), and cyfluthrin (0.15%) were determined. The mortality rates at the lethal time 50% (LT₅₀) and lethal time 90% (LT₉₀) values were calculated by plotting the regression line using Microsoft Office Excel software.
Results: The mortality rates of the Cx. pipiens complex after 1 h exposure to the diagnostic doses of DDT (4%), bendiocarb (0.1%), propoxur (0.1%), malathion (5%), fenitrothion (1.0%), permethrin (0.75%), deltamethrin (0.05%), lambda-cyhalothrin (0.05%), etofenprox (0.5%), and cyfluthrin (0.15%) were 12, 58, 54, 82, 54, 34, 49, 40, 17, and 44%, respectively. According to the WHO classification of susceptibility levels, both field and insectary strains of Cx. pipiens complex in Tehran were resistant to these insecticides.
Interpretation & conclusion: The results of this study showed that field Cx. pipiens complex is resistant to all the groups of insecticides used.

Keywords: Culex pipiens; insecticides; Iran; susceptibility; Tehran

How to cite this article:
Rahimi S, Vatandoost H, Abai MR, Raeisi A, Hanafi-Bojd AA, Rafi F. Resistant status of Culex pipiens complex species to different imagicides in Tehran, Iran. J Vector Borne Dis 2020;57:47-51

How to cite this URL:
Rahimi S, Vatandoost H, Abai MR, Raeisi A, Hanafi-Bojd AA, Rafi F. Resistant status of Culex pipiens complex species to different imagicides in Tehran, Iran. J Vector Borne Dis [serial online] 2020 [cited 2022 May 21];57:47-51. Available from: https://www.jvbd.org/text.asp?2020/57/1/47/308800

Introduction

The most important genus of the culicine mosquitoes is Culex. Culex pipiens (Diptera: culicidae) is known as a species complex including two species and a subspecies, Cx. pipiens, Cx. pipiens pipiens and Cx. quinquefasciatus^[1]. Because of the anthropohilic and endophilic blood feeding habits of the females, Cx. pipiens complex is closely associated with man and human habitations. Culex pipiens pipiens exists in both temperate and tropical areas, whereas Cx. quinquefasciatus can be found in the tropical regions of the world. The Cx. pipiens pipiens can be found in most parts of Iran, while Cx. quinquefasciatus exists in the southern part of the country^[2]. These mosquitoes carry a wide range of pathogens due to their specific behavior and physiology as well as their close relationship with humans. The Cx. pipiens complex has a great medical importance due to its ability to transmit arboviruses, human lymphatic filariasis, and zoonosis including Dirofilaria immitis. Also its biting and nuisance causes severe allergies in humans and other hosts and may lead to discomfort especially in urban areas^[3]. The control of vector-borne diseases strongly depends on the use of insecticides. One of the most important global strategies for reducing mosquito-borne diseases is using chemical insecticides. Chemical insecticides have been used widely since 1940. Four major groups which have been used include organochlorines, organophosphates, carbamates and pyrethroids. Nonetheless, the strong reliance on insecticides to control mosquitoes worldwide and the use of these insecticides in agriculture have created resistance in important mosquito vectors such as Anopheles gambiae, Aedes aegypti, and Cx. pipiens in recent years^[4],[5]. Resistance to insecticides is a great limitation in the implementation of vector control. Constant monitoring of mosquitoes’ resistanc to insecticides is vital in ensuring the sustainability of vector control programs. Due to the fact that there are few studies on the susceptibility of Cx. pipiens complex to insecticides, this study was designed to compare the susceptibility levels of this field strain in the capital city of Tehran with those of laboratory strain to different insecticides.

Material & Methods

Study area and sample collection

Larvae samples of Cx. pipiens complex were collected from Tehran to determine their resistance against different insecticides. For this purpose, larvae of different ages were sampled from rice fields and from the margin of open sewage canals using standard dippers. The larval collection was transported to the insectary of the School of Public Health (SPH) of Tehran University. The larvae were fed daily on some fish food until the adults emerged. The adults were collected from the cages and fed on a 10% glucose-water solution. Laboratory strain of Cx. pipiens (TEH-SPH) which was used as a reference in this study was originally collected from the capital city of Tehran and was colonized in SPH insectary.

Susceptibility tests

Two-to-three days adult female mosquitoes were kept on a 10% glucose-water solution before being subjected to the tests. Insecticide susceptibility tests were carried out according to the guidelines of World Health Organization^[3]. Insecticide susceptibility tests were performed on female mosquitoes only. The 2–3 days old unfed adult females were exposed to the following WHO insecticide-treated papers for 1 h : DDT (4%), bendiocarb (0.1%), propoxur (0.1%), malathion (5%), fenitrothion (1.0%), permethrin (0.75%), deltamethrin (0.05%), lambda-cyhalothrin (0.05%), etofenprox (0.5%), and cyfluthrin (0.15%). The susceptibility of the insects to insecticides was tested by tube test method according to the standard WHO procedure^[3]. Ideally, these diagnostic doses kill 99.9% of homozygous mosquitoes but kill a very small percentage of resistant mosquitoes. The WHO has introduced a diagnostic dose for each kind of insecticide^[6]. Each test was replicated four times for each dose of insecticide, using 20–25 females per replicate, and two tubes were considered as control. If the control mosquitoes had no mortality or their mortality rate was <5%, the test was considered as reliable, and if the control mortality rate was between 5 and 20%, the results were corrected by applying Abbott’s formula. The test was repeated when the mortality rate of the control mosquitoes^[7],[8] was >20%.

Statistical analysis

Based on WHO^[3] recommendations, three resistance classes have been defined: (i) susceptible, when after 24 h, mortality is 98–100%; (ii) possible resistant or tolerant, when mortality is between 90 and 97%; and (iii) resistant, when mortality is lower than 90%. The data were analyzed by probit analysis. Regression lines were measured by the χ² test. The mortality rates at the lethal time 50% (LT₅₀) and 90% (LT₉₀) were calculated by plotting the regression line using Microsoft Excel software, version 2007.

Ethical statement: Not applicable

Results

Susceptibility tests involved the laboratory-reared TEH-SPH strain and the field strain of Cx. pipiens complex. The results of the susceptibility tests revealed that the mortality rates of the field strains of Cx. pipiens complex after 1 h exposure to the diagnostic doses of DDT (4%), bendiocarb (0.1%), propoxur (0.1%), malathion (5%), fenitrothion (1.0%), permethrin (0.75%), deltamethrin (0.05%), lambda-cyhalothrin (0.05%), etofenprox (0.5%), and cyfluthrin (0.15%) were 12, 58, 54, 82, 54, 34, 49, 40, 17, and 44%, respectively. Also, the mortality rates of the insectary strains of Cx. pipiens complex after 1 h exposure to the diagnostic doses of the aforesaid insecticides were 15, 63, 68, 73, 63, 66, 69, 69, 28, and 56%, respectively [Table 1] and [Figure 1]a. The LT₅₀ and LT₉₀ values are presented in [Figure 1]b and [Figure 1]c.

Figure 1: Comparison of: (a) mortality rates of adults Cx. pipiens (field and insectary strains) exposed to diagnostic dose (60 min); (b) LT₅₀ of adult Cx. pipiens (field and insectary strains); and LT₉₀ of adult Cx. pipiens (field and insectary strains) to different insecticides.

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Table 1: Susceptibility levels of Cx. pipiens strains to different insecticides

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Discussion

Insecticides have played an important role in controlling disease vectors, such as mosquitoes, sandflies, fleas and lice. Resistance in important vectors against insecticides is constantly increasing worldwide^[9]. Nowadays >100 species of mosquitoes have showed resistance to at least one insecticide and >50 species are Culicinae^[10]. Resistance to organophosphates and resistance to pyrethroids in Culex vectors have been reported worldwide. Culex quinquefasciatus is an important public health pest which is responsible for the discomfort of people around the world, particularly in Africa and Asia, due to its ability to adapt to different habitats^[11]. As per the WHO report of 1963, Cx. quinquefasciatus is highly tolerant to organophosphate insecticides^[12]. Surprisingly, Cx. quinquefasciatus is becoming resistant to insecticides more quickly than most other mosquitoes^[13]. Also, WHO (1996) reported that Cx. quinquefasciatus has shown resistance to common insecticides in most countries. The Cx. quinquefasciatus, as an urban pest and vector of filariasis, has shown resistance to organochlorines, organophosphates, carbamates, and pyrethroids insecticides around the world. The first resistance of Cx. quinquefasciatus to pyrethroids was observed against permethrin in an insectary strain in California^[14]. The field strain of this mosquito was also demonstrated to be highly resistant to pyrethroid insecticides, largely because of the excessive use of insecticides in agriculture against pests in Africa^[15], Saudi Arabia^[16], and Asia^[17],[18]. However, the results of a previous study which investigated the susceptibility of field and insectary strains of Cx. quinquefasciatus in Tehran during 2004, showed that the field strain was resistant to DDT and bendiocarb, susceptible to cyfluthrin, and tolerant against other insecticides. The insectary strain in their study was susceptible to bendiocarb, malathion, permethrin, deltamethrin, lambda-cyhalothrin, etofenprox, and cyfluthrin, and was tolerant to DDT.

The comparison of the results of this study with the previous study^[19] indicates that in the 10-yr period from 2004 to 2014, Cx. pipiens complex has become resistant to most insecticides. This may be due to the excessive use of insecticides in the agriculture sector. In another study in the southeast of Iran, Cx. pipiens mosquitoes were resistant to DDT, propoxur, lambda-cyhalothrin, and cyfluthrin, while they were susceptible to malathion and tolerant to deltamethrin^[20]. The results of the Feng Cui et al^[21] study revealed that Cx. quinquefasciatus in Baan Suan is highly resistant to DDT, deltamethrin, fenitrothion, permethrin, cypermethrin, and moderately resistant to propoxur and quite susceptible to malathion (100% mortality). Culex quinquefasciatus was more resistant to deltamethrin, permethrin, and fenitrothion than Ae. aegypti, however, both of these species were susceptible to malathion^[21]. In China, Cx. pipiens complex was resistant to organochlorine, organophosphate, and pyrethroid insecticides^[22]. Akiner et al^[23] demonstrated in 2009 that Cx. pipiens was resistant to DDT in five areas in Turkey. They also showed that while the Hatay, Bircik, and Viranchir strains were tolerant against deltamethrin, permethrin, and malathion; Ankara and Antalya strains were quite resistant to the mentioned insecticides. Also Cx. pipiens was found resistant to all common insecticides in the west of the Indian Ocean^[24].

In a study by Karlekr et al^[25] in Nagpur, India, Cx. pipiens was highly resistant to diagnostic doses of DDT, deltamethrin, and lambda-cyhalothrin. In 1971, Brown and Pal^[26] described resistance as follows: resistance is an evolutionary, biochemical, physiological, and behavioral phenomenon and appears as the lack of expected mortality in the adult population which previously had a 100% mortality rate by the same dose of the insecticide. The term ‘resistance’ is not used for insects that are inherently immune to insecticides and in this case it should be said that the insecticide is ineffective^[26]. Different insecticides are directly or indirectly used in large quantities in the fields and households against vector mosquitoes and subsequently resistance occurs in the pests which makes their control more difficult. According to the reports of the Ministry of Health and Medical Education and the Ministry of Agriculture, in Iran, various types of insecticides are used in the households and agriculture to control pests. In order to postpone the development and spread of resistance, rational scientific measures such as using a mosaic of insecticides and regular monitoring of susceptibility levels of mosquitoes should be taken. In addition to these measures, farmers should be educated in relation to inappropriate and excessive use of insecticides.

Conclusion

The results of this study showed that field Cx. pipiens complex is resistant to all the groups of insecticides (DDT, bendiocarb, propoxur, malathion, fenitrothion, permethrin, deltamethrin, lambda-cyhalothrin, etofenprox, and cyfluthrin) used in this study. Therefore, integrated vector control strategies should be used and the susceptibility levels of mosquitoes should be monitored regularly to ensure the sustainability of vector control programs.

Acknowledgements

This study was part of an M.Sc. thesis supported by the Tehran University of Medical Sciences (Grant No. 20149). The authors thank Dr A. Latifi, Ph.D student of Health Education for his kind assistance during the mass collection of immature stages of Culicidae in Tehran. Authors also extend their appreciation and thanks to Mr A.H. Hosseini, and Mrs Z. Abbasi, staff members of the SPH insectary for their great efforts in mass breeding of TEH-SPH strain. This research is partially supported by Ministry of Health and Medical Education under code number of NIMAD 995633.

Conflict of interest: None

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