Science & Innovation

Insecticide Resistance in Mosquito Populations in Central America

In the Americas, selective pressure on mosquito populations through intensive, extensive and indiscriminate applications has led to the development of resistance to one or more major insecticides. This poses a problem for researchers and health experts, as fighting mosquito populations is currently the only way to overcome the transmission of dengue.

Lorenzo Cáceres Carrera MSc. PhD.

The development of synthetic insecticides with a prolonged residual effect during the 1940s and 1950s provided a unique and very affordable model for vector control, becoming the main component of almost all vector control programs and leading to the abandonment of environmental interventions. Overall, the use of insecticides has played an essential role in controlling and reducing diseases; at present it is the most widely-used intervention method for reducing the populations of disease-transmitting mosquitoes. The intensive and indiscriminate use of these chemical agents led to the rapid emergence of resistance to insecticides that had once eliminated the populations of these insects. The appearance of resistance to different groups of chemical agents among pest and medically significant insect populations is one of the most serious undesired effects and technical problems in all parts of the world. However, the impact of insecticide resistance on mosquito populations and its role in the transmission of diseases is difficult to establish given that it has not yet been evaluated in the Americas. In the Americas, the selective pressure exerted on mosquito populations through intensive, extensive and indiscriminate application has led to the development of resistance to one or more insecticides among mosquito populations, making it the main technical problem affecting strategies in vector control programs.

 
The absence of resistance monitoring and management programs puts at risk the effective use of insecticides and the interruption of the transmission of diseases.

 

Little research has been carried out on insecticide resistance in mosquito populations in Central America; this research is needed to establish the behavior of the state of susceptibility to the different insecticides applied and alternated over time in vector control programs carried out in the different countries in this sub region. The earliest known research on resistance was conducted among populations of Anopheles albimanus in El Salvador in the early 1960s, with insecticides dieldrin, malathion and fenthion. During this decade, WHO reported a suspected increased tolerance of malathion in An. albimanus populations in Guatemala and Nicaragua. Research found that the decreased susceptibility to malathion in El Salvador only occurred in areas with intensive cotton cultivation. These areas were subjected to intensive applications of various organophosphorus insecticides, including trichlorfon, parathion, parathion-methyl and malathion, during six months of the year. Later, in the early 70s and 80s, research continued with dieldrin, DDT, among populations of An. albimanus in El Salvador. In recent years, the amount of research on resistance has decreased significantly in the region; only very specific research has been carried out on the resistance in certain countries, mainly related to An. albimanus and Ae. Aegypti. This research is not the result of a resistance monitoring program carried out by vector control programs. The absence of resistance monitoring and management programs in vector control programs in the region means that the effective use of insecticides and the interruption of the transmission of diseases, such as malaria, dengue, Zika and chikungunya by mosquito populations are put at risk.

 

Big Consequences: The Re-Emergence of Dengue

Today, the incidence rate and prevalence of dengue is on the rise in all tropical and subtropical endemic areas; there has been a dramatic increase in cases in the Americas in recent decades. The reduction of Ae. aegypti populations is currently the only viable option available for controlling the transmission of dengue. Vector control programs in Central America are painfully lacking in resistance monitoring, with the only available research consisting of very specific studies carried out by researchers from institutions, centers and universities on the resistance status in Ae. aegypti. Following the re-emergence of dengue in the early 1990s, chikungunya in 2014 and Zika in 2015, most countries have turned their attention to health programs and, mainly, vector control programs in their implementation of effective vector control strategies – the main tool of which is the use of insecticides to prevent the transmission of these arboviruses. However, most of these countries lack effective control over populations of Ae. aegypti, the main vector of these diseases, and it has not been ruled out that resistance to different insecticides is the main cause of this problem.

Little work has been published on the susceptibility of Ae. aegypti from Panama. One study was carried out in 1975 to determine the effectiveness of the 95% technical grade malathion applied with LECO ULV machinery mounted on a light vehicle against Ae. aegypti. The results showed a 100% mortality rate (Echevers et al., 1975). A study is also available about research carried out on Ae. aegypti strains (RIO ABAJO and VICTORIANO LORENZO) from Panama, in which resistance to pirimiphosmethyl was observed in larvae of both strains. However, they were found to be susceptible to the rest of the organophosphorus insecticides - temephos, malathion, fenitrothion, fenthion and chlorpyrifos, and to the pyrethroids deltamethrin, lambda-cyhalothrin, cypermethrin and cyfluthrin. In tests carried out on adult mosquitoes using paper impregnated with pyrethroid insecticides, both populations were found to be fully susceptible to deltamethrin, lambda-cyhalothrin, cypermethrin and beta-cyfluthrin (Bisset et al., 2003). In 2010, another study showed moderate deltamethrin resistance in the Ae. aegypti from Chitré. Tests conducted in 2013 found a high level of resistance to temephos, pirimiphosmethyl, deltamethrin and cyfluthrin (Cáceres, 2013).

Likewise, there is little information available on the behavior of the susceptibility of Ae. aegypti in Nicaragua and Costa Rica. In the latter nation, the susceptibility of Ae. aegypti to ultra-low volume applications of lambda-cyhalothrin and malathion has been proven. Research carried out in 2007 using strains of Ae. aegypti from Panama, Nicaragua and Costa Rica found variable behavior in resistance to insecticides temephos, malathion, fenthion, pirimiphosmethyl, fenitrothion and chlorpyrifos as well as to pyrethroid insecticides deltamethrin, lambda-cyhalothrin, betacypermethrin and cyfluthrin. More recent work carried out in 2013 found the same resistance behavior in Ae. aegypti from Costa Rica to temephos, chlorpyrifos, pirimiphosmethyl, lambda-cyhalothrin and moderate resistance to cypermethrin; this behavior was confirmed in another study carried out in 2014. A study published in 2016 detected resistance to deltamethrin in Ae. aegypti populations from the Caribbean region. In El Salvador, the only printed research available is a recent study in which Ae. aegypti resistance to temephos was detected, resistance to deltamethrin and cypermethrin remained under monitoring.




When it comes to resistance, the cause, intensity and importance of the fight against disease vectors can vary greatly.
 

 

Risk Factors for Insecticide Resistance

As seen in all of the investigations carried out, the Ae. aegypti populations have developed resistance to one or more insecticides; this can be due to different contributing factors. When it comes to resistance, the cause, intensity and importance in the fight against disease vectors can vary greatly. Resistance is a dynamic phenomenon that appears over very variable time lapses. It is well-known that resistance appears more slowly in some species than in others. Even within the same species, under certain conditions, resistance can develop rapidly, while under other conditions it evolves more slowly or simply never appears. This occurs in different species and in the same species when subject to different intensities of insecticide application. Currently, there are three recognized types of factors that affect the development of resistance: genetic, biological and operational. Genetic factors are related to the frequency, number and dominance of the alleles related to resistance, among other important aspects. Biological factors consist of biotic elements, mainly related to the reproduction potential and populational behavior of insects. The operational factors are the chemical agents and their application. The genetic and biological factors are intrinsic to the species and, therefore, cannot be changed. However, it is important to have knowledge of their role as this can be used to evaluate the likelihood of resistance in an insect population. Operational factors can be controlled through insecticide management programs.

 



It is important to develop knowledge about the types of resistance which may be present in a mosquito population in order to make informed decisions about the selection of an effective insecticide.

It is important to point out that the research carried out in the different countries consists of very specific studies and that they do not follow one resistance monitoring policy, program or plan established by the vector control programs in each of the countries. This situation demonstrates a lack of national policies for the correct use of insecticides; a lack of technicians with proper training on the use of susceptibility bio-assay tests; a lack of equipment, supplies, laboratories, insectariums and work with universities; and a lack of participating institutes and research centers to offer their support in all that is related to the best use of insecticides and the monitoring and management of resistance. Similarly, there is a need for health programs, and thus for the companies that produce insecticide molecules and formulas, as well as the companies that manufacture and distribute equipment for the application of insecticides, to develop a closer and continuous relationship in order to optimize the use of the insecticides, in order to protect and prolong the useful life of the efficiency of insecticide molecules.