1. Introduction
Indoor residual spray (IRS) and more recently, long-lasting insecticidal nets (LLINs), have been used as the primary malaria vector control interventions due to their cost-effectiveness and protection against vectors that feed and rest mainly indoors [
1,
2]. In contrast, in Latin America, many vectors feed and rest primarily outdoors (exophagy and exophily, respectively). This behavior, together with the widespread existence of houses with incomplete walls, communities where unprotected travel-related occupations are common [
3,
4,
5], and temporally irregular application of IRS in many communities diminish the effectiveness of insecticide-based interventions throughout Latin America in relation to some other malaria endemic regions [
6,
7].
In 2007, the World Health Organization (WHO) recommendations for LLIN distribution were broadened to include all individuals in malaria endemic areas, rather than solely pregnant women and children under the age of 5 [
8]. Simultaneously, WHO guidelines called for malaria endemic nations to adopt triennial LLIN mass distribution campaigns which would allocate one LLIN for every two people per household in a given region, aspiring to universal coverage [
9]. Following these updates, mass distribution campaigns became more prevalent in Latin America, particularly in the Amazon region. In Peru, the Project for Malaria Control in Andean Border Areas (PAMAFRO) was mainly responsible for a 70% decrease in cases from 2005-2011 [
9]
. Since the program was discontinued, malaria cases have increased fairly steadily due to a decline in international and domestic funding [
10]. The sharpest increase in cases throughout the Latin American region occurred between 2014-2017, when case incidence nearly doubled [
11]; ironically, incidence has apparently decreased with the recent COVID-19 pandemic [
12]. An encouraging sign is that the Ministry Health, Peru, has adopted a new plan as of January 2022 to eliminate malaria by 2030 [
13].
Between 2010-2019, an estimated 28 malaria endemic countries (of 82) have detected insecticide resistance to all four of the most commonly used insecticides (pyrethroids, organochlorines, carbamates, organophosphates), and nearly all (73/82) have reported resistance to at least one class [
14,
15]. Worldwide, the exposure of mosquitoes to any of these classes of insecticides [
16], whether for public health or agricultural use, has the potential to be a strong selective force that favors the survival of resistant populations [
17]. In Latin America there is relatively little entomological surveillance and few published studies on IR, except for
Nyssorhynchus albimanus [
18]. The primary malaria vector in the Amazon,
Ny. darlingi, is generally susceptible to insecticides [
17,
19] and exhibits exo- and endophagic behavior, depending on local circumstances [
20]
. Nevertheless, resistance based on bioassays using WHO paper bioassays [
21] or CDC bottle bioassays [
22,
23] has been reported for
Ny. darlingi to pyrethroids and carbamates (Bolivia), pyrethroids (Brazil), pyrethroids and organochlorines (Colombia), and pyrethroids, carbamates and organophosphates (Peru) [
24].
Insecticide resistance in mosquitoes can be derived from several mechanisms including behavioral (plasticity) [
25], metabolic (enzymes that metabolize (detoxify) or sequester insecticides) [
26,
27], and target-site (genotype) [
28,
29]. Of these, only target-site – considered to be the most accurate indicator of resistance – can be identified using molecular assays [
30]. These assays detect amino acid substitutions that give rise to non-synonymous amino acid changes in insecticide targets, ultimately preventing the insecticide from binding, leading to resistance [
31]. Among the genes of interest are voltage-gated sodium channel (
VGSC)
para and acetylcholinesterase-1 (
Ace-1), which encode for target binding of pyrethroids/organochlorines and carbamates/organophosphates, respectively [
29,
31]. Mutations linked most frequently with IR are L995F and L995S in
VGSC [
32] and G280S in
Ace-1 [
33]. Other mutations include L995C, L995W, V991L, and V994S in
VGSC [
33] and, in
Ace-1, A221T and S216T [
18]. Codon 995 in
VGSC and codon 280 in
Ace-1 were referred to formerly as 1014 and 119, respectively [
18]. Despite the accuracy of molecular assays, susceptibility (or phenotypic) bioassays are advantageous because hundreds of mosquitoes can be tested simultaneously with relatively simple equipment [
24,
30]. Having data from both molecular assays and phenotypic bioassays is ideal for assessing resistance frequency [
31].
The combination of resurgent cases of malaria since 2014 and the widespread increase in the use of insecticides, yet scarce reporting of resistant vectors, suggest a potential knowledge gap in IR throughout the Latin American region. IR detection requires strategic selection of localities in which mosquito samples are collected and analyzed. Commonly used selection criteria from previous reports include accessibility by land or water, history of insecticide use, high malaria prevalence, and a sample size sufficient even for a population with low frequency of resistant vectors [
34,
35]. This study aims to help close the knowledge gap in
Ny. darlingi by using molecular assays for a relatively large sample size covering multiple localities, uncovering novel codon mutations, and providing insight that may assist in improving malaria vector control interventions.
4. Discussion
Molecular analysis of a ~228 bp fragment that encodes for the
kdr target-site resistance region of the
VGSC gene and a ~456 bp fragment of the
Ace-1 gene did not detect any non-synonymous mutations in the specimens of
Ny. darlingi from endemic malaria areas of Brazil and Peru in the current study, similar to results of a recent analysis of
Ny. darlingi from locations in Brazil (Manaus, Unini River, Jau River in Amazonas State and Porto Velho, Rondônia state) and Colombia (Tagachi, Chocó Department) [
42]. An investigation of specimens of
Ny. darlingi from Chocó Department, Colombia, that had been demonstrated to be phenotypically susceptible and resistant, sequenced for the same region of the
VGSC gene, also did not reveal any
kdr mutations [
16]. However, the classic L1014F
kdr mutation has been detected in other important anopheline malaria vectors i.e.,
Ny. albimanus [
16,
43,
44], and
Ny. albitarsis s.s. [
45]. Other species of Latin American malaria vectors evaluated with these molecular assays include
An. vestitipennis and An. pseudopunctipennis, both of which exhibited genotypic susceptibility [
16]. The lack of genotypic evidence of IR in
Ny. darlingi could be a reflection of limited regional data rather than the absence of resistance [
46]. On the other hand, Floch [
47] suggested that frequent reintroduction of wild susceptible populations of
Ny. darlingi from forest into village populations could reduce selection for insecticide resistance. This hypothesis received some support from a study in the Porto Velho area in Rondônia state, Brazil that detected seasonal gene flow between forested and urban populations of
Ny. darlingi [
48].
Several previous studies of
Anopheles malaria vectors attribute a proportion of recent malaria case resurgence to increased outdoor biting and insecticide resistance (IR) following mass distribution campaigns [
49,
50,
51,
52], although there is no evidence in support of this latter trend in
Ny. darlingi across Latin America (scale-up of LLIN distribution has been limited compared with Africa) or after completion of the intensive PAMAFRO project in Peru. Even though daytime biting behavior in members of the
An. gambiae complex has been hypothesized to increase transmission [
53], in Latin America there has been scant investigation of this phenomenon except for observations of
Ny. darlingi biting during the day in forested French Guiana malaria hotspots [
54].
The hot, rainy climate of the Amazon basin is optimal for mosquito habitats [
55]; however, anthropogenic landscape changes – namely, forest fragmentation and increased ecotone density – suggest that vector behavior (mainly exophagy and exophily in
Ny. darlingi) and distribution (i.e., along ecotones for
Ny. darlingi) in one location may not be generalizable to an entire region [
56,
57]. For example, during a recent malaria outbreak in French Guiana,
Ny.
darlingi was the only anopheline collected both outdoors and indoors, and its abundance was exceptionally high, possibly attributed to regional deforestation, and/or the higher than average rainy and dry seasons in 2017 [
58].
Nyssorhynchus darlingi has also been collected biting during the day in French Guiana forest [
59,
60] and along the Maroni River, a former malaria hotspot, in Suriname [
61]. This appears to be a focal phenomenon in
Ny. darlingi, perhaps a behavioural avoidance response to IRS or LLINs.
The majority of IR reports in Latin America are based on bioassay data from the Amazon Basin or Central America [
24,
31]. However, recent genotypic reporting of several Brazilian samples of
Ny. albitarsis s.s. showed heterozygous L995F mutations in
VGSC [
45], and a sample of Guatemalan
Ny. albimanus had a heterozygous G280S mutation in
Ace-1 [
62]. Both these reports infer that agricultural insecticide use was the driver of IR, and a recent review of the contribution of agricultural insecticides and increasing insecticide resistance in malaria vectors found a strong association across Africa, that could be affected by crop type (especially cotton and vegetables), urban development, and strategies undertaken for farm pest management [
63]. Questionnaires and insecticide susceptibility bioassays utilized in a field study in two South Côte d’Ivoire communities determined that local mosquito vectors were resistant to three of four insecticides tested and the authors highlighted the need for collaboration between the public health and agricultural sectors to develop interventions that would benefit both [
64]. Resistance in the important regional malaria vector,
Ny. albimanus, has been detected in Central America, Panama and northwestern coastal Peru, linked mainly to agriculture in general and rice cultivation in coastal Peru in particular [
17].
Public health insecticide use can exert comparable selective pressure on malaria vectors [
65] including, for example, the organophosphate malathion used in Brazilian public health for the arboviral vectors
Aedes aegypti and
Aedes albopictus to reduce transmission of viruses such as dengue, chikungunya and Zika [
66]. Resistance in the vector
Culex quinquefasciatus in Brazil has been detected for organophosphates, carbamate, DDT, pyrethroids and biolarvicides; the concern for such resistance to arise in malaria vectors in Brazil where they co-occur with
Culex quinquefasciatus is limited to Fortaleza, Ceará state and parts of Mato Grosso state [
67].
For control of adult mosquitoes, IRS on interior house walls will kill resting mosquitoes; some also repel mosquitoes such that they modify their behavior and rest outdoors [
17]. Based on an evaluation of the residual effects of four insecticides (deltamethrin, pyrethroids, lambda-cyhalothrin, and etofenprox) on a range of wall materials in Amazonian Brazil [
68], the Brazilian National Malaria Control Plan has been consistently using etofenprox PM 20% for residual spray in houses since 2013 [
19], although a similar study of six insecticides in Amapá, Brazil by Correa et al. [
69] found that deltamethrin WG at 0.025 gm/m
2 had the highest residual effects. In Peru, the pyrethroid deltamethrin 5% is the most commonly applied insecticide for IRS [
38].
Other approaches to tackling insecticide resistance consider biological control in general [
70] or the replacement of synthetic compounds with plant-based compounds formulated as bioinsecticides, reviewed in Demirak and Canpolat [
15]. The classes of compounds described and discussed are phytochemicals, pheromones, microbial pesticides, and plant-incorporated protectants (PIPs); selected candidate compounds demonstrate larvicidal, adulticidal and repellent properties. In general, their advantages are, compared with synthetic compounds, lower toxicity, target-specificity, highly effectivity in small quantities, and they are biodegradable. Despite considerable promise, these products remain in various stages of development, and have not yet been field-tested for use against malaria vectors. As many target insects have evolved successful resistance mechanisms to most classes of insecticide, the evolution of different modes of action against plant-based insecticides could temper the early enthusiasm for such novel products [
26].