Efficacy of insecticides used in indoor residual spraying for malaria control: an experimental trial on various surfaces in a “test house”

Malaria is an avoidable and treatable disease, but it remains one of the most serious public health problems globally. In endemic countries, poor, disadvantaged people with limited access to healthcare facilities are the most affected [1]. Approximately 90% of malaria cases in the Americas are reported in the Amazonian parts of South America, Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Peru, Suriname, and Venezuela, with cases being mainly concentrated in Venezuela and Brazil [2]. According to the Brazilian Malaria Epidemiological Surveillance Information System (SIVEP/Malaria) [3], 99.8% of the cases are reported in the Amazon region, considered the endemic area, with high rates in states such as Amazonas, Acre, Pará, and Amapá [4, 5].

The set of interventions recommended by the World Health Organization (WHO), and adopted by the National Malaria Control Programme (NMCP), proposes: reducing the lethality and severity of cases, reducing the incidence of the disease through the elimination of transmission in urban areas, and maintaining the absence of the disease in places where the transmission has already been interrupted. This approach is understood to involve integrated, selective, and economic control activities that are suitable for the epidemiological scenario and appropriate to the actual conditions in each region [1, 6]. Among the proposed activities, vector control is an essential component and should be implemented based on local entomo-epidemiological data; for this, long-lasting insecticidal nets (LLIN) and indoor residual sprays (IRS) can be widely applied, which have achieved decreases in malaria cases [7, 8] in Africa, Asia, Europe, and Latin America [9‐11].

Some limitations in Anopheles neotropical bioassays include a lack of mosquito colonization in laboratory circumstances and low availability of bioassays, except for colonies of Anopheles aquasalis and Anopheles albitarsis sensu stricto, which were kept in a laboratory by the Oswaldo Cruz Foundation in Rio de Janeiro, Brazil [12, 13]. Therefore, it is a priority to establish colonies of vectors that transmit malaria in Brazil [14]. Although malaria represents a serious public health problem in Brazil, few studies have evaluated the resistance of vectors to insecticides [15, 16]. However, Silva et al. [17] evaluated the susceptibility profile of insects to pyrethroids in the larvae of Anopheles darlingi and Anopheles marajoara, using a simple, fast, and low-cost methodology, as an alternative to traditional, certified tests in the Amazonian states of Brazil, with the results showing susceptibility in the populations in the municipality of Macapá.

The development of insecticides that remain active for long periods was one of the most important advances in insect control in the 20th century. Controlling malaria with insecticides in the Amazon dates back to the 1940s when two-thirds of the Brazilian population lived in endemic areas. Thus, a national campaign for the eradication of the disease with the expressive use of organochlorine DDT (dichlorodiphenyl-trichloroethane) in an organized and systemic manner [18, 19] was initiated, also contributing to the prevention of the epidemic of typhus transmitted by lice [20].

Organophosphates (e.g., malathion, temephos, and fenitrothion.) were developed in the 1940s and have been used ever since as insecticides, herbicides, and plant growth regulators. In the 1970s, organochlorines were replaced by organophosphates [21]; although they are biodegradable and non-cumulative, they present chemical instability and high toxicity in vertebrates [22]. Carbamates present a rapid lethal action; however, as well as organophosphates, their persistence in the environment is short, making more frequent applications necessary [23]. The adoption of pyrethroids in the fight against malaria vectors in Brazil began in the mid-1990s, with cypermethrin in a wettable powder formulation [16], with lambda-cyhalothrin 5% for thermonebulization, and etofenprox WP 20% for BRI [24]. In addition, LLIN is impregnated with pyrethroids. However, the judicious use of chemical insecticides is necessary, avoiding the contamination of the environment and the selection of populations of resistant vectors [25, 26].

Knowledge about the persistence of insecticides (residual effect) is essential to determine the appropriate frequency of insecticide application in dwellings in areas with a high malaria incidence, considering the duration of the malaria transmission season, and to systematize the cycles of application of such products [16, 27‐29]. This activity consists of spraying the walls of residences with insecticides that remain in place on the applied surfaces. The residual efficacy of this is evaluated by performing bioassays as recommended by the WHO [30, 31], which should ideally be conducted in actual human habitations. However, factors such as the operational cost of mobilizing teams to perform this, the difficulty in accessing areas with houses sprayed with insecticides, ethical considerations [32], the variability of dwellings, and the non-use of F₁ generation mosquitoes of a known age compromise the performance of such tests in field conditions [33]. On the other hand, laboratory panel bioassays, under controlled conditions, have demonstrated a more prolonged residual effect [34], which may lead to a longer interval in the spray cycles.

Historically, the use of experimental huts began in the 1940s, with the goal of capturing mosquitoes. Subsequent works adapted the models for studies including the evaluation of IRS and LLIN, repellents, and synthetic attractants [35‐38]; such dwellings may even be transportable, such as the modified experimental Ifakara hut [39], which also exhibited the proven merits of the previously developed huts; however, there are no studies of studies of this nature in the Brazilian Amazon region, despite this being the location of the majority of the cases reported in the country [2]. The use of a “test house” for the field simulation for the wall bioassays can minimize the differences between the laboratory and the field; it can also decrease the operational costs, providing a better solution to define the spray cycles for IRS.

The aim of the present study was to evaluate, for the first time in a field simulation, the residual effect of six insecticide formulations used by the NMCP in an experimental hut called a “test house”, featuring walls composed of four different surfaces in the Brazilian Amazon.

The results presented in the tables and line graphs demonstrate the decay rate of the six different insecticides expressed as the 24 h mortality rate versus the number of days post spraying on the four different surfaces. Here the survival of the mosquitoes is considered as an indicator of the decreased of the residual effect of the insecticide on a given surface. The evaluation of the residual effect of the insecticides was carried out considering the application of different formulations on distinct surfaces: WP1, WP2, CP1, and CP2 (Tables 1, 2).

There was no significant difference between relative humidity (t = 1.453, df = 352.52, p-value = 0.15) and number of rainy days (W = 26,714, p-value = 0.72) when comparing both experimental phases, but there was a significant difference when comparing mean temperature (t = − 6.4421, df = 358.78, p-value < 0.001) (Additional file 1).

The present study demonstrated a wide variation in the residual efficacy of six IRS products from three classes of insecticides in a field simulation (“test house”), applied to four surfaces. The type of sprayed surface and the formulation of the insecticides showed different residual results when compared with those recommended by WHOPES.

IRS can be optimized by adjusting the insecticide formulation depending on the sprayed surface [1]. Wettable powder and water dispersible granule formulations such as etofenprox WP, bendiocarb WP and deltamethrin WG have been shown to have higher residual effects on wood surfaces. The concentrated suspension formulation of alpha-cypermethrin SC was effective on painted wood, while the capsulated suspension of pirimiphos-methyl CS was effective on the wood and masonry surfaces. A mortality rate of 80% or more is the criterion adopted by the WHO to establish the residual effect of insecticides. In this study, in this study, products with residuals of 2 to 6 months were used; the efficacy results of the six insecticides showed that deltamethrin WG and pirimiphos-methyl CS met the WHO recommendation, presenting mortality rates within the established range for up to 240 days [42].

For the other insecticides, the mortality rate did not reach the minimum period of residuality for all the evaluated substrates. The complexity of monitoring the IRS vector control in field situations [51] can be overcome with information obtained from the bioassays performed in field-simulating conditions. In this study the alpha-cypermethrin concentrated suspension formulation (SC) performed better on the wood surfaces than on cement ones; however, the only surface that reached the minimum period with residual efficacy within the WHO parameters for up to 120 days (4 months) was WP1. A similar result was found in the Democratic Republic of Sao Tome and Principe, suggesting that IRS should be applied in three cycles per year [58]. For the WG-SB and WP formulations of alpha-cypermethrin on the clay and cement surfaces, the residual efficacy ranged from 11 to 16 weeks [59]. The data compiled by Dengela et al. [39] about the residual efficacy in African countries showed satisfactory performance of alpha-cypermethrin WP, varying from 4 to 10 months, on surfaces of mud, wood, cement, and other materials in the sprayed dwellings.

The residual efficacy of the etofenprox and lambda-cyhalothrin pyrethroid formulations, according to the WHOPES guidelines, ranged from 3 to 6 months. In Brazil, the recommendation for the IRS with the formulation etofenprox WP was established with a 4-month interval [24], based on the laboratory bioassays performed with WP2 panels. The results in the field simulations presented the same residual efficacy for this surface; however, Santos et al. [16] using this insecticide in the field conditions, observed residuality of up to 3 months for wooden and CP2 surfaces, but on the CP1 surface the effectiveness was lower, supporting the findings of the study. The formulation of lambda-cyhalothrin WP demonstrated a short residual efficacy on the cement and wood surfaces under evaluation in Brazil [16], being in agreement with the results obtained in this study, however studies in African countries showed satisfactory residuality, according to the period recommended by WHOPES on surfaces of cement [60] and wood [61]. Variations in the results were also described for the concentrated suspension formulation of lambda-cyhalothrin CS concerning its effectiveness on the cement surfaces [62].

The residual effect of deltamethrin varied among the different surfaces. With the WP formulation, the results displayed the expected residual efficacy (3 to 6 months) [42, 60, 63]; they surpassed the predicted period when the SC-PE formulation was used [29], and the WG formulation presented results that were below the established [52, 64, 65]. In this study, among the evaluated pyrethroids, deltamethrin WG showed the best performance, demonstrating efficacy for 8 months on the wood surfaces and maintaining a residual effect with mortality ≥ 80% for a period equal to or greater than 90 days on three of the tested surfaces. Similar results were found in previous studies [66, 67].

It was highlighted that some factors affect the residual efficacy and persistence of insecticides; for example, the activity of pyrethroids can be compromised by the rapid degradation on porous surfaces with a high absorption [16, 66]. The low residual bioefficacy of the pyrethroids on the cement surfaces compared with the wooden ones found in the present study confirms previous observations but diverges from the findings of Dunford et al. [64].

The continuous use of pyrethroids has led to an increase in the population of resistant mosquitoes. Few studies have been performed on neotropical Anopheles [68, 69] and although there is a shortage of records in Brazil [17, 70], this is a reality in African countries [62, 71‐73], supporting the importance of using substitute products in IRS rotation schemes [1, 23]. Bendiocarb is an insecticide of the class of carbamates recommended by the WHO, with residual efficacy varying from 2 to 6 months [42]. The WP formulation achieved the expected residual efficacy on different surfaces [51, 74], but some studies reflected a short residuality from 2 to 3 months [51, 75‐78].

These results are similar to those found in this study on the wooden surfaces, but on the masonry surfaces, the effectiveness was less than 30 days. The short residuality of bendiocarb compromises its use as an alternative to replace the pyrethroids in the rotation scheme for malaria control in the Brazilian Amazon since it demands a higher number of spray cycles, not presenting a cost-effective benefit for protection in endemic areas.

In the search for long-lasting insecticides, the microencapsulation technology of pirimiphos-methyl has brought significant benefits in the current context of resistance to pyrethroids. Thus, the CS encapsulation suspension formulation minimizes the limitation of the low residuality found in WP and EC emulsified concentrate formulations [42, 79], prolonging its persistence for up to 10 months on cement surfaces and for 6 to 8 months on the other surface types. The observed residual efficacy is beneficial in areas where there are up to two transmission periods per year [51, 53, 62, 67, 80, 81], corroborating the findings that residual efficacy was 8 months for the masonry and WP1 surfaces and 6 months for the WP2. The pirimiphos-methyl CS presented little variation and good performance among the tested surfaces. However, the bioassay was interrupted before the mortality rate dropped to less than 80% for two consecutive months due to the strong odour, as also reported elsewhere [27, 80].

In areas where there are no records of resistance to pyrethroids, its use should thus be considered with caution, to avoid rejection of its use by human inhabitants and inconsistent control activities (Additional file 2).

The results of this study showed a variation of the residual effects of insecticides on the different tested surfaces. The residuality performance consistent with the WHO guidelines, among the pyrethroids evaluated, was observed for the insecticides deltamethrin WG for WP1, WP2 (240 days), and CP1 (90 days) surfaces and etofenprox WP for both the wood surfaces (90 and 120 days). Within this context, these insecticides would be effective in vector control programmes if applied in quarterly spray cycles. The pyrethroids alpha-cypermethrin SC and lambda-cyhalothrin WP, as well as the carbamate-bendiocarb WP, presented a lower residuality than the other insecticides studied, with a short or no period of effectiveness on the applied surfaces. On the other hand, the organophosphate pirimiphos-methyl CS was shown to be efficient on all of the surfaces so that it could be used in cycles with an interval of up to 6 months. Nevertheless, this insecticide exudes a strong odour and high toxicity in vertebrates; therefore, in countries in which resistance to pyrethroids has not been confirmed, its judicious use is recommended.

This study reveals that the variation in effects among different surfaces and the short residual effect compromises the use of insecticides, to the detriment of the cost–benefit. The persistence of the product is essential, and it should remain effective on the applied surface for sufficient time to cover the malaria transmission period. Against this background, the use of the “test house” for the field simulation is beneficial to evaluate the residual period of insecticides in order to obtain results that are more reflective of those in residences in endemic areas.

This study also recommends the performance of comparative wall bioassays using laboratory panels, field simulations, and directly in the field. The data generated from such studies can serve as an important guide to malaria control programmes, by selecting insecticides for IRS in these environments.