Indoor insecticide spraying (IRS) is a key method to reduce vector-borne transmission of Trypanosoma cruzi, which causes Chagas disease in much of South America. However, the IRS’s success in the Grand Chaco region, which covers Bolivia, Argentina and Paraguay, cannot rival that of other Southern Cone countries.
This study assessed routine IRS practices and pesticide quality control in a typical endemic community in Chaco, Bolivia.
The active ingredient alpha-cypermethrin (ai) was captured on filter paper mounted on the wall surface of the sprayer and measured in prepared spray tank solutions using an adapted Insecticide Quantitative Kit (IQK™) validated for quantitative HPLC methods. Data were analyzed using a negative binomial mixed-effects regression model to examine the relationship between insecticide concentration applied to filter paper and spray wall height, spray coverage (spray surface area/spray time [m2/min]), and observed/expected spray. rate ratio. Differences between health care providers’ and homeowners’ compliance with IRS vacant home requirements were also assessed. The settling rate of alpha-cypermethrin after mixing in prepared spray tanks was quantified in the laboratory.
Significant variations were observed in alpha-cypermethrin AI concentrations, with only 10.4% (50/480) of filters and 8.8% (5/57) of homes achieving the target concentration of 50 mg ± 20% AI/m2. The concentrations indicated are independent of the concentrations found in the respective spray solutions. After mixing alpha-cypermethrin a.i. in the prepared surface solution of the spray tank quickly settled, which led to a linear loss of alpha-cypermethrin a.i. per minute and a loss of 49% after 15 minutes. Only 7.5% (6/80) of houses were treated at the WHO recommended spray rate of 19 m2/min (±10%), while 77.5% (62/80) of houses were treated at a rate lower than expected. The average concentration of active ingredient delivered to the home was not significantly related to observed spray coverage. Household compliance did not significantly affect spray coverage or the average concentration of cypermethrin delivered to homes.
Suboptimal IRS delivery may be due in part to the physical properties of pesticides and the need to review pesticide delivery methods, including training of IRS teams and public education to encourage compliance. IQK™ is an important field-friendly tool that improves the quality of IRS and facilitates training of healthcare providers and decision-making for managers in Chagas vector control.
Chagas disease is caused by infection with the parasite Trypanosoma cruzi (kinetoplastid: Trypanosomatidae), which causes a range of diseases in humans and other animals. In humans, acute symptomatic infection occurs weeks to months after infection and is characterized by fever, malaise, and hepatosplenomegaly. An estimated 20-30% of infections progress to a chronic form, most commonly cardiomyopathy, which is characterized by conduction system defects, cardiac arrhythmias, left ventricular dysfunction, and ultimately congestive heart failure and, less commonly, gastrointestinal disease. These conditions can persist for decades and are difficult to treat [1]. There is no vaccine.
The global burden of Chagas disease in 2017 was estimated at 6.2 million people, resulting in 7900 deaths and 232,000 disability-adjusted life years (DALYs) for all ages [2,3,4]. Triatominus cruzi is transmitted throughout Central and South America, and in parts of southern North America, by Triatominus cruzi (Hemiptera: Reduviidae), accounting for 30,000 (77%) of the total number of new cases in Latin America in 2010 [5] . Other routes of infection in non-endemic regions such as Europe and the United States include congenital transmission and transfusion of infected blood. For example, in Spain, there are approximately 67,500 cases of infection among Latin American immigrants [6], resulting in annual healthcare system costs of US$9.3 million [7]. Between 2004 and 2007, 3.4% of pregnant Latin American immigrant women screened at a Barcelona hospital were seropositive for Trypanosoma cruzi [8]. Therefore, efforts to control vector transmission in endemic countries are critical to reduce the disease burden in triatomine vector-free countries [9]. Current control methods include indoor spraying (IRS) to reduce vector populations in and around homes, maternal screening to identify and eliminate congenital transmission, screening of blood and organ transplant banks, and educational programs [5,10,11,12].
In the Southern Cone of South America, the main vector is the pathogenic triatomine bug. This species is primarily endivoreous and endivoreous and breeds widely in homes and animal sheds. In poorly constructed buildings, cracks in walls and ceilings harbor triatomine bugs, and infestations in households are particularly severe [13, 14]. The Southern Cone Initiative (INCOSUR) promotes coordinated international efforts to combat domestic infections in Tri. Use IRS to detect pathogenic bacteria and other site-specific agents [15, 16]. This led to a significant reduction in the incidence of Chagas disease and subsequent confirmation by the World Health Organization that vector-borne transmission had been eliminated in some countries (Uruguay, Chile, parts of Argentina and Brazil) [10, 15].
Despite the success of INCOSUR, the vector Trypanosoma cruzi persists in the Gran Chaco region of the USA, a seasonally dry forest ecosystem spanning 1.3 million square kilometers across the borders of Bolivia, Argentina and Paraguay [10]. Residents of the region are among the most marginalized groups and live in extreme poverty with limited access to health care [17]. The incidence of T. cruzi infection and vector transmission in these communities is among the highest in the world [5,18,19,20] with 26–72% of homes infested with trypanosomatids. infestans [13, 21] and 40–56% Tri. Pathogenic bacteria infect Trypanosoma cruzi [22, 23]. The majority (>93%) of all cases of vector-borne Chagas disease in the Southern Cone region occur in Bolivia [5].
IRS is currently the only widely accepted method for reducing triacine in humans. infestans is a historically proven strategy to reduce the burden of several human vector-borne diseases [24, 25]. The share of houses in the village of Tri. infestans (infection index) is a key indicator used by health authorities to make decisions about IRS deployment and, importantly, to justify treatment of chronically infected children without the risk of reinfection [16,26,27,28,29]. The effectiveness of IRS and the persistence of vector transmission in the Chaco region are influenced by several factors: poor quality of building construction [19, 21], suboptimal IRS implementation and infestation monitoring methods [30], public uncertainty regarding IRS requirements Low compliance [31], short residual activity of pesticide formulations [32, 33] and Tri. infestans have reduced resistance and/or sensitivity to insecticides [22, 34].
Synthetic pyrethroid insecticides are commonly used in IRS due to their lethality to susceptible populations of triatomine bugs. At low concentrations, pyrethroid insecticides have also been used as irritants to flush vectors out of wall cracks for surveillance purposes [35]. Research on quality control of IRS practices is limited, but elsewhere it has been shown that there are significant variations in the concentrations of pesticide active ingredients (AIs) delivered into homes, with levels often falling below the effective target concentration range [33,36,37,38]. One reason for the lack of quality control research is that high-performance liquid chromatography (HPLC), the gold standard for measuring the concentration of active ingredients in pesticides, is technically complex, expensive, and often not suitable for widespread conditions in society. Recent advances in laboratory testing now provide alternative and relatively inexpensive methods for assessing pesticide delivery and IRS practices [39, 40].
This study was designed to measure changes in pesticide concentrations during routine IRS campaigns targeting Tri. Phytophthora infestans of potatoes in the Chaco region, Bolivia. Concentrations of pesticide active ingredients were measured in formulations prepared in spray tanks and in filter paper samples collected in spray chambers. Factors that may influence the delivery of pesticides to homes were also assessed. To this end, we used a chemical colorimetric assay to quantify the concentration of pyrethroids in these samples.
The study was conducted in Itanambicua, municipality of Camili, department of Santa Cruz, Bolivia (20°1′5.94″ S; 63°30′41″ W) (Fig. 1). This region is part of the Gran Chaco region of the USA and is characterized by seasonally dry forests with temperatures of 0–49 °C and precipitation of 500–1000 mm/year [41]. Itanambicua is one of 19 Guaraní communities in the city, where about 1,200 residents live in 220 houses built primarily from solar brick (adobe), traditional fences and tabiques (known locally as tabique), wood, or mixtures of these materials. Other buildings and structures near the house include animal sheds, storerooms, kitchens and toilets, built from similar materials. The local economy is based on subsistence agriculture, mainly maize and peanuts, as well as small-scale poultry, pigs, goats, ducks and fish, with surplus domestic produce sold in the local market town of Kamili (approximately 12 km away). The town of Kamili also provides a number of employment opportunities to the population, mainly in the construction and domestic services sectors.
In the present study, the T. cruzi infection rate among Itanambiqua children (2–15 years) was 20% [20]. This is similar to the seroprevalence of infection among children reported in the neighboring community of Guarani, which also saw an increase in prevalence with age, with the vast majority of residents over 30 years of age being infected [19]. Vector transmission is considered to be the main route of infection in these communities, with Tri being the main vector. Infestans encroach on houses and outbuildings [21, 22].
The newly elected municipal health authority was unable to provide reports on IRS activities in Itanambicua prior to this study, however reports from nearby communities clearly indicate that IRS operations in the municipality have been sporadic since 2000 and a general spraying of 20% beta cypermethrin; was carried out in 2003, followed by concentrated spraying of infested houses from 2005 to 2009 [22] and systematic spraying from 2009 to 2011 [19].
In this community, IRS was performed by three community-trained health professionals using a 20% formulation of alpha-cypermethrin suspension concentrate [SC] (Alphamost®, Hockley International Ltd., Manchester, UK). The insecticide was formulated with a target delivery concentration of 50 mg a.i./m2 according to the requirements of the Chagas Disease Control Program of the Santa Cruz Administrative Department (Servicio Departamental de Salud-SEDES). Insecticides were applied using a Guarany® backpack sprayer (Guarany Indústria e Comércio Ltda, Itu, São Paulo, Brazil) with an effective capacity of 8.5 l (tank code: 0441.20), equipped with a flat-spray nozzle and a nominal flow rate of 757 ml/min, producing a stream of an angle of 80° at a standard cylinder pressure of 280 kPa. Sanitation workers also mixed up aerosol cans and sprayed houses. The workers had previously been trained by the local city health department to prepare and deliver pesticides, as well as spray pesticides on the interior and exterior walls of homes. They are also advised to require occupants to clear the home of all items, including furniture (except bed frames), at least 24 hours before the IRS takes action to allow full access to the interior of the home for spraying. Compliance with this requirement is measured as described below. Residents are also advised to wait until painted walls are dry before re-entering the home, as recommended [42].
To quantify the concentration of lambda-cypermethrin AI delivered into homes, the researchers installed filter paper (Whatman No. 1; 55 mm diameter) on the wall surfaces of 57 homes in front of the IRS. All homes receiving IRS at that time were involved (25/25 homes in November 2016 and 32/32 homes in January-February 2017). These include 52 adobe houses and 5 tabik houses. Eight to nine pieces of filter paper were installed in each house, divided into three wall heights (0.2, 1.2 and 2 m from the ground), with each of the three walls selected counterclockwise, starting from the main door. This provided three replicates at each wall height, as recommended for monitoring effective pesticide delivery [43]. Immediately after applying the insecticide, the researchers collected the filter paper and dried it away from direct sunlight. Once dry, the filter paper was wrapped with clear tape to protect and hold the insecticide on the coated surface, then wrapped in aluminum foil and stored at 7°C until testing. Of the total 513 filter papers collected, 480 out of 57 houses were available for testing, i.e. 8-9 filter papers per home. The test samples included 437 filter papers from 52 adobe houses and 43 filter papers from 5 tabik houses. The sample is proportional to the relative prevalence of housing types in the community (76.2% [138/181] adobe and 11.6% [21/181] tabika) recorded in the door-to-door surveys of this study. Filter paper analysis using the Insecticide Quantification Kit (IQK™) and its validation using HPLC are described in Additional File 1. The target pesticide concentration is 50 mg ai/m2, which allows a tolerance of ± 20% (i.e. 40–60 mg a.i./m2).
The quantitative concentration of AI was determined in 29 canisters prepared by medical workers. We sampled 1–4 prepared tanks per day, with an average of 1.5 (range: 1–4) tanks prepared per day over an 18-day period. The sampling sequence followed the sampling sequence used by healthcare workers in November 2016 and January 2017. Daily progress from; January February. Immediately after thorough mixing of the composition, 2 ml of solution was collected from the surface of the contents. The 2 mL sample was then mixed in the laboratory by vortexing for 5 minutes before two 5.2 μL subsamples were collected and tested using IQK™ as described (see Additional file 1).
Deposition rates of insecticide active ingredient were measured in four spray tanks specifically selected to represent initial (zero) active ingredient concentrations within the upper, lower, and target ranges. After mixing for 15 consecutive minutes, remove three 5.2 µL samples from the surface layer of each 2 mL vortex sample at 1 minute intervals. The target solution concentration in the tank is 1.2 mg ai/ml ± 20% (i.e. 0.96–1.44 mg ai/ml), which is equivalent to achieving the target concentration delivered to the filter paper , as described above.
To understand the relationship between pesticide spraying activities and pesticide delivery, a researcher (RG) accompanied two local IRS health workers during routine IRS deployments to 87 homes (the 57 homes sampled above and 30 of the 43 homes that were sprayed with pesticides). March 2016). Thirteen of these 43 homes were excluded from the analysis: six owners refused, and seven homes were only partially treated. The total surface area to be sprayed (square meters) inside and outside the home was measured in detail, and the total time spent by health workers spraying (minutes) was secretly recorded. These input data are used to calculate the spray rate, defined as surface area sprayed per minute (m2/min). From these data, the observed/expected spray ratio can also be calculated as a relative measure, with the recommended expected spray rate being 19 m2/min ± 10% for spray equipment specifications [44]. For the observed/expected ratio, the tolerance range is 1 ± 10% (0.8–1.2).
As mentioned above, 57 houses had filter paper installed on their walls. To test whether the visual presence of filter paper affected sanitation workers’ spray rates, spray rates in these 57 homes were compared with spray rates in 30 homes treated in March 2016 without filter paper installed. Pesticide concentrations were measured only in homes equipped with filter paper.
Residents of 55 homes were documented to comply with previous IRS home cleaning requirements, including 30 homes that were sprayed in March 2016 and 25 homes that were sprayed in November 2016. 0–2 (0 = all or most items remain in the house; 1 = most items removed; 2 = house completely emptied). The effect of owner compliance on spray rates and moxa insecticide concentrations was studied.
Statistical power was calculated to detect significant deviations from expected concentrations of alpha-cypermethrin applied to filter paper, and to detect significant differences in insecticide concentrations and spray rates between categorically paired groups of houses. Minimum statistical power (α = 0.05) was calculated for the minimum number of homes sampled for any categorical group (i.e., fixed sample size) determined at baseline. In summary, a comparison of mean pesticide concentrations in one sample across 17 selected properties (classified as non-compliant owners) had a 98.5% power to detect a 20% deviation from the expected mean target concentration of 50 mg ai/m2, where the variance ( SD = 10) is overestimated based on observations published elsewhere [37, 38]. Comparison of insecticide concentrations in home-selected aerosol cans for equivalent effectiveness (n = 21) > 90%.
Comparison of two samples of mean pesticide concentrations in n = 10 and n = 12 houses or mean spray rates in n = 12 and n = 23 houses yielded statistical powers of 66.2% and 86.2% for detection. Expected values for a 20% difference are 50 mg a.i./m2 and 19 m2/min, respectively. Conservatively, it was assumed that there would be large variances in each group for spray rate (SD = 3.5) and insecticide concentration (SD = 10). Statistical power was >90% for equivalent comparisons of spray rates between houses with filter paper (n = 57) and houses without filter paper (n = 30). All power calculations were performed using the SAMPSI program in STATA v15.0 software [45]).
Filter papers collected from the house were examined by fitting the data to a multivariate negative binomial mixed-effects model (MENBREG program in STATA v.15.0) with the location of walls within the house (three levels) as a random effect. Beta radiation concentration. -cypermethrin i.o. Models were used to test changes associated with nebulizer wall height (three levels), nebulization rate (m2/min), IRS filing date, and healthcare provider status (two levels). A generalized linear model (GLM) was used to test the relationship between the average concentration of alpha-cypermethrin on filter paper delivered to each home and the concentration in the corresponding solution in the spray tank. Sedimentation of pesticide concentration in spray tank solution over time was examined in a similar manner by including the initial value (time zero) as the model offset, testing the interaction term of tank ID × time (days). Outlier data points x are identified by applying the standard Tukey boundary rule, where x < Q1 – 1.5 × IQR or x > Q3 + 1.5 × IQR. As indicated, spray rates for seven houses and the median insecticide ai concentration for one house were excluded from the statistical analysis.
The accuracy of the ai IQK™ chemical quantification of alpha-cypermethrin concentration was confirmed by comparing the values of 27 filter paper samples from three poultry houses tested by IQK™ and HPLC (gold standard), and the results showed a strong correlation (r = 0.93; p < 0.001) (Fig. 2).
Correlation of alpha-cypermethrin concentrations in filter paper samples collected from post-IRS poultry houses, quantified by HPLC and IQK™ (n = 27 filter papers from three poultry houses)
IQK™ was tested on 480 filter papers collected from 57 poultry houses. On filter paper, alpha-cypermethrin content ranged from 0.19 to 105.0 mg a.i./m2 (median 17.6, IQR: 11.06-29.78). Of these, only 10.4% (50/480) were within the target concentration range of 40–60 mg ai/m2 (Fig. 3). The majority of samples (84.0% (403/480)) had <40 mg ai/m2 and 5.6% (27/480) had >60 mg ai/m2. The difference in the estimated median concentration per home for the 8-9 test filters collected per home was an order of magnitude, with a mean of 19.6 mg a.i./m2 (IQR: 11.76-28.32, range: 0. 60-67.45). Only 8.8% (5/57) of sites received expected pesticide concentrations; 89.5% (51/57) were below the limits of the target range, and 1.8% (1/57) were above the limits of the target range (Fig. 4).
Frequency distribution of alpha-cypermethrin concentrations on filters collected from IRS-treated homes (n = 57 homes). The vertical line represents the target concentration range of cypermethrin a.i. (50 mg ± 20% a.i./m2).
Median concentration of beta-cypermethrin a.v. on 8-9 filter papers per home, collected from IRS-processed homes (n = 57 homes). The horizontal line represents the target concentration range of alpha-cypermethrin a.i. (50 mg ± 20% a.i./m2). Error bars represent the lower and upper limits of adjacent median values.
Median concentrations delivered to filters with wall heights of 0.2, 1.2 and 2.0 m were 17.7 mg ai/m2 (IQR: 10.70–34.26), 17.3 mg a .i./m2 (IQR: 11.43–26.91) and 17.6 mg a.i./m2. respectively (IQR: 10.85–31.37) (shown in Additional file 2). Controlling for IRS date, the mixed effects model revealed neither a significant difference in concentration between wall heights (z < 1.83, p > 0.067) nor significant changes by spray date (z = 1.84 p = 0.070). The median concentration delivered to the 5 adobe houses was not different from the median concentration delivered to the 52 adobe houses (z = 0.13; p = 0.89).
AI concentrations in 29 independently prepared Guarany® aerosol cans sampled before IRS application varied by 12.1, from 0.16 mg AI/mL to 1.9 mg AI/mL per can (Figure 5). Only 6.9% (2/29) of aerosol cans contained AI concentrations within the target dose range of 0.96–1.44 mg AI/ml, and 3.5% (1/29) of aerosol cans contained AI concentrations >1. 44 mg AI/ml. .
Average concentrations of alpha-cypermethrin a.i. were measured in 29 spray formulations. The horizontal line represents the recommended AI concentration for aerosol cans (0.96–1.44 mg/ml) to achieve the target AI concentration range of 40–60 mg/m2 in the poultry house.
Of the 29 aerosol cans examined, 21 corresponded to 21 houses. The median concentration of ai delivered to the house was not associated with the concentration in the individual spray tanks used to treat the house (z = -0.94, p = 0.345), which was reflected in the low correlation (rSp2 = -0.02) (Fig. .6). ).
Correlation between beta-cypermethrin AI concentration on 8-9 filter papers collected from IRS-treated houses and AI concentration in home-prepared spray solutions used to treat each house (n = 21)
The concentration of AI in the surface solutions of four sprayers collected immediately after shaking (time 0) varied by 3.3 (0.68–2.22 mg AI/ml) (Fig. 7). For one tank the values are within the target range, for one tank the values are above the target, for the other two tanks the values are below the target; Pesticide concentrations then decreased significantly in all four pools during the subsequent 15-min follow-up sampling (b = −0.018 to −0.084; z > 5.58; p < 0.001). Considering individual tank initial values, the Tank ID x Time (minutes) interaction term was not significant (z = -1.52; p = 0.127). In the four pools, the average loss of mg ai/ml insecticide was 3.3% per minute (95% CL 5.25, 1.71), reaching 49.0% (95% CL 25.69, 78.68) after 15 minutes (Fig. 7).
After thoroughly mixing the solutions in the tanks, the precipitation rate of alpha-cypermethrin a.i. was measured. in four spray tanks at 1 minute intervals for 15 minutes. The line representing the best fit to the data is shown for each reservoir. Observations (points) represent the median of three subsamples.
The average wall area per home for potential IRS treatment was 128 m2 (IQR: 99.0–210.0, range: 49.1–480.0) and the average time spent by health care workers was 12 minutes (IQR: 8. 2–17.5, range: 1.5–36.6). ) each house was sprayed (n = 87). Spray coverage observed in these poultry houses ranged from 3.0 to 72.7 m2/min (median: 11.1; IQR: 7.90–18.00) (Figure 8). Outliers were excluded and spray rates were compared to the WHO recommended spray rate range of 19 m2/min ± 10% (17.1–20.9 m2/min). Only 7.5% (6/80) of homes were in this range; 77.5% (62/80) were in the lower range and 15.0% (12/80) were in the upper range. No relationship was found between the average concentration of AI delivered to homes and observed spray coverage (z = -1.59, p = 0.111, n = 52 homes).
Observed spray rate (min/m2) in poultry houses treated with IRS (n = 87). The reference line represents the expected spray rate tolerance range of 19 m2/min (±10%) recommended by spray tank equipment specifications.
80% of 80 houses had an observed/expected spray coverage ratio outside the 1 ± 10% tolerance range, with 71.3% (57/80) of houses being lower, 11.3% (9/80) being higher, and 16 houses fell within the tolerance range within the range. The frequency distribution of observed/expected ratio values is shown in Additional file 3.
There was a significant difference in the mean nebulization rate between the two healthcare workers who routinely performed IRS: 9.7 m2/min (IQR: 6.58–14.85, n = 68) versus 15.5 m2/min (IQR: 13.07–21.17, n = 12). (z = 2.45, p = 0.014, n = 80) (as shown in Additional File 4A) and observed/expected spray rate ratio (z = 2.58, p = 0.010) (as shown in Additional File 4B Show) .
Excluding abnormal conditions, only one health worker sprayed 54 houses where filter paper was installed. The median spray rate in these houses was 9.23 m2/min (IQR: 6.57–13.80) compared to 15.4 m2/min (IQR: 10.40–18.67) in the 26 houses without filter paper (z = -2.38, p = 0.017). ).
Household compliance with the requirement to vacate their homes for IRS deliveries varied: 30.9% (17/55) did not vacate their homes partially and 27.3% (15/55) did not vacate their homes completely; devastated their homes.
Observed spray levels in non-empty houses (17.5 m2/min, IQR: 11.00–22.50) were generally higher than in semi-empty houses (14.8 m2/min, IQR: 10.29–18 .00) and completely empty houses (11.7 m2). /min, IQR: 7.86–15.36), but the difference was not significant (z > -1.58; p > 0.114, n = 48) (shown in Additional file 5A). Similar results were obtained when considering changes associated with the presence or absence of filter paper, which was not found to be a significant covariate in the model.
Across the three groups, the absolute time required to spray houses did not differ between houses (z < -1.90, p > 0.057), while the median surface area did differ: completely empty houses (104 m2 [IQR: 60.0–169, 0 m2) ]) is statistically smaller than non-empty houses (224 m2 [IQR: 174.0–284.0 m2]) and semi-empty houses (132 m2 [IQR: 108.0–384.0 m2]) (z > 2 .17; p < 0.031, n = 48). Completely vacant homes are approximately half the size (area) of homes that are not vacant or semi-vacant.
For the relatively small number of homes (n = 25) with both compliance and pesticide AI data, there were no differences in mean AI concentrations delivered to homes between these compliance categories (z < 0.93, p > 0.351) , as specified in Additional File 5B. Similar results were obtained when controlling for the presence/absence of filter paper and observed spray coverage (n = 22).
This study evaluates IRS practices and procedures in a typical rural community in the Gran Chaco region of Bolivia, an area with a long history of vector transmission [20]. The concentration of alpha-cypermethrin a.i. administered during routine IRS varied significantly between houses, between individual filters within the house, and between individual spray tanks prepared to achieve the same delivered concentration of 50 mg a.i./m2. Only 8.8% of homes (10.4% of filters) had concentrations within the target range of 40–60 mg a.i./m2, with the majority (89.5% and 84% respectively) having concentrations below the lower permissible limit limit.
One potential factor for suboptimal delivery of alpha-cypermethrin into the home is inaccurate dilution of pesticides and inconsistent levels of suspension prepared in spray tanks [38, 46]. In the current study, the researchers’ observations of health care workers confirmed that they followed pesticide preparation recipes and were trained by SEDES to vigorously stir the solution after dilution in the spray tank. However, analysis of the reservoir contents showed that AI concentration varied by a factor of 12, with only 6.9% (2/29) of the test reservoir solutions being within the target range; For further investigation, the solutions on the surface of the sprayer tank were quantified in laboratory conditions. This shows a linear decrease in alpha-cypermethrin ai of 3.3% per minute after mixing and a cumulative loss of ai of 49% after 15 minutes (95% CL 25.7, 78.7). High sedimentation rates due to aggregation of pesticide suspensions formed upon dilution of wettable powder (WP) formulations are not uncommon (e.g., DDT [37, 47]), and the present study further demonstrates this for SA pyrethroid formulations. Suspension concentrates are widely used in IRS and, like all insecticidal preparations, their physical stability depends on many factors, especially the particle size of the active ingredient and other ingredients. Sedimentation may also be affected by the overall hardness of the water used to prepare the slurry, a factor that is difficult to control in the field. For example, in this study site, water access is limited to local rivers that exhibit seasonal variations in flow and suspended soil particles. Methods for monitoring the physical stability of SA compositions are under research [48]. However, subcutaneous drugs have been successfully used to reduce household infections in Tri. pathogenic bacteria in other parts of Latin America [49].
Inadequate insecticidal formulations have also been reported in other vector control programs. For example, in a visceral leishmaniasis control program in India, only 29% of 51 sprayer groups monitored correctly prepared and mixed DDT solutions, and none filled sprayer tanks as recommended [50]. An assessment of villages in Bangladesh showed a similar trend: only 42–43% of IRS divisional teams prepared insecticides and filled canisters as per protocol, while in one sub-district the figure was only 7.7% [46].
The observed changes in the concentration of AI delivered into the home are also not unique. In India, only 7.3% (41 of 560) of treated homes received the target concentration of DDT, with differences within and between homes being equally large [37]. In Nepal, filter paper absorbed an average of 1.74 mg ai/m2 (range: 0.0–17.5 mg/m2), which is only 7% of the target concentration (25 mg ai/m2) [38]. HPLC analysis of filter paper showed large differences in deltamethrin ai concentrations on the walls of houses in Chaco, Paraguay: from 12.8–51.2 mg ai/m2 to 4.6–61.0 mg ai/m2 on roofs [33]. In Tupiza, Bolivia, the Chagas Control Program reported the delivery of deltamethrin to five homes at concentrations of 0.0–59.6 mg/m2, quantified by HPLC [36].
Post time: Apr-16-2024