Field populations of Triatoma infestans Klug were collected during 2002 from four villages in northern Argentina (El Chorro, La Toma, El Sauzal, and Salvador Mazza), after application of deltamethrin and other pyrethroids was ineffective. High levels of resistance to the pyrethroid insecticides deltamethrin, beta-cypermethrin, beta-cyfluthrin, and lambda-cyhalothrin were detected in all of the evaluated populations. The resistance ratio to pyrethroids determined by topical application ranged from 50.5 (deltamethrin, El Sauzal) to 667.6 (beta-cyfluthrin, Salvador Mazza). None of the pyrethroid-resistant insects was resistant to the organophosphorus insecticide fenitrothion. Topical application of piperonyl butoxide to the most deltamethrin-resistant population (Salvador Mazza) led to slight reduction in levels of resistance. Activity of P450 monooxygenase, measured in individual insects through ethoxycoumarine-O-deethylase, showed a slight but noticeable difference in the distribution of activities between susceptible and resistant populations. The total percentage of insects below 0.48 pmol of 7-OH coumarine/min/ insect was 36.4 for Salvador Mazza population and 64.3 pmol of 7-OH coumarine/min/insect for CIPEIN strain. Whereas a low level of resistance to deltamethrin was previously related to monooxygenase activity in T. infestans, the high levels of resistance shown by these populations seem to involve monooxygenase in combination with other resistance mechanisms, for example, insensitivity of nervous membrane. Research on T. infestans resistance is in progress to improve Chagas vector control programs in Latin America and to implement resistance management strategies.

Recently, high resistance to pyrethroid insecticides has been associated with ineffective Þeld treatments against Triatoma infestans (Klug) (Hemiptera: Reduviidae) in northern Argentina. Samples were collected from two areas in Argentina (Salta and La Rioja) and one are in Bolivia (Yacuiba), and they were subjected to toxicological and biochemical assays. All populations were resistant to deltamethrin, but they showed different proÞles to nonpyrethroid insecticides. The Salta population showed high resistance ratios (RRs) to deltamethrin and only slight differences in the susceptibility to fenitrothion and Þpronil compared with the reference strain. Otherwise, the La Rioja population showed a lower RR to deltamethrin and no resistance to fenitrothion or Þpronil. Finally, the Yacuiba population had high a RR to deltamethrin, but it was susceptibility to fenitrothion and Þpronil. In several cases, deltamethrin-resistant populations had higher susceptibility to bendiocarb than the reference strain. Measured activity of P450 monooxygenase in individual insects (based on ethoxycoumarine-O-deethylase), tended to be higher in the deltamethrin-resistant populations, but the differences were not statistically signiÞcant. Activity of speciÞc esterases determined by the hydrolysis of 7-coumaryl permethrate demonstrated an increase in the percentage of insects with higher esterase activity in the Salta and La Rioja populations. Unexpectedly, the Yacuiba population showed lower pyrethroid esterase activity than the reference strain. The different pyrethroid resistance patterns found in T. infestans from three geographical regions within Argentina and in Bolivia suggests that enzyme-based pyrethroid resistance in this species has multiple origins. Nevertheless, because nerve insensitivity (related to the presence of the kdr gene) is also an important mechanism related to pyrethroid resistance, further studies on the kdr gene should be carried to clarify the relative contribution of each pyrethroid-associated mechanism in deltamethrin-resistant populations of T. infestans.

Resistance levels to insecticides used in control of Chagas Disease vectors were assessed in two species of bugs (Hemiptera: Reduviidae): Triatoma infestans (Klug) from Brazil and Rhodnius prolixus Stål from Venezuela. The resistance ratios, compared to susceptible laboratory strains, were determined by topical application bioassays. The T. infestans PA strain exhibited resistance ratios of 7x to deltamethrin, 3.6x to beta-cyfluthrin and 3.3x to cypermethrin, but was susceptible to beta-cypermethrin and lambda-cyhalothrin. Rhodnius prolixus CA strain showed resistance to all the pyrethroids evaluated, the resistance ratios ranging between 4.5x to lambda-cyhalothrin and 12.4x to cypermethrin. Deltamethrin resistance in both strains was decreased by piperonyl butoxide, suggesting oxidative metabolism as cause of resistance.

Effectiveness of the elimination efforts against Triatoma infestans (Klug) in South America through residual application of pyrethroid insecticides has been highly variable in the Gran Chaco region. We investigated apparent vector control failures after a standard community-wide spraying with deltamethrin SC in a rural area of northeastern Argentina encompassing 353 houses. Insecticide spraying reduced house infestation less than expected: from 49.5% at baseline to 12.3 and 6.7% at 4 and 8 mo postspraying, respectively. Persistent infestations were detected in 28.4% of houses, and numerous colonies with late-stage bugs were recorded after the interventions. Laboratory bioassays showed reduced susceptibility to pyrethroids in the local bug populations. Eleven of 14 bug populations showed reduced mortality in diagnostic dose assays (range, 35 ± 5% to 97 ± 8%) whereas the remainder had 100% mortality. A fully enclosed residual bug population in a large chicken coop survived four pyrethroid sprays, including two double-dose applications, and was finally suppressed with malathion. The estimated resistance ratio of this bug population was 7.17 (range, 4.47–11.50). Our field data combined with laboratory bioassays and a residual foci experiment demonstrate that the initial failure to suppress T. infestans was mainly because of the unexpected occurrence of reduced susceptibility to deltamethrin in an area last treated with pyrethroid insecticides 12 yr earlier. Our results underline the need for close monitoring of the impact of insecticide spraying to provide early warning of possible problems due to enhanced resistance or tolerance and determine appropriate responses.

Previous work at our laboratory has indicated high resistance levels to deltamethrin correlated with failures of chemical control in field populations of Triatoma infestans (Klug, 1834) (Hemiptera: Reduviidae) in northern Argentina and southern Bolivia. The aim of the present work was to study the resistance patterns in eggs and first instars of T. infestans in populations from Argentina and Bolivia. At the egg stage, a population from Salvador Mazza, Argentina, showed the highest resistance ratio to deltamethrin and lambda-cyhalothrin, and it was susceptible to fipronil and fenitrothion. A population from Mataral, Bolivia, showed very low resistance ratios to deltamethrin and lambda-cyhalothrin, and it was susceptible to fipronil and fenitrothion. A Sucre population was susceptible to deltamethrin and fenitrothion, and it showed very low resistant ratios to lambdacyhalothrin and fipronil. A Yacuiba population was susceptible to deltamethrin. At the first instar, the Salvador Mazza population was susceptible to fipronil, whereas the Mataral and Sucre populations were susceptible to fenitrothion, and they showed very low resistance ratios to lambda-cyhalothrin but the high resistance to fipronil. The Salvador Mazza population was resistant to deltamethrin at the larval stage. Remarkable differences were found in the resistance profile to fipronil in first instars and eggs from Sucre and Mataral. These indicated that the expression of insecticide resistance in eggs varies between populations and that the pyrethroid resistance diagnosed in T. infestans first instars is not indicative of resistance in the egg stage.

Field populations of Triatoma infestans Klug were collected during 2002 from four villages in northern Argentina (El Chorro, La Toma, El Sauzal, and Salvador Mazza), after application of deltamethrin and other pyrethroids was ineffective. High levels of resistance to the pyrethroid insecticides deltamethrin, beta-cypermethrin, beta-cyfluthrin, and lambda-cyhalothrin were detected in all of the evaluated populations. The resistance ratio to pyrethroids determined by topical application ranged from 50.5 (deltamethrin, El Sauzal) to 667.6 (beta-cyfluthrin, Salvador Mazza). None of the pyrethroid-resistant insects was resistant to the organophosphorus insecticide fenitrothion. Topical application of piperonyl butoxide to the most deltamethrin-resistant population (Salvador Mazza) led to slight reduction in levels of resistance. Activity of P450 monooxygenase, measured in individual insects through ethoxycoumarine-O-deethylase, showed a slight but noticeable difference in the distribution of activities between susceptible and resistant populations. The total percentage of insects below 0.48 pmol of 7-OH coumarine/min/ insect was 36.4 for Salvador Mazza population and 64.3 pmol of 7-OH coumarine/min/insect for CIPEIN strain. Whereas a low level of resistance to deltamethrin was previously related to monooxygenase activity in T. infestans, the high levels of resistance shown by these populations seem to involve monooxygenase in combination with other resistance mechanisms, for example, insensitivity of nervous membrane. Research on T. infestans resistance is in progress to improve Chagas vector control programs in Latin America and to implement resistance management strategies.

BackgroundRapid reinfestation of insecticide-treated dwellings hamper the sustained elimination of Triatoma infestans, the main vector of Chagas disease in the Gran Chaco region. We conducted a seven-year longitudinal study including community-wide spraying with pyrethroid insecticides combined with periodic vector surveillance to investigate the house reinfestation process in connection with baseline pyrethroid resistance, housing quality and household mobility in a rural section of Pampa del Indio mainly inhabited by deprived indigenous people (Qom).Methodology/Principal findingsDespite evidence of moderate pyrethroid resistance in local T. infestans populations, house infestation dropped from 31.9% at baseline to 0.7% at 10 months post-spraying (MPS), with no triatomine found at 59 and 78 MPS. Household-based surveillance corroborated the rare occurrence of T. infestans and the house invasion of other four triatomine species. The annual rates of loss of initially occupied houses and of household mobility were high (4.6–8.0%). Housing improvements did not translate into a significant reduction of mud-walled houses and refuges for triatomines because most households kept the former dwelling or built new ones with mud walls.Conclusions/SignificanceOur results refute the assumption that vector control actions performed in marginalized communities of the Gran Chaco are doomed to fail. The larger-than-expected impacts of the intervention program were likely associated with the combined effects of high-coverage, professional insecticide spraying followed by systematic vector surveillance-and-response, broad geographic coverage creating a buffer zone, frequent housing replacement and residential mobility. The dynamical interactions among housing quality, mobility and insecticide-based control largely affect the chances of vector elimination.

Permethrin-resistant colonies of Pediculus capitis (De Geer) from Buenos Aires were used to establish a resistance profile and to examine resistance mechanisms. All permethrin-resistant head lice (resistance ratio from 52.8 to > 88.7) were also resistant to d-phenothrin (resistance ratio from 40.86 to > 48.39) and deltamethrin (resistance ratio from 16.24 to 38.06). No cross-resistance to carbaryl was found in any of the pyrethroid-resistant P. capitis tested. Otherwise, all resistant colonies showed low to high levels of resistance to beta-cypermethrin. This pyrethroid had never been applied as a pediculicide in Argentina; however, the high level of resistance found in these permethrin-resistant colonies (resistance ratio from 9.74 to 50.97) demonstrated that pyrethroid cross-resistance occurred to this novel insecticide. Treatment with piperonyl butoxide (PBO) or triphenylphosphate (TPP) significantly decreased the toxicity of permethrin in the four colonies tested. The esterase inhibitor TPP produced lower enhancement of toxicity than the multifunction oxidase inhibitor PBO in the colonies having the highest resistance levels. Results presented here concerning the cross-resistance profile and synergism by enzyme inhibitors in permethrin-resistant head lice demonstrated that enhanced metabolism was involved in the pyrethroid resistance. However, the substantial degree of resistance that remained after synergism suggested the presence of another resistance mechanism. Cross-resistance to pyrethroid and susceptibility to the carbamate carbaryl suggested a common action mechanism.