Impact of environmental temperatures on resistance to organophosphate insecticides in Aedes aegypti from Trinidad.

OBJECTIVE: To examine the effects of increasing larval rearing temperatures on the resistance status of Trinidadian populations of Aedes aegypti to organophosphate (OP) insecticides. METHODS: In 2007-2008, bioassays and biochemical assays were conducted on A. aegypti larvae collected in 2006 from eight geographically distinct areas in Trinidad (Trinidad and Tobago). Larval populations were reared at four temperatures (28 ± 2ºC, 32ºC, 34ºC, and 36ºC) prior to bioassays with OP insecticides (fenthion, malathion, and temephos) and biochemical assays for esterase enzymes. RESULTS: Most larval populations reared at 28 ± 2ºC were susceptible to fenthion (>98% mortality) but resistant to malathion and temephos (< 80% mortality). A positive association was found between resistance to OP insecticides and increased activities of α- and ß-esterases in larval populations reared at 28 ± 2ºC. Although larval populations reared at higher temperatures showed variations in resistance to OPs, there was a general increase in susceptibility. However, increases or decreases in activity levels of enzymes did not always correspond with an increase or decrease in the proportion of resistant individuals reared at higher temperatures. CONCLUSIONS: Although global warming may cause an increase in dengue transmission, based on the current results, the use of insecticides for dengue prevention and control may yet be effective if temperatures increase as projected.

Impact of environmental temperatures on resistance to organophosphate insecticides in Aedes aegypti from Trinidad / Repercusión de las temperaturas ambientales sobre la resistencia de Aedes aegypti a los insecticidas organofosforados en Trinidad

OBJECTIVE: To examine the effects of increasing larval rearing temperatures on the resistance status of Trinidadian populations of Aedes aegypti to organophosphate (OP) insecticides. METHODS: In 2007-2008, bioassays and biochemical assays were conducted on A. aegypti larvae collected in 2006 from eight geographically distinct areas in Trinidad (Trinidad and Tobago). Larval populations were reared at four temperatures (28 ± 2ºC, 32ºC, 34ºC, and 36ºC) prior to bioassays with OP insecticides (fenthion, malathion, and temephos) and biochemical assays for esterase enzymes. RESULTS: Most larval populations reared at 28 ± 2ºC were susceptible to fenthion (>98% mortality) but resistant to malathion and temephos (< 80% mortality). A positive association was found between resistance to OP insecticides and increased activities of α- and β-esterases in larval populations reared at 28 ± 2ºC. Although larval populations reared at higher temperatures showed variations in resistance to OPs, there was a general increase in susceptibility. However, increases or decreases in activity levels of enzymes did not always correspond with an increase or decrease in the proportion of resistant individuals reared at higher temperatures. CONCLUSIONS: Although global warming may cause an increase in dengue transmission, based on the current results, the use of insecticides for dengue prevention and control may yet be effective if temperatures increase as projected.

OBJETIVO: Examinar los efectos del aumento de las temperaturas de desarrollo larvario sobre el estado de resistencia a los insecticidas organofosforados de las poblaciones de Aedes aegypti en Trinidad. MÉTODOS: En 2007 y 2008 se llevaron a cabo ensayos biológicos y bioquímicos en larvas de A. aegypti recogidas en el 2006 de ocho áreas geográficamente separadas en Trinidad (Trinidad y Tabago). Las poblaciones larvarias se desarrollaron en cuatro temperaturas (28 ± 2 ºC, 32 ºC, 34 ºC y 36 ºC) antes de los ensayos biológicos con insecticidas organofosforados (fentión, malatión y temefós) y los análisis bioquímicos para las enzimas de esterasa. RESULTADOS: La mayoría de las poblaciones larvarias que se desarrollaron a 28 ± 2 ºC fueron susceptibles al fentión (mortalidad > 98%) pero resistentes al malatión y al temefós (mortalidad < 80%). Se encontró una asociación positiva entre la resistencia a los insecticidas organofosforados y la mayor actividad de αy β-esterasas en las poblaciones larvarias que se desarrollaron a 28 ± 2 ºC. Aunque las poblaciones larvarias que se desarrollaron a temperaturas mayores mostraron variaciones en la resistencia a los organofosforados, hubo un aumento general de la sensibilidad. Sin embargo, los aumentos o las disminuciones en los niveles de actividad de las enzimas no siempre se correspondieron con un aumento o disminución en la proporción de individuos resistentes desarrollados a las temperaturas más altas. CONCLUSIONES: Aunque el recalentamiento del planeta puede causar un aumento de la transmisión del dengue, según los resultados de este estudio el uso de insecticidas para la prevención y el control del dengue todavía puede ser eficaz si las temperaturas aumentan según lo proyectado.

Characterization of insecticide resistance in Trinidadian strains of Aedes aegypti mosquitoes.

Bioassays and biochemical assays were conducted on eight Trinidadian strains of Aedes aegypti larvae to determine the involvement of biochemical mechanisms in resistance to insecticides. Larval strains were assayed to dichlorodiphenyltrichloroethane (DDT), bendiocarb, temephos and permethrin, using the Centers for Disease Control and Prevention (CDC) time-mortality bioassay method. A Resistance Threshold (RT) was calculated for each insecticide in relation to the CAREC reference susceptible Ae. aegypti strain and larval strains with <80% mortality were considered to be resistant. Biochemical assays were performed to determine the activities of nonspecific esterases (α- and ß-), PNPA-esterases, mixed function oxidases (MFO), glutathione-S-transferases (GST) and acetylcholinesterase (AChE) enzymes which are involved in insecticide resistance in mosquitoes. Enzyme profiles of each strain were compared with those of the CAREC reference susceptible strain by Kruskal-Wallis and Dunn's multiple comparison tests (p<0.05). The CAREC 99th percentile was calculated for each enzyme and the percentage of individuals with enzyme activities above that of the CAREC 99th percentile was calculated. Activities were classified as unaltered (<50%), incipiently altered (15-50%) or altered (>50%) for each strain. The established RTs for permethrin and bendiocarb were 30 and 75 min, respectively; and 120 min for DDT and temephos. All strains were resistant to DDT (1.00-40.25% mortality) and temephos (11.50-74.50% mortality) while six strains were resistant to bendiocarb (51.50-78.50% mortality) and five to permethrin (6.50-42.50% mortality). Biochemical assays revealed that the median activity levels for all enzymes varied significantly (p<0.05). The Curepe strain had incipiently altered levels of α-esterase while the other seven strains had altered activity with five of them registering 100%. The St Clair strain showed altered activity levels of ß-esterase while three strains had incipiently altered levels. The majority of strains had altered activity of MFO enzymes but only the St Clair strain showed altered activity of GST. PNPA-esterases activity was unaltered in all strains and only the Haleland Park strain showed altered remaining AChE activity in the presence of propoxur. Elevated levels of enzymes (incipiently altered or altered), except in the case of PNPA-esterases, show that biochemical resistance may play an important role in the manifestation of insecticide resistance in Trinidadian populations of Ae. aegypti. It is therefore important for insecticide resistance surveillance to be ongoing as the detection of resistance before it spreads throughout an entire population makes it possible for early intervention.

Organophosphate resistance in Trinidad and Tobago strains of Aedes aegypti.

Aedes aegypti larvae from 8 sites in Trinidad and 1 in Tobago were assayed against temephos, malathion, and fenthion using the Centers for Disease Control and Prevention time-mortality-based bioassay method. Resistance ratios (RRs) and resistance thresholds (RTs) for each insecticide were calculated in relation to the Caribbean Epidemiology Center reference susceptible strain. Results showed that the Haleland Park and Tobago strains were susceptible to fenthion and malathion, respectively (RRs < 1), while the San Fernando strain had a high RR (33.92) to malathion. All other strains had low-level resistance to fenthion and malathion. Resistance to temephos was more intense with 4 strains showing high-level resistance. The established RT was 60 min for fenthion, 75 min for bendiocarb, and 120 min for temephos and malathion. At the RTs, all Trinidad strains were resistant to temephos (11.50-74.50% mortality), 7 resistant to fenthion (21.25-78.75% mortality), and 5 resistant to malathion (56.25-77.50% mortality). The other strains were incipiently resistant (80-97% mortality). Despite the discrepancies between the RR levels and RT status, it is evident that the organophosphate insecticide resistance is prevalent in Trinidad and Tobago populations of Ae. aegypti. These results suggest that operational failure could soon occur and alternative strategies should be developed and implemented to reduce the probability of further selection pressure on resistant Ae. aegypti populations in Trinidad and Tobago.