Article

Efficacy of pyrethroid‐treated nets against malaria vectors and nuisance‐biting mosquitoes in Tanzania in areas with long‐term insecticide‐treated net use

 National Institute for Medical Research, Amani Research Centre, Muheza, Tanzania
Tropical Medicine & International Health (impact factor: 2.8). 08/2007; 12(9):1061 - 1073. DOI:10.1111/j.1365-3156.2007.01883.x pp.1061 - 1073

ABSTRACT Objective  To measure pyrethroid susceptibility in populations of malaria vectors and nuisance-biting mosquitoes in Tanzania and to test the biological efficacy of current insecticide formulations used for net treatment.Methods Anopheles gambiae Giles s.l., An. funestus Giles s.l. and Culex quinquefasciatus Say were collected during three national surveys and two insecticide-treated net (ITN) studies in Tanzania. Knockdown effect and mortality were measured in standard WHO susceptibility tests and ball-frame bio-efficacy tests. Test results from 1999 to 2004 were compared to determine trends in resistance development.Results Anopheles gambiae s.l. and An. funestus s.l. were highly susceptible to permethrin (range 87–100%) and deltamethrin (consistently 100%) in WHO tests in 1999 and 2004, while Culex quinquefasciatus susceptibility to these pyrethroids was much lower (range 7–100% and 0–84% respectively). Efficacy of pyrethroid-treated nets was similarly high against An. gambiae s.l. and An. funestus s.l. (range 82–100%) while efficacy against Cx. quinquefasciatus was considerably lower (range 2–100%). There was no indication of development of resistance in populations of An. gambiae s.l. or An. funestus s.l. where ITNs have been extensively used; however, susceptibility of nuisance-biting Cx. quinquefasciatus mosquitoes declined in some areas between 1999 and 2004.Conclusion  The sustained pyrethroid susceptibility of malaria vectors in Tanzania is encouraging for successful malaria control with ITNs. Continued monitoring is essential to ensure early resistance detection, particularly in areas with heavy agricultural or public health use of insecticides where resistance is likely to develop. Widespread low susceptibility of nuisance-biting Culex mosquitoes to ITNs raises concern for user acceptance of nets.Objectif  Mesurer la sensibilité aux pyréthroïdes de populations de vecteurs de la malaria et de moustiques piqueurs nuisants en Tanzanie et Examiner l’efficacité biologique des formulations actuelles d’insecticides utilisés pour le traitement des filets.Méthodes  Anopheles gambiae Giles s.l., An. funestus Giles s.l. et Culex quinquefasciatus Say ont été collectés au cours de trois surveillances nationales et de deux études sur les filets traités aux insecticides (ITNs) en Tanzanie. L’effet d’assommage et la mortalité ont été mesurés dans les essais standards de l’OMS pour la sensibilité et les essais de bio-efficacité de «ball-frame». Les résultats des essais de 1999 et de 2004 ont été comparés pour déterminer des tendances dans le développement de résistance.Résultats  An. gambiae s.l. et An. funestus s.l. étaient fortement sensibles au perméthrine (87 à 100%) et au deltaméthrine (tous à 100%) dans les essais de l’OMS en 1999 et 2004, alors que la sensibilité de Culex quinquefasciatus à ces pyréthroïdes était beaucoup faible (de 7 à 100% et 0 à 84% respectivement). L’efficacité des ITNs était pareillement forte sur An. gambiae s.l. et An. funestus s.l. (82 à 100%) tandis que l’efficacité sur Cx. quinquefasciatus était considérablement plus faible (2 à 100%). Il n’y avait aucune indication de développement de résistance dans les populations de An. gambiae s.l. ou An. funestus s.l. sur lesquelles les ITNs ont été intensivement utilisés. Cependant, la sensibilité des moustiques piqueurs nuisants, Cx. quinquefasciatus, déclinait dans certains endroits entre 1999 et 2004.Conclusion  La sensibilité constante des vecteurs de la malaria aux pyréthroïdes en Tanzanie est encourageant pour le contrôle efficace de la malaria avec l’utilisation des ITNs. La surveillance continue est essentielle pour assurer la détection tôt de la résistance, en particulier dans les endroits avec une utilisation massive des insecticides en agriculture et pour la santé publique où la résistance est susceptible de se développer. La faible sensibilité répandue des moustiques Culex piqueurs nuisants souligne le soucis pour l’acceptation des ITNs par les utilisateurs.Objetivo  Medir la susceptibilidad a piretroides en poblaciones de vectores de malaria y otros mosquitos responsables de picaduras molestas en Tanzania y evaluar la eficacia biológica de los métodos actuales de formulación de insecticida utilizados para el tratamiento de redes mosquiteras.Métodos  Se recolectaron Anopheles gambiae Giles s.l., An. funestus Giles s.l. y Culex quinquefasciatus Say durante tres estudios nacionales y dos estudios de redes mosquiteras impregnadas (RMI) en Tanzania. El efecto ‘Knockdown’ y la mortalidad fueron medidas mediante pruebas de susceptibilidad estándar de la OMS y pruebas de bioeficacia tipo ‘ball-frame’. Se compararon los resultados de las pruebas de 1999 con las de 2004, para determinar las tendencias en el desarrollo de resistencias.Resultados Anopheles gambiae s.l. y An. funestus s.l. eran altamente susceptibles frente a la permetrina (rango 87–100%) y la deltametrina (consistentemente 100%) en las pruebas de la OMS en 1999 y 2004, mientras que la susceptibilidad de Culex quinquefasciatus a estos piretroides era mucho menor (rango 7–100% y 0–84%, respectivamente). La eficacia de las RMI con piretroides era similar frente a An. gambiae s.l. y An. funestus s.l. (rango 82–100%) mientras que la eficacia frente a Cx. quinquefasciatus era considerablemente menor (rango 2–100%). No había indicación de desarrollo de resistencias en poblaciones de An. gambiae s.l. o An. funestus s.l. en lugares en los que las RMI habían sido usadas extensivamente; sin embargo, la susceptibilidad de mosquitos Cx. quinquefasciatus disminuyó en algunas áreas entre 1999 y 2004.Conclusión  La susceptibilidad mantenida de los vectores de malaria frente a los piretroides en Tanzania es esperanzadora en lo que respecta a un posible control exitoso de la malaria con RMI. La monitorización continua es esencial para asegurar una detección temprana de resistencias, particularmente en áreas con un uso intensivo de insecticidas, bien sea agrícolas o por motivos de salud pública, en los cuales la resistencia podría desarrollarse. La baja susceptibilidad generalizada a RMI de los mosquitos Culex es preocupante, pudiendo influir en la aceptación que los usuarios tengan de las redes.

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