Abstract:

La pediculosis es una parasitosis externa que afecta con diferentes grados de intensidad las majadas ovinas. Ocasiona pérdidas de difícil cuantificación, tanto en la disminución de la productividad (menor producción de lana, retraso en el crecimiento, etc.) como en el aumento de costos de producción (compra de productos antiparasitarios, instalaciones, etc.). Dado que en la Región Patagónica la explotación ovina es el principal recurso ganadero, la pediculosis constituye una enfermedad de denuncia obligatoria (Senasa, Ley de Policía Sanitaria Animal N.º 3959). En este contexto, el control de la pediculosis mediante productos antiparasitarios reviste particular importancia, resulta así fundamental la evaluación regular de su efectividad. El presente proyecto propone caracterizar la efectividad de los productos antiparasitarios utilizados en el control de la pediculosis ovina causada por el piojo masticador Bovicola ovis. La determinación de dosis letales y posible resistencia permitirá evaluar las estrategias de control usadas hasta el momento y establecer nuevos protocolos de tratamientos antiparasitarios. Se propone establecer una cepa de laboratorio que posibilitará el desarrollo de un protocolo de monitoreo de efectividad cuyo resultado será transferido a las autoridades de Senasa y que podrá ser implementado para optimizar las medidas de control de B. ovis.

Ovine pediculosis is an external parasitic disease that affects sheep flocks with different degrees of intensity, causing losses that are difficult to quantify both by the decrease in productivity (lower wool production, growth delays, etc) and increased production costs (purchase of antiparasitic products, facilities, etc.). Since in the Patagonian region sheep is the main farm livestock resource, pediculosis is a disease of mandatory report (Senasa, Animal Health Police Law No. 3959). In this context, the control of lice with chemical products is of particular importance, and so regular evaluations of effectiveness are necessary. This project proposes characterizing the effectiveness of antiparasitic products used in the control of sheep pediculosis caused by chewing lice Bovicola ovis. The determination of lethal doses and possible resistance will help to assess control strategies used to date and establish new protocols for antiparasitic treatments. It is proposed to establish a laboratory strain that allows the development of an effectiveness monitoring protocol which will be transferred to the authorities of Senasa and which may be implemented to optimize the control measures of B. ovis.

Author affiliation: Germano, Mónica Daniela. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; Argentina

Author affiliation: Larroza, Marcela Patricia. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche; Argentina

Author affiliation: Mougabure Cueto, Gastón Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; Argentina

Abstract:

Triatoma infestans (Heteroptera: Reduviidae) Klug is the main vector of Chagas disease in Latin America. Resistance to deltamethrin was reported in Argentina and recently associated with reproductive and longevity trade‐offs. The objectives of the present study were to describe the demographic consequences of deltamethrin resistance in T. infestans and to establish possible target stages for chemical control in susceptible and resistant colonies. A stage‐classified matrix model was constructed based on the average stage length for susceptible, resistant and reciprocal matings' progeny. The differences between colonies were analysed by prospective and retrospective analysis. The life table parameters indicated reduced fecundity, fertility and population growth in resistant insects. The retrospective analysis suggested the latter was associated with lower reproductive output and increased fifth‐instar nymph stage length. The prospective analysis suggested that the adult stage should be the main target for insecticide control. Although, fifth‐instar nymphs should also be targeted when resistance has been detected. The presented results show demographic effects of deltamethrin resistance in T. infestans. While the older stages could be the main targets for chemical control, this approach is impeded by their higher tolerance to insecticides. It is concluded that the different mode of action insecticides would be more effective than a dose increase for the control of deltamethrin‐resistant T. infestans.

Author affiliation: Germano, Mónica Daniela. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; Argentina

Author affiliation: Picollo, Maria Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; Argentina

Abstract:

Deltamethrin-based campaigns to control Triatoma infestans (Klug) (Hemiptera: Reduviidae) have decreased in success as a result of the development of insecticide resistance. We compared the in vitro effects of the pyrethroid deltamethrin and two doses of the organophosphate fenitrothion, presented on different materials, on T. infestans from La Esperanza, Argentina. Laboratory tests demonstrated a decrease in susceptibility to deltamethrin in the field population [LD50: 30.32 nanograms per insect (ng/i)] compared with the reference population (LD50: 0.13 ng/i), giving a high resistance ratio of 233.42. By contrast, similar susceptibility to fenitrothion was assessed in both the field and reference populations (LD50: 21.65 ng/i and 21.38 ng/i, respectively). The effectiveness of the formulated insecticides varied according to the surfaces to which they were applied. The application of fenitrothion formulations to glass or brick resulted in mortality of 90–100%. The application of fenitrothion formulations to wood or mud caused mortality in the range of 6.7–56.7%. Resistant insects presented low mortality when exposed to the deltamethrin formulation and high mortality when exposed to fenitrothion formulations. Moreover, the insecticides demonstrated residual activity only when applied to glass. The present work demonstrates that fenitrothion is an alternative to pyrethroids for the management of deltamethrin-resistant insects in La Esperanza. However, this effectiveness is not sustained over time.

Author affiliation: Germano, Mónica Daniela. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina

Author affiliation: Picollo, Maria Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina

Author affiliation: Spillmann, Cynthia. Ministerio de Salud de la Nación; Argentina

Author affiliation: Mougabure Cueto, Gastón Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina

Abstract:

Triatoma infestans es un insecto de alta importancia sanitaria en Latinoamérica debido a su rol como vector de la enfermedad de Chagas. Este insecto es controlado mediante el rociado de las viviendas con insecticidas. A partir de las década de 1980, las medidas de control se focalizaron en el uso de la deltametrina, un insecticida de alta efectividad triatomicida y baja toxicidad en humanos. Sin embargo, a fines de la década de 1990 se detectaron fallas de control que fueron correlacionadas con una disminución de la efectividad del compuesto en las poblaciones de insectos. El desarrollo de resistencia a deltametrina fue detectado en diferentes localidades de Argentina y Bolivia, y durante los primeros años de la década 2000 ha sido causal de la disminución de la efectividad de los rociados con deltametrina. En este trabajo de tesis se determinó el alcance de la zona resistente en las provincias de Salta, Santiago del Estero y Chaco. Se evaluaron distintas localidades en cada provincia y se demostró que existen poblaciones de vinchucas con alto y mediano grado de resistencia, como así también prevalecen las áreas con T. infestans susceptibles a la deltametrina. Con el fin de determinar la posible reversión de la resistencia a deltametrina, resultó de interés determinar el modo de herencia de la resistencia a deltametrina en T. infestans. Se condujeron cruzamientos recíprocos de insectos susceptibles y resistentes, y se evaluó el grado de resistencia en la descendencia. Se demostró que la resistencia a este insecticida se hereda de manera autosómica y semidominante, y que se trata de un carácter determinado por al menos un gen mayoritario. Además, se evaluó la respuesta a la deltametrina en generaciones sucesivas de T. infestans resistentes, y se determinó la estabilidad de esta característica en poblaciones de campo y laboratorio. Mediante el uso de bioensayos se determinó la toxicidad de la deltametrina en los distintos estadios de desarrollo de T. infestans. Este ensayo demostró que la cantidad de insecticida necesario para eliminar al cincuenta por ciento de los insectos de una población susceptible aumenta con el estadio, y que la ninfa V es el estadio más tolerante al tratamiento químico. Por otra parte, se verificó que la resistencia a la deltametrina se expresa en todos los estadios de desarrollo. Además, se verificó que la ninfa V representa el estadio más resistente al insecticida en estudio, en comparación con la cepa susceptible. La confección de una tabla de vida permitió determinar las variaciones en el ciclo de vida del vector, en relación a la expresión de la resistencia. La evaluación de la duración de los estadios permitió demostrar que existen diferencias significativas entre la duración de los estadios II, III y V de las cepas susceptible y resistente. La mayor contribución a las diferencias estuvo dada por la duración de la ninfa V, la que presentó una duración significativamente mayor en la cepa resistente. Estos resultados sugirieron que la ninfa V ocupa un lugar clave en la estructura de las poblaciones resistentes a insecticidas mediante un mecanismo doble de resistencia: un alto grado de resistencia toxicológica, y un posible comportamiento de letargo de desarrollo asociado al refugio de la aplicación de insecticida. Además, la estructura poblacional fue evaluada mediante la confección de una matriz poblacional de estadios para las cepas susceptible, resistente, y sus cruzas recíprocas. La proyección de esta matriz demostró una menor tasa reproductiva en las cepas resistentes, en relación a la susceptible. El análisis prospectivo y retrospectivo de los parámetros poblacionales demostró que la causa fundamental de las diferencias en la tasa de crecimiento está asociada a una importante disminución de la fecundidad de las hembras resistentes. Además, se detectó una fuerte contribución de la duración de la ninfa V en este estadio, posiblemente debido a su asociación directa con la muda al estadio reproductivo. Los resultados de este trabajo de tesis indican que la resistencia a la deltametrina está presente en las provincias de Salta y Chaco. Además, se demuestra que es posible el cruzamiento entre insectos de zonas distantes geográficamente, y que el cruzamiento entre insectos susceptibles y resistentes genera una disminución amplia del grado de resistencia. Sin embargo, se observa que existen diferencias reproductivas entre las poblaciones susceptibles y las resistentes a deltametrina. Estas diferencias podrían estar asociadas a la resistencia a deltametrina y manifestarse no sólo con una menor fecundidad, sino como un cambio en la estructura poblacional y las tasas de muda al estadio adulto.

T. infestans is an insect of sanitary importance due to its role in Chagas disease transmission in Latin America. This insect is currently controlled by the use of chemical compounds for house impregnation. Since 1980s, deltamethrin has been the main compound used for its control because of its high triatomicidal action and its low human toxicity. However, campaigns have started to decrease in success since the end of the 1990s, when the development of resistance started to affect its effectiveness in Argentina and Bolivia. In this thesis the spread of deltamethrin resistance was evaluated. It was determined that its range includes Salta and Chaco provinces, where both high and moderated levels of resistance were determined. However, no resistance to this compound was found in Santiago del Estero populations. With the aim of determining a possible reversion of resistance, the way that this character is inherited was evaluated by conducting reciprocal crosses between susceptible and resistant insects. This assay demonstrated that resistance to the insecticide is produced by at least one main gene, and that it is inherited semi‐dominantly. Also, it demonstrated that there are no mother effects affecting resistance. The evaluation of deltamethrin toxicity in successive generations of resistant field and laboratory populations showed that the levels of resistance are not reversed after several years without chemical treatment. Through the use of bioassays the doses necessary to treat each of development stage were evaluated. Lethal doses for eliminating fifty percent of susceptible insects were determined to increase with the age of the insect. Moreover, resistance was expressed in all of the development stages. In fact, both the most tolerant, and the most resistant stage was the fifth nymph. A life table analysis was performed. The length of each stage was determined, and compared between the susceptible and the resistant colonies, including progeny of reciprocal crosses. The length of each development stage was compared, and the result indicated that there are significant differences between the susceptible and resistant colonies, when considering the second, third, and fifth nymph. In particular, the fifth nymph represented the major contribution to the differences between the colonies. The fact that this stage was the most tolerant to deltamethrin and that it represented the major difference between the colonies suggested that it plays a major role in the development of resistance, through two possible mechanisms: the toxicological change in response, and an ecological response related to avoidance of insecticide and possible developmental delays. In addition, life table information was analyzed using a stage structured population model. The population matrix was projected in time and it was determined that the resistant population shows a smaller intrinsic rate of increase. This difference was also detected for the reciprocal crosses progeny, which suggested that the decrease in reproductive success was associated to resistance, although not strictly to the degree of resistance. Prospective and retrospective analysis established that the main cause for this decline in the rate of increase was a lower fecundity of the resistant colony. In addition, the delay in moulting detected for the fifth nymph represented an important contribution to the differences between the colonies, possibly related to the delay in entering the reproductive state. The results presented in this thesis show that deltamethrin resistance is present in Salta and Chaco provinces. Moreover, they demonstrate that insects from distant geographical areas can reproduce effectively, although the crossing of susceptible and resistant insects importantly decreases the level of resistance. On the other hand, it was demonstrated that susceptible and resistant T. infestans present differences in their reproductive output. These differences could be related to deltamethrin resistance and be expressed not only as a fecundity decrease but also as a change in the population structure and rates of moulting to the adult stage.

Author affiliation: Germano, Mónica Daniela. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

Abstract:

Chagas disease is a chronic parasitic infection restricted to America where it is currently estimated that 90 million people are at risk of acquiring the infection. Chemical control with pyrethroid insecticides has been effective to reduce disease transmission in several areas of the Southern Cone, although insecticide resistance has evolved and diminished the campaigns’ results. Considering previous reports on the different levels of resistance between Triatoma infestans from different geographical areas, the objective of this work was to determine if T. infestans populations are toxicologically structured within localities. Response to the insecticide was measured and compared between houses of two Argentine localities. Different toxicity of deltamethrin was detected between dwellings of Chaco province, accounting for both susceptible and resistant houses within the same locality. However no difference was found among houses of Salta province. The results obtained in this work suggest that geographical structure is present not only at the between localities level, but also at the microgeograhical level.

Author affiliation: Germano, Mónica Daniela. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina

Author affiliation: Picollo, Maria Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina

Author affiliation: Mougabure Cueto, Gastón Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina

Abstract:

Chagas disease is a major health concern in Latin America, and Triatoma infestans (Klug, 1834) is responsible for the majority of cases of Chagas disease in the continent. After the discovery of deltamethrin highly resistant populations in the neighboring areas to Yacuiba (Bolivia) and Salvador Mazza (Argentina), we studied T. infestans populations north and southward, with the aim of describing the range of the resistant area. In addition, tests were conducted to describe the susceptibility to fipronil in deltamethrin-resistant populations. Tarija populations were highly resistant to deltamethrin, showing that the resistant area is greater than previously reported. Argentinean and Paraguayan populations were susceptible or presented moderate to low levels of deltamethrin resistance. Resistance to fipronil was found in Bolivian populations, although this insecticide was effective against Argentinean populations. This study shows that the Argentinean and Paraguayan populations are currently under successful control with deltamethrin. However, continuous resistance monitoring is necessary. We found that fipronil is a viable option for Argentina, whereas the resistance found in Bolivia implies that new formulations are needed to control T. infestans in Bolivia. Further research is required to find new alternatives of control in those areas that are currently suffering from high infestation rates.

Author affiliation: Germano, Mónica Daniela. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina

Author affiliation: Roca Acevedo, Gonzalo. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina

Author affiliation: Mougabure Cueto, Gastón Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina

Author affiliation: Toloza, Ariel Ceferino. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina

Author affiliation: Vassena, Claudia Viviana. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina

Author affiliation: Picollo, Maria Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina

Abstract:

Effective chemical control relies on reducing vector population size. However, insecticide selection pressure is often associated with the development of resistant populations that reduce control success. In treated areas, these resistant individuals present an adaptive advantage due to enhanced survival. Resistance can also lead to negative effects when the insecticide pressure ceases. In this study, the biological effects of deltamethrin resistance were assessed in the Chagas disease vector Triatoma infestans. The length of each developmental stage and complete life cycle, mating rate, and fecundity were evaluated. Susceptible and resistant insects presented similar mating rates. A reproductive cost of resistance was expressed as a lower fecundity in the resistant colony. Developmental costs in the resistant colony were in the form of a shortening of the second and third nymph stage duration and an extension of the fifth stage. A maternal effect of deltamethrin resistance is suggested as these effects were identified in resistant females and their progeny independently of the mated male's deltamethrin response. Our results suggest the presence of pleiotropic effects of deltamethrin resistance. Possible associations of these characters to other traits such as developmental delays and behavioral resistance are discussed.

Author affiliation: Germano, Mónica Daniela. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina

Author affiliation: Picollo, Maria Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina

Abstract:

Triatoma infestans Klug (Hemiptera: Reduviidae) is the main vector of Chagas disease in Latin America.This insect has been controlled with pyrethroids since the 1980s, although the emergence of resistance to deltamethrin has decreased control success in some areas of the Gran Chaco ecoregion.The response of T. infestans to deltamethrin was evaluated per developmental stage. In addition, we evaluated the possible stage-dependent expression of deltamethrin resistance.The bioassays were conducted by topical application of the insecticide in acetone.The drop size, age at the time of exposure, and mortality measuring time were standardized per stage. The lethal dose of deltamethrin moderately increased with the developmental stage.The resistance to deltamethrin was expressed in every instar, and was the highest in the fourth- and fifth-instar nymphs.While increasing, weight plays a relevant role in lethal dose stage dependency, a number of contributing factors such as degradative metabolism are probably involved in the variability of insecticide effect and resistance described for different T. infestans developmental stages. Possible explanations for these differences and their implications on resistance management and chemical control are discussed.

Author affiliation: Germano, Mónica Daniela. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche; Argentina

Author affiliation: Picollo, Maria Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; Argentina