The impact of transfluthrin on the spatial repellency of the primary malaria mosquito vectors in Vietnam: Anopheles dirus and Anopheles minimus

Abstract

Background: The complexity of mosquito-borne diseases poses a major challenge to global health efforts to mitigate their impact on people residing in sub-tropical and tropical regions, to travellers and deployed military personnel. To supplement drug- and vaccine-based disease control programmes, other strategies are urgently needed, including the direct control of disease vectors. Modern vector control research generally focuses on identifying novel active ingredients and/or innovative methods to reduce human-mosquito interactions. These efforts include the evaluation of spatial repellents, which are compounds capable of altering mosquito feeding behaviour without direct contact with the chemical source.

Methods: This project examined the impact of airborne transfluthrin from impregnated textile materials on two important malaria vectors, Anopheles dirus and Anopheles minimus. Repellency was measured by movement within taxis cages within a semi-field environment at the National Institute of Hygiene and Epidemiology in Hanoi, Vietnam. Knockdown and mortality were measured in adult mosquito bioassay cages. Metered-volume air samples were collected at a sub-set of points in the mosquito exposure trial.

Results: Significant differences in knockdown/mortality were observed along a gradient from the exposure source with higher rates of knockdown/mortality at 2 m and 4 m when compared with the furthest distance (16 m). Knockdown/mortality was also greater at floor level and 1.5 m when compared to 3 m above the floor. Repellency was not significantly different except when comparing 2 m and 16 m taxis cages. Importantly, the two species reacted differently to transfluthrin, with An. minimus being more susceptible to knockdown and mortality. The measured concentrations of airborne transfluthrin ranged from below the limit of detection to 1.32 ng/L, however there were a limited number of evaluable samples complicating interpretation of these results.

Conclusions: This study, measuring repellency, knockdown and mortality in two malaria vectors in Vietnam demonstrates that both species are sensitive to airborne transfluthrin. The differences in magnitude of response between the two species requires further study before use in large-scale vector control programmes to delineate how spatial repellency would impact the development of insecticide resistance and the disruption of biting behaviour.

Keywords: Anopheles dirus; Anopheles minimus; Malaria elimination; Southeast Asia; Spatial repellent; Transfluthrin; Vietnam.

Fig. 1

Relationship between proportion mortality/knock-down and sampling distance. Mean values across all time points, pooled across replicates (a An. dirus and b An. minimus)

Fig. 2

Relationship between proportion mortality/knock-down and sampling time at each distance, by vector species, pooled across replicates

Fig. 3

Proportion of vectors repelled, attracted or with neutral movement from transfluthrin source at each experimental sampling distance, by species (a An. dirus and b An. minimus)

Fig. 4

Proportion of total vectors showing mortality or knock-down by sampling distance; all time points and heights shown. (N = 25 mosquitoes per replicate) (a An. dirus and b An. minimus)

Fig. 5

Comparison of the proportion of total vectors that showed any activation (either repelled or attracted); and proportion of those that were attracted out of the total that were activated (see “Methods”)