Identification of tungiasis infection hotspots with a low-cost, high-throughput method for extracting Tunga penetrans (Siphonaptera) off-host stages from soil samples-An observational study.

BACKGROUND: The sand flea, Tunga penetrans, is the cause of a severely neglected parasitic skin disease (tungiasis) in the tropics and has received little attention from entomologists to understand its transmission ecology. Like all fleas, T. penetrans has environmental off-host stages presenting a constant source of reinfection. We adapted the Berlese-Tullgren funnel method using heat from light bulbs to extract off-host stages from soil samples to identify the major development sites within rural households in Kenya and Uganda. METHODS AND FINDINGS: Simple, low-cost units of multiple funnels were designed to allow the extraction of >60 soil samples in parallel. We calibrated the method by investigating the impact of different bulb wattage and extraction time on resulting abundance and quality of off-host stages. A cross-sectional field survey was conducted in 49 tungiasis affected households. A total of 238 soil samples from indoor and outdoor living spaces were collected and extracted. Associations between environmental factors, household member infection status and the presence and abundance of off-host stages in the soil samples were explored using generalized models. The impact of heat (bulb wattage) and time (hours) on the efficiency of extraction was demonstrated and, through a stepwise approach, standard operating conditions defined that consistently resulted in the recovery of 75% (95% CI 63-85%) of all present off-host stages from any given soil sample. To extract off-host stages alive, potentially for consecutive laboratory bioassays, a low wattage (15-25 W) and short extraction time (4 h) will be required. The odds of finding off-host stages in indoor samples were 3.7-fold higher than in outdoor samples (95% CI 1.8-7.7). For every one larva outdoors, four (95% CI 1.3-12.7) larvae were found indoors. We collected 67% of all off-host specimen from indoor sleeping locations and the presence of off-host stages in these locations was strongly associated with an infected person sleeping in the room (OR 10.5 95% CI 3.6-28.4). CONCLUSION: The indoor sleeping areas are the transmission hotspots for tungiasis in rural homes in Kenya and Uganda and can be targeted for disease control and prevention measures. The soil extraction methods can be used as a simple tool for monitoring direct impact of such interventions.

A randomized, double-blind placebo-control study assessing the protective efficacy of an odour-based 'push-pull' malaria vector control strategy in reducing human-vector contact.

Novel malaria vector control strategies targeting the odour-orientation of mosquitoes during host-seeking, such as 'attract-and-kill' or 'push-and-pull', have been suggested as complementary tools to indoor residual spraying and long-lasting insecticidal nets. These would be particularly beneficial if they can target vectors in the peri-domestic space where people are unprotected by traditional interventions. A randomized double-blind placebo-control study was implemented in western Kenya to evaluate: a 'push' intervention (spatial repellent) using transfluthrin-treated fabric strips positioned at open eave gaps of houses; a 'pull' intervention placing an odour-baited mosquito trap at a 5 m distance from a house; the combined 'push-pull' package; and the control where houses contained all elements but without active ingredients. Treatments were rotated through 12 houses in a randomized-block design. Outdoor biting was estimated using human landing catches, and indoor mosquito densities using light-traps. None of the interventions provided any protection from outdoor biting malaria vectors. The 'push' reduced indoor vector densities dominated by Anopheles funestus by around two thirds. The 'pull' device did not add any benefit. In the light of the high Anopheles arabiensis biting densities outdoors in the study location, the search for efficient outdoor protection and effective pull components needs to continue.

Predicting the impact of outdoor vector control interventions on malaria transmission intensity from semi-field studies.

BACKGROUND: Semi-field experiments with human landing catch (HLC) measure as the outcome are an important step in the development of novel vector control interventions against outdoor transmission of malaria since they provide good estimates of personal protection. However, it is often infeasible to determine whether the reduction in HLC counts is due to mosquito mortality or repellency, especially considering that spatial repellents based on volatile pyrethroids might induce both. Due to the vastly different impact of repellency and mortality on transmission, the community-level impact of spatial repellents can not be estimated from such semi-field experiments. METHODS: We present a new stochastic model that is able to estimate for any product inhibiting outdoor biting, its repelling effect versus its killing and disarming (preventing host-seeking until the next night) effects, based only on time-stratified HLC data from controlled semi-field experiments. For parameter inference, a Bayesian hierarchical model is used to account for nightly variation of semi-field experimental conditions. We estimate the impact of the products on the vectorial capacity of the given Anopheles species using an existing mathematical model. With this methodology, we analysed data from recent semi-field studies in Kenya and Tanzania on the impact of transfluthrin-treated eave ribbons, the odour-baited Suna trap and their combination (push-pull system) on HLC of Anopheles arabiensis in the peridomestic area. RESULTS: Complementing previous analyses of personal protection, we found that the transfluthrin-treated eave ribbons act mainly by killing or disarming mosquitoes. Depending on the actual ratio of disarming versus killing, the vectorial capacity of An. arabiensis is reduced by 41 to 96% at 70% coverage with the transfluthrin-treated eave ribbons and by 38 to 82% at the same coverage with the push-pull system, under the assumption of a similar impact on biting indoors compared to outdoors. CONCLUSIONS: The results of this analysis of semi-field data suggest that transfluthrin-treated eave ribbons are a promising tool against malaria transmission by An. arabiensis in the peridomestic area, since they provide both personal and community protection. Our modelling framework can estimate the community-level impact of any tool intervening during the mosquito host-seeking state using data from only semi-field experiments with time-stratified HLC.

Exploring the potential of using cattle for malaria vector surveillance and control: a pilot study in western Kenya.

BACKGROUND: Malaria vector mosquitoes with exophilic and zoophilic tendencies, or with a high acceptance of alternative blood meal sources when preferred human blood-hosts are unavailable, may help maintain low but constant malaria transmission in areas where indoor vector control has been scaled up. This residual transmission might be addressed by targeting vectors outside the house. Here we investigated the potential of insecticide-treated cattle, as routinely used for control of tsetse and ticks in East Africa, for mosquito control. METHODS: The malaria vector population in the study area was investigated weekly for 8 months using two different trapping tools: light traps indoors and cattle-baited traps (CBTs) outdoors. The effect of the application of the insecticide deltamethrin and the acaricide amitraz on cattle on host-seeking Anopheles arabiensis was tested experimentally in field-cages and the impact of deltamethrin-treated cattle explored under field conditions on mosquito densities on household level. RESULTS: CBTs collected on average 2.8 (95% CI: 1.8-4.2) primary [Anopheles gambiae (s.s.), An. arabiensis and An. funestus (s.s.)] and 6.3 (95% CI: 3.6-11.3) secondary malaria vectors [An. ivulorum and An. coustani (s.l.)] per trap night and revealed a distinct, complementary seasonality. At the same time on average only 1.4 (95% CI: 0.8-2.3) primary and 1.1 (95% CI: 0.6-2.0) secondary malaria vectors were collected per trap night with light traps indoors. Amitraz had no effect on survival of host-seeking An. arabiensis under experimental conditions but deltamethrin increased mosquito mortality (OR 19, 95% CI: 7-50), but only for 1 week. In the field, vector mortality in association with deltamethrin treatment was detected only with CBTs and only immediately after the treatment (OR 0.25, 95% CI: 0.13-0.52). CONCLUSIONS: Entomological sampling with CBTs highlights that targeting cattle for mosquito control has potential since it would not only target naturally zoophilic malaria vectors but also opportunistic feeders that lack access to human hosts as is expected in residual malaria transmission settings. However, the deltamethrin formulation tested here although used widely to treat cattle for tsetse and tick control, is not suitable for the control of malaria vectors since it causes only moderate initial mortality and has little residual activity.