Wide area spray of bacterial larvicide, Bacillus thuringiensis israelensis strain AM65-52, integrated in the national vector control program impacts dengue transmission in an urban township in Sibu district, Sarawak, Malaysia.

Several sites, Z-7L, Z-5 and Z-14, in Sibu district, Sarawak, Malaysia, experienced intense dengue transmission in 2014 that continued into 2015. A pilot study with Bacillus thuringiensis israelensis (Bti) to control Aedes aegypti (L.) and Ae. albopictus (Skuse) was evaluated in Z-7L, a densely populated site of 12 ha. Bti treatments were conducted weekly from epidemiology week (EW) 24/2015 for 4 weeks, followed by fortnight treatments for 2 months, in addition to the routine control activities. Bti was directly introduced into potable containers and the outdoor artificial and natural containers were treated via a wide area spray application method using a backpack mister. Aedes indices significantly reduced during the treatment and post treatment phases, compared to the control site, Z-5 (p<0.05). A 51 fold reduction in the incidence rate per 100,000 population (IR) was observed, with one case in 25 weeks (EW 29-52). In Z-5 and Z-14, control sites, a 6 fold reduction in the IR was observed from EW 29-52. However, almost every week there were dengue cases in Z-14 and until EW 44 in Z-5. In 2016, dengue cases resurfaced in Z-7L from EW 4. Intensive routine control activities were conducted, but the IR continued to escalate. The wide area Bti spray misting of the outdoor containers was then included from EW 27 on fortnight intervals. A 6 fold reduction in IR was observed in the Bti treatment phase (EW 32-52) with no successive weekly cases after EW 37. However, in the control sites, there were dengue cases throughout the year from EW 1-52, particularly in Z-14. We feel that the wide area Bti spray application method is an integral component in the control program, in conjunction with other control measures carried out, to suppress the vector population in outdoor cryptic containers and to interrupt the disease transmission.

Bacterial Larvicide, Bacillus thuringiensis israelensis Strain AM 65-52 Water Dispersible Granule Formulation Impacts Both Dengue Vector, Aedes aegypti (L.) Population Density and Disease Transmission in Cambodia.

A multi-phased study was conducted in Cambodia from 2005-2011 to measure the impact of larviciding with the bacterial larvicide, Bacillus thuringiensis israelensis (Bti), a water dispersible granule (WG) formulation on the vector, Aedes aegypti (L.) and the epidemiology. In our studies, all in-use containers were treated at 8 g/1000 L, including smaller containers and animal feeders which were found to contribute 23% of Ae aegypti pupae. The treated waters were subjected to routine water exchange activities. Pupal production was suppressed by an average 91% for 8 weeks. Pupal numbers continued to remain significantly lower than the untreated commune (UTC) for 13 weeks post treatment in the peak dengue vector season (p<0.05). Suppression of pupal production was supported by very low adult numbers in the treated commune. An average 70% of the household harbored 0-5 Ae aegypti mosquitoes per home for 8 weeks post treatment, but in the same period of time >50% of the household in the UTC harbored ≥11 mosquitoes per home. The adult population continued to remain at significantly much lower numbers in the Bti treated commune than in the UTC for 10-12 weeks post treatment (p<0.05). In 2011, a pilot operational program was evaluated in Kandal Province, a temephos resistant site. It was concluded that 2 cycles of Bti treatment in the 6 months monsoon season with complete coverage of the target districts achieved an overall dengue case reduction of 48% in the 6 treated districts compared to the previous year, 2010. Five untreated districts in the same province had an overwhelming increase of 352% of dengue cases during the same period of time. The larvicide efficacy, treatment of all in-use containers at the start of the monsoon season, together with treatment coverage of entire districts interrupted disease transmission in the temephos resistant province.

Bacillus thuringiensis var. israelensis misting for control of Aedes in cryptic ground containers in north Queensland, Australia.

In Australia, dengue is not endemic, although the vector mosquito Aedes aegypti is established in far north Queensland (FNQ). Aedes albopictus has recently invaded the Torres Strait region, but is not established on mainland Australia. To maintain dengue-free, public health departments in FNQ closely monitor introduced dengue infections and confine outbreaks through rigorous vector control responses. To safeguard mainland Australia from Ae. albopictus establishment, pre-emptive strategies are required to reduce its breeding in difficult to access habitats. We compare the residual efficacy of VectoBac WDG, Bacillus thuringiensis var. israelensis (Bti) formulation, as a residual treatment when misted across a typical FNQ bushland using a backpack mister (Stihl SR 420 Mist Blower) at two dose rates up to 16 m. Semi-field condition results, over 16 weeks, indicate that Bti provided high mortality rates (> 80%) sustained for 11 weeks. Mist application penetrated 16 m of dense bushland without efficacy decline over distance.

Aedes albopictus control with spray application of Bacillus thuringiensis israelensis, strain AM 65-52.

A Bacillus thuringiensis israelensis (Bti) formulation, VectoBac WG (strain AM 65-52), was evaluated for mosquito control in a training area with dense vegetation. Bti was spray applied to target Aedes albopictus larval habitats of 130 ha once every 2 weeks using a motorized back pack mist blower, Stihl SR420, and a vehicle mounted ultra low volume generator (ULV), IGEBA U40. Ovitrap index (OI) and larval density (LD) were used to measure the efficacy of larviciding. In the Bti treated area the OI and LD significantly decreased with time (p < 0.05); OI decreased from 84.3 +/- 1.7 to 27.5 +/- 2.5 (%) and LD decreased from 27.9 +/- 1.5 to 3.2 +/- 1.8 larvae per ovitrap by 3 months from the start of treatment. During the same period of time there was no significant reduction in OI and LD at the untreated site which was under a conventional mosquito control program. This large scale study indicates larvicidal spraying with Bti of natural breeding sites, was able to reduce Ae. albopictus adult density. This significant reduction was not achieved with conventional manual application methods.

Bacillus thuringiensis var. israelensis (Bti) provides residual control of Aedes aegypti in small containers.

We examined the use of megadoses of VectoBac WG for residual control of Aedes aegypti in 2-L plastic buckets. Doses of 10x, 20x, and 50x the recommended rate of 8 mg/L provided >/= 90% control for 8, 8, and 23 weeks, respectively. There was no significant difference in mortality between dry (neat) or aqueous mixture of VectoBac WG. Pretreatment of dry containers up to 8 weeks before flooding did not significantly decrease efficacy through 11 success weeks. Thus, megadoses of dry formulations of Bti can be used for residual control of Ae. aegypti in small containers. Furthermore, these doses use small amounts of product (0.08-0.4 g/L) that is more practical to measure than the minute amounts (0.008 g/L) required by the recommended rate, and cost US$2.18 to treat 50 Cairns yards containing an average total of 80 containers. This method could also be used to control Aedes albopictus.

Efficacy of a Bacillus thuringiensis israelensis tablet formulation, vectobac DT, for control of dengue mosquito vectors in potable water containers.

VectoBac DT, a tablet formulation of Bacillus thuringiensis israelensis (Bti) was evaluated for the potential control of dengue vectors in various types of potable water containers. On introduction to containers, the tablet sinks to the bottom and the Bti toxins are found concentrated at the sides and the base, while the treated water column is free of Bti toxins within 24 hours after tablet introduction. In a simulated study, earthen, HDPE and plastic containers were kept covered and laboratory-bred larvae were introduced to determine the control by the tablet. The efficacy and persistence of the tablet, with a control of > 90%, was significantly longer in earthen containers in comparison to the HDPE and plastic containers. Efficacy and persistence were observed in earthen containers for a minimum period of 5.5 months (166 days) both without water replenishment and with weekly, 50% water volume, replenishment, and for a maximum period of 2.2 months (66 days) with daily, 50% water volume, replenishment. In plastic and HDPE containers, the tablet activity had a persistence of 2.1 months (63 days) without water replenishment and 1.8 months (54 days) with weekly water replenishment. The efficacy and persistence of the VectoBac DT was significantly longer in the earthen containers, with or without regularly treated water exchange, due to the Bti toxins being embedded in the porous earthen container surfaces, which protects them from rapid degradation. Lesser toxin amounts are removed from the water column during water exchange. The efficacy of VectoBac DT was also evaluated for the control of natural infestation of Aedes larvae which were resistant to temephos at the WHO diagnostic dosage of 0.012 mg/l. The tablet significantly reduced the pupal density by 8 fold in earthen containers for 67 days and 5 fold in HDPE containers for 55 days in comparison to untreated containers (p < 0.05). However, the tablet was effective for a shorter period of 25 days post-tablet-introduction due to fungal infestation in the treated plastic containers. There is a need to determine the capacity of the VectoBac DT to reduce the dengue vector population to a threshold which will prevent dengue outbreaks in dengue endemic areas.

Dengue vector surveillance in urban residential and settlement areas in Selangor, Malaysia.

Ovitrap surveillance was conducted in two urban residential areas (Taman Samudera Timur and Taman Samudera Selatan) and in a settlement area (Kampung Banjar), which is located 16 km from Kuala Lumpur city center, Malaysia. In Taman Samudera, dengue cases were reported monthly in 2003/2004. Thus, a study was initiated to determine the distribution and abundance of dengue vectors, Aedes aegypti and Ae. albopictus. The ovitrap surveillance indicated that Ae. aegypti and Ae. albopictus were present both indoors and outdoors. The residential sites had 73 - 79% of the ovitraps with just Ae. aegypti population and Kg. Banjar had 56% of the ovitraps with just Ae. aegypti. In the indoor and outdoor of the residential areas, together with the settlement area, the Ae. aegypti density was significantly more than Ae. albopictus (p < 0.05) by 3 - 50 folds. There was no significant difference in the larval numbers of Ae. aegypti between indoors and outdoors (p > 0.05), thus implicating that adult gravid female Ae. aegypti are present both indoors and outdoors and they do oviposit indoors and outdoors. Ae. aegypti can be incriminated as the principal dengue vector in the urban residential site, Taman Samudera and in the settlement area, Kg. Banjar.