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Scientists have reported an effective and environmentally sustainable way to block the transmission of mosquito-borne dengue virus, in trials carried out in Malaysia.

Using a strain of the bacteria Wolbachia, which inhibit mosquitoes from transmitting viruses to humans, researchers at the Universities of Glasgow and Melbourne and the Institute for Medical Research in Malaysia were successfully able to reduce cases of dengue at sites in Kuala Lumpur. Their data, published in Current Biology, shows that mosquitoes carrying the wAlbB strain of Wolbachia, when released into the wild, had the effect of reducing the incidence of dengue cases by 40%.

Previously, scientists have carried out successful mosquito releases using a different strain of the Wolbachia bacteria, but while this strain was effective in some conditions, it did not appear to be suitable for use in the very hot conditions experienced in equatorial countries such as Malaysia.

Now, this international team of researchers has shown that the wAlbB strain of Wolbachia is stable and effective, even in daily peak temperatures of 36 degrees and higher, as commonly experienced in areas of Malaysia where dengue is prevalent.

Each year there are around 90 million symptomatic cases of dengue, with severe disease in around 1% of cases, including life-threatening hemorrhage or shock syndrome. In Malaysia alone, over 100,000 dengue cases were reported in 2016, with an annual cost estimated at $175 million.

Researchers released batches of Aedes aegypti mosquitoes carrying the wAlbB strain of Wolbachia into the wild, in six different sites in greater Kuala Lumpur with high levels of dengue transmission. The Wolbachia carrying mosquitoes – both male and female – then went on to mate with the wild mosquito population, resulting in the spread and establishment of the virus-inhibiting bacteria. In some sites, Wolbachia-carrying mosquitoes were measured at over 90% frequency more than a year after the mosquito releases ended.

The success of lowering dengue cases at these sites has led to a cessation of insecticide fogging in these areas, highlighting both the environmental and economic benefits of this method.

Professor Steven Sinkins, lead contact for the study and Professor at the MRC-University of Glasgow Centre for Virus Research, said: “We are excited by these findings, which show that we have a strain of Wolbachia that can be used to effectively reduce the number of dengue cases in very hot climates. The next step is to deploy this strain in more and larger sites, but we are now confident that this will become an effective way to control dengue on a large scale.” The Malaysian Ministry of Health is leading this expansion in and around Kuala Lumpur.

The wAlbB strain of Wolbachia is also effective for blocking transmission of other viruses including Zika, chikungunya, and yellow fever.

Michael Chew, Infection and Immunobiology Portfolio Manager at Wellcome said: “These findings mark important progress in the future of dengue fever control. It is exciting to see, for the first time, a strain of Wolbachia successfully reduce the number of dengue cases in very hot climates in the wild.

“The unprecedented rise of dengue worldwide make control methods such as these a vital addition to the tools we currently have to tackle one of the fastest-spreading mosquito-borne viral diseases. It is crucial that local communities and the Malaysian government continue to provide their support, which will be key for the long-term sustainability in controlling dengue on a large scale.”

Professor Ary Hoffmann, one of the three lead contacts for the study from Melbourne’s Bio21 Institute, indicated that: “This study provides us with a new Wolbachia strain for field release and highlights disease impact within a complex urban setting where dengue incidence rates are high. The intervention succeeded despite ongoing pesticide applications and other challenges that can make it hard for the Wolbachia to become established, and the approach holds promise not only in Malaysia but also in other countries.”

The paper, ‘Establishment of Wolbachia strain wAlbB in Malaysian populations of 2 Aedes aegypti for dengue control’ is published in Current Biology. The work was funded by the Wellcome Trust.

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