Bacteria keeping up the fight against dengue

By Natalie Parletta

An international team of scientists has successfully deployed bacteria to reduce dengue cases by 40% in areas of Kuala Lumpur, Malaysia, where the disease is rife.

They achieved this using a new strain of the Wolbachia bacteria, known as wAlbB, that was able to survive the temperatures of 36 degrees Celsius and higher commonly experienced in the area. 

This is good news for other regions with hot climates in the South Pacific, Saudi Arabia, Africa and the Americas, where dengue and other mosquito-borne diseases are endemic. 

The resilient bacteria thrived despite the odds, notes Ary Hoffmann from the University of Melbourne, Australia, with promising applications and spin-off benefits.

“This study provides us with a new Wolbachia strain for field release and highlights disease impact within a complex urban setting where dengue rates are high,” he says. 

“The intervention succeeded despite ongoing pesticide applications and other challenges that can make it hard for the Wolbachia to become established.”

The successful lowering of dengue cases put an end to insecticide fogging in those areas, which is quite ineffective anyway – more good news from an economic, environmental and public health perspective.

Around half the world’s population is now at risk from dengue, which has spread rapidly throughout urban and semi-urban regions with tropical and sub-tropical climates due to increased human traffic and climate change.

Now second to malaria as a fever-causing disease, dengue infects nearly 400 million people in more than 100 countries each year, including outbreaks reported in areas of Europe and Australia. 

Around half a million people with severe dengue, causing haemorrhage or shock syndrome, need hospitalisation each year and around 2.5% die from the disease – although many countries have kept fatalities below 1% through improved case management. {%recommended 8785%}

Wolbachia bacteria infect an estimated 40% to 60% of insect species including mosquitos, but not those that spread human pathogens. Because the bacteria shorten the insects’ lifespan, researchers postulate this could thwart the incubation of viruses and parasites.

Other mechanisms that protect mosquitoes from viral infection seem to be involved as well, according to Hoffmann, including competition between the virus and bacteria for host resources and immune priming of host genes.

Previous trials infected Aedes aegypti mosquitoes – the primary dengue vector – with the wMel strain of Wolbachia, showing promise in hindering the disease’s spread, and recent evidence suggests the biocontrol approach has long-term viability. But those bacteria were not very stable in hot conditions.

In this study, researchers from the University of Melbourne, the Univeristy of Glasgow and the Institute for Medical Research in Malaysia released batches of A. aegypti mosquitoes carrying wAlbB into six different regions of Kuala Lumpur with high levels of disease transmission.

As the mosquitos mated with their wild counterparts, the virus-inhibiting bacteria spread rapidly to more than 80% at all sites, and in one area were found at 98% frequency a year after the last mosquitoes were released.

Glasgow’s Steven Sinkins says the positive results are encouraging for regions where dengue is endemic.

“These findings show that we have a strain of Wolbachia that can be used to effectively reduce the number of dengue cases in very hot climates,” he says.

“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.”

Hoffmann says it is currently being rolled out to more Malaysian sites and other countries are starting to show interest. 

Wolbachia also block other arboviruses carried by A. aegypti, such as Zika, chikungunya and yellow fever. Some preliminary work is exploring its potential for controlling malaria, caused by a parasite transmitted through Anopheles mosquito species.

The study is published in the journal Current Biology.