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MANILA] Deploying mosquitoes carrying the Wolbachia bacteria can significantly reduce the incidence of dengue, the results of a trial conducted in Indonesia by the World Mosquito Program (WMP)Monash University, showed. 
 
Wolbachia, a common bacterium, occurs naturally in about 60 per cent of insect species, including mosquitoes. However, Aedes aegypti, the mosquito that transmits dengue, is one of those species that do not naturally carry the bacteria. 

“As we expand to larger cities around the world, we think it has the potential to eliminate dengue as a major public health problem where Wolbachia is deployed”

Dale Amtsberg, World Mosquito Program

“After successfully transferring Wolbachia from fruit flies into mosquitoes, WMP realised that it prevented replication of dengue within the mosquito and limited its ability to transmit the virus,” Dale Amtsberg, senior media and public relations advisor of the WMP, tells SciDev.Net. “This shifted our focus from shortening mosquito lifespans to infecting mosquitoes with Wolbachia to reduce their ability to transmit disease.”


Video credit: World Mosquito Program.

According to WMP, which announced the results of the trials in Yogyakarta last month, Wolbachia boosts mosquitoes’ immunity, making it harder for these to get infected with dengue and stopping transmission to humans. The bacteria also compete with viruses for key molecules, making it harder for the viruses to thrive in mosquitoes.
 
Besides dengue, the Wolbachia method can also make it harder for Zika, chikungunya and yellow fever, other viral diseases spread by A. aegypti, to be transmitted. 
 
WMP claims that the “first ever gold-standard trial to successfully target the A. aegypti mosquito and reduce dengue cases” showed a 77 per cent reduction in the incidence of dengue in the Wolbachia-treated areas when compared to untreated areas.
 
Adi Utarini, professor at the Universitas Gadjah Mada and principal investigator, WMP Indonesia, said in a statement: “This exciting result of the trial is a great success for the people of Yogyakarta. Indonesia has seven million dengue cases every year. This trial result shows the significant impact the Wolbachia method can have in reducing dengue in urban populations.”

Mosquito testing
A local community member gathering sample mosquitoes for testing within her home. Image credit: World Mosquito Program. 

“As we expand to larger cities around the world, we think it has the potential to eliminate dengue as a major public health problem where Wolbachia is deployed,” says Amtsberg.
 
Wolbachia-treated mosquitoes were first released by WMP ten years ago in North Queensland, Australia. According to Amtsberg, the bacteria was sustained by the local mosquito population with no negative results to date. Besides Yogyakarta and North Queensland, the WMP has similar projects in Brazil, Colombia, Fiji, India, Kiribati, Mexico, New Caledonia, Sri Lanka, Vanuatu and Vietnam.
 
Dengue is a leading cause of serious illness and death in some countries in Asia and Latin America, according to WHO, which reports that half of the world’s population is at risk of getting infected. Around 100 to 400 million contract dengue yearly.
 
“The incidence of non-fatal dengue cases has continued to increase in the recent decades,” Raman Velayudhan, unit head of WHO’s department of control of neglected tropical diseases. Rapid urbanisation, increased international travel and trade, and climate and environmental changes are factors in the rising incidence, he says.  
 
“Unfortunately, dengue continues to be neglected, with a lack of resources at country level and declining operational funds in recent years,” says Velayudhan. “There is a need to refocus efforts on building country capacity and address dengue control as a programme with year-round activity, working across sectors, and using locally adapted interventions to implement sustainable dengue prevention and control.”
 
Singapore also recently deployed Wolbachia but with a different strategy that involves release of male Wolbachia-carrying A. aegypti to mate with the wild female mosquitoes. The eggs they produce do not hatch, leading to a decline in the A. aegypti population. “If you can release enough infected male mosquitoes into the population to mate with the majority of females, the population will crash,” says Duane Gubler, emeritus professor and founder of the emerging infectious diseases programme at the Duke-NUS Medical School and chair of the Global Dengue and Aedes-Transmitted Diseases Consortium. “If successful, dengue and other viruses transmitted by this mosquito, i.e. Zika, yellow fever and chikungunya, will be effectively controlled.”
 
Gubler, who is the chair of the Dengue Expert Advisory Panel of Singapore, says there is no evidence suggesting that the release of Wolbachia-treated mosquitoes will have ecological or other impacts. “The only negative implication of this method is that it must be continued indefinitely to maintain control,” Gubler says.
 
Velayudhan recommends a “whole-of-system” approach to control the spread of dengue. “Stepped-up personal and population-wide measures for control of vector mosquitoes and interruption of human-vector contact are needed. This requires a whole-of-system approach and enhanced community engagement.” 
 
While WHO encourages affected countries to boost its current mosquito control interventions, it also welcomes the new initiatives being undertaken to regulate dengue. “As there are few effective, sustainable tools available to combat Aedes-borne diseases, all new tools that demonstrate public health value against dengue and similar viruses, will be a welcome addition to the vector control arsenal.”
 
Dengue expert and former president of the American Society of Tropical Medicine and Hygiene Scott Halstead tells SciDev.Net that a dengue vaccine remains the best bet to combat the disease. “This [Wolbachia method] is a step forward, but, not yet a dramatic breakthrough.” 
 
This piece was produced by SciDev.Net’s Asia & Pacific desk.
 

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