Last April, researchers released the dengue-blocking mosquitoes on Tri Nguyen Island with an aim to field test the mosquitoes implanted with the Wolbachia bacterium.
Wolbachia can be transmitted by an infected female mosquito to her offspring. Uninfected females that mate with infected males rarely produce fertile eggs. This gives infected females an advantage and helps the bacteria spread quickly through a mosquito population — at least in theory.
The researchers had calculated that if the introduced mosquitoes grew in number to 80 per cent of the island’s mosquito population, the 3,200 people living there would be protected from dengue.
But after reaching close to 80 per cent in August, the percentage of mosquitoes with the bacteria has fallen to below 65 per cent in early December, the study’s lead researcher, Scott O’Neill, a professor at Monash University in Australia, tells SciDev.Net.
O’Neill says the trend suggests the mosquitoes infected with the bacteria are dying possibly from the effect of bacteria, which do not occur naturally in mosquitoes, and that the bacteria will not stay in the island’s mosquito population for the long term.
But he insists the project is going well despite the setback.
“We’re in a process of optimising different strains of the [bacterium] Wolbachia and different approaches to deploying [the dengue-blocking mosquitoes], so it is just the first step in a series of trials,” he says.
O’Neill adds that the next field trial may begin in Vietnam in April or May, pending government approval.
The Wolbachia bacterium functions as a ‘vaccine’ for the mosquito by protecting it against many types of viruses including dengue virus possibly reducing the spread to humans. According to the WHO dengue infects up to 100 million people worldwide annually.
Field trials to introduce mosquitoes that block the spread of dengue fever have been under way in Australia for the past three years as part of the Eliminate Dengue Programme.
The Vietnam field trial is the programme’s first outside Australia, and there are plans to conduct field trials next year in Brazil, Colombia and Indonesia, says O’Neill.
“Success for us is not measured in a single trial,” he adds. “It’s measured over a number of trials in different locations, and overall the research is going very well at the moment despite the Wolbachia dropping out of that particular trial in Vietnam.”
Duane Gubler, a professor at the Emerging Infectious Diseases programme at the Duke-NUS Graduate Medical School in Singapore, says the results of the Vietnam study are “disappointing” but “not surprising”.
Even before the Wolbachia strain was introduced on the island, scientists suspected field trials would be less effective than it had been in the laboratory because of too many variables such as weather, he adds.
“These results should not discourage us as they are typical of the successes and failures that occur when a control approach developed in the laboratory is introduced to the field,” says Gubler.
This article has been produced by SciDev.Net's South-East Asia & Pacific desk.