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It's time for bacterial warfare against the Aedes mosquito

NEA's $3m budget for use of Wolbachia bacteria bodes well, as reducing the Aedes population is best defence against dengue, Zika virus

Dengue has been plaguing Singapore for decades, with epidemics getting more severe with each cycle of infection.

Before the 1990s, dengue infections here had gone above 1,000 cases a year only once - in 1973.

In 2005, numbers soared to five-digits for the first time, with more than 14,000 people infected that year. It reached a new high in 2013 when more than 22,000 people were diagnosed with the disease.

The number of infections has not gone below five-digits since then, and given the high numbers this month already, 2016 is not shaping up well.

More than 2,000 people have come down with dengue this month alone, with a 47-year-old man dying from it. And this time of the year is traditionally a low dengue period.


So it is obvious that, in spite of the many millions of dollars spent and all the efforts made, dengue is not just here to stay, but can be expected to get worse.

According to two Nanyang Technological University dons, dengue cost the country more than $220 million in 2007 when more than 8,800 people were infected.

About two-thirds of the money was spent on attempts to eradicate the mosquito, and the rest comprised cost of treatment and man hours lost to the disease.

According to two Nanyang Technological University dons, dengue cost the country more than $220 million in 2007 when more than 8,800 people were infected. About two-thirds of the money was spent on attempts to eradicate the mosquito, and the rest comprised cost of treatment and man hours lost to the disease.

Generally, one in five people diagnosed with the disease is hospitalised. Given the higher number of dengue infections today, the cost now is likely to be significantly higher.

What is needed are new tools to fight this scourge, and not just more of the same.


Professor Duane Gubler of Duke-NUS Medical School, who specialises in emerging infectious diseases, said some of these new tools are already available and more will become available in the next two to five years.

The first vaccine against dengue has been commercially available in several countries since late last year. Two more vaccines are being developed which should hit the mass market in a couple of years.

Another tool available is the use of the bacteria, Wolbachia, to turn the male Aedes mosquito sterile. This has been used successfully by several countries, including Australia and Brazil, for many years.

Both have yet to make their appearance here.

Another weapon in the pipeline is a mosquito spray that has long residual effects, but which is ecologically friendly. Such insecticides can be sprayed on surfaces mosquitoes rest on. It can kill mosquitoes that land on such surfaces for many months after being sprayed.

But it will be some years before this hits the market.

Of the tools available, the current vaccine by Sanofi Pasteur is not a perfect match for Singapore, as it is most effective against Den-3 and Den-4 dengue strains, while the most active strains here are Den-1 and Den-2.

There are four strains of dengue virus, and an infection protects against only that particular strain and not the remainder.

This vaccine also works better on people who have been previously infected with dengue.

Because of decades of dengue suppression, more than half of people here have never been infected.

Usually, a second infection tends to be more serious. The vaccine has shown to provide more than 90 per cent protection against severe dengue illness caused by any viral strain.

So though it might not protect a person against getting infected with the Den-2 strain, which is the current dominant strain here, it would protect him against severe illness if infected.

There are still some unknowns about the vaccine - such as how long it remains effective and whether booster shots are needed.

Mr Masagos Zulkifli, Minister for the Environment and Water Resources, said in Parliament yesterday that the Government "will carefully evaluate the vaccine to ensure that it is safe, or high quality and effective, before it is made available in Singapore".

The vaccine has gone through years of testing. Yes, there is still a question of how effective it is, but nevertheless, it would be good to make it available for people who have been infected before.

It will then be up to the individual to decide whether to have the vaccine. But unless it is available here, they do not have that choice.


Another tool that has been used by people in other countries - and which Singapore has been studying for some time - is the release of male mosquitoes made sterile by the introduction of the Wolbachia bacteria.

Female mosquitoes they breed with lay eggs that do not hatch, thus bringing down the mosquito population.

The National Environment Agency (NEA) has been warning of the rising mosquito population in recent months, citing that as one reason for the sharp rise in dengue cases.

In 2014, an NEA expert panel recommended exploring this technique. Asked for an update this month as dengue cases started to soar, the NEA said it is doing a "comprehensive study and assessment of the novel technology".

The spokesman added: "Though Wolbachia is a naturally occurring bacterium and known to be safe, NEA is taking additional steps to assess the acceptability and suitability in Singapore."

Yesterday, Mr Masagos said his ministry is putting aside $3 million for the next three years to develop this "novel method of suppressing the Aedes mosquito population".

But this method is not really that novel with countries, including Australia, Brazil, Vietnam and Indonesia, already using it for several years.

As Prof Gubler noted, there are two ways of dealing with new tools: "Wait till we get all the answers, or learn by doing and monitor its use."

Singapore appears to be taking the first route.

The fact is that both the vaccine and Wolbachia are safe. The question is how effective they might be.

Wolbachia, while not found naturally in mosquitoes, is part of the ecology, being found in many insects including butterflies, dragonflies and fruit flies.

Aedes mosquitoes carrying Wolbachia are also not genetically modified. They merely carry a bacteria that prevents their sperm from fertilising eggs.

Using these sterile male mosquitoes cannot be a one-off attempt. Fresh batches of infected male mosquitoes need to be released periodically. This doesn't come cheap.

But Singapore is already spending a lot of effort and money in this fight, with limited results. Trying out new tools might just do the trick.

It is time for Singapore to take the more daring option of discovering by doing, rather than waiting for all answers before trying out new methods to bust dengue.

This is especially crucial now, given the danger of the newly emerging threat of the Zika virus which is transmitted by the same Aedes mosquito. It has already appeared in neighbouring Malaysia and Thailand.

As the population has never been exposed to this virus, people here have no immunity to this infectious disease. If it does come, and experts say it is just a matter of time, the situation here is ripe for its spread.

Historically, illness caused by the Zika is mild. But Brazil blames it for causing more than 4,000 babies to be born with microcephaly, or abnormally small heads, since last October.

It might also be linked to an increased incidence of the devastating Guillain-Barre syndrome, and other autoimmune and neurological disorders.

Several South American countries, where Zika is spreading, are now urging women to delay pregnancy.

The best protection Singapore can have is to reduce the number of Aedes mosquitoes, without which, neither dengue nor Zika can spread.

That should be the priority today.


A version of this article appeared in the print edition of The Straits Times on January 28, 2016, with the headline 'HomeFront It's time for bacterial warfare against the Aedes mosquito'. Print Edition | Subscribe