Act against dengue now with tools that exist

Act against dengue now with tools that exist
An NEA officer checking the roof of a house for mosquito-breeding sites. Vector control remains a key strategy in the fight against dengue.

Sir Robert Watson Watt, one of the inventors of radar, once said: "Give them the third best to go on with; the second best comes too late, the best never comes."

In the public health arena we frequently, in our quest for perfection, lose sight of tools that we have in hand which can make a big impact on the multitude of public health problems we face.

We also often forget that no single tool can solve these complex problems. The control of dengue is a prime example.

The incidence of dengue, caused by viruses spread by the Aedes mosquito, has increased 30-fold in the past 50 years.

An estimated 390 million cases occur every year in more than 100 countries, mostly in the developing world, and 40 per cent of the world's population is at risk.

Each year, an estimated half a million people contract severe dengue, often accompanied by bleeding and shock, and require hospitalisation.

The annual economic cost of dengue illness in Asia is estimated at US$2 billion (S$2.7 billion), not including the costs of preventive and vector control efforts.

Unfortunately, the situation is likely to remain grim.

Increased urbanisation, travel and migration, the pressures of globalisation, and global warming are likely to maintain dengue transmission at high levels and continue to result in major outbreaks in affected countries.

In Singapore, dengue is considered "hyperendemic" because all four strains or serotypes circulate all year round.

Singapore witnessed large outbreaks of dengue in 2013 and 2014 with 22,170 and 18,338 cases respectively, and, while cases are down this year, continued vigilance and preparedness is clearly needed.

Dengue outbreaks come in cycles and it is difficult to predict when the next one will happen, or if the predominant serotype will change. The economic burden of dengue in Singapore from 2000 to 2009 is estimated each year to range between US$0.85 billion and US$1.15 billion, of which 41 per cent to 58 per cent is direct medical, direct non-medical and indirect costs such as loss in work productivity, and reduction of household services. The remainder is the cost of vector control efforts.

Dengue control remains both a challenge and an enigma.

In the absence of effective treatment for severe dengue disease, the mainstays of dengue control have been to reduce the mosquito vectors which transmit the virus, and a high level of public awareness and community involvement in the fight against dengue.

While vector control remains a key strategy, it has not always been effective.

Singapore spent about half a billion US dollars over the past decade on control programmes and has successfully reduced the Aedes premises index to very low levels, yet it still experiences major outbreaks of dengue.

And despite more than 40 years of research, we continue to wait for an effective vaccine.

Because of all these challenges, it is clear that we need to fight dengue on many fronts through an integrated and inter-sectoral approach, which includes good clinical case management, surveillance with effective field-laboratory-clinic coordination, as well as community participation, engagement and ownership.

Singapore has successfully implemented the Dengue Community Alert System, to alert residents in areas where there is heightened dengue transmission, so that they can take the necessary steps.

But as part of such a strategy we must also continue our search for new and better tools, and to deploy those promising ones which already exist.

First, continued basic research to identify the critical cells in the body which mount a protective immune response to dengue should be pursued, as it may eventually result in novel vaccines.

Basic research to develop effective treatments for dengue disease should similarly be a priority. Several leading institutions in Singapore are vigorously pursuing these lines of research.

Indeed, one group announced a breakthrough here last week - it had created an artificial antibody that is equally effective against all four serotypes, which is expected to go on human trials next year.

Second, novel methods for suppressing mosquito populations, such as the use of Wolbachia-infected Aedes, should be fully evaluated and tested.

Wolbachia is a naturally-occurring bacterium which, when spread among mosquitoes, leads to sterility and reduction in numbers.

The National Environment Agency (NEA) of Singapore is evaluating this novel and promising approach.

Third, serious and urgent consideration should be given to deploying the recently-developed dengue vaccine as an important complementary tool in the fight against dengue.

Manufactured by Sanofi-Pasteur, the vaccine has completed extensive testing in all phases of clinical trials and has reached the review process by national regulatory authorities for licensing.

Underscoring the importance of the problem, several other vaccine manufacturers also have dengue vaccines which are being evaluated in various phases of testing, and which may become available in the coming years.

At the same time, concerns have been voiced about some of these promising new tools.

Wolbachia-infected Aedes, as a means of controlling the Aedes mosquito, has been tested only in fairly limited pilot studies in Australia, Vietnam and Indonesia, where no adverse effects were observed.

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