Despite five decades of battling dengue, the fight in Singapore remains a Sisyphean task. However, the city-state might soon gain the higher ground in this onerous fight.
Singapore’s National Environment Agency (NEA), on 18 October, released around 3,000 male Aedes Aegypti mosquitoes carrying the wolbachia bacteria into a small estate. Aedes Aegypti — or Aedes mosquitoes — are transmitters of viruses like Dengue, Chikungunya, and Zika. Singapore is host to a large population of Aedes mosquitoes. When the infected males mate with wild females, the resulting eggs do not hatch. Male mosquitoes do not bite humans and therefore do not transmit any viruses.
The wolbachia-males would be released in intervals over the next six months at three locations in Singapore: Braddell Heights, Tampines West, and Nee Soon.
“If we were to have regular releases of these mosquitoes, theoretically, there should be a reduction of mosquito population in the field,” said Dr Ng Lee Chin, director of environmental health institute, NEA.
In essence, wolbachia is the subterfuge NEA is deploying in this 50-year war. And the infected males will play the role of the Trojan horse.
However, this launch’s primary objective is not to suppress the Aedes mosquitoes. Rather, it’s part of a six-month study to analyse the flight range, behaviour, and lifespan of infected male mosquitoes. The data will help scientists design better strategies for the next phase — the suppression trial, which, according to Dr Ng, is the “ultimate aim.”
History of dengue in Singapore
Dengue has existed for over 200 years. The first recorded case of dengue in Singapore was in 1960. The number of cases climbed quickly thereafter prompting the government’s establishment of the Vector Control and Research Branch in June of 1966.
Vector control refers to regular ridding of mosquitoes (known as vectors) and their breeding.
There is no vaccine for dengue fever. So vector control, according to the World Health Organization (WHO), is our best defence against Aedes mosquitoes.
Over the next 50 years, Singapore responsively stepped up vector controls. That notwithstanding, the number of cases continued to rise, with peaks in each subsequent decade significantly higher than its prior.
1,324 cases were recorded in 1973. That swelled to 1,733 in 1990s. By the end of 2013, there were 10,257 of recorded cases. Singapore’s health ministry, together with the NEA, have warned that the number of dengue cases this year could go as high as 30,000, or higher.
Undebatable effectiveness of vector controls
The growing number of dengue cases in Singapore seems to indicate an indiscriminate job on the government’s end. However, there is a case to be made.
“I think Singapore has been doing a decent job,” said Dr Leong Hoe Nam, an infectious disease specialist in Singapore, “[But] the irony is that the more successful we become, the worse the dengue outbreak will be.”
To enjoy the irony, you’d first need to know the term, “Herd Immunity.”
At the outset, understand that though there are no known vaccines for dengue, recovering from the virus inoculates you — sort of like the chickenpox. Immunised individuals customarily shields small pockets of the population from the virus. Basically, the more of them sandwiched between you and the virus, the thicker the protection.
And as it happens, the success of vector controls weakens this shield by reducing the number of people getting, and recovering from, the virus.
That, compounded with dengue-virgin foreigners diluting and increasing Singapore’s population, reduces herd immunity, according to Dr Leong.
Statistically speaking, an increased population in a compact society alone would explain the rise in case numbers.
On top of that, effective waves of vector controls also delay the age someone experiences their first dengue attack.
“The older you are when you acquire dengue, the more symptomatic the person is likely to be,” said Dr Leong. In other words, dengue-related deaths are likelier to occur with age.
Currently, the average age of a Singaporean’s first brush with dengue is anchored between 60 to 70 years old.
Enter the Trojan horses
Countries like Australia, Brazil, Indonesia, and Vietnam have adopted the wolbachia technology as trial tests since 2011. According to the WHO, wolbachia is a natural bacteria found in 60 percent of common insects, so effects on the ecosystem would be next to zero.
The NEA began experimenting with the technology in 2010. Six years of hard work, vigorous risk assessments, and extensive research in the lab finally culminated in the release of 3,000 Trojan Aedes on one breezy Tuesday morning.
And at the right time, too.
Just two months before, Zika — a virus almost as old as dengue though not as potent — punctured Singapore. The virus gained profound attention when news that Zika in pregnant women causes babies to be born with microcephaly; underdeveloped brains and abnormally small heads.
“Zika is transmitted by the same Aedes Aegypti mosquitoes [as dengue],” said Dr Ng.
“So if we are successful with this technology, it should have an impact on Zika as well. It will reduce the risk of Zika transmissions.”
If it all works out according to plan, the uphill battle might soon conclude. Or, at the very least, it’ll level the playing field.
Derek is a journalist by day and a fast-food junkie by night. He enjoys political debate with a side of fries.