Dengue, a major mosquito-borne viral infection, continues to pose a serious public health threat in tropical and subtropical regions such as Malaysia.
With no specific cure, early detection and prevention remain the most effective strategies to curb this infectious disease.
Common measures include eliminating mosquito breeding sites (i.e. any stagnant water, even tiny puddles), avoiding mosquito bites (by using mosquito repellent, screens and nets) and fogging.
Another lesser-known method is through mosquito trapping.
“This method was first explored by Australian researchers, who showed that mosquito traps alone could reduce mosquito density,” shares Prof Datin Dr Indra Vythilingam from Universiti Malaya’s dengue and zoonotic malaria research group.
“Singapore later adopted it, and we saw potential in doing the same.”
The idea is to provide residents in dengue hotspots with a low-cost monitoring system for mosquitoes infected with the dengue virus.
Prof Indra and her team used gravid oviposition sticky (GOS) traps, which cost only 40sen to RM1 each when bought in bulk online.
These traps are designed to attract female mosquitoes with a diluted hay infusion (made by soaking dried grass for 10 days) as bait.
Only female Aedes aegypti or Aedes albopictus mosquitoes bite humans and transmit dengue, as the male mosquitoes don’t bite.
Insect glue and netting ensures mosquitoes can’t escape the trap.
In order to determine whether or not these mosquitoes were carrying the dengue virus, the scientists repurposed the NS1 antigen test kit, which is used in diagnosing dengue in humans.
The NS1 antigen is a protein on the dengue virus.
The captured mosquitoes were brought to the lab, identified and ground up, with the resulting fluids tested using the kits.
Prof Indra and her team first tested this method at the Mentari Court apartment complex in Petaling Jaya, Selangor, which has a history of dengue outbreaks.
Their aim was to determine how many mosquito traps were needed for effective monitoring.
They deployed traps in phases, starting with two blocks of the complex, before slowly expanding to all seven apartment blocks.
The researchers found that three traps per floor on alternate floors were sufficient for efficient monitoring.
In addition, Prof Indra shares that: “When a mosquito tested positive, human cases often followed within one to two weeks.
“This gave us time to intervene before outbreaks spread.”
Following the success of this first study, the team then conducted a randomised controlled trial (RCT) at Damansara Damai, Petaling Jaya, involving eight apartment blocks.
The blocks with mosquito traps showed a 40% reduction in dengue cases, compared to the blocks that didn’t have traps.
It was also found that as few as 20 traps per block were sufficient for early infected-mosquito detection.
The trial also involved testing residents for the dengue virus, following positive mosquito detection.
Of the 407 volunteers, 14 (3.43%) tested positive via the NS1 kit, while 34 (8.35%) tested positive through RT-qPCR (a more sensitive test than the rapid antigen kit), despite having no symptoms.
Their findings were published in the Scientific Reports journal in January 2022.
Prof Indra notes that: “The trap method was far more efficient than traditional larval surveys, which cover about 40 houses a day.
“Our method could monitor up to 3,000 homes daily with reduced manpower and lower costs.”
Larval surveys involve public health officials inspecting houses and other premises for potential mosquito breeding areas and counting the number of Aedes larvae present to assess the risk of dengue virus transmission.
When virus-positive mosquitoes were detected, the local council (i.e. the Petaling Jaya City Council) was alerted for fogging and other control efforts.
This method, Prof Indra says, offers a promising and cost-effective tool for early dengue detection in high-risk communities, providing valuable time for residents to act before infections surge.
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(From left) An NS1 antigen test kit and the two components of the GOS trap: a black container (a colour mosquitoes are attracted to) and a white container containing sticky paper.
Mentari Court served as the first test site for the mosquito trap-based dengue surveillance project. — Photos: Prof Datin Dr INDRA VYTHILINGAM
The researchers preparing to deploy the mosquito traps in Mentari Court.
The GOS traps were often placed next to potted plants as that is one of the places mosquitoes can be frequently found.
Caught! An Aedes mosquito trapped by the sticky paper in a GOS trap.
Some of the NS1 antigen kits used to test (human) volunteers during the trial at the Damansara Damai apartments. Two bands means that the person has the dengue virus.
