Dr. Ong Song Quan, Lecturer And Head of Department, UOW Malaysia KDU

The Aedes Mosquito is now quietly breeding and we may well be faced with a spike in dengue in May-June.

By Dr Ong Song Quan, Head of Department-Centre for Pre-University Studies in Penang

Blazing sun, dull sky and threatening rain is the typical weather in Malaysia. This intermittent sunny then rainy weather pattern creates the perfect breeding conditions for the vector of Dengue – Aedes mosquitoes.

Dengue cases dropped during the Movement Control Order (MCO) 2.0 however we should not be complacent as they are set to surge this year in the months of May to June.

According to a model simulation study in 2020 which did not expect the implementation of MCO 1.0, it was found that the actual trend in dengue cases was lower in the beginning, but later it demonstrated a significant spike in transmission.

If the same forecast model were to be applied in 2021, the trend for dengue cases may show a surge later in May and June (Figure 1).

This “U” trend in dengue incidences, and the low levels of dengue reported last year, could be due to several factors. In the period between March to May we typically see a decline in reported dengue cases, but meanwhile in the background Aedes mosquitoes are quietly breeding as it is the transition of the Northeast monsoon season – presenting the ideal breeding conditions for these mosquitos.

Meanwhile, in May-June we typically see an increase in dengue reported, however in year 2020 this was inhibited somewhat by movement controls restrictions leading to less people movement and resulting in greater attentiveness to home environments. This led to less opportunities for mosquitoes to find a suitable breeding spot.

In addition, there are indications that dengue infected patients may have been afraid to visit hospitals for treatment due to their concern of contracting COVID-19. Finally, there is also the possibility in the midst of the pandemic there may have been under reporting of dengue incidences due to the overwhelming number of COVID-19 cases on the healthcare system.

Looking ahead to May-June 2021, we are not yet in a national lock down, and the Northeast monsoon season is in progress, we may see a spike in reported dengue cases similar to that which we had in year 2019. The rakyat are focused on safely moving around to avoid COVID-19 transmission, but we are moving more than we were a year ago. Meanwhile the hospital system is beginning to strain under the pandemic. Now is the time for us to be mobilized to take action against the Aedes mosquito.

Source: World Health Organization

Figure 1: Comparison of dengue incidences for the years 2020, 2021, and the forecast and mean for 2015-2019

What can we do to protect ourselves if the dengue incidence surges in the coming weeks?

The ultimate solution is to reduce the potential breeding sites for Aedes mosquitoes, as in the Ministry of Health’s “10-minit Hapuskan Aedes” campaign to eliminate the potential breeding places. In this regard, household insecticides can be helpful in situations where complete cleaning of the housing area is not possible. The table below shows the pros and cons of some of the common household insecticides available in the market.

Aerosols
A self-contained system with the operation of a nozzle that emits space-spray droplets of insecticide into the air.
Pros – Convenient to use. Usually, consist of active ingredients that bring knock-down and repellant effect.
Cons – Failure to comply with the instructions on its application may lead to inhalation injury. Hazardous if punctured, overheated, or used near an open flame.

Mosquito coil
A formulation of insecticide, wood powder, dye, and other additives.
Pros – Easy to use. Provides continuous protection for 8 hours with consistent performance.
Cons – Generates smoke that may lead to respiratory health complication (although some products contain “smokeless” formulation).

Mosquito mat
A cardboard mat that is infused with insecticide solution.
Pros – Clean (smokeless).
Cons – Inconsistent performance.

Electric liquid vaporizer
A device that consists of an electrical heater with a reservoir of liquid insecticides.
Pros – Clean. Provides protection that lasts for 30, 60, and 90 hours.
Cons – The heater needs to be switched on 2 hours prior to bedtime and requires brand- specific replaceable reservoir. More expensive.

Repellent
Chemicals that are applied to the skin, clothing, or other surfaces which discourage mosquitoes.
Pros – Provide more personal protection, especially in the outdoor environment.
Cons – Duration may be an issue. At the moment DEET is still the most effective repellent.

The selection of a suitable insecticide formulation depends on several factors such as the convenience of use, particular conditions surrounding the treatment, price, duration of protection, and so on. A basic understanding of formulation before purchasing the insecticides helps us to avoid mistakes in using the product, and most importantly protects us and our family from the fatal dengue infection.

Just as Malaysians have become accustomed to being part of the public health solution for COVID-19, now is the time for us to identify potential mosquito breeding grounds and taken action.