Dengue fever is a re-emerging public health problem with two-fifths of the world population being at risk of infection. This infectious illness is the most rapidly spreading vectorborne disease in the world.
It is estimated that there are over 50-100 million cases of dengue worldwide each year, with three billion people living in dengue-endemic countries. Dengue is estimated to cause approximately 20,000 deaths a year globally.
The disease is transmitted by the bite of a mosquito infected with one of four dengue virus serotypes: DENV-1, DENV-2, DENV-3 and DENV-4. A fifth variant, DENV-5, was isolated in October 2013 from a 2007 blood sample of a Sarawakian farmer who was hospitalised with dengue fever then.
The likely causes of the emergence of this new serotype are genetic recombination, natural selection and genetic bottlenecks. However, DENV-5 seems to circulate mainly within non-human primates and rarely jumps into humans.
The symptoms of this disease range from mild to incapacitating high fever, with severe headache, pain behind the eyes, muscle and joint pain, and rash.
There is no specific medicine to treat dengue. Severe dengue, previously known as dengue haemorrhagic fever, is a potentially lethal complication. Dengue cases have an upward trend in Malaysia, having increased from 2001 to 2014.
We are one of the worst affected countries globally, with recorded cases increasing from 7,103 cases in 2000 to 108,698 in 2014, with 215 deaths and over 31,000 notified cases in any given year.
The control of dengue depends solely on the suppression of the two most important vectors, namely, the Aedes aegypti and Aedes albopictus mosquitoes. These mosquitoes are the ones that carry and transmit the dengue virus from an infected human host to an uninfected one.
A local study indicated that the incidence of dengue is likely to increase to four times the number of cases in 2010 by 2020 and six times the number of 2010 cases by 2040.
Despite close monitoring and continuous efforts from the Health Ministry and municipal councils in conducting prevention and control activities, the number of dengue cases continues to increase due to multiple uncontrolled factors.
These factors are the dengue virus, the human as the host, environmental conditions such as cleanliness, the mosquito vectors and their behaviour, and climate change.
The ministry has implemented an integrated strategy for dengue prevention and control in the National Dengue Strategic Plan (NDSP). There are seven strategies in this plan, which are:
● Strengthening of dengue surveillance
● Promoting cleanliness and practising integrated vector management
● Emphasising dengue case management
● Social and community mobilisation for prevention activities
● Ensuring rapid response in managing the dengue outbreak
● Developing new innovative methods through dengue research, and
● Reducing dengue cases in the Klang Valley, where more than half (57%) of the cases occur.
The primary preventive measure to reduce dengue infections is the control of mosquito populations, as the transmission of dengue requires mosquitoes as vectors.
One strategy is to eliminate unnecessary containers that collect water, such as plastic jars, bottles, cans, tyres and buckets, in which the Aedes mosquitoes can lay their eggs. This strategy, i.e. source reduction, can be effective when performed regularly, especially when members of a community are mobilised and educated about vector control.
For example, mosquito populations can be reduced when all members of a community clear blocked gutters and street drains, and keep their gardens free of containers with standing water. Any open containers should be emptied and cleaned every week to eliminate mosquito eggs and larvae.
Another method of reducing the number of Aedes mosquitoes is to use an ovitrap. This is a black, cylindrical container filled with water and covered with a mesh.
The female A. aegypti lays her eggs in the ovitrap and when the adult mosquitoes emerge, they are trapped beneath the mesh and are unable to escape from the ovitrap. Some ovitraps have been adapted to include sticky surfaces that entrap adult mosquitoes, and others include the use of a pesticide on the device.
Ovitraps can also be used for mosquito surveillance. When sufficient numbers of ovi-traps are used and frequently maintained, the vector population can be decreased.
One successful example is in Singapore, where ovitraps were used to eliminate mosquitoes at Changi Airport. However, these traps have limitations, in that they require constant supervision and monitoring to prevent them from becoming productive breeding habitats.
Insecticides can be used to kill both mosquito larvae and adult mosquitoes. The use of insecticides is recommended in emergency situations, such as during dengue epidemics or when an epidemic is emerging.
On a regular basis however, sustainable, coordinated, community-based environmental approaches are favoured over chemical methods for controlling mosquitoes, and limited reliance on these chemicals is preferred.
One reason is because mosquitoes can develop resistance to insecticides. In addition, insecticides are expensive, and high doses can be toxic to humans and other species. Therefore, it is best to be cautious about applying these chemicals.
These are a combination of biological controls and insecticides. One example of a bioinsecticide is Bacillus thuringiensis israelensis (Bti), which can effectively kill mosquito larvae present in water.
Other bioinsecticides like pyriproxyfen and methoprene, act as juvenile hormone analogues that prevent mosquito larvae from metamorphosing into adults.
Recently, researchers used mosquitoes to transfer insecticides to larval habitats. They noticed that after taking a blood meal, female A. aegypti mosquitoes enjoy resting in damp and dark areas.
To take advantage of this behaviour, the researchers set up dark, damp stations dusted with a bioinsecticide that target larvae.
When the mosquitoes came to rest on the stations, their legs picked up the bioinsecticide and transferred it to the water containers where they lay their eggs. This method was effective in killing the mosquito larvae and reducing the number of adult mosquitoes.
Predatory crustaceans called copepods and many varieties of fish, including mosquitofish (Gambusia affinis) and goldfish, eat mosquito larvae. When these organisms are placed in containers that contain water, decorative ponds and pools, they prey on mosquito larvae, effectively preventing mosquito development.
The addition of copepods into large water-storage tanks was successful in limiting dengue transmissions in Vietnam. Other live predators, such as dragonflies, small aquatic turtles and beetle larvae, have also been shown to be effective in killing the Aedes mosquito.
New genetic approaches are also being considered as ways to control mosquito populations. Researchers at the University of Oxford and Oxitec in the United Kingdom, and University of California, Irvine, in the United States, genetically engineered male mosquitoes whose female offspring cannot fly.
The British researchers also genetically modified male mosquitoes to produce offspring that will die young.
A trial releasing these mosquitoes in Bentong, Pahang, was successfully conducted in December 2010. However, the Government abandoned the idea as it was too costly and impractical. Both these genetically-engineered mosquitoes could be used to control mosquito populations and reduce dengue transmission.
Researchers at the University of Queensland, Australia, came up with another innovative approach to control dengue transmission. These scientists infected mosquitoes with bacteria called Wolbachia pipientis, which can shorten the lives of dengue-infected mosquitoes.
When a mosquito is infected with the dengue virus, eight to 12 days must pass before it can infect a healthy person. After that period, the mosquito can infect people for the rest of its life, which is generally three to four weeks.
If its life span is shorter, an infected mosquito would have fewer opportunities to transmit dengue.
Currently, the researchers are testing another Wolbachia strain that seems to kill the dengue virus inside the Aedes mosquito. This could serve as a dengue vaccine for mosquitoes, indirectly protecting humans as well.
A 2017 report assessing the effects of interventions for A. aegypti control published in the BMC Public Health journal, suggests that community participation programmes are an effective intervention in reducing the mosquitoes.
Another systematic review of community-based dengue control programmes suggests that coordinated intersectoral cooperation of local health services, trained vector control personnel, civil authorities and the community could encourage communities to take over prevention and control measures.
The globally-endemic dengue poses a serious public health and economic challenge. Efficient, cost-effective and environmentally-sustainable prevention and control techniques need to be practised diligently in dengue-endemic regions.