Dengue, Zika and Chikungunya Scenario in Malaysia
16 Jan 2017
Aedes aegypti is not indigenous to Malaysia. It is believed to originate from tropical Africa and introduced to Malaysia via India at the turn of the 20th century. On the other hand, Aedes albopictus is native to Southeast Asia including Malaysia. Both species are prolific breeders in artificial & natural containers and are vectors of human viruses, such as dengue, Zika, yellow fever, chikungunya etc. One of the significant biting behaviour of both these species is that they are multiple feeders whereby they can bite several people when their feeding is being disturbed. Ae. aegypti is the primary vector for dengue and Zika in Malaysia. Ae. albopictus has also been recognized as a secondary vector and is important in the maintenance of dengue and chikungunya viruses in places where Ae. aegypti is absent or not well established.
In recent years, dengue outbreaks have attained epidemic proportions, causing significant public health impact by causing high morbidity and mortality. Presently dengue is the most rapidly spreading virus infection globally, with transmission occurring in at least 128 countries and almost 4 billion people are at risk. The number of dengue cases reported to World Health Organisation (WHO) has increased steadily from an average of less than a thousand cases per year globally in the 1950s to more than 3 million cases in 2013. Dengue is practically the only serious mosquito borne virus in Malaysia, with 101,357 dengue cases and 237 deaths reported in 2016. The virus that causes dengue is an arthropod-borne virus (arbovirus) with four antigenically distinct serotypes (DEN-1, DEN 2, DEN 3 and DEN 4) that offer no long-term cross-protective immunity against each other. All 4 serotypes are endemic to Malaysia. Recently DEN-1, DEN-2 and DEN-3 are the dominant circulating serotypes in the country.
In Malaysia, chikungunya infection was first recorded in 1998-1999 in Port Klang affecting more than 51 people, then followed by the outbreak in Bagan Panchor, Perak in 2006. After a lapse of almost 2 years, the re-emergence of chikungunya infection occurred in July 2008 in Tangkak, Johor which then spread to other states in peninsular Malaysia such as Negeri Sembilan and Malacca with more than 2000 cases reported. Until March 2009, more than 1600 cases had been reported in Kuala Lumpur, and the state of Selangor. In subsequent years, few chikungunya cases were reported, until March 2017, when a total of 50 cases were reported from Baling, Kedah and Gombak, Selangor, indicating a possible resurgence of the disease.
On the 31 August 2016, Malaysia recorded the first human case of Zika. The patient was infected after a visit to Singapore, subsequently the first locally acquired Zika case was reported in Sabah on the 3rd September 2016. Another case involving a pregnant woman was also reported later. To date, a total of eight cases have been reported.
There is no specific treatment for dengue, Zika and chikungunya infection. A safe and highly effective dengue tetravalent vaccine is still in development. As Aedes mosquito is a day-biting mosquito, bed nets are also not effective against Aedes-borne diseases like dengue, chikungunya or Zika.
The current strategy is to control the mosquito vectors, Aedes aegypti and Aedes albopictus using conventional methods such as fogging with insecticides and breeding site reduction. However, these methods cannot prevent dengue outbreaks by themselves as they are not sustainable and as seen from examples around the world. An important reason for this is the low vector threshold for dengue transmission. As low as 2-3 adult female mosquitoes emerging every day in a locality of 100 people is sufficient to start an outbreak.
Because of this, public commitment and involvement is also of paramount importance and this has been adapted to continue public education and ‘COMmunication for Behavioural Impact’ (COMBI) for sustained breeding site reduction involving the public. Legally-enforced larval control, whether by breeding site elimination or larviciding, is neither practical for a large and diverse country like Malaysia, nor has it stopped the resurgence of dengue in other countries. Because of these limitations, there is an urgent need to research other innovative control methods, such as use of Wolbachia, thereby employing the mosquitoes themselves to control the disease.