In the hot zone – preparing for dengue in Singapore

Dengue is an infectious disease which has become a feature of tropical and sub-tropical urban and semi-urban areas. Infected patients can suffer from fevers, severe headaches, joint and muscular pains, vomiting, diarrhoea and rashes. Worse than a severe flu by far and the more severe form, Dengue Haemorrhagic Fever (DHF) can cause fatalities in adults and children.

No treatment is available, and the only means to battle this is through the control of the mosquito vector. Two fifths of the world’s population are now at risk according to this WHO Dengue and dengue haemorrhagic fever fact sheet (2009).

The dengue virus-carrying mosquito, the Asian tiger mosquito is a day-flying mosquito whose larvae will breed in any pocket of freshwater. So it is well suited to tropical urban environments.

Aedes albopictus (wikipedia)
Aedes albopictus, isn’t she a beauty?
Abdomen distended with her blood meal (CDC/Wikipedia)

With global warming and more than half the world’s population living in cities since 2008, the incidence of dengue will only increase. For an introduction to dengue, see Scott Halstead’s introductory chapter from the 2008 book he edited on dengue.

DengueMap | A CDC-HealthMap Collaboration
Dengue Map
indicating areas of ongoing transmission risk

Vector control has been a significant part of Singapore city, since we became independent, starting with the successful battle to make the country malaria-free in the 60’s. Undergrads up to the early-90’s would have heard war stories from A/Prof Chan Kai Lok who was formerly the head of the Vector Control and Research Department at the Ministry of the Environment and at the forefront of the battle against malaria.

Prof Chan had joined the Department of Zoology in NUS and taught a course on Vector Control which emphasised an integrated approach. It was fascinating for those of us who had been doing lots of travel in the region’s rainforests for he also explored the diversity of diseases therein!

By then, dengue was already rearing its head. In 1990 I cut out a newspaper article for my lab wall which announced that we had reached epidemic proportions with more than 1,000 cases in Singapore. These days even 5,000 cases doesn’t raise eyebrows as much for Singapore exceeds 10,000 cases during epidemics, the last of which was in 2007.

Recently I visited Pulau Tekong for a survey. The incidence of mosquitoes was much lower than during visits I made with Adrian Loo in 1997. The difference was due to the fact the army had dealt with the problem aggressively to eliminate the need for soldiers and recruits to consume chemoprophylaxis which have side-effects.

Read about mosquito control in Pulau Tekong in Lee, et al., 2010. Elimination of malaria risk through integrated combination strategies in a tropical military training island. Am. J. Trop. Med. Hyg., 82(6): 1024–1029. A powerpoint presentation is available here.

Dengue Control in Pulau Tekong JITMM-3-12-2009-E13-Damian-Hong.pdf (page 42 of 47) Adult surveillance data Mar 2007

Prevention is better than cure
I regularly send students into the field for practicals and research work. Monitoring dengue news is essential in case our destination include high incident areas. In such instances, students are asked to apply insect repellent at the start of the practical and we provide the repellent.

I’ve cleaned out the Holland Village Guardian’s stock of insect repellent cans several times before a practical. This is when I get a report from Pasir Panjang and am about to head out to Kent Ridge. I had observed from public walks that even individuals susceptible to mosquito bites will forget about bringing and applying repellent, until they start getting bitten when its too late.

In reality, the chances of getting bitten in your neighbourhood is sometimes higher than in many of our field sites! Freshwater, a container (plastic litter is very helpful) and warm temperatures and human blood supply encourage mosquito breeding – that’s all our neighbourhoods.

So trash, particularly plastics provide breeding grounds for mosquitoes. As I am interested in the use of Pandan mangrove for teaching of undergrads and school students, I identified it as a site for the International Coastal Cleanup Singapore in 2008.


Fighting dengue through the International Coastal Cleanup Singapore
– the Pandan Mangrove cleanup is in its fourth year

Neglected for years, trash had accumulated and was providing artificial breeding grounds for mosquitoes. Although the population might not have been harbouring the dengue virus, multiple visits by asymptomatic students might start a cycle.

After the three years of the cleanup anchored by students and staff in my own Department of Biological Sciences and staff of Wildlife Reserves Singapore, the trash load is now low and it is a safer place to all of us to use for teaching.

Dengue updates
The Singapore government has current information about dengue in Singapore – a “Campaign against Dengue” webpage at dengue.gov.sg. Recently the popular weather site, Weatherlah has produced automated projections of local cases on a map at weatherlah.com/denguemap – I love this as it is very useful for me to look in on before field practicals.

~ Dengue | WeatherLah ~

On 29 June there was a spike in Marsiling where an outbreak of the Dengue type 3 was just reported. This dengue virus is known from South Asia but uncommon here, and its sporadic appearance has been of concern for some time now since Singaporeans have very little immunity against this virus. We have been exposed mainly to dengue types 1 and 2 have been predominant in the past decade.

Dengue Virus Surveillance, Singapore, Figure 1 | CDC EID
“Trends of monthly dengue cases in Singapore, 2005–2008, showing a switch in predominant serotype from dengue virus serotype 1(DENV-1) to DENV-2 in January 2007 and cocirculation of all 4 serotypes with general dominance of DENV-1 and DENV-2 and lesser circulation of DENV-3 and DENV-4.” From Lee K-S, Lai Y-L, Lo S, Barkham T, Aw P, Ooi P-L, et al, 2010. Dengue virus surveillance for early warning, Singapore. Emerg Infect Dis, 16(5).

Early warning?

Well even Google has pitched in, using the Ministry of Health’s “weekly publication of statistics on local infectious disease situation” to plot Singapore data. Based on the successful Google Flu Trends in which aggregate search query trends mirrored estimates of flu seasons provided by traditional surveillance systems, Google Dengue Trends was launched in May this year [see BBC News Technology] in the hope of developing an early-warning system.

Participating countries providing dengue case counts besides Singapore are Bolivia, Brazil, India and Indonesia.

Google Dengue Trends | Singapore

Singapore’s data since 2005 reveals three search peaks, in 2005 and 2007. Perhaps more interestingly, there have been no search peaks since 2007 mirroring our relatively disease-free situation. With little or no exposure for several years, our population immunity levels are low once again and the chances for an outbreak are high, certainly something the government is bracing itself for.

Early indicators are already up and about for while this post languished in my draft folder for a week, a report about dengue reaching warning levels emerged, amplified by the spike of 66 Dengue-3 cases in Marsiling (“Weekly dengue cases hit new high,” by Wayne Chan. Chanel News Asia, 05 Jul 2011).

Based on previous epidemics, we can expect a spike between July to September, which is now!

Epidemiological characteristics of the 2005 and 2007 dengue epidemics in Singapore 2013 similarities and distinctions
Dengue peaks in the last two epidemics appeared in the latter half of the year – which means now! Source: Ler et al. , 2011. Epidemiological characteristics of the 2005 and 2007 dengue epidemics in Singapore – similarities and distinctions. Western Pacific Surveillance and Response Journal, 2(2).

Get vigilant!

So this is a fact of life in urban living, and especially so in warmer climates. We need to step up vigilance for this predicted spike in cases.

I remember when an NEA officer visited residences in my area for checks due to the emergence of a few dengue cases in the neighbourhood. After examining a few likely places, I chatted with the officer and found out the typical culprit areas were under the sink, under flower pots and in toilet brush holders.

I told this story to a friend who returned home and took a look – sure enough, several mosquitoes emerged from his toilet brush holder. I disposed of mine since.

So its up to all of us to deny mosquitoes breeding sites in our hoes and work places. Start by refreshing your mind about the disease and prevention by reading the NEA mosquito breeding prevention FAQ and the CDC Dengue page.

Dengue
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