Dust flux, Vostok ice core

Dust flux, Vostok ice core
Two dimensional phase space reconstruction of dust flux from the Vostok core over the period 186-4 ka using the time derivative method. Dust flux on the x-axis, rate of change is on the y-axis. From Gipp (2001).

Friday, January 17, 2014

Cycles of dengue in Singapore

The last time I posted on this, the numbers afflicted were in decline, during the the normal seasonal drop-off. I assumed the number of cases would return to about 100 per week.

Weekly dengue cases in Singapore, 2011-2014. Adapted from here.

I am a little surprised to find that the outbreak is still raging. The rate of new cases is running at more than double this time last year, which was already a record. At first glance it looks like something new might be happening.

Only three years of records is too short to support such a hypothesis. Can we do better?

Dengue in Singapore, 1966-2005. DHF - Dengue hemorrhagic fever; 
DF - Dengue fever (only reported from 1977). Screen cap from here.

Given the population of Singapore in 2005 (4.4 million), the number of cases per 100k would give us an estimate of 14,300 cases that year. The number of cases of dengue reported in 2013 was over 22,000, and given the population estimated to be about 5.4 million, that would equate to over 400 cases per 100,000, which would be off the top of the above graph.

There are two things we note on the above graph: first is the extreme variability of the numbers of cases of dengue fever over a cycle length of approximately 8 years; secondly, the incidence of cases appears to be increasing since the late 1980s. The cycle lows are generally before 1985, around 1993, 2000, and (from the recent data) perhaps around 2011.

When I see cyclicity in epidemiological data, I look for climate oscillations. For the Pacific, we have the Pacific decadal oscillation (PDO), which alternately warms and cools the western Pacific (and many other areas, as well as changing the distribution of rainfall).

Schematic of PDO and effects on sea surface temperature from here.

During the positive phase of the PDO, we see the sea surface temperature around Singapore is a little warmer than usual. A common epidemiological argument is that warmer temperatures favour mosquitos, so if the PDO is influencing dengue cases, then we would expect to see more cases during the positive phase, and fewer cases during the negative phase. So let's combine the record from 1966 with the PDO oscillations over the same interval.

Although there is some agreement between the curves, particularly over the last 20 years, the small peak in the early 1990s corresponds with generally cooler surface temperatures--and the incidence of dengue was very low during the warm spell from the late 70s through to the 1980s.

One factor may be overturn of the strain of dengue encountered--for instance, the dominant strain in 2007 was different than that of 2005 (pdf). The strain in 2005 was similarly different from the dominant strain previously.

But I think the major factor in the general increase is the changing culture in Singapore. Look at the population growth of the place.

A snip from Google.

Despite only producing 1.2 babies per woman, the population of Singapore has grown by over a million in the last seven years. Obviously, this has been through immigration.

Singaporeans will no doubt complain that these newcomers don't share their cultural values--in particular, their cultural values regarding cleanliness and public order. I know, I married one. In Singaporean culture, the vacuum cleaner is not a labour-saving device, it is a device that allows you to use the same amount of labour to make the house cleaner than before.

When we were there last year, we stayed with one of my wife's relatives. They were having troubles with their new neighbours, who had recently immigrated from China, and were in the habit of tossing used diapers and tampons off their balcony.

But the reason may be more basic than that.

Image of Singapore.

That snaking body of water near the top of the image is the straits of Johor, which separates Singapore from Malaysia.

Singapore is a small place. To cram in more people requires dense construction in increasingly marginal lands, which are mainly in the north and western portions of the island, and on reclaimed land, which is primarily on the southern margin. Common to all of these marginal lands is that they tend to be wet and low-lying.

The population graph has a notably higher slope (population growth) starting in the late 1980s. I assume this is the beginnings of the major influx of immigrants. Also note that this period correlates to the interval where dengue fever outbreaks begin on the chart of dengue vs the PDO. Even though conditions at the time favoured cooling, if you bring in a lot of people (a few of whom have dengue) and place them in mosquito-infested swampland, you soon have a lot of people with dengue.

The increasing incidence of dengue in Singapore looks to be a reflection of government policy, which requires greater numbers of people to be stationed in close contact with mosquitos. And given that the Singapore government is pushing to increase the population by another million or so, we can only expect the trend of dengue infections to increase.

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