Randomness of Dengue Outbreaks on the Equator - Volume 21, Number 9—September 2015 - Emerging Infectious Disease journal - CDC
Volume 21, Number 9—September 2015
Randomness of Dengue Outbreaks on the Equator
Dengue, a vectorborne infectious disease, has complex epidemiologic dynamics (1). The recent expansion of the range of dengue makes this disease a considerable public health concern worldwide (2). In the city-state of Singapore, the number of dengue cases has increased dramatically since the 1990s, and all 4 serotypes of the dengue virus are endemic (3). Cyclical outbreaks of dengue of increasing magnitude have been observed with a cycle of 5–6 years (4), but this pattern appeared to cease in 2005, and no obvious cycle has occurred since then. Although other tropical and subtropical countries in Southeast Asia have distinct seasonality (5) so that dengue epidemics occur at distinct and predictable times of the year (6), Singapore’s proximity to the equator gives it an aseasonal climate, and the timing of dengue epidemics is irregular (7,8).
Many factors have been postulated to contribute to dengue’s spread in Singapore, such as a consistently warm and humid climate that favors year-round vector proliferation, high urbanization, and a tendency for vectors to live in human residences (9). The extent to which these factors affect dengue epidemics in aseasonal Singapore, if they do at all, is unclear. Competing explanations for the timing of large dengue outbreaks in Singapore can be found in the literature. One study attributes dengue epidemics to conducive temperatures and precipitation variations (10); another attributes them to variable maximum and minimum temperatures (11). Rainfall and temperature have been shown to be related to dengue outbreaks in Brazil, another equatorial country (12).
The tendency to see patterns where none exists has been well recognized. When 2 events happen contemporarily and a plausible story connects the events, the tendency to assume that 1 causes the other is strong (13). Cancer cases cluster around mobile phone masts (base stations), not because the radiation from a mast is carcinogenic at typical exposures but because numerous masts exist and occasionally cancer cases cluster together, similarly to spilled grains of rice (14). A study in the heuristics and biases program discusses a famous example from sports (15), which are notorious for stories being concocted around essentially chance outcomes. Basketball fans, coaches, and pundits often believe that players have “hot hand” streaks when they have a run of good form, making many shots in succession and playing above their usual level during a match. The study systematically deconstructed this belief by a series of statistical tests that showed that the patterns of actual hits and misses was consistent with mere chance—analogous to sequences of coin tosses rather than an illusory hot hand (15).
In probabilistic models, chance is represented by error terms, or noise, encompassing all the many complicating factors that are not worth including in the systematic signal. Past models for dengue in Singapore have accounted for chance alongside systematic effects of the weather and other factors (10,11). However, is chance alone sufficient to explain the frequent, large, and ostensibly chaotic outbreaks we observe? We sought to assess whether the rise and fall of dengue outbreaks from week to week in Singapore come in runs or are indistinguishable from random noise and thereby whether it is necessary to consider other possible drivers of these epidemics.
Ms. Chen is a research assistant and doctoral student at the National University of Singapore. Her main research interest is modelling of endemic diseases such as dengue and hand, foot and mouth disease.
This study was funded by the Center for Infectious Disease Epidemiology and Research in the Saw Swee Hock School of Public Health, and additional funding was provided by Singapore's Health Services Research grant number HSRG-0040-2013.
Data used in this paper are available at http://www.moh.gov.sg.
- Cummings DA, Irizarry RA, Huang NE, Endy TP, Nisalak A, Ungchusak K, Travelling waves in the occurrence of dengue haemorrhagic fever in Thailand. Nature. 2004;427:344–7.
- Bhatt S, Gething PW, Brady OJ, Messina JP, Farlow AW, Moyes CL, The global distribution and burden of dengue. Nature. 2013;496:504–7.
- World Health Organization. Dengue and severe dengue. 2014 Mar [cited 2015 Apr 1].http://www.who.int/mediacentre/factsheets/fs117/en/index.html
- Ooi EE, Goh KT, Gubler DJ. Dengue prevention and 35 years of vector control in Singapore. Emerg Infect Dis. 2006;12:887–93.
- Wongkoon S, Jaroensutasinee M, Jaroensutasinee K. Distribution, seasonal variation & dengue transmission prediction in Sisaket, Thailand. Indian J Med Res. 2013;138:347–53.
- Bhatia R, Dash AP, Sunyoto T. Changing epidemiology of dengue in South-East Asia. World Health Organ South East Asia J Public Health.2013;2:23–7.
- Ministry of Health Singapore. Communicable diseases surveillance in Singapore 2011. 2012 [updated 2013 Jul 26] [cited 2014 Jan 17].http://www.moh.gov.sg/content/moh_web/home/Publications/Reports/2012/_communicable_diseasessurveillanceinsingapore2011.html
- Ministry of Health Singapore. Communicable diseases surveillance in Singapore 2012. 2013 [updated 2014 Jan 22] [cited 2014 Jan 17].http://www.moh.gov.sg/content/moh_web/home/Publications/Reports/2013/Communicable_Diseases_Surveillance_in_Singapore_2012.html
- Gubler DJ. Dengue, urbanization and globalization: the unholy trinity of the 21(st) century. Trop Med Health. 2011;39(Suppl):S3–11.
- Hii YL, Rocklöv J, Nawi N, Tang CS, Pang FY, Sauerborn R. Climate variability and increase in intensity and magnitude of dengue incidence in Singapore. Glob Health Action. 2009;2.
- Pinto E, Coelho M, Oliver L, Massad E. The influence of climate variables on dengue in Singapore. Int J Environ Health Res. 2011;21:415–26.
- Horta MA, Bruniera R, Ker F, Catita C, Ferreira AP. Temporal relationship between environmental factors and the occurrence of dengue fever. Int J Environ Health Res. 2014;24:471–81.
- Fung K. Numbers rule your world: the hidden influence of probabilities and statistics on everything you do. New York: McGraw Hill; 2010.
- Blastland M, Dilnot A. The numbers game: the commonsense guide to understanding numbers in the news, in politics, and in life. New York: Gotham;2010.
- Gilovich T, Vallone R, Tversky A. The hot hand in basketball: on the misperception of random sequences. Cogn Psychol. 1985;17:295–314.
Suggested citation for this article: Chen Y, Cook AR, Lim AXL. Randomness and dengue outbreaks on the equator. Emerg Infect Dis. 2015 Sep [date cited].http://dx.doi.org/10.3201/eid2109.141030