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. 2010 Jul 13;4(7):e747.
doi: 10.1371/journal.pntd.0000747.

Dengue dynamics in Binh Thuan province, southern Vietnam: periodicity, synchronicity and climate variability

Khoa T D Thai  1 , Bernard Cazelles, Nam Van Nguyen, Long Thi Vo, Maciej F Boni, Jeremy Farrar, Cameron P Simmons, H Rogier van Doorn, Peter J de Vries
Affiliations

Dengue dynamics in Binh Thuan province, southern Vietnam: periodicity, synchronicity and climate variability

Khoa T D Thai et al. PLoS Negl Trop Dis. .

Abstract

Background: Dengue is a major global public health problem with increasing incidence and geographic spread. The epidemiology is complex with long inter-epidemic intervals and endemic with seasonal fluctuations. This study was initiated to investigate dengue transmission dynamics in Binh Thuan province, southern Vietnam.

Methodology: Wavelet analyses were performed on time series of monthly notified dengue cases from January 1994 to June 2009 (i) to detect and quantify dengue periodicity, (ii) to describe synchrony patterns in both time and space, (iii) to investigate the spatio-temporal waves and (iv) to associate the relationship between dengue incidence and El Niño-Southern Oscillation (ENSO) indices in Binh Thuan province, southern Vietnam.

Principal findings: We demonstrate a continuous annual mode of oscillation and a multi-annual cycle of around 2-3-years was solely observed from 1996-2001. Synchrony in time and between districts was detected for both the annual and 2-3-year cycle. Phase differences used to describe the spatio-temporal patterns suggested that the seasonal wave of infection was either synchronous among all districts or moving away from Phan Thiet district. The 2-3-year periodic wave was moving towards, rather than away from Phan Thiet district. A strong non-stationary association between ENSO indices and climate variables with dengue incidence in the 2-3-year periodic band was found.

Conclusions: A multi-annual mode of oscillation was observed and these 2-3-year waves of infection probably started outside Binh Thuan province. Associations with climatic variables were observed with dengue incidence. Here, we have provided insight in dengue population transmission dynamics over the past 14.5 years. Further studies on an extensive time series dataset are needed to test the hypothesis that epidemics emanate from larger cities in southern Vietnam.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Wavelet analyses of dengue time series with monthly data from 1994 to 2009 in 9 districts of Binh Thuan province.
(A) Duc Linh, (B) Tanh Linh, (C) Ham Tan, (D) Ham Thuan Nam, (E) Ham Thuan Bac, (F) Phan Thiet, (G) Bac Binh, (H) Tuy Phong, (I) Phu Quy, (J) Binh Thuan province. For each district (i) left panel: the time series of cases in the district (ii) middle panel: the wavelet power spectrum of dengue cases (square rooted and normalized and trend suppressed); colors code for increasing spectrum intensity, from blue to red; dotted lines show statistically significant area (threshold of 5% confidence interval); the black curve delimits the cone of influence (region not influenced by edge effects (iii) right panel: to the mean spectrum (solid line) with its threshold value of 5% (dotted line).
Figure 2
Figure 2. Wavelet coherence and phase analyses of dengue time series between neighboring districts in Binh Thuan province.
The left panel represents the wavelet coherence. Blue, low coherence; red, high coherence. The dotted lines show α = 5% significance level. The cone of influence (black curve) indicates the region not influenced by edge effects. The right panels represent the phase analyses between two districts (in blue and red), based on wavelets for 2–3-year periodic band. Green boxes represents the period of time where coherency is significant, when interpretation of analysis was possible. Red lines: first district; blue lines: second district; dashed black lines: phase difference between the two oscillating components.
Figure 3
Figure 3. Phase differences from Phan Thiet district.
(A) Map of Vietnam (B) Mean phase differences for 1996–2001 data at the 2–3-year period band. (C) Mean phase differences for 1995–2002 data at the 1-year period band. (D) as in C, but for 2004–2008 data.
Figure 4
Figure 4. Wavelet coherence analyses of dengue incidence with ENSO indices.
(A) MEI, (B) NIÑO 1+2, (C) NIÑO 3, (D) NIÑO 4 and (E) NIÑO 3.4. Blue, low coherence; red, high coherence. The dotted lines show α = 5% and 10% significance levels. The cone of influence (black curve) indicates the region not influenced by edge effects.
Figure 5
Figure 5. Wavelet coherence analyses of dengue incidence with local climate variables.
(A) mean temperature, (B) humidity and (C) rainfall. Blue, low coherence; red, high coherence. The dotted lines show α = 5% significance levels. The cone of influence (black curve) indicates the region not influenced by edge effects.
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