Estimation of the reproduction number of dengue fever from spatial epidemic data

Dengue, a vector-borne disease, thrives in tropical and subtropical regions worldwide. A retrospective analysis of the 2002 dengue epidemic in Colima located on the Mexican central Pacific coast is carried out. We estimate the reproduction number from spatial epidemic data at the level of municipalities using two different methods: (1) Using a standard dengue epidemic model and assuming pure exponential initial epidemic growth and (2) Fitting a more realistic epidemic model to the initial phase of the dengue epidemic curve. UsingMethod I, we estimate an overall mean reproduction number of 3.09 (95% CI: 2.34, 3.84) as well as local reproduction numbers whose values range from 1.24 (1.15, 1.33) to 4.22 (2.90, 5.54). Using Method II, the overall mean reproduction number is estimated to be 2.0 (1.75, 2.23) and local reproduction numbers ranging from 0.49 (0.0, 1.0) to 3.30 (1.63, 4.97). Method I systematically overestimates the reproduction number relative to the refined Method II, and hence it would overestimate the intensity of interventions required for containment. Moreover, optimal intervention with defined resources demands different levels of locally tailored mitigation. Local epidemic peaks occur between the 24th and 35th week of the year, and correlate positively with the final local epidemic sizes (q = 0.92, P-value < 0.001). Moreover, final local epidemic sizes are found to be linearly related to the local population size (P-value < 0.001). This observation supports a roughly constant number of female mosquitoes per person across urban and rural regions.

References:

• G. Chowell, P. Diaz-Duenas, J.C. Miller, P.W. Fenimore, J.M. Hyman, C. Castillo-Chavez. Estimation of the reproduction number of dengue fever from spatial epidemic data. Math. Biosci. 2007 Aug;208(2):571-89 (pdf)
• G. Chowell, F. Sanchez. Climate-based descriptive models of dengue fever. J. Env. Health 68 (10) Jun, 60-3 (2006) (pdf)
• G. Chowell , C.A. Torre, C. Munyaco-Escate, L. Suárez -Ognio, R. López-Cruz, J.M. Hyman, C. Castillo-Chavez. Spatial and temporal dynamics of dengue fever in Peru: 1994-2006.  Epidemiology and Infection, (to appear 2008).

Figure 1. The weekly number of dengue cases per 1000 individuals during the 2002 dengue epidemic diagnosed at the hospitals of the Mexican Institute of Public Health (IMSS) in each of the 10 municipalities in which the state of Colima, Mexico is divided .

Figure 2. The best fits (solid lines) provided by the stage progression model (Method II) to the initial epidemic phase of the state-wide and local dengue epidemics (circles) in logarithmic scale. The duration of the initial epidemic phase was determined by the X2 goodness-of-fit statistic [20] as in Method I.