Extent of flood damage increased cerebrovascular disease incidences in Iwate prefecture after the great East Japan earthquake and tsunami of 2011.

Background: Several studies have reported on increases in the incidence of cardiovascular and cerebrovascular diseases after huge earthquakes. An increase in the incidence of cerebrovascular diseases was observed after the Great East Japan Earthquake and Tsunami of 2011. To assess whether tsunami damage or the earthquake was responsible for this trend, we assessed the relative impact of earthquake magnitude and flood damage on cerebrovascular disease.

Methods: A total of 12 coastal municipalities facing the epicenter were divided into 4 flood severity groups according to the percentage of people living in the flooded areas (<20, 20-40, 40-60, and ≥60%) and 3 groups according to the Japanese Meteorological Agency seismic intensity of the main shock (<4.5, 4.5-5.0 and ≥5.0). The standard incidence ratios (SIRs) of cerebrovascular diseases in the first 4 weeks after the disaster compared with the same periods in 2008-2010 were calculated for each flood severity group and each earthquake severity group. Odds ratios (ORs) of disease incidence and the adjusted ORs for seismic intensity (using the Mantel-Haenszel method) between the higher (≥40%) and the lower flooded area (<40%) were compared with the same periods in 2008-2010. Likewise, ORs and adjusted ORs for flood severity in the high seismic intensity area (≥4.9) were compared with those in the low seismic intensity area (<4.9).

Results: SIRs increased with the increased flood severity: 0.94 (0.59-1.30) at <20%, 1.02 (0.70-1.34) at 20-40%, 1.26 (0.66-1.86) at 40-60% and 1.98 (1.25-2.72) at ≥60%. However, SIRs did not increase with increased seismic intensity: 0.95 (0.60-1.29) at <4.5, 1.52 (1.07-1.98) at 4.5-5.0 and 1.17 (0.80-1.54) at ≥5.0. ORs and adjusted ORs for seismic intensity in the high flood area compared with the low flood area were significant: 1.68 (1.07-2.65) and 1.78 (1.08-2.96), respectively. However, ORs and adjusted ORs for flood severity in the high seismic intensity area compared with the low intensity area were not significant: 1.33 (0.82-2.17) and 1.19 (0.62-2.31), respectively.

Conclusions: Cerebrovascular disease incidences after the Great East Japan Earthquake and Tsunami of 2011 increased because of tsunami damage and not because of the earthquake magnitude.