Incidence of Minamata disease in communities along the Agano river, Niigata, Japan--patterns of the exposure and official diagnosis of patients

Abstract

Data obtained from all residents, who lived in the Agano river villages where Minamata disease occurred as of June 1965, were used to analyze its incidence patterns and to evaluate validity of the official recognition of the disease. The residents totaled 18,253 (8,911 males and 9,342 females), including 262 (151 males and 111 females) recognized patients and 378 (186 males and 192 females) rejected applicants. Consumption of the contaminated river fish was associated with hair Hg measurements (ppm), but there were few residents who denied the consumption but showed elevated Hg values, probably due to false-negative answers in the census survey in 1965. Incidence rates of the recognized patients increased with the upgrading codes for fish ingestion, but analysis of the association of fish ingestion among the recognized applicants indicated that there were ca. 50% false-positive diagnosis. This analysis showed that The Government made best efforts to help very mild cases despite the fact that such a generous attitude inevitably causes overdiagnosis. Incidence rates of the rejected applicants also showed a dose-response. This is extremely important because a rejection meant that compensation was refused by the polluting company. Protest groups insisted the existence of Minamata disease showing only sensory symptoms, but this concept has not been accepted by the Government and the polluters. The present study disclosed a dose-response relationship in the rejected cases giving positive ground for the opinions proposed by protest groups. Proportions of such "Hg-associated sensory disorders" (cases with unexplained sensory disorders which are associated with river fish ingestion) were 15-30% among the exposed applicants, 6.6-15% among the all rejected applicants, and 3.9-10% among the unexplained sensory disorders in the regional population. Owing to the river pollution, there was 5-11% increase of the unexplained sensory disorders among the local residents, in addition to recognizable Minamata disease.