Epidemiological Studies on Subacute myelo-optico-neuropathy (SMON) at Yubara Town (Report 1)

In a restricted area of the northern part of Okayama Prefecture, Yubara Town, an outbreak of SMON (subacute myelo-optico-neuropathy) was observed from the beginning of 1967. An epidemiological investigation has been made on this outbreak and the results are as follows:(1) Concentration of cases occurred in the summer of 1968, though cases have been reported sporadically in the area from the beginning of 1967. The incidence ratio against the population was 659/100, 000 during 22 months.(2) The incidence was the highest in summer and the ratio in females was 3 times higher than in males. Concerning age group, males showed a peak in the thirties, whereas in females many cases were evident between the twenties and sixties.(3) Relatively enclosed districts are apt to expand over a period of time. Cases which occurred in neighboring families as well as those within the same families tend to give the impression that the disease coule be infectious.(4) Among the cases, a close contact relation was observed.(5) Physical exhaustion before the onset of the disease was observed to be 43.2% among the total cases.(6) In occupational analysis, a higher rate was revealed among workers who had close human relations such as hospital workers and public service personnel.(7) The tendency to other diseases of the nervous system as well as those of the digestive organs was checked by inspecting receipts of the National Health Insurance from the beginning of 1965. Nothing related to SMON was recognized before the outbreak.(8) Diseases of the intestinal tract and tonsillitis were observed in higher rates in the history of the patients.(9) The investigation of environmental conditions has revealed the fact that there is a higher rate of incidence in families who do not use service water compared to those who do.