あま美諸島におけるハブこう傷危険人口の推定

The “habu” (Trimeresurus flavoviridis) is a venomous snake which has a distribution among certain islands of the Amami and Ryukyu archipelagoes, and the present study was designed to provide an estimate of the population which is in danger of being bitten by “habu” in the Amami Islands. The results were as follows:<br>(1) Over the period 1967-1976, a total of 2, 673 cases of “habu” bite occurred in Tokunoshima and Amamiohshima (Amamiohshima proper, Yorojima, Kakeromajima, Ukejima and other small islands). The number of those who had at least one bite in the 2, 673 cases was 2, 583, of whom 2, 502 had been attacked once, 73 twice, 7 three times and one four times. Although we have information only on those who have at least one bite, provided that the total number of cases was derived from the total population combined for Amamiohshima and Tokunoshima, the distribution for the frequency of “habu” bite per head containing those who were not bitten conformed approximately to the negative binomial distribution rather than to the Poisson series. Assuming that the frequency per head for those who have at least one bite is a truncated Poisson distribution rather than a complete series, the size of the population at risk can be estimated by trial and error. The value obtained was approximately 31% of the total population combined for Amamiohshima and Tokunoshima.<br>(2) The age specific incidence rate was lowest for the youngest age group, increased as age advanced to reach adulthood and then declined again among the most elderly. The estimated age specific incidence rate is introduced by the quadratic function defined by the equation: f(t)=bt+ct² where f (t) is the incidence of bites per year at age t, and b and c are the coefficients which can be estimated by the method of least squares. The total number of cases among the total population can be estimated as the sum of the product of the cumulative value of the estimated incidence rate at each age multiplied by the population corresponding to that age. The value obtained for the total population combined for Amamiohshima and Tokunoshima is 10, 859.16.<br>(3) If it is assumed that the decimal fraction of those who have at least one bite approached a limit 1, the speed at which this increases can be expressed by the differential equation: dy/dt=(bt+ct²)(1-y) where y is the decimal fraction of those who have at least one bite at age t. Under the condition that y is 0 when t is 0, the equation has the solution y=1-1/exp(bt²/2+ct³/3) The total number of those who have at least one bite can be estimated as the sum of the product of the value of y at each age multiplied by the population corresponding to that age. The value obtained was 9, 181. 12, i. e. 7.3% of the total population combined for Amamiohshima and Tokunoshima.<br>(4) On the basis that the frequency of bite per capita follows the negative binomial distribution, the number of people corresponding to each class of the frequencies of bites was estimated for the total population combined for Amamiohshima and Tokunoshima.