Motor neuron disease on Guam: geographic and familial occurrence, 1956-85.

We investigated the geographic and familial occurrence of motor neuron disease (MND) on Guam, and then considered etiologic hypotheses related to cycad use and metal intoxication. The research was based on 303 Chamorros from Guam and 3 Chamorros from other Mariana Islands, all with MND onset on Guam during 1956-85. Inarajan and Umatac, two southern districts, each had, for both sexes combined, an average incidence rate significantly higher than the corresponding overall rate for Guam. Also, for each sex, geographic patterns of incidence were significantly related to 1) socioeconomic level (men only), 2) cycasin concentrations in cycad flour samples (men and women), 3) iron concentrations in water samples (men and women), 4) silicon concentrations in water samples (men only), and 5) cobalt and nickel concentrations in soil samples (men and women). The MND risk in susceptible sibships was about 7-28 times greater than that in the general population. The cycad hypothesis conforms somewhat better than the metal intoxication hypothesis with the data presented.