During the period from July, 1957, to September, 1958, epidemiological studies were carried out, with the specific object of investigating familial prevalence, on Ascaris and hookworm infection among the inhabitants of 10 agricultural communities and 1 residential area (the control) in and around Chiba City. The rural areas where the investigation was carried out had been so selected that they included lands with different geographical features-the reclaimed land, the land where paddy-fields predominte, and the land where fields prodominate. With over 90 per cent of the inhabitants participating in the program, tests for infection with Ascarides and hookworms were conducted by means of smear method, suspension method and culture method.
1. The smear method proved superior to the suspension method in detecting unfertilized Ascaris eggs, whereas the culture method was better than the suspension method in detecting hookworm eggs.
2. Farmers and part-time farmers had a higher incidence of positive reactions to tests for Ascaris and hookworm infection than did non-farmers. The farmers cultivating fields ranging in area from 0.1 to 2.0 tan (1 tan corresponds to 0.245 acre) had the highest rate of Ascaris infection.Hookworm infection had no relation to the area of land cultivated. Both Ascaris and hookworm infection rates were low among the farmers using night soils fertilizer by mixing it with manure, than among the farmers employing other methods of night soil disposal.
3. Among adults, those who were engaged in agriculture had the highest incidence of both Ascaris and hookworm infection. Among those engaged in agriculture, males had a higher incidence of infection with Ancylostoma duodenale, and females with Ascarides and Necator americanus.
4. A. duodenale' showed the highest familial prevalence, followed by Ascarides and N. americanus in the indicated order. The prevalence of A. duodenale and N.americanus among those members of the family who were engaged in agriculture was as high as the familial prevalence, but the prevalence of Ascarides among those members of the family who were engaged in agriculture was lower than familial prevalence. Familial infection of these parasite worms in terms of the relations between husband and wife and parent and child gave the following picture: husband had a close relation with wife in infection with Ascarides; parent and husband had a close relation with child and wife, respectively, in infection with A. duodenale; and husband had a close relation with wife in infection with N.americanus.
5. The following picture of mixed infection was obtained: Ascarides were closely related to A. duodenale in infecting children; Ascarides and A. duodenale were closely related to A. duodenale and N. americanus, respectively, in infecting adults. No close relation was found between Ascarides and N. americanus.
6. In the light of the result of present investigation, it would seem that A. duodenale has larger possibilities of invading the human body by mouth than N. americanus.