With the purpose of investigating the major diseases and the main problems in health management among the Japanese staying for a long period in tropical and subtropical countries, an epidemiological survey was made on the diseases and clinical laboratory data of the Japan Overseas Cooperation Volunteers (JOCV) who lived in and extended technical cooperation to the developing countries. Of the major health problems found in a total of 1, 469 subjects (1, 085 males and 384 females), malaria (101 cases), acute hepatitis (94 cases), chronic diarrhea/enteritis (78 cases) and traffic accidents (71 cases) ranked high. Suspecting that many cases of malaria and dengue fever were not reported, further investigation was carried out and it was revealed that the incidence of clinical malaria ranged from 38.1 to 66.7 per cent in tropical African countries. The incidence of malaria was 36.5 per cent in those who regularly took a combination of sulfamonomethoxine-pyrimethamine while it was 50.4 per cent in those who took the combination irregularly or not at all. Many cases with intestinal parasites (30.7%), eosinophilia (5.9%), and abnormal hepatic function (5.4%) were detected upon their return to Japan. The incidence of mental disorders and venereal diseases was suspected to be considerably higher than the number of cases reported. Among the so-called “tropical diseases”, a high incidence of malaria and infectious diseases of the digestive organs was observed. Prevention of those diseases is, needless to say, important but in view of the high rates of hepatitis, mental diseases, venereal diseases, traffic accidents and aggravation or recurrence of chronic diseases which were present before their overseas assignment, it is also important to take steps to prevent such problems when Japanese, especially young people, stay in tropical or subtropical countries.
The diseases and accidents which the Japan Overseas Cooperation Volunteers suffered from during their two years of assignment overseas in tropical and subtropical countries were analyzed and the following conclusions were obtained.
1) Endemic diseases such as malaria, hepatitis, diarrheal diseases, dengue and other arbo-virus diseases are important and frequent diseases among the Japanese staying in tropical or subtropical countries. Efforts should be made to give the youths instructions in preventing them before their departure to those countries.
2) Knowledge about and ways of preventing traffic accidents, mental health and the recurrence of other non-tropical diseases which the youths had suffered in the past, such as peptic ulcer, dental caries, hypertension, and neurotic diseases should be given equal emphasis.
3) The conclusions and suggestions apply not only to the JOCV but also to other Japanese or subjects in non-endemic, well developed countries who go to tropical and subtropical countries and stay there for some time.

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The degree of susceptibility of Simulium metallicum to infection with Onchocerca volvulus was studied in Venezuela. Wild flies which had fed on the infected volunteer were maintained at a temperature varying between 22°C and 28°C, and all flies dying every 24 hours were dissected for larval development. Third-stage larvae were first seen in the head of flies dying between five and six days. Few abnormal and deformed larvae were observed. The larval development was asynchronous; only 41 of 117 larvae recovered from 32 flies examined on days 6-11 were in the third stage, the remaining larvae being still in earlier stages. However, the proportion of third-stage larvae among all larvae recovered increased from 6 to 69 per cent, with the passage of time from days 6-8 to 9-11. Likewise, the percentage of third-stage larvae found in the head during the same period rose from 25 to 54 per cent. These results indicate that despite the asynchronous and retarded development, many larvae can develop to the infective stage in S. metallicum and the possibility of infective larvae capable of being eventually inoculated into man is high, when flies could survive more. It is suggested that the susceptibility of S. metallicum in Venezuela to the indigenous strain of O. volvulus is relatively high, as compared with that of the same species in Guatemala or Colombia.

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