Longitudinal study on Schistosoma japonicum infections in the Philippines. 3. Incidence and environmental modification at Dagami, Leyte.

ABSTRACT

The yearly change of incidence of schistosomiasis japonica was observed among 1800 children enrolled in 9 schools situated within a 5 km radius around Dagami Poblacion, Leyte, Philippines from School Year (SY) 1974/75 to 1979/80. The purpose of this observation was to know the effect of environmental modification undertaken in about 50 ha of snail-infested and abandoned rice field which is the most depressed and central portion of this project area. Children were examined by egg detection from stool and by circumoval precipitin test. Coverage of examination among the school population ranged from 40 to 80% yearly and the ratio of re-examination after about 1 year was from 30 to 60%. The annual incidence in the different schools differed greatly from 12.15 to 42.86% in contrast to the annual prevalence which differed only slightly. There was corresponding increase of incidence with age from 18.6% at age 7 to 33.9% at age 12 while no significant difference was observed between sexes. The overall annual incidence rates for 9 schools during the 5-year follow-up period (SY 1975/76 to 1979/80) were 22.2, 24.2, 26.9, 9.6 and 28.4%, respectively. Those of Dagami Central II, the nearest school to the reclaimed site, were 15.7, 18.2, 19.3, 5.2 and 11.6%, respectively, for the same period. Obviously, the incidence was increasing while the reclamation was going on except in SY 1978/79. Analysis of the sudden drop of incidence in that year in all the 9 schools showed that it was not due to the effect of reclamation, but was most likely due to scanty rainfall in the previous year (1977). These observations indicated that small scale environmental modification had doubtful effect on outlying endemic areas in reducing the danger of Schistosoma infection but may have a little effect on the immediate surroundings as shown by a lesser elevation of incidence after the abrupt decrease at the nearest school as compared to those of schools located farther from the reclamation area.