V. Evaluation of Cross-Immunity against Type 1 Dengue Fever in Human Subjects Convalescent from Subclinical Natural Japanese Encephalitis Virus Infection and Vaccinated with 17D Strain Yellow Fever Vaccine
University of the Philippines College of Medicine, Ministry of Health, Division of Geographic Medicine and Department of Biometry, Case Western Reserve University School of Medicine, Rockefeller Foundation, Manila, Philippines
Of the 1,098 individuals 1 year of age or older in the village of Santa Rosa, 1,010 (92%) had complete parasitological examinations, histories and physicals. Schistosoma japonicum infection was found in 32.4% of this population. Prevalence was 14% in age group 5–9; it peaked at 57% at age 15–19, and declined to about 30–50% from age 30–60 years. While 68% of the population was uninfected, 21% had light infections (1–100 eggs/g feces), 8% had moderate infections (101–400), and 3% had heavy infections (>400). The mean intensity of infection in males was 186 eggs/g feces, and in females 105. Seven percent of the overall population described some weakness, and about 7% had colicky abdominal pain; there were no changes in these percentages with either the presence or intensity of infection. Four individuals had a history of convulsions, but none of them had S. japonicum eggs in their feces. On physical examination hepatomegaly equal to or greater than 3 cm in the midsternal line was seen in 15% of the infected population and in 21% of those with heavy infections, while only 8% of uninfected individuals had this degree of liver enlargement. A striking difference in age prevalence of hepatomegaly between males and females was observed. The much earlier onset of liver enlargement in males correlated with high levels of egg output in childhood. Females did not achieve high egg outputs until adulthood, and this was later followed by a peak in the prevalence of hepatomegaly. An enlarged spleen of Hackett grade 2 or greater was found in 2.7% of the population. There was no relationship between splenomegaly and the presence or intensity of infection. Thus, it appears that S. japonicum infection of a moderate prevalence and intensity was associated with low morbidity. Contrary to general belief, schistosomiasis japonica did not appear to be a significantly more pathogenic infection than schistosomiasis mansoni, as seen in similarly studied communities in Kenya.