Immuno-epidemiological studies on subclinical infection in leprosy. III. Yearly observations and follow-up studies of schoolchildren by using FLA-ABS and lepromin tests.

The schoolchildren in three regions of Miyako Islands, Okinawa Prefecture, were surveyed annually, from 1978 to 1984, by using the fluorescent leprosy antibody absorption (FLA-ABS) test and the lepromin reaction with the Dharmendra's antigen, for detecting the individuals at high risk of leprosy and for evaluating predictive value of these immunological tests. Constant potency of these tests was confirmed by the percentage of positive reactions among the children in the first grade of elementary schools (5 or 6 years old) surveyed from 1980 to 1984. A temporal rise or drop of the percentages during this period seemed to associate with the yearly reported number of new leprosy cases in each region. Among 1,168 schoolchildren tested with FLA-ABS once or twice or more, the percentage of positive reactions was significantly higher in the children with the enlargement of peripheral nerve without sensory loss than in those without this signs and symptoms. The longer the duration of this signs and symptoms, the higher the percentage of positive FLA-ABS tests. Lepromin test did not show any significant correlation with this signs and symptoms. A concordant persistence or change between the FLA-ABS tests and neural signs or symptoms was observed in 133 out of 331 children examined twice or more. Discordant changes in the remaining were mainly attributed to a conversion to seronegativity before the disappearance of neural signs and symptoms. Changes of FLA-ABS and lepromin reactivities between the initial and final tests suggested a spontaneous cure of subclinical infection with Mycobacterium leprae among the children who were FLA-ABS positive but lepromin negative at the initial test and therefore considered to be a group at high risk of leprosy. Although neural signs and symptoms were found in the majority of these children, none of them so far developed overt symptom of the disease. Based on these findings, predictive value of FLA-ABS test together with lepromin was discussed.