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
Intracutaneous tests were performed with saline, lipid-free and Melcher extracts of Onchocerca and Dirofilaria on persons living in areas endemic for onchocerciasis in Guatemala and bancroftian filariasis in American Samoa. When measured by their N concentration, coupled with standardized methods of extraction, administration and measurement of reactivity, a logarithmic relationship appeared to exist between the wheal area and nitrogen content of injected antigen. Antigens prepared by saline extraction were generally more reactive than were antigens prepared by the other methods.
Concentrations of antigens from 3 to 4 µg N/ml elicited high percentages of positive reactions in persons from endemic areas, while normal controls and persons with intestinal nematodes from non-endemic areas gave negative reactions. Antigens with a strength of 0.8 µg N/ml yielded a lower reactivity, and may be nearer an antigen concentration that will distinguish between currently infected and non-infected persons than the higher concentrations.
Dirofilarial extracts appeared to elicit group reactions in the two populations while onchocercal extracts elicited more specific reactions in persons infected with Onchocerca. The frequency of reactions by age-groups in both populations increased with age and with the nitrogen concentration of the antigen employed. No variations by sex were observed.