Antibodies to Yellow Fever and Other Arthropod-Borne Viruses in Human Residents of San Vicente de Chucuri, Santander, Colombia *

Abstract

Summary The Municipio of San Vicente de Chucuri, Santander, Colombia, is situated partly in the Foothills of the Andes and partly in the Valley Bottom of the Magdalena River. The climate is tropical and the original climax vegetation was tropical rain forest. Since 1936, 78 fatal cases of jungle yellow fever (JYF) have been diagnosed in the municipio by viscerotomy. This serological study of 1,189 human sera from residents of the Foothills and of 96 sera from residents of the Valley Bottom is the first phase in a long-term study of the epidemiology of JYF in the municipio, which is typical of the JYF area in the Magdalena Valley. Of the 1,189 sera tested from Foothill residents by yellow fever (YF) neutralization tests (NT) 73.6% were positive, and of the 96 sera from Valley Bottom residents 76.0% were positive. All age groups were about equally positive, as would be expected as the result of the repeated and intensive YF vaccination which has geen done in the municipio. However, about a fourth of the population is still without demonstrable protection against YF, as judged by very strictly interpreted NT results. Thus, it is clear that the ideal procedures for applying 17D vaccine have not yet been put into practice in San Vicente. Quite different results were obtained by the hemagglutination-inhibition (HI) test for YF using an antigen prepared from the pantropic JSS strain of virus. Of 609 representative Foothill sera tested only 24.6% were positive, and of 93 Valley Bottom sera only 38.7% were positive. Not only was the proportion of positives markedly lower in the HI test, but the distribution of the positives was significantly different. The proportion of positives increased with age and increasing length of residence. The HI results on the sample of 90 sera tested with a 17D HI antigen parallel very closely the NT results on the same sera. The significance of these differences has not been ascertained. Since there is a poor correlation between JSS HI results and Ilheus HI and dengue HI results, we conclude that the HI antibodies for Ilheus and dengue are not exclusively the result of YF and 17D virus infections. The positive NT results with dengue Tr 1751, Ilheus and St. Louis are interpreted as not necessarily indicative of past infection with those viruses because only three of the sera studied with all four viruses by NT were single positives: one each of Ilheus, dengue Tr 1751 and St. Louis. It would appear that unless and until dengue, Ilheus and St. Louis viruses, or possibly some other group B virus, are isolated in San Vicente, this doubt cannot be cleared up. With VEE virus, HI tests were positive in 2.1% of 864 Foothill sera and in 34.1% of 94 Valley Bottom sera. This difference is striking and is interpreted to mean that VEE virus probably does not invade the Foothills area. An HI titer of 1:80 or higher is taken as diagnostic of VEE, and of other group A viruses; lower titers are interpreted as heterologous reactions with other group A viruses. With Mayaro virus three of 90 Foothill sera were positive by HI. No Valley Bottom sera were tested with this virus. No evidence of activity of either EEE or WEE virus was obtained. Chikungunya is a group A virus from Tanganyika which has been found to be broadly antigenic in other geographical areas. The number of sera tested is too small to give significance to the negative results. The significance of the two positive results by HI with Bunyamwera virus is not known. The virus is “ungrouped” by Casals and is of Central African origin. Low proportions of positives are frequently found in groups of sera from South America.