Secondary serologic responses to dengue virus during the 1998 outbreak in Salts, Argentina, where other flavivirvses co-circulate.

In 1998, a dengue outbreak (serotype 2) occurred in Salta province in North ern Argentina, following the first detection of dengue in the same area in 1997. We classified the serologic response of cases from 1998 as primary or secondary, since the risk of severe disease is greater for secondary cases . We studied 154 cases by plaque reduction neutralization and hemagglutinat ion inhibition tests. Thirty-eight cases (25%) were classified as primary s erologic responses and 84 cases (54%) as secondary responses. Thirty-two ca ses (21%) with borderline IgG titers could not be classified. Previous expo sure to potentially cross-reacting flaviviruses (Saint Louis Encephalitis [ SLE] and Yellow Fever [YF] viruses) was analyzed, as a possible cause of th e secondary response pattern. Our results indicated that among cases classi fied as dengue secondary response, 83% could be attributed to previous SLE or YF exposure or serologic cross-reactivity. Vaccination against YF virus was at most a minor contributor to the secondary response pattern. The find ing of a positive YF serologic result among persons not vaccinated may indi cate silent circulation of YF in a region that can support both urban and j ungle cycles. Other cases showing dengue secondary responses remained unexp lained, suggesting the unrecognized occurrence of a previous infection with other dengue serotypes or of flaviviruses other than SLE or YF.