AN EPIDEMIC OF SYLVATIC YELLOW-FEVER IN THE SOUTHEAST REGION OF MARANHAO STATE, BRAZIL, 1993-1994 - EPIDEMIOLOGIC AND ENTOMOLOGIC FINDINGS

Yellow fever virus transmission was very active in Maranhao State in B razil in 1993 and 1994. An investigation was carried out to evaluate t he magnitude of the epidemic. In 1993, a total of 932 people was exami ned for yellow fever from Maranhao: 70 were positive serologically, hi stopathologically, and/or by virus isolation, and another four cases w ere diagnosed clinically and epidemiologically. In Mirador (17,565 inh abitants), the incidence was 3.5 per 1,000 people (case fatality rate [number of deaths/number of cases diagnosed] = 16.4%), while in a rura l yellow fever risk area (14,659 inhabitants), the incidence was 4.2 a nd the case-fatality rate was 16.1% (10 of 62). A total of 45.2% (28 o f 62) asymptomatic infections were registered. In 1994, 49 serum sampl es were obtained and 16 cases were confirmed (two by virus isolation, two by seroconversion, and 12 by serology). No fatal cases were report ed. In 1993, 936 potential yellow fever vectors were captured in Mirad or and a single strain was isolated from a pool of Haemagogus janthino mys (infection rate = 0.16%). In 1994, 16 strains were isolated from 1 ,318 Hg. janthinomys (infection rate = 1.34%) and one Sabethes chlorop terus (infection rate = 1.67%). Our results suggest that this was the most extensive outbreak of yellow fever in the last 20 years in Brazil . It is also clear that the lack of vaccination was the principal reas on for the epidemic, which occurred between April and June, during the rainy season, a period in which the mosquito population in the forest increases.