UNSTABLE HYPOENDEMIC MALARIA IN RONDONIA (WESTERN AMAZON REGION, BRAZIL) - EPIDEMIC OUTBREAKS AND WORK-ASSOCIATED INCIDENCE IN AN AGROINDUSTRIAL RURAL SETTLEMENT

A longitudinal study was conducted from January 1991 to January 1992 o n the Urupa farm, a rural agro-industrial forestry settlement in Rondo nia state (Western Amazon Region, Brazil) to define the parasitologic and clinical profile of malaria. Three cross-sectional, parasitologic, and clinical surveys were performed. In the intervals between surveys , malaria cases were monitored by twice a week medical visits to the f arm and permanent local surveillance. The population of residents was approximately 170 and was characterized by high mobility. The slide po sitive rates found in the cross-sectional surveys were 0.5, 4.2 and 2. 1, respectively, for the total population (Plasmodium vivax plus P. fa lciparum). Spleen rate values in children 2-9 years old were always le ss than 1%. However, this basically hypoendemic malaria situation was unstable, with occurrence of a typical epidemic outbreak at the end of the dry season. The total number of malaria cases recorded from Janua ry to December 1991 was 163, giving an annual parasite index of 970 pe r 1,000 inhabitants. However, sex and age distribution of cases showed rare incidence of malaria in infants and low incidence in children le ss than the age of 10. Male adults 16-40 years of age represented the main risk group. The observed clustering of cases allowed us to identi fy the place of work as a factor responsible for high incidence of mal aria among adults. The general epidemiologic profile indicated that in doors transmission of malaria by the local Anopheles vector was low or absent.