Urbanisation of yellow fever in Santa Cruz, Bolivia

Background Reinvasion by Aedes aegypti of cities in the Americas poses a th reat of urbanisation of yellow fever. After detection of yellow-fever infec tion in a resident of the city of Santa Cruz, Bolivia, in December, 1997, w e investigated all subsequent suspected cases. Methods We introduced active surveillance of yellow fever in the Santa Cruz area. Hospitals and selected urban and rural health centres reported all s uspected cases. Patients were serologically screened for yellow fever, deng ue, hepatitis A and B, and leptospirosis. We collected clinical and epidemi ological information from patients' records and through interviews. We also carried out a population-based serosurvey in the neighbourhood of one case . Findings Between December, 1997, and June, 1998, symptomatic yellow-fever i nfection was confirmed in six residents of Santa Cruz, five of whom died. F ive lived in the southern sector of the city. Two had not left the city dur ing the incubation period, and one had visited only an area in which sylvat ic transmission was deemed impossible. Of the 281 people covered in the ser osurvey 16 (6%) were positive for IgM antibody to yellow fever. Among five people for whom this result could not be explained by recent vaccination, t here were two pairs of neighbours. Interpretation Urban transmission of yellow fever in Santa Cruz was limited in space and time. Low yellow-fever immunisation coverage and high infesta tion with A aegypti in the city, and the existence of endemic areas in the region present a risk for future urban outbreaks. We recommend immediate la rge-scale immunisation of the urban population, as well as tightened survei llance and appropriate vector control.