Urbanisation of yellow fever in Santa Cruz, Bolivia.

Background: Reinvasion by Aedes aegypti of cities in the Americas poses a threat of urbanisation of yellow fever. After detection of yellow-fever infection in a resident of the city of Santa Cruz, Bolivia, in December, 1997, we 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 suspected cases. Patients were serologically screened for yellow fever, dengue, hepatitis A and B, and leptospirosis. We collected clinical and epidemiological 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 infection was confirmed in six residents of Santa Cruz, five of whom died. Five lived in the southern sector of the city. Two had not left the city during the incubation period, and one had visited only an area in which sylvatic transmission was deemed impossible. Of the 281 people covered in the serosurvey 16 (6%) were positive for IgM antibody to yellow fever. Among five people for whom this result could not be explained by recent vaccination, there 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 infestation 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 large-scale immunisation of the urban population, as well as tightened surveillance and appropriate vector control.