Field epidemiology of an outbreak of dengue fever in Charters Towers, Queensland: are insect screens protective?

Between March and July 1993 a dengue virus epidemic swept through Charters Towers, a rural North Queensland mining community of 10,000. This clearly delineated outbreak provided an ideal opportunity to carry out one of the few field epidemiological studies of dengue in Australia. The epicurve was consistent with that of a point source outbreak, 18 weeks in duration and peaking at four weeks. A basic reproduction number for the dengue epidemic of 1.99 indicates a similar rate of spread to that found in dengue epidemics overseas. A female-to-male sex ratio of 1.7:1 was obtained for the 238 cases identified. An age- and sex-matched retrospective case-control study showed that cases were significantly more likely to live in unscreened houses than were controls (McNemar chi 2 = 56.1 df, P < 0.0001). Despite being generally accepted, an association between insect screens and a reduced incidence of mosquito-borne diseases has not previously been demonstrated in Australia. We speculate that unscreened housing facilitates the initial spread of a dengue epidemic.