An epidemic of dengue 3 in far north Queensland, 1997-1999

Objectives: To describe an epidemic of dengue type 3 that occurred in far n orth Queensland in 1997-1999 and its influence on the further development o f dengue prevention and control strategies. Design: Epidemiological and laboratory investigation of cases, entomologica l surveys and phylogenetic analysis of dengue virus isolates. Main outcome measures: Numbers and characteristics of confirmed cases; Bret eau Index (BI; number of containers breeding Aedes aegypti per 100 premises ); effect of control measures on mosquito populations; genetic homology of epidemic virus with other dengue virus isolates. Results: The epidemic lasted 70 weeks and comprised 498 confirmed cases in three towns (Cairns, Port Douglas and Mossman); 101 patients (20%) were adm itted to hospital. Median interval between symptom onset and notification w as seven days (range, 0-53 days), and cumulative duration of viraemia of pu blic health significance was 2072 days. Bls in affected areas were high, pa rticularly in Mossman (45) and Port Douglas (31), Control measures signific antly reduced mosquito populations (assessed as number of ovitraps containi ng Ae. aegypti eggs and mean number of eggs per trap [P < 0.05 for both]). However, transmission persisted in several foci, in part due to undetected waterfilled containers breeding Ae, aegypti. The epidemic virus belonged to serotype 3; phylogenetic analysis suggested it was imported from Thailand. Conclusions: The epidemic had greater morbidity than other recent Queenslan d epidemics of dengue and was harder to control, necessitating substantial revision of the Dengue Fever Management Plan for North Queensland. The epid emic's severity supports the hypothesis that dengue viruses from South East Asia are more virulent than others.