2 CONTIGUOUS OUTBREAKS OF DENGUE TYPE-2 IN NORTH QUEENSLAND

Objectives: To investigate two outbreaks of dengue type 2 in north Que ensland, one in the Torres Strait beginning in late 1996, the other in a Cairns suburb in early 1997. Design: Epidemiological investigation of all laboratory-confinned cases of dengue, entomological investigati on of the local environment, and laboratory analysis of the isolated d engue viruses. Main outcome measures: Numbers of confirmed and of loca lly acquired cases; virus serotype; comparison of nucleotide sequences between viruses isolated from the two outbreaks; and Breteau Index (B I = number of containers with larvae of the mosquito vector Aedes aegy pti found per 100 houses investigated) on the affected islands and in the Cairns suburb. Results: There were 201 confirmed cases of dengue i n the Torres Strait outbreak, which lasted nearly seven months, and se ven confirmed cases in the Cairns outbreak, which lasted about nearly 11 weeks. Most (190) were confirmed as dengue type 2. Nucleotide seque ncing of viruses isolated from the two outbreaks showed they were iden tical. Ae. aegypti breeding sites were very common on the five Torres Strait islands surveyed (Bls, 73-219 - high risk), but less so in the Cairns suburb (Bl, 23). The most common breeding sites were water stor age reservoirs, particularly rainwater tanks, on the outer Torres Stra it islands, discarded containers (such as plastic containers, buckets and tyres) on Thursday Island, and garden items (such as flowerpot bas es and jars) in Cairns. Conclusions: The virus responsible for the Cai rns outbreak was most probably introduced from the Torres Strait, wher eas the virus responsible for the Torres Strait outbreak was imported from Papua New Guinea. Preventive strategies tailored to specific loca tions are needed to reduce breeding of Ae. aegypti in north Queensland , and the consequent risk of future outbreaks of dengue.