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Disease WAtch

Bali travel behind spike in dengue fever

Dengue fever is a mosquito-borne acute viral infection caused by the dengue virus. It is endemic in most tropical and subtropical regions of the world, particularly Southeast Asia and Central and South America, reflecting distribution of the vectors Aedes aegypti and Aedes albopictus.

Dengue is not endemic in Australia but outbreaks occur in north Queensland where A. aegypti mosquitoes are established.

Signs and symptoms of dengue fever include fever, headache, retro-orbital pain, myalgia, arthralgia, rash, nausea and vomiting. The differential diagnosis in a returned traveler includes malaria, typhoid fever, measles, rubella, leptospirosis, scrub typhus and other arthropod-borne viral infections, such as chikungunya virus.

Dengue virus has 4 distinct serotypes. Infection with one serotype confers lifelong immunity to that serotype but a subsequent infection with another serotype increases the risk of dengue haemorrhagic fever.

Dengue fever in returned travelers

From January to September 2012, there was a dramatic increase in dengue fever notifications among WA residents with 484 cases reported. This was 1.5 times the number of cases reported in 2011 (318 cases) and almost equivalent to the record number in 2010 (494 cases). Of the more recent 484 cases notified in WA, just over half were males (55%), the median age was 40 years (range: 8-77 years), and 84 (17%) were hospitalised. The vast majority of cases (83%) were associated with travel to Indonesia. Of the Indonesian-acquired cases, 395 (96% overall) reported travel to Bali. The next most frequently reported countries of acquisition were Thailand, with 43 cases (9%), the Philippines (6), Malaysia (5), Vietnam (4), East Timor (3), Sri Lanka (2), Cambodia (2) and Singapore (2), as well as single cases associated with a range of other mostly South-East Asian countries.

An upward trend in notifications of dengue fever commenced in 2007 when 52 cases were reported following a 5-year annual average of 14 cases per year. Most of the infections were acquired in Bali. Official data on the incidence of dengue in Bali are lacking, however, local newspaper reports indicate periods of significant transmission of the disease, including a surge in cases in early 2012.

Figure 1 shows seasonal variation in dengue notifications by place of acquisition. Peaks in the Bali-acquired cases are likely a reflection of periods of increased mosquito activity.

Figure 1 is graph showing seasonal variation in dengue notification by place of acquisition.

Figure 1: Seasonal variation in dengue notifications by place of acquisition. (* Denotes January to September 2012.)

Laboratory data from the Bali-acquired dengue cases show that all 4 serotypes circulate in Bali, highlighting the risk of dengue haemorrhagic fever occurring in WA travelers with previous infections returning to Bali or other endemic areas.

In the year ending June 2012, Indonesia (primarily Bali) was the most popular holiday destination (328,000 visits) for Western Australians (Australian Bureau of Statistics data). It is therefore important that doctors advise people planning travel to Indonesia of the risks of contracting dengue fever and other mosquito-borne diseases. Travelers should be advised to use insect repellent during the day, including whilst indoors, because A. aegypti mosquitoes are day-time feeders and prefer to rest in cool dark areas. Likewise, doctors should consider dengue fever in persons with febrile illnesses who have recently travelled to Bali or other dengue-endemic regions.

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