Useful Information

16 June 2004

Dear Doctor,

Dengue Fever Alert - Surge in Ovitrap Indices

The Food and Environmental Hygiene Department recorded a sharp increase in ovitrap index during the month of May. The ovitrap index is a measure of the prevalence of the mosquito vector ( Aedes albopictus ) for dengue fever (DF) transmission. The index was 31.6% in May 2004, compared with 11.9% in April 2004 and 17.1% in May 2003. This rise signals an increased risk of local transmission of DF in Hong Kong.

The first local DF case in Hong Kong was reported in September 2002 since it became statutorily notifiable in 1994. Year 2002 saw 20 local cases and 24 imported cases. In 2003, one local case and 48 imported cases were confirmed. Fourteen imported cases were notified as of June 15, 2004. The 14 cases in 2004 comprised 9 males and 5 females, aged 10-47. Travel histories included Indonesia (4), Thailand (3), Philippines (2), Malaysia (1), Singapore (1), Cambodia (1), Sri Lanka (1), and Tahiti (1). According to the World Health Organization, Indonesia experienced an upsurge of dengue fever with 58,301 cases reported during January through April 2004. At the end of April the situation has returned to normal with all provinces reporting cases at a low level.

Dengue fever presents with a spectrum of clinical illness ranging from a non-specific viral illness to severe and fatal haemorrhagic disease. The incubation period is usually 3 to 14 days. The disease is characterized by sudden onset of fever for 3-5 days, intense headache, myalgia, arthralgia, retroorbital pain, anorexia, GI disturbance, and a maculopapular rash. Thrombocytopenia is frequently seen and leucopenia is sometimes observed. In severe cases, the patient may present with bleeding and shock. Children may have milder symptoms than adults.

For serological confirmation, IgM antibody is usually detectable by day 5 after onset of illness. Demonstration of a fourfold or greater rise in total antibody titre to any dengue serotype in paired sera is also diagnostic. Diagnosis by RT-PCR requires a serum specimen within the first week after onset of symptoms.

Dengue fever should form a differential diagnosis in a patient with travel history to dengue endemic areas (e.g., SE Asia) within 14 days and presenting with febrile flu-like illness, especially when a rash is present. Please report patients with suspected DF to the central notification office of the Surveillance and Epidemiology Branch (SEB) under the Centre for Health Protection. The Virology Division, Public Health Laboratory Services Branch of the CHP can offer diagnostic support for suspected DF cases. The contact details of the SEB central notification office are:

More information is available at http://www.chp.gov.hk , and http://www.fehd.gov.hk ).