Dengue fever continues relentless climb

NewScientist.com news service

Outbreaks of dengue fever have risen around the world since the start of 2003, continuing the relentless spread of the once rare tropical disease.

Sharp increases in dengue fever, and its deadly complication dengue haemorrhagic fever (DHF), have been seen across the globe, from Vietnam, Malaysia, Indonesia, Australia to South America.

Akib Kamaluddin, head of South Sulawesi's health office in Indonesia, says the mosquito-borne disease has caused 23 deaths in the first two months of 2003, compared to 38 in the whole of 2002.

In Malaysia, a new strain has caused a massive jump in cases. In 2002, there were over 11,000 cases of dengue fever or DHF. But by 22 February there had already been 3410 recorded cases, with 11 deaths.

And in Vietnam, over 1700 people in Vietnam contracted in the January and February - a 66 per cent rise on the same months in 2002. Three people have died.

Mosquito control

Dengue fever was first recognised in the 1950s, and its dramatic rise since has been blamed on inadequate mosquito control in tropical and subtropical regions. The main culprit is city-dwelling Aedes aegypti, which has become more prevalent as nations have rapidly urbanised.

The disease is now endemic in over 100 countries, and according to the World Health Organization, 40 per cent of the world's population is now at risk. There are estimated to be as many as 50 million cases a year. Although it rarely kills, dengue fever causes a severe flu-like illness that can lead to hospitalisation.

There are four closely-related strains of the virus that causes dengue fever. Contracting the disease confers lifelong immunity, but only to that strain. The rise in cases in Malaysia is partly because the DEN3 strain is new to the country.

Paradoxically, people with immunity to one strain have an increased risk of developing the more lethal DHF if they then catch a different strain. This has added to the difficulties in developing a vaccine, because such a drug must not increase the risk of more serious disease.

Rainwater receptacles

"The big problem with dengue is that the lack of a vaccine. Therefore we have to use vector control - mobilise communities to restrict mosquito breeding sites," says Harold Townson, president of the Royal Society for Tropical Medicine and Hygiene, in London, UK.

A. aegypti breeding was encouraged by discarded containers such as tin cans and tyres which collect water, as well as receptacles left to collect rainwater, he told New Scientist.

Malaysian health minister Chua Jui Meng agrees, saying the rise in public apathy in keeping the environment clean, as well as an exceptionally heavy rainy season, has worsened the dengue outbreak. He told AFP news the government had served 30,000 notices to people to stop mosquitoes breeding on their premises and taken 550 people to court.

In Cairns, Australia, there have been 16 confirmed cases, with 60 more suspected cases reported. Twenty cases have been reported in the Paraguayan city of Pedro Juan Caballero.

Townson says it is too early to say if the 2003 increases represent a major new wave of the disease, but he adds: "It certainly shows no major sign of abating."