Asian countries already burdened by increases in dengue fever and chikungunya may have to step up their disease-control efforts as the climate in the region changes. Mary Ann Benitez reports.
Climate change could worsen mosquito-borne diseases in Asia, where dengue fever and chikungunya fever have taken their toll, according to a leading expert.
“Climate change will make the situation of dengue and chikungunya diseases more acute, especially when the present control measures concentrating on the Aedes mosquito vectors have not been effective”, said Lam Sai-kit, professor emeritus datuk and former head of the Department of Medical Microbiology at the University of Malaya in Kuala Lumpur, Malaysia, at the 8th Asia Pacific Congress of Medical Virology in Hong Kong at the end of February.
Studies have shown that an increase in 1—2°C results in “quicker and higher replication” of viruses. But Lam said that the jury is still out on whether global warming is spreading vector-borne diseases in tropical and subtropical regions of Asia.
Dengue fever is ranked as the most important mosquito-borne viral disease in the world, with incidence increasing 30-fold in the past 50 years. The Caribbean, South America, Asia, Pacific Islands, and Africa have seen increased disease burden since 1980. “Global warming resulting in drought may lead to increase of cases due to households storing water”, says Lam. But historical patterns did not support a direct correlation, he cautioned, citing studies in his own country and other places.
The Aedes aegypti mosquito—the primary vector for the spread of dengue fever
Dengue fever has increased dramatically in Malaysia from less than 1000 cases in 1973 to about 46 000 cases in 2007, Lam said, citing figures from the country's ministry of health. In that same 34-year period, average rainfall had been 2833·5mm whereas mean temperature was 27·4°C.
“We have shown that in the last 10 years the number of dengue fever cases has increased tremendously and yet when you look at the rainfall patterns, over the past 30 years and also the temperature change, we do not think those changes could account for the increase of dengue”, he said. “We still do not have good evidence. Maybe you need 100 years [of monitoring]. We still need to look at this continuously and other countries should also”, he said. Data from Thailand over 33 years also did not show increased transmission of dengue in warmer months.
Through surveillance and intense mosquito control, Hong Kong has so far avoided local introduction of dengue, since a local spread at a construction site in 2002 when at least 20 people were infected. The principal vector, Aedes aegypti, is not found in Hong Kong but the prevailing species Aedes albopictus can also spread the disease. There were 42 imported cases of dengue fever in Hong Kong last year, compared with 58 in 2007 and 31 in 2006.
Lam said simulation models for predicting outbreaks of arboviruses that cause diseases such as dengue and chikungunya have been described but many of these models did not take into account social factors such as population increases or interactions between climate variables.
Chikungunya virus, spread by the Aedes aegypti vector, is the latest mosquito-borne disease that is spreading fast in the Asia region. Symptoms can include sudden fever, chills, headache, nausea, vomiting, joint pain with or without swelling, lower back pain, and a rash. “Chikungunya has become a new phenomenon in Malaysia. We found that this virus caused its worst outbreak in Malaysia in early 1999”, said Lam. 27 people were infected at that time. At least 4000 suspected cases were detected last year.
Lam said that vector control was not sustainable. “You have to keep on doing [mosquito control] year in and year out, especially when there is no outbreak. You have to spend a lot of money and effort on control. That is why we say it is not sustainable in developing countries where there are other health problems”, he said.
New developments may help control efforts in the future. Malaysia is working with a biotech company, Oxitec Ltd, on using genetically modified mosquitoes to prevent breeding of Aedes mosquitoes. Meanwhile, five companies are developing a dengue vaccine. And a Paediatric Dengue Vaccination Initiative of the Bill & Melinda Gates Foundation, of whom Lam is a member, is committed to having a commercial dengue vaccine available by 2012.