Four types of the dengue virus currently in circulation in Sri Lanka: DENV1, DENV2, DENV3, and DENV4.
Sri Lanka's worst dengue outbreak, which occurred in 2009 and claimed 346 deaths, was attributed to a new strain of DENV1, says a review led by Faseeha Noordeen, a virologist at the University of Peradeniya.
Earlier in 2003, different strains of DENV3 were seen to have swapped their genetic material to produce new clades (group of organisms with a common ancestor), Noordeen tells SciDev.Net.
DENV2 and DENV3 were predominantly circulating in the island nation until 2009 and were responsible for 86 per cent of dengue infections.
But, new genetic variants of DENV1 and new clades (group of organisms with a common ancestor) of DENV3 are the reasons for epidemics in the last decade, according to the review in the January 2014 in International Journal of Infectious Diseases.
"Our review emphasises that the virus is continuously changing and is unstable in nature due to its RNA genome," Noordeen says.
"Perhaps similar strains of the virus in the environment can co-infect one patient or a vector mosquito," Noordeen says.
In 2008, the main dengue-transmitting mosquitoes in Sri Lanka, Aedes aegypti and A. albopictus which were thought to breed only in fresh waters, were found to have adapted to laying eggs in brackish waters, posing a threat to dense human habitations in coastal areas.
Sinnathamby Surendran, professor of zoology at the University of Jaffna, who led the team that reported the mosquitoes' adaptability, showed through laboratory studies conducted in 2011 that the larvae could thrive in varying concentrations of salt solutions.
“This may well be an early alarm but will definitely pose a threat in days to come,” says Sinnathamby Surendran.
These findings may call for changes in local policy as well as in WHO vector control guidelines that presently target only freshwater containers as having the potential to breed dengue-spreading mosquitoes, Noordeen says.
Urbanisation in tropical countries has favoured the spread of dengue. Nearly half of the 2.5 billion people facing dengue threat live in 10 countries that fall in WHO's South-East Asia region. There is currently no drug or vaccine against dengue.
> Link to the paper in the International Journal of Infectious Diseases
This article has been produced by SciDev.Net's South Asia desk.