KUALA LUMPUR: Some Aedes aegypti mosquitoes have developed resistance to insecticides, adding to the challenges in combating vector-borne diseases like dengue, Zika and yellow fever, medical researchers revealed.
Institute for Medical Research Malaysia’s Infectious Diseases Research Centre (IDRC) medical entomology unit head Dr Lee Han Lim said there was evidence that Aedes mosquitoes were becoming less susceptible to chemical insecticides because of extensive use .
“However, it is highly localised and limited to several areas only.
“Hence, it will not affect insecticide efficacy in other areas,” he told the New Straits Times.
Dr Lee said the problem could be countered by rotating, every six months, the use of insecticides that had different mechanisms of killing mosquitoes.
He said one should use synergist (chemicals that increased the killing power of insecticides), such as Piperonyl butoxide, or non-chemical control agents, such as Bacillus thuringiensis israelensis, or BTI, a bacterium that killed mosquito larvae, but has no effects on other organisms, and mosquito larvae would not develop resistance, even after more than 20 years of use.
Asked if dengue strains had mutated into a new, more contagious strain, IDRC virology unit research officer Dr Ravindran Thayan said based on sequencing of selected genetic regions of local dengue virus strains, there was no evidence of new dengue serotypes or genotypes.
However, Dr Ravindran said there were possibilities of new clades (a group of organisms believed to have evolved from a common ancestor) within the same dengue genotype.
“Whether this results in a more contagious strain is not known. There is no evidence that these strains can result in more severe manifestations of dengue.”
Dr Ravindran said there was no scientific evidence that the Aedes aegypti mosquitoes had changed its breeding habits as they still preferred to breed in still, clear and clean water.
“Dirty and polluted water, such as those in clogged drains, only breeds Culex mosquitoes, which are not dengue vectors.”
Culex mosquitoes carry arbovirus, West Nile virus, filariasis, Japanese encephalitis, St Louis encephalitis and avian malaria.
However, recent research found that Aedes mosquitoes are able to breed in the clear upper layer of sewage waters as well.
Universiti Malaysia Sabah (UMS) professor Dr Chua Tock Hing said Aedes were adaptable insects and able to colonise all sorts breeding sites containing stagnant water.
He said there was ample evidence to show that Aedes mosquitoes were resistant to pyrethroids insecticides (such as permethrin) and organophosphates (such as teme-phos and malathion).
Another scientist from UMS, Dr Sylvia Daim, said there was no evidence to indicate that dengue viruses had mutated to become more contagious from human to human, human to mosquito or mosquito to human.
However, she said, there were reports of periodical emergence of dengue viruses with new genetic types that became easily transmissible by mosquitoes and caused an outbreak under favourable conditions, such as concurrent high mosquito densities, with emergence of new dengue virus genetic type.
This phenomenon, she said was attributed to the unique evolutionary dynamics of dengue virus, in which genetic lineages within the same serotypes regularly emerge and extinct.
“Such events occur on a regular basis where dengue is endemic,” said Dr Sylvia.
She cited a research article, titled “Dengue virus type 1 clade replacement in recurring homotypic outbreaks” published on BMC Evolutionary Biology in 2013, that speculated the next major DEN-1 dengue virus serotype outbreak in Malaysia would occur some time in 2019.
“The researchers had analysed the genes of DEN-1 isolated between 1997 and 2011.
“The study reported that the recurring major DEN-1 outbreaks in 1987, 1997 and 2004 were associated with virus lineage replacements within this serotype and that these were due to random chance events.
“With a seven- to 10-year cyclical pattern, they speculated that the next major DEN-1 outbreak in Malaysia could perhaps take place three years from now.”
Dr Sylvia said organisms evolved to adapt to the changing environment and perpetuate the existence of their species.
However, micro-organisms, such as viruses, mutate much more easily and at a faster rate because of their simpler genetic machinery compared with animals and plants.
For infectious agents like the dengue virus, they evolve mainly to better evade a host’s immune response and to enhance their transmissibility, she added.