Deadly dengue mosquitoes learn 'dirty' tricks
They are now breeding in polluted water bodies in urban areas, including soakage pits, septic tanks and even domestic run-offs studies show Need to revise the vector-control methods to include the ‘tradition al’ as well as ‘newly-found’ breeding grounds
The ‘dealers’ of dengue bringing illness and death to many a man, woman and child are ‘clever’ little insects. All this while, Sri Lankans were peering into flower vases, fridge trays, discarded yoghurt cups, rambutan peel, thambili shells, tyres and gutters et al having even a drop of clean water to check out the breeding of the dengue vectors. Over and over again, we heard that the dengue primary vector Aedes aegypti and secondary vector Aedes albopictus love clean water to lay their eggs.
Await some disturbing findings – both these mosquitoes have cleverly adapted and changed their egg-laying behaviour. They are now also breeding in polluted water bodies in urban Sri Lanka, the Sunday Times learns.
“They now lay their eggs in organically-polluted or dirty water, including soakage pits, septic tanks and even domestic run-offs,” says Senior Lecturer Dr. Nayana Gunathilaka of the Department of Parasitology, Faculty of Medicine, Kelaniya University, who has carried out in-depth research on the changes in the ‘bionomics’ of both the Ae. aegypti and Ae. albopictus.
Explaining that bionomics is the study of the mode of life of organisms in their natural habitat and their adaptations to their surroundings, he says that his research found a change in the oviposition behaviour – the depositing or laying of eggs through the ovipositor, an organ at the end of the abdomen by the female mosquitoes.
In his laboratory, Dr. Gunathilaka experimented not only with the type of water the mosquitoes preferred to lay their eggs in but also the size of the containers and the colour of the containers. He had collected water from different sources such as drains in towns; de-chlorinated water; chlorinated water; ponds; and wells; while in the salt-concentration (salinity) study a range of 0.2, 0.5, 1, 2 and 3% of Sodium Chloride (NaCl) in water had been utilised, with pure distilled water being the control.
To check out what colour attracted these female mosquitoes, he had used black, white, yellow, red, green and blue coloured paper for the ovitraps.
The following findings should drastical- ly change vector-control activities, which are the only way to curb dengue: Both Ae. aegypti and Ae. albopictus are able to breed in waste water with low dissolved oxygen and high turbidity. So drains in towns entice them. Both species preferred black ovitraps to any other colour, with the lowest preference being white. The highest oviposition was detected at 0.2% of NaCl concentration, with no oviposition observed at 3% of concentration. “The culprit for the dengue epidemics reported from Trincomalee and Kinniya may be the brackish water rampant in those areas,” says Dr. Gunathilaka, adding that the first evidence of the dengue female mosquitoes’ breeding behavioural changes came from a research team at the Jaffna University. The team had found that these mosquitoes could breed in brackish water with a salinity range of 2 to 15 ppt in discarded plastic or glass containers, abandoned fishing boats and also unused wells in coastal as well as peri-urban environments.
Before going into detail on how he set about his study in 2016, he says that mosquitoes are a group of Nematocerous Diptera that are of medical importance because many species transmit diseases to humans and animals.
The water quality in a potential breeding site will determine whether or not the female mosquitoes will lay their eggs and the resulting larvae and pupae will survive to become adult mosquitoes. While some mosquitoes prefer clear water, others thrive in polluted water, depending on their bio-ecological tolerance. More and more evidence is pointing to some mosquito species adapting to survive in alternative water bodies due to rapid urbanisation, industrialisation and global warming, he says.
Dipping into literature published earlier, Dr. Gunathilaka takes a close look at the dengue female mosquitoes: The selection of an oviposition site is an important behavioural component of their survival; they choose oviposition sites through a combination of visual and chemical cues; they may also select oviposition sites by checking the availability of larval food; and they do not lay all their eggs in one location, following a ‘skipping’ pattern to distribute the eggs over a wider area.
Did you know that the female mosquitoes “taste” the water in a potential site to detect the chemical cues which would give an indication whether the site is suitable or not for them to leave their precious eggs, he asks.
Dr. Gunathilaka studied 3,000 Ae aegypti and 3,000 Ae. albopictus, starting with 1,000 each and repeating the studies not once but thrice.
Having collected immature and adultstage mosquitoes of Ae. aegypti and Ae. albopictus from the Narangodapaluwa Public Health Inspector’s division, representing the highest reported dengue cases in the Ragama Medical Officer of Health area, mass rearing had been carried out at the Molecular Medicine Unit, Faculty of Medicine, University of Kelaniya.
When asked why the Aedes mosquitoes are adapting, he says that it may be because the urban setting is replete with polluted water bodies which have a high nutrient content.
“Now we have to re-think the advice that we give people to add salt to any indoor containers with water to reduce mosquito breeding because they have adapted to that,” says Dr. Gunathilaka, urging that with the bionomics of the Aedes mosquitoes changing to attain wider dissemination in the environment, the current dengue-control activities need to be changed as they rarely focus on polluted water habitats.
There is an urgent need to take into consideration both the Jaffna and Kelaniya studies and revise the vector-control methods to include the ‘traditional’ as well as ‘newly-found’ breeding grounds.