News
Deadly dengue mosquitoes learn ‘dirty’ tricks
View(s):- 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
By Kumudini Hettiarachchi
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.
Mosquito studies in the laboratory
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 drastically 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 adult-stage 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.
| Clearing up operations: Schoolchildren can spread the message It is a ‘lesson’ from schools that can be learnt by homes, government and private sector institutions and anyone else willing to take heed. Regular inspections, general vector and breeding-spot surveillance and destruction of such spots immediately bring down the risk of the spread of dengue, a project in 60 schools in four educational zones in the Gampaha district has shown. Schoolchildren can spread the message, has become crystal clear.
With an earlier Health Ministry research finding that 66.7% schools in Colombo city had at least one container positive for dengue larvae and pupae, it was more or less certain that schools are at risk. However, as no routine entomological work is carried out, the entomological risk is still unknown, says Dr. Gunathilaka. Therefore, to fill this gap, they carried out entomological surveys in the 60 chosen schools in the Gampaha district which comprised clusters from the Gampaha, Minuwangoda, Kelaniya and Negombo educational zones, over seven months, from June to December, last year. In its wake followed the identification of 12 mosquito-breeding site categories — leaf axils; tree-holes; decaying material such as banana leaves; coconut shells; discarded plastic containers; clay-containers; flower-pots; discarded tyres; tyre-prints; discarded metal containers; drains; and blocked toilets. The findings are revealing: Setting up focus groups which included representatives from schools such as the principal, several teachers, students and parents and also officials such as those from the local government authority, they had been mobilized by the research team not only to search for breeding habitats but also potential ones and destroy them. With schools having issues in managing their solid waste, the wide-ranging programme had included green practices such as vegetable plots, a compost yard and waste segregation on the 3R-basis (reduce, re-use and recycle). Strict surveillance, according to a stringent checklist, in different settings will bring about long-term monitoring and sustainability in Sri Lanka’s quest to battle the dengue mosquito, adds Dr. Gunathilaka, citing the excellent example of the Gampaha district schools. “Dengue Prevention Committees should be established in schools and given maximum support to maintain a dengue-free environment. Conducting awareness programmes, shramadana campaigns and regular field inspections, at least once a week, are vital,” he says, adding that this would prevent schoolchildren taking dengue to their homes and communities. | |
While the breeding habitat study was ongoing, Dr. Nayana Gunathilaka along with the Central Environmental Authority’s Senior Environment Officer, Lakmini Radhika who is a Ph.D candidate under Dr. Gunathilaka’s supervision, also conducted a surveillance programme in schools in the Gampaha district which is badly affected by dengue.