The floods have come and gone and a dengue expert from Australia has urged that a thorough clean-up should be ensured after the waters recede to remove mosquito breeding sites. “Flooding can actually sweep mosquito breeding sites and larvae away. However, this is only temporary,” points out Prof. Scott Ritchie from the James Cook University [...]


‘Clean sweep’ post floods vital to eliminate dengue breeding places: Aussie expert


Prof. Scott Ritchie and Colombo East Medical Officer of Health Dr. D.A. Adikariwattage look for likely mosquito-breeding spots during the field visit

The floods have come and gone and a dengue expert from Australia has urged that a thorough clean-up should be ensured after the waters recede to remove mosquito breeding sites.

“Flooding can actually sweep mosquito breeding sites and larvae away. However, this is only temporary,” points out Prof. Scott Ritchie from the James Cook University of Australia, citing neighbour India’s Chennai floods where dengue cases dropped substantially soon after but was “only temporary”.

While Prof. Ritchie was in Sri Lanka for a day’s workshop on the scourge of dengue, it was also the day, May 16, when flood waters from the Kelani River inundated large areas, leaving thousands of men, women and children without home and hearth.

It was Prof. Ritchie’s first time in Sri Lanka and he laments that “the rain was a bit much!” even though he “loved” the country.

When asked about fogging as a preventive measure against dengue, Prof. Ritchie whom the Sunday Times contacted on e-mail after he returned home to Australia, says that many countries rely on outdoor fogging to control dengue. This does kill mosquitoes, but only as long as the “fog” is present, which is not very long. Then new mosquitoes enter the area and you are back to the same problem within days.

Likening Aedes aegypti, the dengue mosquito, to the ‘cockroach’ of mosquitoes due to its preference to hide and bite indoors, he points out that as such the recommendation is spraying, with a residual pesticide, dark shady areas, under tables and beds to kill mosquitoes when they land on it. This “surface spray” can kill mosquitoes for one or two months and has been shown to stop dengue transmission in a house or neighbourhood.

He urged Sri Lanka to consider a pilot trial of Wolbachia in the control of dengue mosquitoes and speaks with information in hand as he has been involved in a successful pilot of this programme in Cairns in North Queensland.

This initiative, ‘Eliminate Dengue Program’, part of an international collaboration, involves the release of mosquitoes which carry Wolbachia, a naturally-occurring bacterium, into the environment. These mosquitoes then breed with wild mosquitoes and pass Wolbachia to future generations of mosquitoes through their eggs, it is understood.

Prof. Ritchie points out that the “remarkable effect” has been that this bacterium prevents the dengue vector mosquitoes from transmitting the dengue viruses.

Although Wolbachia is present in up to 60% of all the different species of insects including some mosquitoes, it is unfortunately not in the primary species of mosquitoes involved in the transmission of dengue. As the same species of mosquitoes transmit other viruses such as chikungunya and Zika, this could also help curb these diseases, it is learnt.

Comparing and contrasting North Queensland to Sri Lanka, although he concedes that he did not really get a chance to look at Sri Lankan housing as “it was too wet!” he says both areas are tropical and quite wet. The dengue mosquitoes often have access indoors. “I imagine that there are poor areas in Sri Lanka that do not have a reliable water supply and store water in drums and tanks. We do not have that in Cairns.”

How in Sri Lanka gutters are checked for water collection which could in turn become ideal breeding grounds for mosquitoes, is demonstrated to Prof. Scott Ritchie at the office of the Colombo East Medical Officer of Health

When asked about the green ‘Lure and Kill’ dengue programme, he says that it is a method in which plastic buckets treated with pesticide are used to attract and kill egg-laying mosquitoes. Although used to some extent by Queensland health staff, the results have been mixed. He suggests that Sri Lanka should try adult mosquito trapping as a means of prevention which is a better measure of the impact of control than larval sampling.

Prof. Ritchie, meanwhile, commended the clinical management of dengue as impressive as it had been able to deal with severe dengue cases and bring death rates down.

His visit to the country was part of the Australian High Commission’s support to Sri Lanka’s National Dengue Control Unit (NDCU) in facilitating a workshop on May 16 to share experiences and knowledge on dengue control.

The workshop with around 40 doctors and entomologists from across the country participating was conducted jointly by Prof. Ritchie and NDCU’s National Coordinator Dr. Hasitha Tissera. The others involved in the workshop were the Deputy Director-General (Public Health Services), Dr. Sarath Amunugama and NDCU Director Dr. A.M. Thowfeek. It had also entailed a field visit to the Colombo East Municipal Council office of the Public Health Department where discussions were held on dengue surveillance and control activities.

While Dr. Tissera pointed out that poor drainage systems and ineffective solid waste management were the main reasons in the creation of mosquito breeding places, Prof. Ritchie emphasised that key measures in the prevention of the spread of dengue are good sanitation, a reliable water supply and the adoption of targeted indoor residual spraying rather than outdoor fogging.

Meanwhile, at the workshop, Australian High Commissioner Bryce Hutchesson held up Prof. Ritchie’s visit as another example of the excellent technical cooperation between the experts of the two countries. “Together we can fight dengue and ensure a better quality of life for our people.”

With the workshop drawing on research conducted in several other countries including Colombia, India and the Asia-Pacific region, Mr. Hutchesson also underscored the need for countries with experience in this field to share their knowledge and build networks among experts.

Dengue bigger threat than Zika

Zika will obviously have a huge impact in some areas such as South America, but dengue will ultimately have a more sustained risk to public health in Asia, says Prof. Scott Ritchie when asked what the ‘bigger threat’ would be.

“Dengue outbreaks occur every year across all of the tropics. Thus, an ongoing programme will be necessary and require funding,” he says, adding that there is no Zika in Sri Lanka or India yet. “Hopefully, it won’t be a big issue in Asia.”

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