Dengue crisis bigger than Bti

Experts call for comprehensive eradication plan
By Kumudini Hettiarachchi

Cuba is to send some experts to Sri Lanka for discussions and relevant studies with authorities after which a decision would be taken over the type of ‘bug’ or bacteria needed against the dengue menace which has caused the deaths of 163 and affected nearly 15,000 others.

Two Cuban experts are expected soon, said a high-level Health Ministry source, explaining that once that happens, the bacteria – Bacillus thuringiensis sub-species israelensis (Bti) –which they are planning to use as a biological pest control agent against the mosquito larvae will be imported to Sri Lanka in the many-pronged battle against dengue. (See box for how the bug works and what approvals are needed)

While the authorities were awaiting such expertise, mosquito-control was among many issues raised at an urgent, top-level meeting summoned by the ministry on Friday, where detailed discussions were held among experts and professors from the fields of entomology, parasitology, zoology and epidemiology. A technical committee is being formed, it is learnt.

The most-important aspects of dengue control such as keeping the environment free of small containers including yoghurt cups, coconut shells, rigi-foam containers, pieces of polythene etc, proper garbage collection and disposal, and the need for fogging in dengue-affected areas were also stressed, the Sunday Times understands.

Explaining the different stages at which the dengue mosquito could be contained, an expert said that the most urgent requirement would be to deal with “source reduction” or the breeding grounds not only during an epidemic but all the time.

“If you remove the breeding places (in Sri Lanka the dengue mosquito is known to be mainly a ‘container breeder’), the mosquito or vector will not be able to lay its eggs and the dengue threat could be contained, he said, adding that next would come steps to destroy the larvae followed by measures to deal with the adult mosquito.

While the application of biological agents such as Bti could prevent the larvae from growing to be adults, fogging, in turn, would destroy this tiny but potent mosquito which has brought death to many people, it is learnt.

The people are expecting a “miracle” in the form of Bti in controlling dengue but this is a “complementary” measure which has to be done parallel with all others such as destroying breeding places, garbage control and also fogging, another source stressed.

For, Bti has been used in trials in Sri Lanka as far back as 2004, the Sunday Times learns, where its application was found to have “limited usefulness” and that too only for generally large water bodies such as water tanks.

Do you think people will go around their homes putting Bti into small containers or even bird baths and vases, queried another source, adding that it will be easier to clear domestic premises of such containers.

Explaining how it works, the source said Bti must be at the water surface for the mosquito larvae which is also suspended there to feed on it and die. What happens is that the Bti, after several days, goes down or precipitates at the bottom of the tank, while the larvae remain at the surface. “So to ensure success, Bti will have to be introduced to these tanks every seven days or so,” he said.

Another question raised by a source was whether the Cuban Bti (in liquid form) has been evaluated and passed by the World Health Organization (WHO) for use in public health programmes, while another pointed out that Bti from a different country (in powder form) is already being used in the Colombo Municipal Council (CMC) area.

The WHO Pesticide Evaluation Scheme (WHOPES) collects, consolidates, evaluates and disseminates information on the use of pesticides for public health. Its recommendations facilitate the registration of pesticides by member states, according to its website.

Health sources claimed that as far as they knew the Cuban Bti has not been put up for evaluation to WHOPES and efforts to contact the WHO Colombo office were unsuccessful.

The Sunday Times was also unable to contact CMC officers to check whether they were using Bti from another country and how successful it was in curbing dengue. If the CMC is using Bti, the number of dengue patients in its area would point otherwise, another source said.

The consensus among the experts, however, was that a concerted effort was needed to battle dengue not only on one front but on all – while larvae control with biological agents was important as would be fogging for the adult mosquito, emphasis should be on ridding homes, offices, roads, playgrounds and parks of breeding grounds by cleaning-up the environment and having proper garbage disposal.

That is the long-term answer, echoed by many over the years, but not implemented leading to the unnecessary deaths of men, women and children from a preventable disease.

What is Bti

Bti, a bacterium found naturally in soil, has been used as a biological pest control agent to combat both, mosquitoes and black-flies experts have found. Bacterium is the simplest and smallest form of life.

During the spore-forming stage of its lifecycle, the Bti bacterium produces a protein crystal which is toxic only to the mosquito and black-fly larvae. When the crystals are ingested by the insect larva when feeding, the alkaline environment of its digestive system makes them dissolve and convert into toxic protein molecules that destroy the walls of its stomach. The larva dies within days, it is learnt.

The bacteria are not harmful to humans and animals because their acidic stomachs do not activate the Bti toxins, sources said.

Bti bacteria are applied directly to the water where mosquito and black-fly larvae are found, the Sunday Times understands. However, none of the products containing Bti are generally applied to treated, finished drinking water for human consumption, a source pointed out because during the production process, if contaminated, when fermenting Bti, could release other bacteria which could be harmful to humans.

That’s why WHO evaluation is of paramount importance, sources pointed out.

Procedure for bringing in a biological agent

As Bti would be brought in the form of a pesticide, it has to be mandatorily registered with the Registrar of Pesticides who would look into aspects of safety, check out pest-risk analysis and specific certification etc., which process would take about six months to a year, it is disclosed.

However, based on “exigencies of need” the process could be expedited if there is a Health Ministry request, it is learnt. In this case, there would be no problem, because it is already registered after 2004 trials, a source said. Meanwhile, environmentalist Jagath Gunawardena said Bti was a harmless, species specific bacteria which would only affect the mosquito larvae.

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