A better method to control dengue

By Carolyn Hong

KUALA LUMPUR, Sat. - Dengue control will be more effective with a new biological larvae killer developed by the Institute for Medical Research and jointly commercialised with a local company.

Called Mosbac, this product is the first IMR biotechnology research finding to be commercialised, and the second success for the institute. The first was a computer software on nutrition which was marketed last year.

The head of IMR's medical entomology division Lee Han Lim said tests had shown Mosbac to be very effective, with test areas consistently dengue-free.

Dengue is a serious problem, with 11,814 cases reported nationwide from January to July last year, up from 6,628 cases for the same period in 1996, a 78.2 per cent increase. Forty-five people died of dengue last year.

The use of Mosbac is also efficient as it does not rely on public co-operation, and unlike with chemicals, it will be difficult for mosquitoes to develop resistance.

According to Lee, Mosbac uses the bacteria Bacillus thuringiensis H-14 to kill mosquito larvae and is especially effective on the aedes mosquito.

The product, commercialised in a joint venture with Inbiotech Sdn Bhd, was sent to the Pesticides Board for approval in October. Clearance is expected in a few months.

Inbiotech has built a RM500,000 plant in Klang, using manufacturing technology from Russia. IMR will initially be responsible for quality control but will eventually only provide technical support to the company's lab.

"The commercialisation is the result of 10 years of research by IMR. It's an entirely local product, except for the Russian technology which will be transferred to us in a year. We are confident of success as it's a good product. We also hope to export it to neighbouring countries," Lee said in an interview.

Bacteria have been used for mosquito control in Western countries for many years but the imported product is very expensive, costing about RM50 a litre, with the bacteria used not very suitable for the tropical climate.

IMR began hunting for a local strain from soil and water samples nationwide. The bacteria which are being commercialised were isolated from a forest soil sample taken in Ulu Lui in 1988.

Each bacterium contains two or three crystal toxins. When the bacteria solution is sprayed onto the water surface, the mosquito larvae, a filter feeder, will swallow the bacteria.

The bacterium breaks up in the intestine, releasing the toxins which destroy the cells. As the intestine also functions as a kidney, waste cannot be expelled and the larva dies of something similar to blood poisoning.

The bacteria are harmless to humans and other animals, plant, fish or insect because the toxin is only activated in a pH10 alkaline condition. The intestine of mosquito larvae is alkaline while that of humans is acidic.

The toxin also requires a special enzyme called proteolytic enzyme, which is found only in mosquitoes and black fly larvae.

"Extensive tests have proven that the bacteria have no effect on any organism except the black fly and mosquito. This method is also attractive because it's difficult for the mosquito to develop resistance. The toxin is made up of five types of protein and it would take a long, long time before resistance can be developed."

The bacteria are also easily produced using waste material and remain stable for a year at room temperature.

However, they only kill larvae and have no effect on adult mosquitoes.

Mosbac will be used together with insecticide fogging. The Health Ministry has been using the insecticide Malathion but has recently switched to synthetic pyrethroids which are odourless, leave no oily residue and have very low toxicity.

Sprayed into the air, the insecticide kills adult mosquitoes while the bacteria settle, within half an hour, into stagnant water, the droplets being able to find their way into nooks and crannies.

Lee said this was the first integrated mosquito control programme and was expected to be effective as it did not rely on public co-operation, unlike current practices which required the people to control breeding areas.

"Studies have shown less than 30 per cent make the effort," he said.Field tests, using one part insecticide to nine parts bacteria, carried out since 1995, have been very successful."It is important to kill larvae because IMR's research in 1995 showed for the first time that larvae could harbour the dengue virus," he said.

IMR's next project with Inbiotech is commercialisation of a bacterium discovered by Lee, called Bacillus thuringiensis H-28 subspecies jegathesan. The bacterium is named after former IMR director and current Health Ministry deputy director-general (research and technical support) Datuk Dr M. Jegathesan.

It has been found to be effective against the malaria-carrying anopheles mosquito. Malaria is now confined to rural areas and is hoped to be eradicated within a few years.

New Sunday Times 1 February 1998