Battle against dengue fever: science or artefact?

Dr. P. M. Wijeratne Senior Entomologist, Department of Agriculture

There is a huge public outcry over the widespread prevalence of the dengue epidemic which is in the spree of claiming public lives, and the allegations are directed towards the Health Ministry for their failure to contain the plague which is vectored by a species of mosquito scientifically identified as Aedes aegypti.

Spraying chemicals to destroy mosquito breeding places. File photo

In an attempt to overcome the disease epidemic, the Health Ministry has gone to the extent of obtaining expertise from Cuba where the epidemic is said to have been wiped out. The two Cuban scientists who paid an official visit to Sri Lanka to investigate the problem, enlightened the Health Ministry officials on a bacterium known as Bacillus thurengiensis (Bt) capable of eliminating the larval populations of the vector when the breeding sites of the mosquitoes are treated with the bacterium.

Vector populations

I am not aware whether the Health Ministry officials have taken a decision to import the bacterium. The pertinent question that may strike the mind of any citizen in this country is that “Could this be a successful attempt in eliminating the vector populations and suppressing the spread of disease epidemic?” As an entomologist in the Department of Agriculture who was engaged in a constant battle for the past 29 years in the management of insect pests of crops grown in Sri Lanka, I would like to air my view point on this whole episode.

At the first instance the elimination of an insect population is out of context in the insect management strategies; We are rather inclined to manage the insects at very low population levels so that the menace caused by them can be minimized.

This is simply because the 860,000 species of insect species including mosquitoes present on earth are highly successful organisms who are capable of adapting to various environmental situations; Within few generations they are capable of developing the ability to overcome the control strategies adapted by man. Hence their elimination is rather difficult if not impossible.

The insects may suffer a temporary set back at the cruel hands of the most intelligent animal on earth. Yet in return they will adapt a new strategy for their survival and emerge victorious. However, some countries have been able to eliminate certain diseases vectored by insects through the elimination of disease organism but not through the elimination of vector populations. For example, the US has been able to eliminate malaria from their homeland.

Mosquito species

Mosquitoes belong to the Family Culicidae in the Order Diptera in the Class Insecta. The most salient feature of the Order Diptera is that all the members have only a single pair of wings with the hind pair of wings modified to form a club like structure called halteres. As such mosquitoes also have a single pair of wings with fringes along the margin and the veins of the wings. The mouthparts of these tiny animals form a beak like structure with which they pierce the skin of the host and suck the blood. Presently more than 1,600 species of mosquitoes have been discovered across the world with majority of them recorded in the tropics.

Economically, mosquitoes are of utmost significance owing to their functioning as the intermediary host of a number of virus diseases including malaria, yellow fever, elephantiasis and dengue. The family Culicidae contains three Genera which are of medical importance: Anopleles, Culex and Aedes. The members of the Genus Anopleles are capable of spreading malaria, while those in the Genus Culex act as the vectors of elephantiasis. Aedes aegypti and the related species are able to transmit the disease organisms of elephantiasis, yellow fever and dengue. The life cycle of these mosquitoes consists of 4 stages viz, egg-larvae-pupa - adult, and the larval stage is spent in the water.

The females of Aedes aegypti lays eggs singly in stagnant water, old tins, broken bottles coconut shells and holes in trees where the eggs can remain viable on the substrate for a period of 3-4 months.

The female always prefers to lay eggs in places where little amounts of water is available but not on ground pools. The length of the life cycle is normally 15-20 days, but may be as short as 11 days depending on the environment.

The survival capacity of their eggs for a comparatively long period of time and also the capacity to survive in undetectable places where little water is available makes it harder to control them. The adult mosquito of Aedes aegypti is more easily identified by the lyre-shaped white mark on the thorax.

The miraculous Bt the Cuban scientists boasted of is not at all a miracle or a new introduction to Sri Lanka.

This is the product which was imported to Sri Lanka under the trade name of Thuricide and registered as an insecticide for the control of the caterpillar of Helicoverpa armigera that damage a number of crops grown in Sri Lanka. However, it is pertinent at this juncture to look at the history of the product and its present position in the Insect Pest Management programs in the world.

Bt is an aerobic spore-forming bacterium found rather commonly in the environment. This bacterium was first discovered by Berliner in 1911. The potential of this bacterium as an insecticide was discovered relatively early and the commercial formulations were developed in 1950s in the US to be used as an insecticides against the insect pests of crops and the mosquitoes. This bacterium is capable of producing a crystalline protein during sporulation and it is this protein which act as an insecticide, in contrast to many other insecticides available in the market. Bt is not a contact poison but a stomach poison and releases its toxic effect in the mid gut of the alimentary canal of the insect. Under the circumstances the Bt will become toxic only when the crystalline proteins are ingested by the larvae of mosquito. The crystalline protein of Bt dissolve in the mid gut and as a result protoxins are released. These protoxins bind with the receptors of the mid gut epithelial cells, and generate pores in the cell membrane disturbing the cellular osmotic balance. This phenomenon results in the death of insect.

DNA technique

The most extraordinary value is that Bt is amenable to genetic manipulation using recombinant DNA technique because the genes that encode the crystalline protein are borne on plasmids. Subsequently, techniques were discovered for transferring the genes to crop plants. The gene has been successfully transferred to maize, soybean and tomato to develop transgenic crop varieties resistant to major insect pests.

Currently there are 34 subspecies of Bt with widely differing insect specificities. However, all of them tend to be toxic to larvae of butterflies and moths. Few subspecies are toxic to mosquitoes and beetles. Most of the formulations currently used in US are derived from three subspecies, viz, Bacillus thurengiensis kurstaki, Bacillus thurengiensis israelensis and Bacillus thurengiensis tenebrionis. In the US the subspecies Bacillus thurengiensis israelensis is used widely for the control of mosquitoes. It is my understanding that scientists from Cuba made reference to the formulation of Bacillus thurengiensis israelensis or perhaps any other formulation they have developed themselves out of the 34 subspecies mentioned above.

Effect on humans

Cuban epidemiologists Dr. Aramis Martinez, Dr. Yelina and Cuban Ambassador Nirsia Castro Guevara look on while Director General Health Dr. P. G. Maheepala examines the BTI bacteria.

The desirability of Bt toxin and the impetus for their use in mosquito control result primarily from the safety to non-target organisms and the environment. Unlike many of the insecticides currently used for the control of insects pests of crops, the toxins of Bt has no detrimental effect on humans, wild or domestic animals and they tend to be short lived in the environment.

The point to be considered here is that whether it is possible to overcome the dengue catastrophe with the mere introduction of this product? When we look at the life history details it may be clear that the control of an insect like mosquito which is capable of breeding in places where stagnant waters are found cannot be achieved only with the spraying of a chemical. Even if 80 percent of the population is eliminated with spraying the remaining 20 percent of the population can make a rapid multiplication to reacquire the nitch if the breeding sites are available. Hence, it is extremely important to formulate an integrated control strategy with public sector and private sector participation.

The most important component of such a program should be to destroy the breeding places of mosquitoes where the active involvement of the citizens of this country is imperative. The Bt toxin can be used to supplement such an integrated control program. In other words what we have to do is to make a combined effort to maintain a clean environment devoid of breeding places of mosquitoes. All the countries that have conquered diseases vectored by insects have achieved their success through maintenance of a clean environment through the public awareness programs and their active participation.