Health Notes

W.H.O. presents-

Insecticide Resistance

Problem For Research

WE all know how DDT has done wonders in reducing the great insect-borne pestilences such as malaria, yellow fever, typhus, plague and many types of dysen tery. Now, to our dismay, we are finding that, in one part or other of the world, DDT is no longer con trolling the mosquitoes, lice, fleas and houseflies that transmit these diseases, and it is difficult to realise that it is because the insects them selves have changed, to become re sistant to DDT.

It was in agriculture that the phenomenon of insecticide resis tance was first observed. In 1913 it was proved that the San Jose scale, an insect pest of apple orchards, was no longer controlled by lime sulphur in the state of Washington.

USA, because the insect itself had changed to lose its former suscepti bility. In Canada and Australia wheats resistant to the attacks of rust fungus have been bred but in the passage of a few years a new rust fungus has appeared that can attack the resistant wheat. The cattle tick and blue tick have be come resistant to sodium arsenite dips in Australia, South America and South Africa.

Laboratory experiments have shown that if susceptible houseflies are exposed to DDT in each genera tion and bred from the sur vivors, the susceptibility steadily decreases until the flies become practically resistant.

New “wonder insecticides” were produced, such as BHC, chlordane and dieldrin, with encouraging re sults at first; but generally within the space of a single year the houseflies became resistant to them also.

So the insects are fighting back in their own way, as dictated by the inexorable laws of nature.

Through what qualities have they become resistant where they were susceptible before?

One possibility is that they are just tougher generally, and can better withstand inimical condi tions. Instances where flies or mos quitoes accumulate moderate resis tance to insecticides in general, not just to the one being used, fall in this category, but generally resis tance is much more specific than this, and there are many examples to-day of intense DDT-resistance without any increase in general toughness or decreased suscepti bility to other insecticides. (Over)

Biochemical research workers have shown that DDT-resistant houseflies contain an enzyme which normal susceptible strains do not.

An enzyme is an organic com pound whose presence causes change in the composition of an other compound, or compounds, while the enzyme itself remains chemically unaltered.

This particular enzyme is present in all tissues of the fly, and especially in its body-wall. As the DDT enters the body through the body-wall, this enzyme dehydro chlorinates it, leaving a harmless compound. A similar mechanism is found in some mosquitoes.

Research workers have been forced to the conclusion that in the housefly the genes for resistance are present in normal populations in very low frequency, ofily awaiting selection by the insecticide—DDT in this case —to increase that fre quency in successive generations.

Successive elimination of indivi duals with genes for DDT susceptibility concentrates the DDT-resistance genes more and more, so that ultimately the popu lation is entirely DDT-resistant.

In the race against the develop ment of insecticide-resistance, there is an urgent need for increased re search on many fronts. For only research provides the knowledge and understanding which may pro duce countermeasures against the onset of resistance. —Compiled from bulletins issued by the World Health Organisation.