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15
The pre-war methods had to be severely curtailed on the outbreak of war.
The use of hydrogen cyanide was limited to a few special cases, owing to the dangers
arising from storage of large stocks of this highly poisonous gas under war-time conditions.
Staff shortages severely limited both inspections and treatment, and
circumstances arose which made it appear that the problem would become even
more widespread. In the great majority of cases, the only disinfestant used was
lethane, which was imported from the United States of America. Pyrethrum
insecticides were practically unobtainable, as the limited supplies were strictly
controlled by the Government, mainly for use by the Services. It can be said,
however, that in spite of all the difficulties, the bug problem remained under control,
and the post-war period has not shown any increase in infestation.
One important result of the war was the impetus given to scientific research
in the field of disinfestation. The use of D.D.T. in typhus epidemics such as that in
Naples in 1943, together with the use of the new substances in the Services, has
drawn public attention to the merits of the important " residual " effects of these
insecticides.
D.D.T. did not become available outside the Services until 1945, when experiments
were commenced by various authorities in the application of the methods
which had been evolved in war-time. From the published results, and from
discussions with persons concerned in the experimental work, it appeared that
considerable progress had been made in the search for the ideal disinfestant and
its practical applications in the municipal field.
It was felt that the time was now ripe for a change over to the newer substances
and methods, and arrangements were put in hand for the preparation of a new scheme
of disinfestation of houses, embodying the use of these substances and methods.
D.D.T. (dichlorodiphenyltrichloroethane). D.D.T. is a white crystalline substance
when pure. It is practically insoluble in water, moderately soluble in petroleum,
and readily soluble in a number of organic solvents, the most usual being
cyclohexanone. The dissolved substance is then diluted in kerosene to obtain the
required strength.
Research on the use of D.D.T. in laboratories and Army field tests had shown
the substance to be lethal to a large number of insect pests. It attacks the nervous
system and is toxic both by ingestion and by penetrating the surface of the insects
through nerve endings and between body segments. D.D.T. will not kill the eggs
of insects, neither does it act as a repellent.
A careful assessment of its toxicity to animals and man was carried out by
leading toxicologists both in this country and in the U.S.A., and it was found that
although it was absorbed through the skin from oil solutions, it was sufficiently
non-toxic to be used in the concentration necessary to kill the common insect pests.
Provided reasonable precautions are taken when spraying, a 5 per cent. solution
in kerosene is accounted safe, and a 10 per cent. powder can be used without danger
if direct inhalation is avoided.
D.D.T. is slow-acting and does not possess the knock-down effect such as is
obtained with pyrethrum. The latter, however, is very unstable and can be
broken down by exposure to light, heat, or air, whereas D.D.T. is very stable and
has persistent lethal effect over long periods after application.