London's Pulse: Medical Officer of Health reports 1848-1972

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City of London 1952

[Report of the Medical Officer of Health for Port of London]

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A REVIEW OF METHODS EMPLOYED IN THE DESTRUCTION OF RODENTS.*
So much has been said and written about rats and mice, their depredations, their danger to health,
and so many "infallible" methods for their destruction have been advocated, that some very good
reason should be forthcoming before one more contribution on the subject can be contemplated. My
excuse is that I wish to sing the praises of two remarkable new rodenticides which are in common use
in the United States but have not yet received the attention they deserve in this country.
First, let me briefly discuss the results that we seek to obtain by any method we employ for
exterminating rats or mice and the conditions that should be fulfilled before it can be regarded as
really satisfactory. The word "exterminate" is used advisedly—we are not out to destroy a few rats
in a haphazard manner but to eliminate the whole colony and, if possible, to prevent any subsequent
reinfestation. There must be no compromise in this since failure to exterminate means that in a short
time—a very short time—we shall find ourselves once again faced with the same problem and in the
same dimensions.
What are the conditions that the ideal method should fulfil ?
1. Properly used it should be 100 per cent, effective in eliminating the whole colony;
2. It should kill rapidly and preferably painlessly;
3. It must not be dangerous to other animals, including humans, and should preferably be
specific to rats and mice;
4. Given a modicum of intelligence it can be applied by anyone, provided instructions are
carefully followed out;
5. It must not be costly either in material, time or labour.
No known method fulfils all these conditions; many that are widely advertised fulfil none of
them, and only a few of them fulfil more than two or three.
The methods at present in vogue can be divided broadly into four categories:—
Direct Violence. This is, of course, in no way new and indeed is as old as contact between rats
and man. It can be very effective and appeals to some because it contains an element of sport. Thus
occasionally a whole colony of rats can be killed by direct violence, as for instance, when a rick or a
stack of bagged grain that is known to be infested with rats is broken down. Every available person
and dog is called in to join in the mass slaughter of the rats as they are driven out into the open.
But it seldom happens that the last rat is done in by this means for a certain number will inevitably
escape to start their domestic life afresh elsewhere. Nevertheless, it should be possible, with good
organisation, to get something approaching a 100 per cent. kill.
Trapping. Trapping is, of course, a form of violence and can be very effective, particularly in
small, newly established colonies, in farm buildings, warehouses, and in localised infestations, in a ship
or in the home. It has the disadvantage that it is a skilled art requiring considerable knowledge of
rats and mice and their habits, without which the result may be a failure, judged by the capture of
only a proportion of the established colony.
The best trap in my experience is the spring treadle or "break-back" trap, and of the various
types the best of them is the one in which the treadle takes the form of a moveable platform held in
position by the tongue and loop which sets the spring and on which the bait is laid. In this type of
trap the rat or mouse must put the forefeet on the platform or at least stretch the neck well over in
order to get to the bait, with the result that it springs the wire across the shoulders or neck resulting
in instant death or at least so firm a hold that there is no question of escape. The skill in the use of
this trap lies in the proper adjustment of the wire rod and loop so that the slightest touch will spring
the trap, in using an appropriate bait, and in laying the trap in runs and other places which are
frequented by the rats as they move about.
There is another essential to success which is not usually appreciated—that it is of little use laying
one or two traps as is so often done, particularly in the house. Efficient trapping calls for a mass
attack using a large number of traps in the hope that most of, if not all, the rats will be caught in a
very short time. Rats become trap-shy very quickly and once nipped or frightened by the springing
of the trap will not come near it again, however attractive the bait may be. In effect, as many
traps should be employed as the estimated number of rats in the colony and in any event not less than
six traps even to catch one rat or mouse. Rats, and to a lesser degree mice, have what is called a
"new object" reaction—they are inclined to avoid any new object until they have become accustomed
to it. A trap is certainly a "new object" and it takes some little time before rats will approach, and
much more, feed from it. It is quite a good plan, therefore, when laying baited traps, not to set the
spring for two or three days so that the rats may become accustomed to them and feed on the bait
with some confidence. Then one day set the spring and your kill will be the more satisfactory.
Most baits used with traps are too elaborate and are allowed to remain too long and so become
stale. The advantage of the "break-back" trap with platform is that loose bait can be sprinkled on
the platform, with the result that the rat will sooner or later put his forefeet on the platform and so
spring the trap. An excellent bait both for rats and mice consists simply of a mixture of rolled oats
five parts and fine sugar one part sprinkled on the platform. Obviously a bait of this kind cannot be
used in the open where rain is likely to spoil it and in any case it is doubtful whether traps are of much
use in the open. In brief, traps of whatever kind—gin, break-back, cage, etc.—have been superseded
by better methods for the total elimination of colonies.
POISONS.
Poisonous Gases. The only poisonous gas at present in general use in this country is hydrocyanic
acid gas (HCN). It can be used either in the form of a liquid which rapidly evaporates on contact with
the air, or absorbed in a neutral substance which gives off the gas when exposed to the air. Under
suitable conditions HCN is as nearly an ideal poison as is known, for the reason that the whole colony
can definitely be exterminated by this means. Nevertheless, it has the great disadvantage that it is
intensely poisonous to man and animals and most dangerous to use unless strict precautions are
taken. It can be used for the destruction of colonies of rats in ships, in burrys in the hedgerows, in the
farmyard, and in warehouses and other buildings where the space occupied by the rats can be effectively
isolated from the outer atmosphere so that a good lethal concentration of the gas can be built
* A note submitted by Dr. Morgan at the Annual Meeting of the Association of Sea and Air Port Health
Authorities of the British Isles.
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