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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up and penetrate into all nooks and corners that offer harbourage. It is seldom that this can be done
unless the gas is pumped into the enclosed space under pressure. HCN is rapidly absorbed by moisture
so that much more gas may be required than is calculated. In any case, HCN should only be employed
by skilled operators who are fully aware of the dangers and will take all the necessary precautions
laid down in the Fumigation with HCN Regulations for Ships and Buildings.
Chemical and Vegetable Poisons. Before referring briefly to the range of poisons of this type, a
word must be said on certain principles which must be observed if their use is to be effective. Poisons
are generally mixed with an attractive bait and then laid in a number of stragetic points where there is
evidence that rats are moving about, e.g., near holes and rat runs, or in places where there is an
abundance of rat droppings. Poison bait is also a "new object" and rats must become accustomed
to it before they will feed freely.
A system has, therefore, been devised that is known as "pre-baiting." It simply consists in laying
unpoisoned bait at the various points selected, for a period of three to five days or more until it is
found that the rats are taking the bait freely, the bait taken being replenished day by day. At the
end of the pre-baiting period, the same bait, to which the poison has been added in the appropriate
proportion, is laid and a kill secured. It is more than probable that only a proportion of the rats killed
will be found, so that in order to ascertain that the kill has been successful, it is necessary to "postbait"
with an entirely different bait (on the assumption that the remaining rats, if any, will have
become shy to the poison bait) and to ascertain the take, if any, of the new bait.
All this pre-baiting, poisoned baiting and post-baiting business takes time and labour and is,
therefore, costly. Nevertheless, if carefully done, it can be highly successful.
Turning to the poisons, the following are in common use at present: zinc phosphide, arsenic, red
phosphorus, "Antu" and red squill. The first four of these poisons are dangerous to all warm-blooded
animals and should, therefore, only be used in warehouses, farms, and domestic buildings with strict
precautions to ensure that all domestic animals are kept well away from the site of operations, or that
the poison baits are in any case quite inaccessible to them, and that at the end of the operations all
poison baits are swept up and burned.
A word about Red Squill. Red Squill has the property of causing vomiting and consequently
dogs, cats and such like, if they consume a red squill bait will escape serious harm by vomiting it up.
Rats die of red squill poison for the simple reason that they are unable to vomit and so eliminate the
poison from their stomach.
There is another class of poison employing bacteria or so-called viruses, the intention being to
give the rats a fatal transmissible disease which will spread throughout the colony. Such poisons are
not to be advocated since they may cause symptoms of the disease in varying degree in humans.
One final point in regard to the use of these poisons. Rats, and of course, mice, poisoned with
zinc phosphide, arsenic, phosphorus, or "Antu" can, if eaten by dogs or cats, cause their death. It is
of great importance, therefore, that following a poisoning operation, dead rats should be carefully
sought for and, if found, promptly destroyed, preferably by burning.
We can now turn to the two recently discovered rodent poisons which are the principal subject
of this article. They are Sodium Fluoroacetate ("1080") and "Warfarin." The former may be
regarded as a short term poison and the latter as a long term poison for reasons which will appear as
they are described below.
Sodium Fluoroacetate, popularly known as "1080," was first employed as a poison in the United
States during the War and has been considerably developed since. It is, with the exception of HCN,
the most effective poison now available, but it is also among the most dangerous to warm-blooded
animals. It is a light, white crystalline powder, odourless and tasteless (so I am told !), very soluble in
water but practically insoluble in oils. One or two cubic centimetres of a quarter per cent. solution in
water will put the rat out of action in twenty minutes or less, and it is not uncommon to see the bodies
of rats lying within a few feet of the solution that they have been drinking. It appears to have no
repellent effect and, in fact, experience shows that it must have some attractive taste to rats since
they have been found to drink it in preference to plain water nearby. It kills by acting on the heart
and nervous system.
In theory, therefore, and indeed in practice, all that is required is a suitable form of small unspillable
container holding, say, twenty to thirty cubic centimetres of the watery solution of "1080"
placed at a number of strategic points. The use of a solution rather than a solid bait is of great
advantage since rats (much less so mice) are great water drinkers and will not establish themselves
anywhere where there is not an abundance of water at their disposal. The mechanics of the method
are therefore of the simplest; there is no need for pre-baiting and, the poison solution once laid,
results can be expected in a matter of hours.
The poison would therefore be ideal were it not for the fact that it is, to repeat, intensely poisonous
to all warm-blooded animals, including human beings, and there is no known antidote. The most strict
and stringent precautions must, therefore, be taken to ensure that unauthorised persons cannot come
into contact with it and that effective means are taken to avoid any possibility of other animals, of
whatever kind, either drinking the bait or, what is equally important, consuming the whole or part of
a poisoned rat. A number of fatalities have occurred in the United States among animals, cats, dogs,
poultry, pigs and even cattle, as a result of their access to "1080" bait or eating "1080" poisoned
rats, and it must be confessed that human fatalities have also occurred due principally to carelessness
in the preparation of the poison solution, such as allowing stocks of solution made up in unlabelled
bottles and such like to lie about where children and others can get at it and inadvertently drink it.
Obviously also every precaution must be taken against contamination of foodstuffs by "1080."
It can now quite readily be deduced that the indications and opportunities for the use of this
poison are strictly limited. It should obviously not be available to the general public for use in the
home nor in places such as restaurants or, indeed, in any space where there is not the strictest control