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Each infestation has its own peculiarities, and the nature and number of treatments depends
on the features present. /
The technique described above, while fundamentally the same for both species of rat, differs in
respect of numbers of baits used, weight and sites of baiting points for Ship rat infestations. Four
ounce pre-baits and 6 ounce poison baits are generally used in the case of Common rat and floor-baiting
is usually sufficient, but for Ship rat 1 ounce pre-baits and 2 ounce poison baits in greater numbers
must be placed on runways, beams, stanchions and offsets, in order to get between the living quarters
and the food supply.
On the survey side of the work, 2,636 premises have been inspected. Of this number, 320 have
received treatment by your Officers. This entailed 4,388 revisits for operational purposes, and a further
1,112 visits by the Rodent Officer or his foreman during the course of the work or afterwards, making
a total of 5,500 revisits.
Poison control has resulted in an estimated kill of 11,388 rats, and of this number nearly 14 per
cent, or 1,558 bodies have been recovered. The latter number does not include those found subsequently
by occupiers when harbourage has been removed or goods providing nesting facilities delivered.
Trapping as a secondary control was the means of destroying 423 rats, and this figure added to
that of bodies found after poisoning, gives a total of 1,981 bodies taken from premises by this department.
While this report is, in the main, a review of overground work, the treatment against the Common
rat in the City sewers must be included in assessing the net result. A campaign which started on the
13th December, 1943, was finally concluded on the 22nd January, 1944. This work resulted in an
estimated kill of 4,977 rats.
The method of estimating the number of rats killed in an infestation is based on the amount
of poison bait consumed, and is arrived at in accordance with a formula evolved by the Scientific
Officers of the Infestation Branch, Ministry of Food. The calculation is, however, based on Common
'
rat extermination, and as 75 per cent, of overground infestations dealt with were of Ship rat character,
the estimated kill of 11,388 rats is conservative and could reasonably be increased by 20 per cent.,
as this rat being much smaller and finer built than the Common rat, naturally consumes less food,
and consequently a smaller quantity of poison bait constitutes a lethal dose.
While the estimated kill and the bodies found are a source of satisfaction to your Officers, the
real value of the service lies in the number of premises freed or greatly relieved for at least a long
period. To enlarge on this aspect, of 320 premises treated 270 were considered clear after one treatment,
and no further action taken or complairts received from the tenants. 31 premises were the
subject of two courses of treatment; the second, after a long period of freedom. The remainder
received three or more courses during the year, and here the premises and nature of the businesses
are conducive to re-infestation. Further, the term premises is elastic and may refer to a building
of many floors and sections, or on the other hand a small shop and basement.
Much of our work has been in dealing with infestations which in the past have been the subject
of unorganised and haphazard control measures. In many cases, the occupier, before seeking expert
advice, has expended a considerable sum and much labour on various methods, without any regard
as to how the infestation will react to the control. Often several proprietary poisons are put down or a
couple of traps set where dozens are necessary, and although a number of immature rats may be
destroyed, the mature and productive adults often remain, and generally the net result is a poison
prejudiced or trap shy population.
During the year many such cases have been brought to a satisfactory conclusion by your Officers,
and tenants, who after failure had become resigned to the nuisance, were given welcome relief or
freedom from the pest.
Your Officers are frequently asked two questions : (l)will the dead rats cause an offensive smell,
and (2) is it possible to use a poison which will prevent the nuisance.
As to the first question, occupiers are warned that ' corpse smell may develop as the result of
numbers of rats dying, and should this occur it must be accepted as part of rodent control. In instances
where this nuisance has resulted, assistance has been given in the appropriate action to clear it as
quickly as possible. Though unpleasant while it lasts, in the majority of instances occupiers have been
more than satisfied by the knowledge that there has been an entire clearance of live rats, in most
cases achieved at one blow.
Regarding the second question, it is doubtful if any poison suitable for rat extermination will have
such action as to cause the animals to die without the possibility of smell arising. Smell is dependant
on the action of the poison and where the creature dies. If atmospheric conditions are favourable
and there is plenty of ventilation and air currents, the smell is often carried away and not detected
by the occupier.
Of the poisons used, Arsenious Oxide is least likely to cause smell by reason of its preservative
properties, but even so, if the animals die near hot-water pipes, the heat accelerates the development
of micro-organisms, setting up rapid decomposition.
A complete recording of any treatment is available should the result, number of hours or charge
at any time be challenged.
A set of large-scale maps are notated with coloured inks, showing areas surveyed marked in
red and sites of infestations in green. The maps are open to inspection by the Divisional Rodent
Inspector, Ministry of Food, who visits periodically to ascertain progress.
It is submitted that the 'Direction ' and procedure advised, although standard for all areas, is
being adhered to as far as practicable, taking into consideration the problems peculiar to the City,
the character of the infestations and the type of premises.'
I have the honour to be,
Gentlemen,
Your obedient Servant,
CHAS. F. WHITE,
Medical Officer of Health,
City of London.