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Pre-baiting followed by poisoning has again been the principal means of extermination with
trapping as a secondary measure, and the methods officially recognised have been adhered to.
During the period under review, 3,545 premises were inspected and these comprised blocks
of offices, riverside warehouses, markets, restaurants and kitchens, workshops and domestic
dwellings.
A total of 651 infestations were treated and these consisted of 82 major and 266 minor rat
infestations, and 182 major and 121 minor infestations by mice. This work entailed 9,529 revisits
for operational purposes.
In the process of pre-baiting, 33,712 initial points were selected, 124,789 pre-baits and
25,291 poison baits were laid. These measures resulted in a minimum estimated kill of 3,436 rats,
and of this number 1,308 bodies were found. Trapping was the means of destroying 775 rats, and
adding those found as the result of poison, a total of 2,083 bodies were removed from premises.
In addition 643 mice were found as the result of poison and traps.
In 85 instances structural rat-proofing was carried out by occupiers or owners, and in four
instances defective drainage resulting in rat infestation was remedied.
During the latter part of the year the use of Warfarin as a rodenticide was, with reservations,
officially recognised by the Infestation Division, Ministry of Agriculture and Fisheries. Warfarin
(3 Alpha-phenyl-beta-acetylethyl-4-hydroxycoumarin) is a blood anti-coagulant which, when
ingested regularly in small doses over a period of time, induces fatal haemorrhage in rats and mice.
It is slow acting and a sufficient time must elapse for the animals to suffer capillary damage
and to develop a hæmorrhagic condition. During this time they seem neither to feel pain nor to
undergo physiological disturbance with which they could associate resulting illness. The time
between the commencement of feeding and death varies, but is usually within 5-10 days. Both
sexes appear to be equally susceptible, but pregnant females and young rats usually die first.
The toxicity of Warfarin varies for the different species of rodents. The Ship (black) rat and
House mouse are said to be some 3.4 times more resistant than the Common (Sewer) rat. The
concentrations recommended are 0.025 per cent. and 0.005 per cent. for use against Ship rat and
Common rat respectively. Owing to the difficulty of hand-mixing such small quantities, a
master mix of fine oatmeal containing 0.5 per cent. and 0.1 per cent. by weight is obtainable and
when mixed with the bait base to be used at 5 per cent. by weight, produces the concentrations
advised.
Pre-baiting is unnecessary and the bait should be left in position undisturbed, more being
added as required until feeding ceases. Since the bait must be left down for a relatively long
period, it is necessary to use a dry bait and medium oatmeal is advised. If this bait is readily
taken it is not necessary to visit daily, and usually three times a week to replenish is sufficient.
If, however, the dry bait is not acceptable and a moist bait is used, these must be replaced daily.
Although the use of Warfarin against mice has not yet been approved by the Ministry of
Agriculture and Fisheries, many infestations of this nature have been treated in the City and the
results appear satisfactory.
The introduction of Warfaren does not mean that the methods employed in the past are
obsolete; its use provides an additional means of rodent control.
Following the initial treatment for the destruction of rats in sewers, December, 1943, it has
been the practice, in accordance with legislation, and in conjunction with Metropolitan Boroughs
contiguous to the City, to carry out a six-monthly maintenance treatment. It is the purpose of
these treatments to check the build-up of the common rat population in the sewers, and thereby
assist in preventing infestations of surface properties from this source via defective drainage.
Such treatments comprised a two-day pre-bait, followed by poisoning at entrances where
pre-bait was taken, and recording of the result on the fourth day.
On completion of maintenance treatment No. 13 December, 1950, analysis of the result
showed that the upward trend of the pre-bait 'takes' had continued. Unfortunately little is
known scientifically of the movement of rats in sewers, but it appeared to indicate that the low
basic level of the residual rat population attained by maintenance controls up to May, 1948, was
not being maintained, and that there was increased distribution over a wider area, plus the breeding
increase during the intervening period.
It was considered that efforts should be made to effect a further reduction, and discussions
were held with the Technical Officers, Infestation Division, Ministry of Agriculture and Fisheries.
As a result, it was suggested by them that a series of four treatments at quarterly intervals be
carried out; such treatments to comprise direct poisoning with Sodium Fluoro-acetate technically
known as Compound '1080' This compound is produced in the U.S.A. and was supplied by the
Ministry, who, it is understood, hold the entire stock in this country.
By using the direct poisoning method the time taken for a treatment would be halved by
reason of the elimination of two days pre-baiting, so that four controls per year would involve
approximately the same number of man-hours as was required by the two six-monthly treatments.
The compound is in powder form and as a precautionary measure is tinted blue with nigrosene
dye. It is extremely lethal to man and animals, there being no known antidote. It should never
be handled without rubber gloves being worn, and its use should be controlled by a responsible
person, both in its dispensing and distribution.
It has been used experimentally against surface infestations on several occasions by the
Ministry, and to a larger extent by the Port of London Health Authority, but the treatments
recently concluded are the first to be undertaken in the entire sewer system of any area.