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9
For the application of the treatments the sewer area was divided into sewer sections each
having approximately 63 entrances. It was decided not to bait every entrance in each sector,
and 306 of a total of 427 usually pre-baited were selected. This number comprised 225 entrances
where pre-bait 'takes' were recorded during the December, 1950, treatment, plus 81 entrances
which showed a negative result at that time, but where ' takes ' were recorded on either of the
two previous treatments.
Every precaution necessary was taken during the mixing of the bait and rubber gloves were
worn. The bait comprised sausage rusk and water in equal parts by weight, evenly mixed with
'1080' which is soluble and odourless, at a concentration of 1 per cent. by weight. The bait
was mixed in batches of 100 ozs., the required amount of rusk being added to the solution and
well stirred to obtain an even mix. After each mix every utensil was thoroughly washed under
a running tap.
The bait was carried in large tins with lids, and the operator depositing the bait wore rubber
gloves which he put on before leaving the depot. Each bait was 4 ozs., this amount being
measured by means of a mug holding that amount.
As each sector took approximately 5½ hours it was obvious that a break would be necessary,
and the operators were instructed to adjourn to a public convenience, where the recording
operator who had not touched the poison would turn on the wash basin tap to enable the depositing
operator to wash his gloves before removing them and so avoid touching any object before doing so.
The labour needed to work a sector comprised a sewerman to raise the covers and two
operators. One operator deposited and picked up the baits, and the other recorded, lowered and
raised the container when the poison was cleared up. The baiting and examination of four
sectors was observed by officers of the Ministry.
In the treatments prior to this series, when Zinc Phosphide and Arsenious Oxide were used,
it was the practice to take a recording from the top of the entrance, and leave the bait to decompose,
but in the instance of this series it was imperative that the poison be cleared up. This
entailed descent of the shafts, and also enabled a reasonably correct recording to be made.
The method of treating was to bait three sectors on Monday, examine and pick up on
Thursday ; three sectors on Tuesday, examine and pick up on Friday ; and one sector on
Wednesday, examine and pick up on Saturday, thus covering the whole area in the week.
The poison 'takes' of the half-yearly pre-bait treatments from May, 1949-December, 1950,
were 126, 109, 158 and 153 respectively, each bait being 8 ozs. and the 'takes' ranging from
good to small.
In the May, 1951, treatment by direct poisoning with '1080,' 150 'takes' (3 complete,
12,X¾, 25X½, 103X¼, total 150) were recorded. In the second of the series September, 1951,
142 'takes' were recorded (6x¾, 25X½ and 111X¼). The third in December, 1951, produced
89 'takes' (1 complete, 2X¾, 7X½ and 79X¼). The final treatment completed on the 15th March,
1952, produced only 53 'takes' (1 complete, 2X¾, 6X0½ and 44X¼ or less). Of the 44X¼ or less
'takes,' 20 were so small as to be hardly perceptible and could only be seen on close examination.
During these treatments, 121 bodies were found at the base of the shafts, or in the flow close to the
base. This does not include those found subsequently by the sewermen along the lengths of the
sewers and in dead ends during their routine work. In one instance, 15 bodies in one group were
found, close to the entrance where a complete 'take' of poison was recorded in the final treatment.
The final result appears to be very satisfactory, there being a reduction of 97 poison 'takes'
from the May, 1951, '1080' control, and 100 poison 'takes' from the normal pre-bait control
of December, 1950.
From these facts it can be reasonably assumed that the entrances and lengths of sewers near
the entrances where no 'takes' were recorded are free of rats. This is to some extent corroborated
by the sewermen who state that they see only an occasional rat, also by the fact that few surface
infestations of common or sewer rat character are found in the City today, and these are usually
of a minor nature and mainly in the Billingsgate area among the older properties.
In one recent instance in this area, a small infestation estimated at eight rats was found
in a basement where food is stored. Considerable damage had been done, and from borings
and runs in the concrete floor it appeared that the source was defective drainage. Subsequent
tests proved this correct, and major work costing several hundreds of pounds was necessary to
remedy the nuisance. In this case, the property was open to rats in the sewers, the basement
offering food and harbourage, and it can only be assumed that the small number of rats to find
access and take advantage of the defects was due to the reduction of the population in the sewers.
Consideration has been given to the question of test-baiting all the entrances poisoned in
this series of controls, and it appears that little useful information would be gained for the additional
costs this would incur. It is, however, proposed to apply a treatment late in May, 1952,
again using '1080' and including all entrances not baited in this series.
Graph No. 1 illustrates the breakdown of the pre-bait 'takes' from December, 1943, to
May, 1948, and the progressive rise from that point up to December, 1950. The taller columns
represent the pre-bait ' takes ' and the smaller columns the poison ' takes.'
Summary No. 1 gives full details of the results of these treatments, and summary 1A gives
details of the pre-bait and poison ' takes ' of four of these treatments prior to the ' 1080' series.
Graph No. 2 indicates the progressive breakdown of the '1080' poison 'takes,' and summary
No. 2 gives full details of the results obtained during the series.