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Analysis of the sector recordings and overall test result indicated that the previous low
level had not been maintained. The upward trend and distribution indicated by the acceptance
of the non-poison test baits coincided with the observations of the sewermen who had noticed
an increase in the number of rats seen during their routine duties in the sewers between these
controls, and in particular the last quarter. The low poison acceptance and increase in complete
test bait "takes", plus the visual evidence indicated the possibility that some poison
prejudice had been produced by the continued use of Zinc Phosphide at a 10% concentration.
It was therefore considered that a return to Sodium Fluoroacetate was necessary and during
the period September, 1955 and June, 1956, three "direct" poison controls and one pre-bait
treatment was applied.
In the September control 397 entrances were poison baited, and after three days 38 "takes"
(one complete, 9 x ½ and 28 x ¼ or less) were recorded and 52 bodies were recovered. In the
two subsequent "direct" controls the "takes" were 12 and 10 respectively, and with two
exceptions these were in the ¼ or less category. These controls produced 21 bodies.
For the final "1080" control it was decided to apply a pre-bait treatment on the 1-3-5
system, the pre-bait substituting for the annual test-baiting. Unfortunately bad weather was
experienced, and thunderstorms and prolonged rain made recording difficult. The pre-bait result
was 49 "takes" (36 complete, 9x ¾ and 4 x ½). Some of the "takes" recorded as complete were
doubtful and could have been washed away by charged up flows. The poison result was 25
"takes" (2 x ½, 23 x ¼ or less) and 10 bodies were found.
Having carried out 21 treatments over a period of five years, principally with Sodium Fluoroacetate,
it was suggested that a new poison, Fluoracetamide, technically known as "1081"
might prove advantageous as an alternative to the former, and as the result of discussions with
the Scientific Officers, Infestation Control Division, it was agreed that a series of "direct"
controls be applied in a similar manner to the "1080" treatments.
The median lethal dose (LD50) for Common rat is about 15MG/KG of body weight as
against 1 MG/KG for "1080". Undiluted Fluoracetamide is approximately 10 - 15 times less
toxic to Common rat than Sodium Fluoroacetate and about three times more toxic than Zinc
Phosphide. At 2% in bait therefore it is about half as toxic to rats as bait containing 0.25%
Sodium Fluoroacetate and about three times as toxic as 2.5% Zinc Phosphide. After a lethal
dose the speed of onset of warning symptoms is said to be slower than Zinc Phosphide for which
the LD50 is 40 MG./KG.
It was considered desirable to obtain by overall test-baiting a basis on which the effectiveness
of Fluoracetamide could be assessed after four "direct" controls, and this was carried
out during the period 10th - 13th September, 1956, three months after the final Sodium Fluoroacetate
control, and repeated the 19th - 22nd November, 1956, ten weeks later, and three weeks
prior to the institution of the new series.
For the first test, 402 sewer entrances were baited and the baits each 4 ounces of damp
sausage rusk were left down three days, and on examination 103 "takes" (84 complete, 6 x
8 x ½ and 5 x ¼) were recorded. The weather was favourable, and 299 test baits recorded as
negative were in good condition.
For the purpose of the second overall test, ten weeks later, 403 entrances were baited, and
on examination, 174 "takes" (165 complete and 9 x ½) were recorded. The weather was again
favourable, the sewer entrances being dry and the conditions ideal for the test.
The significant feature of the test baiting results was the formidable increase in the number
of the "takes", indicating the rapid build-up and distribution in the intervening period of nearly
six months from the final control of the former series.
These did provide a sound basis on which to judge, and the first Fluoracetamide "1081"
control of such a nature to be applied in this country was carried out in December, 1956 two
weeks after the second test, the procedure being similar to the "1080" controls, in dispensing
and distribution.
A total of 394 sewer entrances were treated, each bait being 4 ounces, comprising sausage
rusk and water at equal parts by weight, evenly mixed with "1081" at a concentration of 1%.
The baits were left down 3-4 days before clearance and on examination 87 "takes" (8 complete,
7 x ¾, 39 x ½ and 33 x ¼ or less) were recorded. A total of 57 bodies were recovered,
and no doubt a good many were washed away in fast flows, as during the treatment heavy and
prolonged rain was experienced on several occasions. The majority of the dead rats were found
in shallow flows a fair distance from the bait sites.
The second treatment was applied in March, 1957 when 401 entrances were baited and the
recorded result on examination was 60 "takes" (11 complete, 7 x ¾, 18x ½ and 24 x ¼ or less).
In this instance 39 bodies were found in shallow and slow flows.
At this juncture the procedure was altered to allow the baits to remain down longer, the
timing to include a weekend when the discharge of waste from the many catering premises is at
a minimum.
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