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Periodical treatment at 3 monthly intervals for the control of common rats in sewers is a
primary factor in the control of this species in surface properties. It is the purpose of these treatments
to keep the rat population of the sewers down to the lowest possible level, thereby preventing
the infestation of surface properties via defective drainage.
During the period under review, four "Direct" poison treatments with Fluoracetamide commonly
known as "1081", were applied in March, June, September and December, and an average
of 390 sewer manholes distributed over 34 miles of traversable sewers were poison-baited on
each occasion except in June.
The annual overall test baiting which provides an indication of the results of the previous
treatments was applied prior to the June control and a total of 390 manholes were baited, the test
baits being left for two days. These baits which in effect act as pre-baits were then examined
and where taken replaced by poison bait.
The test bait result was 48 "takes" (30 complete, 13 x 3/4 and 5 x 1/2) each bait being 4 ozs
and comprising sausage rusk and water in equal parts by weight. Taking into account only the
complete takes recorded the summarised result is 7.5% complete "takes" of the total number
test baited.
The comparable test in 1961 produced 85 "takes" of which 71 were complete which is 17.6%
of the total number baited and in the years 1962-1964 the result was 11.6%, 8.9% and 6.7% respectively.
Subsequent to the initial eewer treatment in December, 1943, it was the practice to carry
out six-monthly controls, comprising a two day pre-bait followed by poison baiting where pre-bait
was taken. The poisons alternately used were Zinc Phosphide, arsenious Oxide and Alpha—
naphthyl-Thio-urea
The recorded result of the initial treatment was 355 pre-bait "takes" from 427 entrances
baited of which 185 or 43% were complete "takes" indicating the numerical density and distribution
of the rat population of the sewers at that time.
Continued six-monthly controls resulted in a gradual reduction of pre-bait "takes" and the
recording in the May 1948 treatment was 118"takes" of which 63 or 15.5% werecomplete "takes".
From then onwards an upward trend of pre-bait takes was experienced until in December 1950,
the pre-bait takes were 225 of which 185 or 45.3% were complete "takes", thus equalling the
number obtained in the initial control December, 1943.
This result indicated that the use of the above mentioned poisons and six-monthly controls
was no longer maintaining the low level of residual rat population and that there was increased
distribution over a wider area and a possibility of some poison prejudice.
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