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In this respect, the Port Health Authority by exercise of its functions relating to control of
rat infestation in ships and lighters, make a contribution of primary importance by minimising
the problem of re-infestation of Ship (Black) rats from vessels and lighters discharging cargo to
riverside wharves, Likewise, by their efforts the Rodent Control organisations of contiguous
Local Authorities have similarly contributed to the general overall plan of Rodent Control; and
to this must be added the contribution of the commercial servicing undertakings.
Infestation of Common (Sewer) rat character in surface properties was again on a minor scale,
and this is, in no small measure, due to the systematic periodical maintenance control measures
applied in the City sewers. 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 infestation of surface
properties from this source via defective drainage.
Following the initial sewer treatment in December, 1943, it was the practice in accordance
with the legislation, and in conjunction with contiguous Metropolitan Boroughs to carry out sixmonthly
controls. These measures comprised a two-day pre-bait, followed by poison baiting in
all entrances where pre-bait acceptance was recorded, and examining the result on the fourth
day. The poisons used were Zinc Phosphide, Arsenious Oxide, and Alpha-Napthyl-Thio-Urea.
The recorded result of the initial treatment was 355 pre-bait "takes" of which 185 were
complete "takes", this result indicating the numerical quality and distribution of the rat population
of the sewers at that time.
Continued six-monthly maintenance controls from January 1945, resulted in a gradual reduction
of the pre-bait "takes", and the recorded result in May, 1948, was 118 "takes" of which
63 were complete. From then onwards an upward trend of pre-bait "takes" was experienced, with
a proportional increase in complete "takes", until in December, 1950, the pre-bait "takes" were
225, of which 185 were complete, thus equalling the number of complete "takes" obtained in the
initial control in December, 1943.
Little was known scientifically of the movement of rats in sewers, but the position obtaining
at the time of the December, 1950, control appeared to indicate that a wider distribution had
developed, due to the breeding build-up during the intervening period of six months between
controls.
It was considered that in view of the time and effort expended on the maintenance controls,
further action was necessary to effect a drastic reduction of the residual rat population and to
this end discussions were held with the Technical Officers of the Ministry of Agriculture, Fisheries
and Food. The utmost co-operation was received from the Ministry Officers and it was
agreed that as from May, 1951, a series of treatments comprising "Direct" poisoning with Sodium
Fluoroacetate, technically known as "1080", be carried out at three-monthly intervals. As
mentioned in previous reports, this poison is extremely toxic and its use should be controlled
by a responsible person, both in dispensing and distribution.
In applying the "direct" poison method the time per treatment was halved by reason of the
elimination of the two days required for pre-baiting, so that four treatments annually involved
approximately the same number of man hours, as was needed by the six-monthly controls.
A total of 17 treatments at quarterly intervals in four stages have been applied since May,
1951, the final control of the series being carried out in June, 1956. During the period September,
1954, and June, 1955, in agreement with the Ministry, Zinc Phosphide at a 10% concentration
was substituted for "1080".
The first stage of the series comprised five treatments during the period May, 1951 — May,
1952. In the first control in May, 1951, 150 poison "takes" (3 complete, 12 x 32 x ½ and
103 x ¼ or less) were recorded, each bait being 4 ozs. The second control in September, 1951,
produced 142 poison "takes" (6 x ¾, 25 x ½ and 111 x ¼ or less). In the third treatment in
December, 1951, 89 "takes" (1 complete, 2 x ¾, 7 x ½ and 79 x ¼ or less) wsre recorded. The
fourth treatment in March, 1952, showed a continued reduction in the poison "takes", which
were (1 complete, 2 x 6 x ½ and 44 x ¼ or less) a total of 53. In the fifth control in May, 1952,
424 sewer entrances were baited as against 306 in the previous controls, thus including 118
entrances not treated for 18 months. The result of this treatment was 20 "takes" only (3 x ½ and
17 x ¼ or less). A feature of this control was that only three "takes" were obtained from the
additional 118 entrances baited.
The decline in the number of poison "takes" was very satisfactory, but the possibility of
poison prejudice was visualized, and in order to be reasonably satisfied on this point, and at the
same time confirm the results which the "1080" controls had apparently obtained, a non-poison
test baiting of all available sewer entrances after a lapse of three weeks was applied. A total of
417 entrances were treated and the baits left down for two days, and the recorded result on.
examination was 35 "takes" (7 complete, 3 x ¾, 10 x ½ and 15 x ¼ or less). Altogether 104 lbs.
of test bait was put down, and the estimated amount taken was 72 ozs + .
This is a great reduction on the final pre-bait recording of December, 1950, when 225 "takes"
(185 complete, 29 x ½ and 11 x ¼)ere obtained and the estimated amount then taken was 67 lbs+.
15.