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any appreciable carrying of food can be discounted. It may be that the stray
rat does come to the surface in such circumstances and is observed scavenging
in gardens and open spaces, particularly in the region of "top end" and branch
sewers.
A three-monthly analysis for 1959 gives the number of surface infestations
by one or more rats as follows
January - March 59
April - June 74
July - September 50
October - December 24
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In my Annual Report last year there is a reference to the first use in
Hornsey of sodium fluoroacetate as a poison. This was carried out in November,
1958, using the services of a contractor. During 1959 the sewers were treated
in January, May and August with sodium fluoroacetate, and in March and
November with "Warfarin 5." The March treatment was on a very reduced
scale in an endeavour to assess the reduction in the residual rat population of
the sewers. It was found that whereas the "take" at the baiting points was
between 60 and 70 per cent during 1957 and 1958 when zinc phosphide, arsenic
and Warfarin had been used, after two treatments with sodium fluoroacetate in
November, 1958 and January, 1959, the Warfarin check treatment on 600 soil
and surface water sewer manholes showed only "takes" in 7 per cent of them.
Two further treatments with sodium fluoroacetate in May and August, 1959,
were carried out in co-operation with the staff of the Borough Engineer &
Surveyor's Department, and followed in November 1959 by a treatment of 968
soil and surface water manholes with Warfarin - the "take" was between 3 and
5. 6 per cent only. The difference in these figures refers to 24 feeding points
where the poison bait appeared to have been disturbed and where the reading was
uncertain.
It appears that a considerable reduction in the rat population has resulted
from the use of sodium fluoroacetate and Warfarin in treatments of the sewers.
This is indicated by the decreased "take" of the Warfarin bait from about 70 per
cent in previous years and 3 to 5. 6 per cent in 1959. At the same time there
has been a reduction in the number of surface infestations reported.
During the last four months of the year the use of Warfarin 1 was discontinued
in favour of Warfarin 5 which is considerably more potent, and the
poison made more attractive to rats by supplementing the oatmeal base with
sugar and oil. It is interesting to note that when taking poisons such as zinc
phosphide and arsenic rats develop a "poison prejudice;" this is not so with
Warfarin.
With sodium fluoroacetate the action is more rapid than with Warfarin,
even in the smallest quantities, and the complication of "poison prejudice" does
not arise. Because such small quantities are effective it is necessary to use
some other form of baiting and poisoning at least once a year in order to assess
the effectiveness of the treatment and Warfarin appears to be the best answer
to this at present, as the amount of bait taken by the rats is measurable.
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