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over access to the baited area, be it ship, warehouse, barn, outbuilding, or in the open hedgerow.
Nevertheless, it is of the greatest value where time is an essential factor, limiting the operation to
overnight or weekend baiting. Thus, it has been found of great use in the destruction of rats in ships
making only a short stay in port, and in warehouses and similar premises which are only unoccupied
overnight or at the weekend. Used in this way it has replaced expensive fumigations with HCN. It
may or may not have extensive use in the countryside or even in towns and it certainly should not be
employed other than by trained men under strict control.
Warfarin is a complicated synthetic product and in normal circumstances is a colourless crystalline
solid without odour or taste in the concentrations used for the destruction of rats and mice. It
was first developed in this country and then perfected by Dr. K. P. Link of the University of Wisconsin,
U.S.A. He got a hint of a unique poison in nature when he watched some cattle eating spoiled sweetclover
hay. He was struck, he says, by the element of repose prevailing in the barn. Dead cows
looked as if they were only sleeping. Cows too weak to stand kept eating the deadly hay, not realising
that what they ate was making them bleed to death.
Spoiled sweet-clover hay has been found to contain an element known as Dicoumarin which has
the property of slowing down the rate of clotting of the blood to a point when, after progressive doses
of the substance, wounds will bleed and continue to bleed and spontaneous bleeding will occur without
any apparent injury to the blood vessels. It is used in certain human diseases to prevent the clotting
of the blood, but the dosage and the number of doses must be strictly controlled if dangerous bleeding
is to be avoided. It is, therefore, a long term poison to the extent that it must be taken regularly,
not intermittently, for a period of days in order to secure a state when spontaneous and fatal haemorrhages
will occur.
Synthetic Warfarin compound is infinitely more effective than Dicoumarin and in the case of
rats and mice so small an amount as 0.025 per cent. Warfarin mixed with an appropriate bait and
eaten by rats steadily over a period of five to ten days will so reduce the coagulant property of the
blood that the rats will die of spontaneous hæmorrhage as well as, of course, the parturient doe at
the time of the birth of her young, resulting not only in her death but the death of the whole of her
litter.
There need be no pre-baiting with Warfarin. It need only be mixed with a bait more attractive
than the bait on which the rats are principally feeding and be laid at a number of points, being
replenished for a period of five to ten days until there is no further evidence of take.
What will be the effect on other domestic animals? Warfarin taken in this way will, of course,
kill other domestic animals and it is necessary therefore that they should be kept away from the
baited site, but in the concentration employed one or even two large helpings of the bait taken by an
animal will have little if any effect on it since it is the continued consumption of Warfarin that is
essential to produce a state of spontaneous hæmorrhage. Its danger, therefore, is in quite a different
category to the commonly used poisons, such as "1080," zinc phosphide, arsenic, etc., a single dose
of which is fatal.
Rats and mice eating the poison bait are quite unaware that it is the bait which is causing their
symptoms and is steadily killing them; consequently, they continue to eat the bait until the end.
Whole colonies can in this way be exterminated, but there is more to it than that. Baits laid and
properly protected will be eaten by fresh invaders before they have had any time to establish themselves
and, consequently, Warfarin can be regarded as a permanent method of rat control.
One of the simplest and the best types of permanent baiting point is the agricultural pot-drain
consisting of a simple tube about a foot long by four inches or so in diameter, into which a rat can
easily penetrate, but not dogs, cats, poultry, pigeons, etc. A number of pot-drains, therefore, laid at
stragetic points, into which a heap of Warfarin bait is inserted, can lay for an indefinite period, being
visited as often as necessary according to the amount and nature of the bait being taken.
There is a great variety of baits with which Warfarin can be mixed, and it is not possible in a
short article of this kind to suggest the most suitable baits for the simple reason that it depends
entirely on the type of food on which the rat colony is feeding. It is obviously of little use to lay baits
of wheat in a granary where wheat is being stored, or a fish bait in proximity to fish manure. Nevertheless,
it may be found that soaked wheat (wheat soaked in water overnight and the water then
drained off) may be taken by the rats in preference to hard wheat. Ground desiccated kipper may be
found an attractive bait as also sausage rusk (unleavened bread, which is the principal constituent
of our present-day so-called meat sausage!), sweet crumbled biscuit, sugar meal (rolled oats 5 parts,
sugar 1 part) or coarsely ground maize in which the germ is retained. In other cases oil bread mash
(ground stale bread 16 ozs., crude cod-liver oil 3½ ozs.) may also be found to be readily taken, but care
must be taken to replenish this and any other oil or fatty bait before rancidity occurs. Other baits
that may be used are soya flour, soaked crushed barley, palm nut flour, horse meat, or boiled blood
(obtainable from any abattoir).
Warfarin is now obtainable in this country in a concentrated form for mixture with appropriate
baits and there is every likelihood that it will become widely used in the near future.
Only occasional reference has been made to mice but the principles underlying the destruction
of mice are the same as those for rats. One of the best baits for mice is certainly not ration cheese nor
any old lump of solid substance, but rather a flour or oatmeal sugar mixture which they will take
readily. They have much less "new object" reaction than have rats. There is another, quite different
but at times spectacular method of killing mice which dispenses with traps and poison baits. Obtain
some 20 per cent D.D.T. powder and dust it freely around the holes and along the tracks used by
the mice. The mice run over the powder, pick it up on the feet and fur, clean themselves by licking
it off and die as a result. This method is no use for rats which don't pick up enough to kill them.
A word of warning. Whatever method is used for the elimination of rat or mouse infestations, it
w ill not succeed if carried out in a careless or haphazard manner. A study of the site, the type and
extent of the infestation, and indeed of the active life of the colony, must first be carried out and a
well prepared and thought-out plan of campaign devised.
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