London's Pulse: Medical Officer of Health reports 1848-1972

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City of London 1933

[Report of the Medical Officer of Health for Port of London]

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"Rat-proofing means eliminating enclosed spaces in which rats might nest and breed, or at least
making such spaces inaccessible to rats, and preventing the access of rats to supplies of food and water.
"Potential nesting or breeding places are termed rat-harbourage. A house is still a house even if
it is unoccupied, and so rat-harbourage is still rat-harbourage even if at the time of inspection no rats are
making use of it. This statement appears to be so obvious as to be unnecessary, but I have found difficulty
in getting Inspectors to report rat-harbourage unless there is actual evidence of the presence of rats.
"It is usually easier, particularly in ships, to eliminate or protect rat-harbourage than to cut off supplies
of food and water. Sometimes it is easier to prevent rats obtaining water than to deny them access to
food. The complete programme of rat-proofing comprises the prevention of nesting, feeding and drinking,
but if it is not practicable to do all three it is worth while depriving the rat of two, or even only one, of these
necessities of life.
"I think it is generally agreed that rat-proofing is the only rat-repressive measure which is permanent
in its effects. It is very difficult completely to annihilate a rat colony by trapping, poisoning or even
fumigation. Usually there are a few survivors, and rats are so prolific that they quickly breed up to the
limits of the nesting accommodation and the food supply. But even if every rat in a building or a ship is
destroyed it is certain that good homes for rats will not remain long untenanted, and the methods of
deratisation must be repeated again and again at regular intervals.
"I think it is true to say that the majority of rats in ships are born on board and that every ship has
a certain maximum rat capacity. I am aware that from time to time we do get sudden invasions of ships
by large numbers of rats, but this is the exception and not the rule. I know, too, that there is a certain
interchange between rats ashore and those afloat. But at the same time I feel sure that rats do
not voluntarily leave a comfortable home. Increase of population or scarcity of food supply may enforce
migration. I suppose there are some rats who are naturally wanderers, and perhaps romance or domestic
difficulties may cause others to leave home. I understand also that on the occurrence of a severe epizootic
of rodent plague rats will flee from the infected area. But I think that rats who are well housed and well
fed will settle down to a comfortable family life and have large litters at frequent intervals. Our best method
of waging war on rats is therefore to present them with an acute housing problem and a food shortage.
Then we may be sure that their birth rate will go down and their infant mortality and general death rates
will go up.
"In ships rat-proofing has an additional advantage in that it increases the efficiency of fumigation.
There are in many ships, spaces which are accessible to rats, but to which no fumigating gas, not even
hydrogen cyanide, can penetrate in lethal concentration during any ordinary fumigation. So much has
heen said of the toxicity of hydrogen cyanide and of the danger to human life in cyanide fumigations that
I find a tendency on the part of shipping people to believe that the gas will penetrate everywhere and kill
everything. But in actual fact no fumigant will get through small openings into comparatively large dead
spaces where there is no movement of air, for the physical process of diffusion is relatively slow and the
distribution of a fumigating gas through a ship depends very largely on internal air currents or draughts
inside the ship, set up by differences of temperature and wind pressures. Such dead spaces are precisely
those chosen by rats for nesting and for hiding when they are disturbed, as they frequently are during the
daytime when a vessel is in port. For effective fumigation such spaces must be opened up to enable the
gas to enter. It is obviously better to eliminate these harbourages or to make it impossible for rats to get
into them.
"There is another important advantage in rat-proofing. It has been shown by Eskey, of the United
States Public Health Service, during plague investigations in Peru and Ecuador, that the degree of flea
infestation of rats is proportional to the density of the rat population and that the ' cheopis index ' is
highest where there is the best rat-harbourage. Thus rat-proofing is a double insurance against bubonic
plague, not only reducing the number of rats, but also reducing the number of fleas, and particularly of
X. cheopis, on such rats as may survive.
"The broad principles of rat-proofing are quite simple. They are:—
"(1) Eliminate all enclosed spaces which might be used by rats for hiding or nesting, or if it is
impracticable to eliminate such spaces, protect them, with galvanised sheet metal or
expanded metal of not more than half-inch mesh, so that rats cannot get into them.
"(2) Close all openings through which rats might pass from one compartment to another in search
of food or water or to escape from fumigating gases.
"Rats seldom gnaw on flat surfaces, but nearly always at edges, corners or angles, and therefore it is
these points that need attention in protective work.
"Before I proceed to consider the detail of the application of these principles I must acknowledge the
main source of such information as I possess. Some eighteen months ago I went to New York and spent
several days with Mr. B. E. Holsendorf, who is the officer in charge of the Rat-proofing division of the
Quarantine Department. Mr. Holsendorf has spent years in the study of the rat-proofing of ships and has
worked out all the practical details in co-operation with naval architects and shipbuilders. I visited with
Mr. Holsendorf, ships which had been rat-proofed in commission and ships which were being rat-proofed
during construction. He kindly gave me copies of his book," The Rat-proofing of Vessels," and a number
of photographs. Such of the photographs as could be reproduced have been made into lantern slides by
Dr. Newham, of the London School of Hygiene and Tropical Medicine. Mr. Gillis, of my own office staff,
has enlarged some of the most useful drawings from Mr. Holsendorf's book and slides have been made from
these by Dr. Newham. So far as the text of the rest of this Paper is concerned I have endeavoured to pick
out the principal recommendations from Mr. Holsendorf's book and to explain them to the best of my

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