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23
The Table given below shows the annual number of fumigations during the last
ten years, the number of certificates of exemption from deratisation issued annually
since Article 28 of the International Sanitary Convention, 1926, was put into operation ;
the total number of rats and mice destroyed in ships by fumigation and the average
number of each per vessel. The latter figures have been plotted in a graph on page 28d.

Bat-Proofing.
Your Medical Officer understands that a Sub-Committee of the Joint Sanitary
Committee of the Chamber of Shipping of the United Kingdom and the Liverpool
Steamship Owners' Association has been set up to enquire into the question of ratproofing,
from the point of view both of new vessels and of existing vessels.
There is no doubt that rat-proofing is the only measure of rat-repression which
will give results of permanent value. Trapping, poisoning and even fumigation
frequently leave a few live rats behind from which a new colony rapidly grows. But
even if these methods are completely effective, re-infestation will inevitably occur
if there exist on board good nesting-places and an accessible supply of food and water.
Good homes for rats do not long remain untenanted. Afloat as well as ashore it is
essential to build against the rat.
The broad principles of rat-proofing are simple:—
1. Dead spaces in which rats may nest and breed undisturbed must be eliminated,
or, if it is not possible to eliminate them, they must be made inaccessible
to rats. In this connection it is important to appreciate how small an
opening will admit a rat and how strong are their teeth. Consequently,
if openings are not blocked by sheet metal it is necessary to use expanded
metal of not more than half-inch mesh.
2. Openings through which rats may pass from one compartment to another
must be closed in order to prevent rats from travelling about the ship
in search of food and water.
3. Rats do not usually gnaw on flat surfaces, but on edges and angles to which
special attention is therefore necessary.
Though these principles are simple and even obvious their application to the
varied conditions in ships requires a knowledge of ship construction, imagination
and an eye for detail.
Mr. B. G. Holsendorf. of the United States Public Health Service, is thus equipped
and in association with Dr. S. B. Grubbs has compiled a report on " The Bat-Proofing
of Vessels," which contains not only the theory of rat-proofing, but practical instructions
for the execution of the work together with a large number of illustrative
diagrams and photographs.
During the visit of your Medical Officer to New York he had the advantage of
spending several days with Mr. Holsendorf, and of visiting ships under construction
and ships in commission which had been rat-proofed in accordance with his specifications.
Your Medical Officer was already convinced of the value of rat-proofing,