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but he was very greatly impressed by Mr. Holsendorf's enthusiasm and by the
thoroughness with which he had tackled every aspect of the problem. Some of those
who have studied Mr. Holsendorf's recommendations critically may consider that it is
unnecessary to rat-proof spaces which, though they provide potential rat-harbourage,
are, from their situation, unlikely to become rat-infested. But experience has shown
that rats do establish themselves in such places, and though, in considering
any particular case, the risk of rat-infestation may be weighed against the cost of
rat-proofing, care must be taken not to under-estimate the former.
Others may observe that it is no use attempting the rat-proofing of existing ships.
But it is well worth while dealing even with the more obvious defects because every
bit of rat-proofing increases the efficiency of fumigation by reducing the possibilities
of rats escaping from lethal concentrations of the poisonous gas employed. Thus
the inter-fumigation period is prolonged and the cost of rat-proofing is repaid.
Your Medical Officer recently visited a ship in which two insulated holds were
reported to be rat-infested despite several fumigations at intervals of six months.
It was found that rats had burrowed into the insulation and thus escaped the
effects of the hydrocyanic acid gas which had been the fumigant employed. The
insulation was protected by strong wire screening of small mesh covered with
cement, but in the corners and angles, just the parts which would be attacked by rats,
the protection was not complete, showing the necessity of paying special attention
to these points by overlapping at the edges and interlocking at the corners or flashing
with sheet metal.
Objection may be raised to the cost of rat-proofing, but there are some respects
in which rat-proof construction is actually more economical than the usual practice,
and these at least should be studied and adopted forthwith.
The recommendations of the Committee on rat-proofing will be eagerly awaited
by all Port Sanitary Authorities.
Bat-Flea Index.
Bubonic Plague is primarily a disease of rats which is conveyed from rat to rat
or rat to man by fleas. The more numerous the fleas on the rats the more readily is
Plague spread. Moreover, though there are a number of different types of fleas
which are capable of carrying the infection, the species Xenopsylla cheopis is the
principal one concerned. It therefore appears that both the extent to which the
shore rats are flea-infested and the types of fleas they harbour are to some extent
indications of the susceptibility of the Port to infection by rat Plague.
Your Worshipful Committee directed that during the year both the "Bat-flea
Index" and the "Cheopis Index" (i.e., the average number of all fleas per rat and the
average number of Xenopsylla cheopis fleas per rat) should be worked out in the
Port of London.
Dr. Kean, one of your Boarding Medical Officers, undertook this investigation
and his report appears below. The extent of the Port necessitated special arrangements
for killing the rats and their fleas at the same time and as soon as possible after
trapping. Special portable "lethal chambers" were therefore designed, to which the
Assistant Bat Officers could transfer the rats from the cages in which they were
caught. Only rats trapped alive and singly were combed. At first Dr. Kean's
classification of the fleas was checked by Dr. P. A. Buxton, Dr. V. B. Wigglesworth
and Mr. H. S. Leeson, of the Department of Entomology of the London School of
Hygiene and Tropical Medicine, but, as he was consistently correct in his identification,
periodical checking of all the fleas collected during a month or so was substituted for
checking the fleas combed from each rat. The strict conditions of the investigation
limited the number of rats examined, because it not infrequently happens that more
than one rat is caught in a cage trap and also many rats are caught in snap traps, and
these were not examined. It is, however, proposed to continue the investigation
for another year, and to extend it to rats obtained from ships in the Port.