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45
Conditions in the Mediterranean and Black Sea; coasts determine these to be
endemic plague areas, though partially or wholly outside the 40° parallel; moreover,
London, and England, in general, from time to time in history have been swept by
epidemics of plague or constituted endemic plague regions. There are, therefore,
other factors than the meteorological in determining infectibility; these are suggested
for the past as dirt, and in general that stage of civilisation which is tolerant of the
rat and breeds out his fleas to excess, from which England, not unaided by " weather,"
has possibly emerged on to the fractional and more or less safe side of the critical
cheopis index. Moreover, the rat is not always and everywhere the rodent concerned
in plague spread.
The theory, however well advanced, is not absolute knowledge, and will need
further research.
An extended series of observations on rat fleas is necessary in ports and places.
Unlike the specific mosquito vector of dengue and yellow fever, which leads a free life,
the X. cheopis is more or less attached to a travelling, warm-blooded animal, and thus
is travelled outside the limits of one of the meteorological conditions of its life and
reproduction—temperature.
A seasonal flea prevalence is a complication which might render a place alternately
infectible and non-infectible, according to the rise and fall to above or below the critical
cheopis index. Further, a zone such as the waterside of a port, receiving constant
small additions of X. cheopis, will possibly have a higher index, and thus a greater
infectibility than its perizonal region,
Information as to the cheopis index and its variations is needed from every port,
first as a contribution to the pool of world knowledge,, and then as probably essential
to well-directed plague preventive measures on ships in their voyage to or from ports
of higher or lower infectibility. Practical measures which are those directed against
the rat, would remain unchanged, unless in the direction of stringency; the higher
the numbers of rats the more ready is the spread of plague and the greater the
infectibility of the place harbouring them. But the subject of plague has yielded its
inner secrets to research so far that there is a distinct value in determining through
the cheopis index the status of the Port of London in the scale of plague infectibility
through the rat.
The index of the Port being known, that for some surrounding area should be
determined.
The list of observers is long, and records of their observations over many years,
on which this report is based, are a large study.
Practically the determination of the index might be initiated with the recovery
and examination of the fleas of about 80 rats per month. A microscopic examination
of each rat for plague should be concurrent with the flea examination, and these
should be supervised by an expert bacteriologist and entomologist respectively.
A scheme for this work and as to the probable expenditure which would be incurred
should include provision for a systematic rat and rat destruction survey in the Port.