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(5) Rat-proofing :—
(a) To what extent are clocks, wharves, warehouses, &c., rat-proof?
A very steady rat-proofing of the docks and warehouses is in progress;
the reduction of the normal rat population over 25 years is estimated
in one dock at no less than by 80 per cent., and in another dock bv
66 per cent.
(b) Action taken to extend rat-proofing :—
(i.) In ships—
The chief Shipping Companies using the Port of London have
been circulated as to the benefits and nature of rat-proofing
of ships.
(ii.) The Port of London Authority, whose area as regards the docks is
largely coincident with that of the Port Sanitary Authority,
is active as to (1) (b), (3) (b) and (5) (b) (ii.).
In Report No. 82, I noted the elucidation of the infectious disease—Dengue. The
vector of this disease has been proved to be a mosquito (Stegomyica fasciata); the
same mosquito conveys yellow fever, and the limitation of these two diseases Northward
and Southward is the degree of cold outside the tropics at which this mosquito could
no further survive, i.e., at about latitude 35°; England, therefore, is free from these
infections.
I have now to record a similar, if somewhat modified, set of observations in the
spread of rat-borne bubonic plague. If these are less defined, they are more interesting
from the fact that this want of definition is associated with the double nature of the
vector of plague; the primary vector— the rat, bearing a further and communicating
vector— the flea, but far more because London has in the past been visited by epidemic
and endemic plague, whereas it appears that London may be now less infectible.
In India extended observations and close experiment, with deductions therefrom,
show that there are areas free from endemic plague, surrounded by or neighbouring
to endemic plague areas, that the free areas are often invaded by plague, but the
disease gets no lasting hold, and that as between plague incidences in these areas the
constant and deciding factor of differentiation is a difference of flea on the respective
rat populations. The Indian Rat Flea (Xenopsilla cheopis) in India, in Ceylon and in
America, has been recognised as the flea most concerned with the carriage and spread
of rat plague and its human sequela. Other fleas, e.g., the European Rat Flea
(Ceratophyllus fasciains) and even the human flea (Pulex irritans) to a limited extent
may spread the disease when once it gets a foothold in a country, but it appears that
they do not initiate or perpetuate epidemics of plague.
In England small rat plague infections die out. Certain ports over the world,
though bombarded by plague, are beginning to be recognised as non-infectible. As
this fact becomes apparent the reason is persistently queried and sought, but naturally
without plague in England, comparative observation on plague cannot be made or the
answer found here.
Hence the value of research into the Indian conditions mentioned above, and of
comparative observations made in ports on the North American coast line, of which
the following is a mere sketch, but sufficient to the present purpose.
The Indian Rat Flea (X. cheopis) is found on rats in increasing numbers in the
ports the further towards the Equator these are situated. At a Northern hemisphere
port plague gets no hold; to the South is a port where it gets a precarious hold only
to disappear; further South is a place where plague rats are found and from time to
time a human case of plague; further South again rat plague appears and spreads, with
consequent human plague, possibly in epidemic form. Coincident with these varying
plague conditions is the increasing cheopis flea index, i.e., numbers of this flea found
on the rat.
It is now tentatively suggested that the critical cheopis flea index, or that number
of this flea per rat which can sustain plague in a rat community, lies at or about 1. For
instance, with cheopis index .2 or .6 there is not likely to be sustained rat-borne plague
in a place when this is introduced; with index 2 there is sustained rat plague, while
index 6 may be coincident with epidemic plague.
A deciding factor in the index for coast lines bordering the oceans appears to be
meteorological, and the latitude of critical index is at or about 40° on either side of
the Equator.