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These rats may have been infected from the north side. It is as probable that
these were separate escapes from ships berthed on the south side, and that their
detection is due to the efficiency of the Port of London Authority's system in
combination with the routine examination carried out under the direction of your
Worshipful Committee in meeting the danger arising from plague-infected rats.
Rat plague is said to smoulder in large ports and now and then break out into an
epizootic ; with regard to the plague rats found from time to time in the Port of London,
these only occur in areas and at wharves where there is close and constant shipping
connection with the East. If smouldering, the rat plague should extend to other parts
of the Port. I cannot find any evidence that this occurs in the recorded returns of rat
examinations. Again, smouldering rat plague if present in the Port would be discovered
continuously in routine rat examination. I am of opinion that these small time-spaced
outbreaks are severally due to introductions of infected rats from unnoticed epizootics
of Eastern origin on shipboard.
Dr. Newham's examinations probably represent the proportions in which the
varieties of rat are found in the Port district.

These figures are for 1916.
Of infected rats (four in all), two were of species decumanus, and two rattus.
The significance of the large proportion of mus rattus in the Upper Docks lies
possibly in the amount of warehousing in these Docks, which affords undisturbed
nesting for rats, under favourable conditions as to food, that is, a nearer approach to
conditions of domestication than is afforded in transit sheds.
There is also less chance of decumanus invasion in the Upper Docks, which are
remote from rubbish tips and other conditions of wildness favourable to the persistence
of the wilder species.
The species rattus—the black rat (domestic rat of India) is sometimes said to be
extinct in England. As shown above this is not the case in the Port, where, in places,
the black rattus, with its variety alexandrinus, is found in excess of the wild " sewer "
or brown rat decumanus, and the black and brown species carry on their depredation
side by side, where food is plentiful, without apparent discord.