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So far 728 rats have been examined, of which 497 were black and 231 brown;
2,246 fleas were collected, of which 1,862 were Ceratophyllus fasciatus, 370 were
Leptopsylla musculi, 9 were Ctenophthalmus agyrtes and 5 Ctenocephalus felis. Not a
single Xenopsylla chopis was found. The total flea index rose from about 1.5 in the
winter to about 4 in the summer.
From the absence of Xenopsylla cheopis and the low total flea index it would
seem that there is little danger of any imported rat Plague giving rise to a serious
epizootic in the Port of London.
Report on Rat-flea Survey by Dr. E. Kean.—In the early part of 1932 it
was decided to institute a systematic examination of rats trapped in the Port with a
view to determining—
(a) The "Cheopis Index" for the area.
(5) The degree of flea-infestation of the rat population.
After the project had been taken in hand it was considered advisable to have the
scope of the enquiry extended so as to include a recorded classification of the various
types of fleas encountered.
Acting on the advice of Dr. P. A. Buxton, of the London School of Hygiene and
Tropical Medicine, it was decided that this survey should be presented in accordance
with a standard method devised to ensure, as far as possible—
(1) That each rodent when captured was harbouring its natural complement
of parasitic fleas.
(2) That each rodent was transferred alive into a specially constructed lethal
chamber of a portable type.
(3) That each rodent and its attendant parasites were killed and the numbers,
species and sexes of the latter determined by a standard technique.
A card-index file was maintained, and the result of the examination of each rodent
was separately recorded thereon.
Owing to the distances separating the several dock areas in the Port it was decided
that the killing and examination of all rodents should be conducted at a central
station.
All the rats examined were caught alive in cage traps. For the purpose of carriage
of the rats several portable lethal chambers, each divided into four separate compartments,
were constructed of plywood. Each compartment was lined with block tin
and the entire inside was coated with white enamel. To each compartment was fitted
a sliding door, an observation window and a small aperture for the introduction of
chloroform.
To ensure that each rodent when trapped harboured its natural complement of
parasitic fleas, it was essential to examine only rodents which had been caught singly.
When two or more rats are caged together, fighting or friction against the bars of the
cage will result in a diminution in the numbers of fleas normally carried by the rats.
Such escaped fleas will attach themselves to other rodents prospecting in the vicinity
of the cage and the combing of such rats may give an entirely erroneous flea count.
On the apposition of the open entrance of the lethal chamber to the opened cage
door the rat quickly passed into the darkened interior of the lethal chamber and was
there secured. One rat only was carried in each compartment of the chamber.
The lethal agent used during this survey was chloroform. It was found that one
drachm of chloroform on a pledget of cotton-wool was lethal for a large rat, brown or
black, when the rat was confined in a space of small dimensions and was subjected to
the influence of the vapour for a period of five to eight minutes. All attendant fleas
were likewise killed in ten minutes.