View report page

Report of the County Medical Officer—General. 57
Vermin Inquiry.
Study has been made during the last six years of the variation in prevalence of three kinds of
body vermin which, unfortunately, are of common occurrence. The results are shown in the form of
curves in the accompanying diagram (Z) which exhibits the variations shown month by month for the
entire period. In the case of lice and bugs the records were obtained by the inspection of beds in common
lodging-houses ; in the case of fleas the earlier records relate to the common lodging-house population,
but during the last two years use has also been made of the figures obtained by the school nurses from
their examination of school children. It is very gratifying to note that the accompanying diagram
shows a very marked improvement as regards the prevalence of lice and bugs, a result doubtless in
large part attributable to the efforts made to improve the conditions of cleanliness in common lodging
houses. The curve relating to fleas does not show a corresponding fall, and, indeed, experience shows
that the problem of dealing with flea prevalence is a more difficult one than that of coping with the
other species of body parasites. A special study has been made in previous annual reports (see report
for 1911, pp. 62 and 63) of the seasonal prevalence of fleas in relation to scarlet fever, and further reference
is made to the subject in Part II. of this report, pp. 127-8.
In studying the relation of flea prevalence to distribution of scarlet fever in London in recent years,
two main preliminary considerations should be borne in mind :—
1. Scarlet fever is wont to display major waves of prevalence in which the interval between
crest and crest is some 6-8 years; thus wave crests are shown by the mortality curve to have occurred
in 1863, 1870, 1875, 1880, 1887, 1893, 1901 and 1907. A wave crest was therefore, due to appear about
1914, and such a crest has been developed in accordance with anticipation. Comparison between the
curve giving scarlet fever notifications for the last 6 years (diagram K) and that giving flea prevalence
(diagram Z) shows a general correspondence; in both, there is a tendency to rise on passing from
the earlier to the later years ; in both, 1913 and 1914 yield the highest figures; in both, 1909, 1911 and
1912 yield comparatively low figures; the year 1910 is, however, anomalous, it is a low year as far as
scarlet fever is concerned and not low so far as fleas are concerned.
2. Scarlet fever displays, as Dr. Longstaff pointed out several years ago, remarkable inverse
correlation with rainfall. Dry years and particularly cycles of dry years correspond with cycles of
scarlet fever prevalence. If the humidity of the six years be examined month by month it will be found
that, taking the months of June, July, August and September, 1911,1913 and 1914, were dry years, 1909
and 1912 were wet, and in 1910 June, July, and August were above and September below the average.
Thus, on the whole, there is some general correspondence between the annual prevalence waves
of dryness and scarlet fever; though it might, perhaps, have been predicted, looking at the scarlet fever
curve, that, assuming any close correlation to exist, the flea results for 1910 would have given the lower
and those for 1909 the higher figure.
In India, moreover, considerable attention has been devoted to study of the influence of temperature
and humidity upon flea prevalence. In Vol. 8, No. 2 (Fourth Plague Report) of The Journal of
Hygiene (p. 241) the statement appears : " Dampness in the surroundings of a breeding place hinders the
development of the insects. Adult fleas, also, appear to dislike wetness. A number of our failures in our
earlier experiments to transmit plague from animal to animal by means of fleas, and possibly in the
experiments of others, can be attributed to this cause, viz., the death of the fleas because of the wet
surroundings." This observation would, of course, assuming the correctness of the flea hypothesis,
tally with the observed relation of scarlet fever to dry years in England. It needs to be borne in mind,
however, that the plague commission's reports relate to India and not to England, and to the rat flea
and not to the human flea.
The result of experiments with the rat flea (Pulex Cheopis) at Poona are further considered in
The Journal of Hygiene, Vol. 12, Seventh Report on Plague. These experiments show generally that
the propagation of rat fleas is markedly influenced by conditions of temperature and humidity, the
latter factor being on the whole more important. A comparsion with seasonal variations in humidity
shows, during the years 1909-1913, some suggestion of a general correlation between a favourable
spring or early summer humidity and high autumnal flea prevalence; the year 1914 appears,
however, to be a marked exception.
On the whole, the conclusion may be formulated that broadly speaking the curves of scarlet
fever and flea prevalence for the period of six years show a tendency to rise together from a low level
at the beginning to a high level at the end of the period, but the results for a longer series of years need
to be examined before any very precise conclusions can be arrived at. It must always be remembered,
moreover, that it is possible there may be no direct relation between flea prevalence and scarlet fever
incidence, and that both the coincidence of the autumnal maxima, and the observed correspondences
in behaviour of the curves over a series of years, may rather be due to the fact that the conditions
favouring the propagation of the causa causans of scarlet fever may be closely similar to those favouring
the prevalence of fleas. It is interesting to note here that from observations made by Dr. Tidswell
in Sydney, New South Wales, during 1909, there was a maximum seasonal prevalence of rat fleas in
March and April of that year ; and at about this period scarlet fever also appears to have attaiaed a
maximum prevalence in Australia.
In commenting upon the first flea curve, obtained in 1909, it was pointed out that in addition
to the correspondence between the seasonal curves of both fleas and scarlet fever, there must be borne in
mind the well-known special predilection of both for children, and the fact that infection tends to cling to
particular houses and rooms and in schools to particular departments or even classrooms. Subsequent
enquiry has tended to strengthen materially the case tentatively put forward in 1909; in particular
27715 I