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the possibility is perhaps not wholly precluded that there may be some mistake about its existence*
There was, however, a distinct interval in 1906. As Dr. Niven says, The impression produced by
the two sets of figures in 1905 is rather one of simultaneity than of cause and effect, and if we take the
figures as indicating that flies do cause rise in the number of cases, we must also assume that their action
is very rapid." (Annual Report, 1905.) In 1906 it was clear that the increase of flies commenced "before
the increase in fatal cases." Dr. Niven says, "It is probable, therefore, that the relation is one of cause
and effect. These observations, however, fall far short of absolute proof." (Annual Report, 1906.)
It should be pointed out that there are certain difficulties in the way of accepting the thesis that
the correspondence exhibited in the curves affords reason for concluding that flies and diarrhœa stand
to one another in relation of cause and effect. At the commencement of the hot summer weeks, when
the number of flies has begun to show marked increase, the diarrhoea curve is rapidly rising. After
some weeks the number of flies reaches the maximum, and then diminishes, and so, in almost precise
correspondence, does the amount of diarrhoea. A period is later reached, towards the close of the hot
weeks, at which the number of flies is still as markedly excessive as at the earlier period when the amount
of diarrhoea was increasing, but at the later period the amount of diarrhoea is declining ; it even anticipates
decline in the number of flies. If the fly is to be regarded as the carrier of the organism which
causes diarrhœa, it might perhaps have been anticipated that at the later period—the number of flies
still being excessive and infective material being then presumably more widely distributed than ever
before—the amount of diarrhœa, instead of showing early and rapid decline, would still be increasing.
It would almost appear that the advocate of the "fly-borne diarrhoea hypothesis" must necessarily fall
back in support of his theory upon the hypothetical organism, conveyed by the fly, which he may claim
is affected by temperature in such a way as to bring about correspondence between the diarrhoea curve
and the fly curve. The very closeness of the correspondence between these curves may indeed
from this point of view be thought of as constituting a difficulty rather than a point in favour of the
hypothesis that summer diarrhoea is caused by flies.
The attempt has, however, been made to explain the fact that the diarrhœal curve has already
begun to decline, while the number of flies still remains excessive, by the hypothesis that so many persons
have at that time been attacked by diarrhoea that then at length, the "susceptible material"
having all been exhausted, a decline in the diarrhœal curve has become inevitable. Thus Dr. Niven
writes (Annual Report for 1904), "It is noticeable that the descent in the number of deaths precedes
the diminution in the number of flies. It is possible that this is due to exhaustion of material."
Against this exhaustion hypothesis two considerations must be set. First, that the comparatively
early fall in the amount of diarrhoea is observed not only in years of excessively high mortality, but also,
as, for example, in London last year, in years of very low mortality. In the case of the former, an
hypothesis of "exhaustion of susceptible material" may hold good, but it fails to explain the phenomena in
the latter instance. The second consideration is that the "exhaustion of susceptible material" hypothesis
does not adequately account for the almost identically similar behaviour of the two curves—for even
small variations in the one, the fly curve, are generally accurately reflected in the other, the diarrhœa
curve—and this not only in their ascent but also in their descent. This close correspondence of the
variations in these curves better accords with the view that both are dependent on temperature variations
than with the hypothesis that diarrhœa stands in direct causal relation to fly prevalence.
The difficulty caused by the fact that the diarrhœa curve follows so closely upon the fly curve
may perhaps be in part met by an assumption that only the newly fledged fly can transmit diarrhœa—
the disease being an infantile disorder, so to speak, in the fly—and by further assuming that immediately
upon its becoming a perfect insect the fly is infected, and in its turn infects; for the exigencies of
time, having in view the close correspondence of the fly and diarrhœal curves, admit of little delay.
Even in the event of no time whatever being lost, it is, however, difficult to understand why, despite
the large increase in the amount of infective material which presumably occurs during the hot weeks,
the fall in the diarrhoea curve so closely corresponds with arrest of increase of flies. It might be thought
that a comparatively small number of flies would serve to maintain a high level of diarrhoea prevalence
at this period, if it be legitimate to assume that there is then especially widespread distribution of
infective matter.
But, on turning from the left-hand side of the curves, i.e., from the ascending side, to the righthand
side, i.e. to the descending curves, it will be seen that the time difficulties are now greater than
before, for the descent of the diarrhœal curve actually precedes that of the fly curve (see Diagram III.).
On the whole, therefore, while it is not easy to reconcile the behaviour of the curves with an hypothesis
that flies transmit diarrhoea, it must be admitted that the phenomena accord quite well with the supposition
that both flies and diarrhoea stand in close relation with temperature. On this latter hypothesis
the delayed fall of the fly curve, as compared with that of the diarrhœal curve, presents no difficulty.
In North America and South Africa it has been pointed out that enteric fever outbreaks have a
tendency to occur at times when there are large numbers of flies in an affected locality. It is impossible
here to enter into detailed discussion of this subject. A brief reference may, however, be made to a carefully
studied outbreak in this country in which question arose as to whether the spread of enteric fever
had been caused by fly prevalence. The circumstances of this outbreak, which occurred near London,
were investigated by the Medical Officer of Health (Dr. Ransome) and by Dr. Young (County Medical
Officer of Middlesex), and are briefly recounted by the latter in his Annual Report for 1906. I have had
the opportunity of fully discussing the circumstances with Dr. Young. Into the possible causes of the
outbreak, other than flies, it is here unnecessary to enter. The only point of present interest is whether
this strictly localised explosive outbreak of some 50 or 60 cases of enteric fever was attributable to
flies bred in deposits of house refuse situated some 200 or 300 yards to the south-west of the affected
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