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37
secondary," finds that of 65 "primary" cases 51 received and 14 did not receive
the particular milk in question. An estimate of the probable number of sporadic
cases, i.e., of cases unassociated with the special cause in the affected area at the
time of occurrence of the "primary cases," cannot give, having regard to the size
cf the population at risk and the distribution of typhoid fever in London at the
time, more than 2 or 3 at the most, and will certainly not account for as many as
14. Thus a milk hypothesis of origin of the outbreak leaves 11 or 12 "primary
cases," and some 30 per cent, of the 66 confirmed cases of typhoid fever, to be
accounted for otherwise than by the suspected milk.
The number of those attacked who gave a history of eating ice-cream at about
the time when infection presumably took place was quite small, but it was suggested
that some of the 30 per cent, of cases negative as to milk may have obtained milk
from source X indirectly, through the medium of ice-cream; and it was pointed out
that one of these patients belonged to a family occupying premises where ice-cream
was made; this patient, it should be added, was not among the very early sufferers
in the outbreak, and thus could not have been the original source of infection of the
milk. The enquiries made as to ice-cream consumption in the affected area showed
a considerable percentage of early sufferers giving a history of consuming ice-cream
at some time or other (mainly in the previous summer), but of those whose illness
commenced subsequent to the 24th September (some 56) only 9 gave such a history,
and these had obtained ice-cream from different sources. It is to be noted that the
ice-cream consumed in the affected area consisted of frozen custard, and thus had
been initially sterilised by boiling.
The percentage of milk negative cases cannot in any case be regarded as being
materially lessened by including, in the category milk, any ice-cream which may
have been made with milk from source X. On the other hand, it should be pointed
out that the percentage given of milk negatives (30 per cent.) is almost certainly
underestimated. In 20 instances, at least, the consumption of X milk was only very
occasional; and inasmuch as the dates of consumption of such milk were not
definitely ascertained, it is possible that in most of the 20 instances the date of
consuming milk from source X did not correspond with the time when infection
was contracted. On the whole, the fact that at least 30 per cent, of the sufferers
denied having consumed milk from source X suggests that a milk hypothesis
per se will not account for the special prevalence.
The main consideration, however, as regards possible milk causation, arises
in connection with detailed analysis from a geographical and sociological point of
view of the customers obtaining their milk from source X. As has been already
mentioned, the affected area may be divided into four nearly equal sections, separated
from one another by two main roads running north and south and east and west
respectively. The location of source X is in the main road dividing the northeastern
and south-eastern sections of the area, the milk premises being placed, however,
quite excentrically, in fact definitely to the east of the intersection of the two
main cross-roads, and at some distance from the centre of gravity, so to speak, of the
cases constituting the special prevalence.