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41
It is generally held that the incubation period in typhoid fever is about a
fortnight, but the need for assuming the possibility of the apparent date of onset
of illness being exceptionally within a day or two of the ingestion of infected oysters
was demonstrated by Bulstrode in 1904; and the probability that the period of
incubation may on the other hand be prolonged even for four or five weeks was
also pointed out by him. (See Transactions of the Royal Society of Medicine,
Vol. IV., 1910-11, Epidemiological Section, p. 131, on the question of prolonged
incubation period.)
On arranging cases in order of onset of illness as stated by the patient or
relatives, it appears that the first two undoubted cases of typhoid in the outbreak
of 1924 commenced on 13th and 14th September. The cases occurring in the week
ended 20th September and following weeks numbered 9, 20, 15, 5, 5, 8, 4, 3, and 2,
respectively; in the remaining 6 weeks of the year, 5 cases occurred. It will be
seen that the large majority of the patients sickened during the first three or four
weeks, but the increase in the numbers of those sickening in the 5th, 6th and the
three next succeeding weeks suggests the possibility that a subsidiary outbreak or
recrudescence, was then developed.
The facts may be accounted for by case-to-case infection or by a second
introduction of infected food material. There is a third possibility, namely, that
in the latter part of the prevalence cases of a slight and doubtful character were
included as cases of typhoid. In this connection it is noteworthy that 2 or 3 of the
later groups of multiple cases included cases in which the bacteriological results
were indefinite. Moreover influenza was prevalent in London at the time.
Study of the
anomalous
time distribution
of the
multiple
cases in
households
occurring in
this outbreak.
There were in all six instances of multiple cases in a household (see the
annexed chart), viz., four of 3, one of 4 and one of 7.
In one family two cases, Nos. 35 and 36, occurred on September 30th, but
as the attacks were practically simultaneous they are not included among the six
"multiple case attacks." In only one instance did the first case occur in September,
and here the diagnosis of the illness of the original sufferer was not free from doubt.
In two instances the first case occurred in the first week of October, and in the
remaining three the first illness did not commence until the 15th, 21st, and 24th
October. It is of special interest to note that all save one (No. 71) of the cases of
typhoid fever with onset after the end of October (i.e., 10 cases in 4 families) occurred
in households in which there had been previous cases.
It is of considerable interest to note that two distributions of infected fish, one
early in September and one early in October, would (allowing for the incubation
period and allowing in one or two instances a small amount of latitude as regards
date of onset) account for all the cases. On such a hypothesis of two infections
it is to be observed that the main groups of multiple cases in households might
result from the later of the two infections. The suggestion that the earlier
infection was by fried fish from a fried fish shop, and the later infection by fish
bought at a coster's stall and cooked at home (both consignments of fish being
conveyed presumably from market by the same coster), would a fiord a comparatively
simple explanation of the curious phenomena both as regards the age and sex
incidence and the grouping of multiple cases observed in this outbreak.
The hypothesis that all the multiple cases were to be explained by case to
case infection is difficult of acceptance; thus, in all the groups taken together, the
length of time separating the multiple cases or groups of cases in households averaged
nearly 4 weeks, and in one instance it was 9 weeks; again, the removal of a first
case to hospital together with the taking of precautions against spread of the disease
was nevertheless in some instances followed by the occurrence of later cases in the
same house. The difficulty in deciding in the case of such groups whether later
attacks in families should be regarded as resulting from case-to-case infection,
or as being due to a continued operation of the cause which produced the original
case was discussed twenty years ago by Bulstrode in connection with an outbreak