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The ducks were then killed and a post mortem examination performed
on each. The findings were as follows:—
Salmonella typhi-murium isolated from 5 eggs (3 birds),
from faeces of 1 duck, from ovaries of 6 ducks and from the
liver of 1 duck. Eleven of these cultures were sent to the
Central Reference Laboratory for phage-typing, where they
were all shown to belong to the same strain as those from the
patients and eggs sent previously, viz., Vi Strain, Group 2.
Discussion.
This investigation raises interesting considerations as to the route
by which infected duck eggs can convey infection and as to the risk
from ducks with a positive blood reaction to S. typhi-murium. There
can be no doubt that the origin of this outbreak was the flock of ducks
kept on the premises. It is generally believed that hard-boiled duck
eggs are safe, but that those cooked otherwise are not free from risk,
and it would be convenient and satisfying to explain 23 of the first
series of 25 cases which occurred in girl boarders by the fact that they
all had poached eggs on the evening of the 7th June. The incubation
period, however, is generally accepted to be between 8 and 48 hours,
possibly up to 72 hours. Whilst, therefore, it is possible that eggs
eaten on the 7th June gave rise to cases which occurred on the 9th, or
even the 10th June, it is highly improbable that they caused illness up
to 7 days afterwards. If those cases which occurred on the 9th and
10th June (17 out of 25) were regarded as primary cases and the
remaining 8 spread over 4 further days as secondary, then the incubation
period was from 40 to 67 hours with an average of 55 hours. Moreover,
examination showed that three weeks later only one duck was laying
infected eggs, and those intermittently, and at no time throughout
the lengthy investigation of the ducks were more than 3 found to do so.
As the eggs served on the 7th June would represent about 4 days'
layings which had accumulated over Whitsuntide the number of infected,
eggs among the 60 to 70 served on the 7th June could scarcely have
been as high as 17. Moreover, it leaves unexplained two of the three
patients in the second series who only had hard-boiled duck eggs in
salad within 48 hours of onset. It is much more probable that the
cases were infected by another route, among which must be considered:—
Dishes made with infected duck eggs;
A missed case or carrier originally infected from
the ducks;
Contamination of food by members of the
kitchen staff after handling duck eggs
soiled with infected excreta.
With regard to the first, shell eggs were not used for cooking,
dried egg powder being bought for this purpose. Shell eggs were
either served whole or hard-boiled in salad. In this connection it is
known that the girls and the community ate salad containing hardboiled
egg on several occasions between the 7th and 14th June and
that the members of the community, three of whom comprised the
second series of cases, ate salads during the period preceding the second
phase of the outbreak.
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