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The second channel of infection is a more probable one. W.D., of
the kitchen staff, was ill with diarrhoea on the 7th and 8th June and
continued on duty although S. typhi-murium was not recovered from
her rectal swab. More significant is the fact that S.J., who was in
charge of the large dining hall, had a positive rectal swab when first
examined, but swabs on the following three days failed to confirm
it. She may in common with the cases have been a non-suffering victim
or a cause or both.
With regard to the three cases which occurred after an interval of
a fortnight, K.B.'s illness was almost certainly due to infection in the
duck egg she ate a few hours before, but the mode of infection in the
other two, S.R. and S.A.M., is not so obvious. Neither had whole eggs,
but did have hard-boiled duck eggs in salad. Strict hygienic precautions
had been impressed on everyone in the establishment and there was
a resident infirmarian who was a trained nurse with many years of
hospital experience. It is extremely unlikely that a missed case
in a food handler could have occurred at this stage and, as is known,
the outbreak ended completely with elimination of duck eggs from the
diet. S.R. and S.A.M. were either infected by supposedly hard-boiled
eggs in salad or more likely by the third of the possible routes mentioned
above which may also have accounted for the first series of cases.
The investigation into the serum reactions of the ducks in this
outbreak shows that in the positive reactors the titre is not a reliable
guide to the capacity of the duck to lay an infected egg. Blood testing
of the 31 ducks (excluding the drake) for S. typhi-murium showed that
5 were negative, 6 slightly doubtful, and 20 positive, 8 of the latter
being in high titre. Laboratory investigation carried out at the Epsom
Public Health Laboratory and prolonged after the outbreak at Weybridge
until the ducks went out of production, detected in all 10 infected eggs
from 3 birds out of 192 eggs examined. One of these three ducks, the
only one identified before the flock left the school premises, gave only
a doubtful serum reaction, but was subsequently found to have infected
faeces, to be the only one with intestinal infection and, post mortem,
to have infection of the ovary. None of the other 5 ducks with a
doubtful serum reaction or the 5 with a negative reaction gave any other
evidence of infection. The other 2 ducks which produced infected eggs
were among the 8 high-titre reactors, one of which at autopsy had
infection of the ovary. The remaining 6 with a positive serum in high
titre were not found to lay infected eggs. Three of these at autopsy
had infection of the ovary and one infection of the liver.
Outbreaks of S. typhi-murium due to a purely local and exclusive
supply of duck eggs are not common and this occurrence is of interest
in that circumstances made it possible not only to trace the infection
to its source, but to secure confirmation by phage-typing of the
responsible salmonella.
Acknowledgments.
Grateful acknowledgment is made to Dr. Stone of the Epsom
Public Health Laboratory for her valuable assistance in the examination
of no less than 142 duck eggs and many other articles of food ; to
Mr. Blaxland of the Ministry of Agriculture and Fisheries Veterinary
Laboratory, Weybridge, for his willing co-operation in his investigation
of the offending ducks, and to the School Infirmarian and Authorities
for their help and ready provision of every facility for prosecuting this
investigation.
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