View report page

between the outbreak and flies could be established, although flies in the district
were found to be contaminated with this germ at the same period of time.
Comparison of Organisms from Insects and Patients.
The only organism common to both flies and patient was Pr. morgani, which
was found 18 times on four species of fly (15 times on either green or blue-bottles)
and was isolated from eight patients (one baby, five children, two adults).
Lactose fermenting coliform organisms were often found and certain strains
are thought to cause infantile diarrhoea.1
In a high proportion of cases of gastro-enteritis it seems difficult or impossible
to isolate a pathogen.
The most important observation of the whole experiment is, we think,
the surprising number of organisms carried by insects. About three quarters
of the insects examined bacteriologically were infected with organisms—many
highly pathogenic. If this represents the average rate of infection generally
amongst the insect population, it is not surprising that infections are liable to
be conveyed to persons by insects, especially on food.
Planning the Experiment.
The planning of an investigation such as this on a statistical basis is difficult.
Our experience has shown the necessity for attention to the following points.
In the part dealing with the insects we should consider and try to balance the
many and varied conditions in which traps may be placed ; whether inside or
outside premises ; the amount and proximity of breeding facilities, e.g. within
so many yards of food, decomposing material, scraps of food lying about, dustbins,
refuse works, pig-bins, open spaces, etc., the standard of domestic catering
or industrial cleanliness. The bait used in the traps is of importance—species
vary in their likes and dislikes. The times of placing and collecting traps should
be as uniform as possible. We noted the maximum and minimum temperature
each day. Each trap should have a record including the number of insects
caught, alive or dead, and the species. Of the live insects a known proportion
should be selected for bacteriological examination. In the part dealing with
the patients the points requiring care about illnesses are the source of the
information ; allowance for the variation in zeal of practitioners and staff
notifying ; the variability of conditions classed as illnesses ; the date and time
of onset and the duration ; the age, sex, vocation, school, and address ; the
symptoms, diagnosis, and bacteriological proof ; the article and place responsible
for infection, and the bacteriological proof.
Acknowledgments.
I wish to thank Dr. Doris M. Stone and the staff of the Public Health
Laboratory, Epsom, by whom the bacteriological examinations were made.
The British Museum (Natural History) Entomological Department,
Diptera Section, co-operated fully and thanks are due to Mr. Riley (Keeper),
and particularly to H. Oldroyd, Esq., M.A., and Dr. F. van Emden (Commonwealth
Inst, of Entomology), who identified the many specimens sent to them.
I wish also to thank Mr. Nuttall, Divisional Education Officer, Twicken-
1 B.M.J. 1949. 16th July. Page 141.
18