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The Danger of Infection Spread by Insects.
By T. Williams, Chief Sanitary Inspector.
Routes of Infection.
The route by which a person receives an infection is often unknown.
Many infections are carried by insects. The Medical Officer of Health and I
thought that as we had little knowledge of the extent of this danger in our own
locality, it would be useful if we tried to find out what organisms were carried
by insects, and if any illnesses which occurred could be attributed to such a
mode of spread. We decided to collect insects from various parts of the
Borough, to have them identified and examined bacteriologically; and to
collect information about as many cases of infectious disease as we could,
especially the food poisoning group.
Insect Traps.
We experimented with various forms of insect traps and eventually
decided to use a conical pattern which was made out of celluloid and wire
mesh. With the assistance of the district sanitary inspectors, traps were set
in various parts of the Borough during the period 18th June to 31st December,
1948, and insects were collected as long as any were about. The number of
traps laid was 331, and 1,044 insects were caught. We laid traps in places
as varied as possible in and around houses, restaurants, canteens, shops,
schools, open spaces, refuse works, etc. We tried out 26 different types of
baiting material, such as fish, fish paste, meat, hash, sausage meat, corned beef,
soup, milk, fruit, beer, compost and animal excreta. We found that more
flies per trap were obtained with fish as a bait than with any other material.
We were not able to plan the experiment by laying two or more traps with
different baits in the same place, nor could we ensure that the placing of the
traps in the various parts of the district was " balanced " for similarity of
environment, temperature, or time of exposure, as should be done in a " block "
experiment.
Identifying the Organisms and Insects.
The traps were taken to the Public Health Laboratory at Epsom, where
some of the live insects from certain traps were examined for organisms.
It was not possible to examine all the insects for organisms, partly because of
the great amount of work entailed in their isolation and identification, and
partly because the insects die so quickly; nor was it possible to ensure that
the specimens examined formed a random sample, having regard to place and
circumstances of collection. In all, 172 insects were examined for organisms,
and these, together with the remainder (872) were sent to the British Museum
(Natural History) Entomological Department, Diptera Section, where they
were identified.
Species and Organisms.
The 1,044 insects identified were spread over no less than 45 different
species (Table 35). Admitting the deficiencies of our sampling technique,
we found that 488—by far the greatest number—belonged to the species
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