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67
The Spread of Disease by House Flies.
We may take it that flies are the principal agents in the spread of
summer diarrhoea in young children. We are absolutely dependent on
outside sources for the supply of milk, and it is readily conceivable
that flies contaminate some of the milk even before it arrives here, and
this fly-infected milk may then give rise to infantile diarrhoea in a
district which itself is comparatively free from flies. If warm weather
conditions prevail, the growth of the bacteria in milk is favoured, and
the greater number there are of flies the greater likelihood there is that
unprotected milk will be contaminated. Luckily we have taken special
pains to do away with collections of manure and house refuse, which
are the favourite breeding grounds of the house fly.
If flies are present in noticeable numbers in a crowded district, it
only needs the ignorance and carelessness of one mother, whose child
has summer diarrhoea, to cause a rapid spread in the number of cases;
the flies acting as carriers of germs from the infective excreta, and
infecting houses where there is a free access to food.
Flies may therefore be a nuisance and injurious to health by their
mere numbers, or may also be a specific and dangerous nuisance by
being capable of carrying all sorts of disease germs. A consumptive,
for instance, who has not been instructed, may expectorate on to the
ground or into an open vessel, flies settle on it, ingest some of the
tubercle bacilli, and then may immediately settle on to food suitable for
the growth of the germ, and convert it, especially milk, from being a
wholesome food into a virulent dangerous poison.
Life History of the Domestic Fly.—Flies begin breeding in the
months of June and July, and continue to breed until October, the
greatest activity being in the months of August and September, this
being greatly influenced by heat, the higher the temperature the more
rapid the development. Their chief breeding place is stable manure,
but they will breed equally well in any collections of manure, or
excrement, or any collections of waste animal or vegetable substances,
provided there is a certain amount of moisture present, and the
temperature is favourable.
The fly is an extremely prolific insect, a single female being able to
lay from 120 to 150 eggs at one time, which it usually deposits in dark
crevices, and this may be repeated five or six times during its short life.
Development of the Fly.—It must here be explained that in the
development of the fly from the egg, the insect goes through certain
changes of form and habit which are known as a metamorphosis. The
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