DISSEMINATORS OF DISEASE
The house-fly may seem at first much less to be dreaded than any one of
the painfully "biting" or (to be correct) skin-piercing and
blood-sucking flies; yet it should be regarded as a much greater enemy
to humanity and a more dangerous peril than any of those other flies, of
which some short mention and description has now been given. Its
life-history and its fecundity have been already alluded to; its rapid
growth and maturity counterbalance the fact that it is short lived.
From ancient times there has been a consensus of opinion that there was
in some way a connection of cause and effect between swarms of flies and
the spread of disease. In the plagues of Egypt, in the reign of Pharaoh
of the Exodus, it will be remembered how, after "the land was corrupted
by reason of the swarms of flies," Exodus viii. 24, there came "a very
grievous murrain" upon cattle, Exodus ix. 3, followed by a "plague of
boils and blains" upon man and beast.
In our present day insect life is being scientifically investigated with
the view of establishing the connection, and of discovering fully the
serious _rôle_ of disseminating disease, of which the house-fly has long
been suspect. The microscope reveals much, and the art of bacterial
culture now explains how it is true that the superabundant creature,
which has persistently followed civilised man into every quarter of the
globe, has ever had a share in conveying contagion beyond that of any
other household pest.
That the house-fly is bred in filth matters not much. After emerging
from the puparium its first voidance of fæcal matter may be contaminated
with live baneful germs, but it voids itself before its first flight.
Having six legs it stands upon two pairs, whilst with the other pairs,
at one time the front pair and at another time the hind pair, it works
frequently and vigorously at brushing and stroking down every part of
its body. Though it starts its new life quite a wholesome newborn
creature, and though it must be credited with being a most assiduous
remover of dirt from its own body, yet from the human point of view its
subsequent life is a persistently disgraceful career.
It is the evil course of the newly-hatched and self-cleaned fly not to
restrict its diet to the honey of flowers, as do some of its relations.
Its food includes excrement, sputum, and every kind of putrefying
organic matter likely to be swarming with micro-organisms of a character
deleterious to humanity. It is certain that, when only a few days old, a
fly will practically abound internally and externally (on the feet) with
dangerous germs, as amply proved by methods of laboratory culture. As it
feeds, it walks over the food; and the hairy joints of its feet, when
microscopically viewed, appear conspicuously liable to carry germs in
spite of frequent attempts at self-cleaning. Wherever it alights and
walks, it prospects with a touch of its trunk, which is the main
instrument of evil. It has a very filthy habit, from time to time, of
depositing pale vomit spots as well as dark-coloured fæcal droppings.
These defilements are visible, wherever it may alight on walls, windows,
ceilings, and especially on pendent ornaments, whereon the males delight
to rest.
Its manner of feeding upon solid food is to pour forth a copious supply
of saliva, to regurgitate some previously imbibed fluid draught, and
then to re-imbibe; thus, besides devouring soft food, it dissolves,
befouls, and feeds on crystalline sugar and other hard dry food
materials.
Its regurgitated fluid commonly swarms with bacteria, microbes, and the
like. Imbibed bacteria are not inevitably killed in the digestive
process of the fly, for its excrement has been found to abound with well
recognisable infective germs. In the market, the shop, the larder, and
on our tables, the house-fly seeks every opportunity of befouling and
contaminating human food.
The varieties of micro-organisms are multitudinous, doubtless many more
in number than the microscopist and the bacteriologist expert have yet
isolated and registered as capable of identification. Granted that the
majority of these are non-pathogenic to humanity, still a formidable
number, including some which are very generally disseminated, are
virulently pathogenic, and many are suspect. There is no need to give a
list of all the infectious diseases which man and beast are liable to
contract, but the germs of nearly all may be carried from place to
place, from creature to creature, and from person to person, through the
intervention or agency of the house-fly. The medical profession are
convinced that infantile mortality from epidemic diarrhœa must be
attributable to summer flies.
In the matter of food which becomes fly-infected after having been
cooked, or of food like milk, butter, and fruit, which are consumed raw,
it should be known that a single pathogenic germ of ultra-microscopic
dimensions, having obtained lodgment in the body, may there multiply and
originate a fatal disease. On the other hand, raw meat which has been
infected may, after the bacteria have been all killed by cooking,
contain excreted poison in deleterious quantity. The decomposition of
infected meat begins ten or twelve hours before the bad odour is
perceptible.
Fortunately the omnipresent germs which most commonly deteriorate our
food are not very actively deleterious, or are only slightly
debilitating; yet wherever such less obnoxious germs get lodgment, there
the ready prepared and most favourable breeding place for the worser
kind is to be found. The various species of these evil things are not
always exterminating competitors; they sometimes flourish in company,
and dwell together, like the seven devils within the exorcised and sane
man after his relapse, as mentioned in the Scripture.
That food gets fly-blown and maggot-infected is a very disgusting fact,
but the plainly visible result is of little hygienic significance apart
from the more concealed facts of the fly-borne conveyance of zymotic
diseases.
Internal protozoal parasites and parasitic worms breed in and are
disseminated by the house-fly; so also are the fungic spores of
fermentive yeasts, of moulds, and the like, but these latter are mainly
disseminated by mere air currents. The eggs of tape-worms and the like
are carried by dung-frequenting flies to food, especially to semi-putrid
food devoured by dogs and pigs.
Some of the skin-piercing and blood-sucking flies are pestiferous in a
more direct way than any of the tribes of filth and sweat-flies. They
are the usual or suspected agents whereby anthrax, cattle-plague, swine
fever, glanders, and other diseases are spread far and wide. Some of
these last blood-sucking flies will travel with _and on_ the bodies of
transported animals for long distances; of course there can be no doubt
also as to the capability of disease dissemination by the direct
independent flight of flies to long distances with favouring weather and
breeze. Such evils are prevalent throughout the temperate zones, but
circumstances are far worse in the tropics, where _Glossina morsitans_,
considered by some to be a near relation of our _Stomoxys calcitrans_,
transmits the microscopic trypansomes which cause the devastating
"sleeping sickness" of mid-Africa. This last reference, and other
discoveries of the fly-borne germs of recurrent fevers, should bring
into prominent notice a very pertinent fact; which has not yet received
adequate scientific investigation. All the bites of our common
blood-sucking insects, flies, gnats, midges, fleas, etc., are each kind
of them wounds, sometimes very inflammatory, sometimes but little or not
at all so; furthermore, the worst inflammatory wounds not uncommonly
show feverish symptoms of a well marked periodic character, quiescent
intervals being followed by revived inflammation in the same spot. These
facts almost prove, or at least strongly suggest, an explanation that in
the latter cases the source of pathogenic trouble is of a microbic
character, and that periodic recurrence of feverish and inflammatory
symptoms is the time of the spore-swarming of breeding microbes; the
difference between these latter and those of a more severe and often
fatal kind in the tropics being that the one class finds the human body
a fit host in which to multiply but the other class does not, and
accordingly the latter grow weaker until their breed dies out. Similar
effects can be observed in cattle and other animals, but all creatures,
after suffering much from fly-bites at first, afterwards become for a
time more or less immune.