MYIASIS AND THE _ŒSTRIDÆ_
The family of the _Œstridæ_ is the most curious and horrific of all the
different tribes of flies; it is very limited in species, of which five
or six are prevalent throughout Great Britain. The worst of these could
be almost exterminated with ease, but unfortunately mistaken ideas have
prevailed, and graziers commonly believe that though the sheep's nostril
fly is conspicuously harmful and dangerous, the horse's bot-fly and its
congeners are negligible as regards the practical health of the host.
The bot-fly and the worble-flies are all of a largish size, only the
sheep's nostril fly and _Œstrus hæmorrhoidalis_, which latter infests
the throat and rectum of the horse, are of a medium size.
It has been known from very ancient times that man himself was not
exempt from some fly, which was imagined to resemble the horse's
bot-fly, and it has been wrongly surmised that many different creatures
and all ruminant animals were more or less subject to the attacks, each
one of its own kind, of œstrid fly. It is undeniable that man is
sometimes internally afflicted with dipterid larvæ, but it is most
certain that the fly to be incriminated is not a congener of the horse's
bot-fly.
An old illustrated French encyclopædic work gives coloured pictures of
the flies and larvæ of _Œstrus bovis_ (the worble-fly of the ox) and of
_Œstrus equi_ (the bot-fly of the horse), but only the larvæ of a
so-called _Œstrus hominis_ is figured. Recently, however, new attempts
have been made to identify the species causing intestinal myiasis, of
which the larvæ are observable from time to time in the course of
post-mortem examinations and during anatomical study. Of recent years it
has been suggested that the lesser house-fly is addicted to such a
manner of breeding; then later that another species of the same genus
has been found to be the real culprit. However, the peculiar larvæ of
these last-mentioned flies do not in the least resemble the fat round
larvæ of the true bot-fly or of the worble-fly, which are correctly
represented in the above-mentioned French work, nor the round and rather
smooth maggots which were observed in Westminster Hospital nearly fifty
years ago, and at other places from time to time both before and since,
giving rise to much wonder and discussion, and also to very incredible
tales.
Another more credible surmise attributes the offence of human intestinal
myiasis to _Muscina stabulans_; if this be correct, the infliction would
be probably due to the subject having eaten damaged and egg-laden plums
or similar fruit, for _M. stabulans_ is credited with being normally,
though not exclusively, fruitarian or vegetarian.
If any one of the above suppositions be true, it does not exclude any
other one, amongst many explanatory surmises, from being possible.
Judging from the remarkable attractiveness of the odour of humanity to
the common house-fly, and from the fact of the maggots possessing well
developed tenter-hooks on their heads (somewhat like those which the
bot-fly maggots use for internal attachment), it is just as likely, nay
more likely, that this species (as the writer stated for the information
of the authorities of Westminster Hospital nearly fifty years ago) is
more than any other capable of adopting such a life-cycle existence;
these maggots would mature after five or six days feeding and then
emerge. If there were a veritable "_Œstrus hominis_," however rare, the
hairy and peculiar female would be conspicuously observable, a
persistent hoverer about the person of her victim until she had attached
eggs to his body, from which the maggots would not emerge until after
nine months. Most of the tropical flies, which are said to similarly
attack humanity, may be rather compared to the green-bottle flies which
infest sheep, but the latest medical records and reports profess to
identify ten or twelve species of very different genera as having
myiasic capabilities.
The family of _Œstrida_ has been fitly divided into three sections,
namely, the _Gastrophilinæ_ (the larvæ living in the gullet, the
stomach, or the intestines), the _Hypoderminæ_ (worble-flies), and the
_Œstrinæ_ (nasal or nostril flies); all the species are hairy or furry,
and the gravid females fly slowly with loud buzzing, in a characteristic
attitude peculiar by the bending downwards of abdomen and tail, with a
much extruded ovipositor.
The sheep's nostril-fly, _Œstrus ovis_, has a chequered abdomen and is
less hairy than others; it is the type of the section to which the
generic term _Cephalomyia_ is given in some books; species of this
section attack deer and other animals.
The section termed _Hypoderminæ_ comprises the "worble" flies or
"marble" flies. One may imagine that the latter name indicates in the
mind of the cowherd the appearance of the round pustulent boils on the
hide of the suffering animal, and that the former name is a corruption
of "worm-hole," originating with the tanner, observant of the
deterioration of injured hides. A mixing of the terms worm-hole and
marble probably originated the name "warble." The maggots live under the
skin on the back of oxen, and breathe externally through openings in the
boil-like excrescences. The discoloured flesh of infected oxen is called
"flecked." Two species of worble-flies are prevalent, one or the other,
in many parts of England.
The third section, to which the sub-family termed _Gastrophilina_ is
sometimes applied, comprises the "bot-fly," which commonly infects the
horse; it is the imperfect knowledge of this latter which has led to
erroneous surmises explanatory of the horribly disgusting fact of human
intestinal myiasis.
All the species of all the three sections are single-brooded. Although
the flies themselves can inflict no immediate pain, at their mere sight
all the animals out at grass on the farm are seized with an instinctive
terror, conspicuously greater than when attacked and copiously bled by
any "blinden" breeze flies, which, however, fly more silently and settle
on their victims very furtively. One can understand the violent efforts
of the horse to free himself from the exceedingly painful bites of a
newly attached forest-fly, but one can only wonder at the frantic
galloping of oxen and horses to and fro when a non-biting œstrid fly
buzzes about like a harmless fat bumble bee and slowly approaches.
The females of all the worble-flies, the nostril-flies, and the bot-fly
are short-lived, appearing on the wing in August, possibly seen a few
days earlier. In the act of ovipositing they make themselves very
conspicuous; they lay their eggs whilst hovering in the air, their
extruded ovipositors attaching glutinous eggs to their victims. The
hatching of the eggs of the bot-fly is assisted by the habit of animals
to lick themselves and each other, when certainly their warm, moist
tongues will convey into their mouths the newly emerged bot-fly's
maggots, which many months later are to be found attached to the
internal lining of the unwilling host's stomach. When fully grown in
June, these maggots loosen their hold, are discharged with the dung, and
pupate in the soil.
No satisfactory account has yet been given as to the early stages of the
maggots of the worble-flies. The eggs, having been attached to hairs on
the host's hide in August, the prominent round pustulent swellings,
called worbles, wherein the maggots dwell, do not become conspicuous
until the following months of April and May. It is a reasonable surmise
that the obscure and long first-period of the maggot's existence may
more or less conform to that of some of those flies which are also
single-brooded but are predaceous or parasitic on insects. The newly
hatched maggot perhaps can crawl, but does not feed until after several
moults; at each moulting the strange creature becomes smaller and
smaller, but probably at the same time is provided with a new head well
suited for the purpose of that period; firstly, with a burrowing or
grappling head, and in due time with a feeding suctorial mouth, and then
only does practical growth begin. No dipterid flies, at all events,
known to be native to Great Britain, possess skin-piercing ovipositors.
I have been astonished to read in current literature much about œstrid
flies which is not in agreement with my long course of personal
observation; for instance, one high authority (F. R. S.) writes that
œstrid "_flies_" appear from May until October, and hints that their
egg-laying aggressions upon their victims are not conspicuously
observable. I feel confident that the facts are quite otherwise.
That the bot-flies normally (and a few others abnormally, but for short
periods only) pass a very long larval stage in the stomach and
alimentary canal of herbivorous animals is one of the greatest marvels
of insect life. All other growing creatures, which normally breathe in
free air, require a certain large amount of breathable oxygen; and they
would be stupefied or killed by a much smaller percentage of carbonic
dioxide and other fermentive gases of digestion than undoubtedly exist
in the strange abode wherein the bot-fly maggots dwell during the entire
period of their feeding career. It has been stated that fly maggots
artificially ingested into the human system have emerged alive in a
normal condition, but the repulsive and objectionable experiment is not
stated to have procured well nourished and full grown normally pupating
larvæ. Some of the maggots of human intestinal myiasis are not perhaps
amenable to artificial culture up to the stage of final metamorphosis;
and they do not appear to have developed a breed or new species with a
distinct habit of life. All the credible accounts of human intestinal
myiasis point towards some fly which is plural-brooded, and of which the
larvæ develop rapidly and promptly quit the body all at once; otherwise
more than one infection must have occurred. The tales of prolonged
continuous breeding, with slow and prodigiously copious emergings at
intervals, should be altogether discredited.
It is an amply warranted criticism to say that recently published
records by authorities, in an endeavour to comprise every reported
instance of myiasic infection, seem to countenance mere coarse
Gargantuan jokes. On the other hand, it is painful to read such a
"cock-and-bull" story as that of the doctor about his elderly lady
patient, up whose nostril a gravid female blue-bottle flew and
successfully performed the prolonged and delicate operation of laying
therein a large batch of eggs, in spite of all attempts to expel the
invader by violent sneezing. Day by day the said doctor observed the
terrible injury, and the symptoms accompanying the growth of the feeding
maggots, whilst the injection of a spoonful of paraffin would have
effected an instantaneous cure.