PARASITES.
=Parasites= (Greek, _para_, upon, and _siteo_, I feed), in the broadest
sense of the word, are living organisms, which derive their nourishment
from other living organisms. They may belong to the vegetable or
animal kingdoms, and may live on the skin, in the alimentary canal,
or in some one of the internal organs. Some, like the fungus causing
ringworm, feed on the living tissues of the animal infested; others,
like tape-worms, on the partly digested food; while other parasites,
like fleas, only pay temporary visits to the surface of the body, for
the purpose of obtaining food.
=Vegetable Parasites.=—Vegetable parasites all belong to the class of
fungi, and more accurately to the two lowest divisions of this class
which have been provisionally formed, viz.—Schizophyta, and Zygophyta.
The Schizophyta include two orders, =Schizomycetes= and Saccharomycetes.
BACTERIA.
=Bacterium= is the generic name given to the micro-organisms
belonging to the schizomycetes, whether a bacillus (rod-shaped),
coccus (rounded), spiral-formed (spirillum or vibrio), or filamentous
(leptothrix and spirochœta). All these are destitute of chlorophyll and
multiply by fission.[9] They are all extremely small, the width usually
not exceeding 1 µ = 1∕25000 inch. Various names are given to them,
which are synonymous, thus: germs,
microbes,
micro-organisms,
microzymes,
bacteria (_singular_ bacterium).
When they cause disease they are called _contagia_. They multiply
rapidly, and may reach maturity in 20 to 30 minutes. One bacterium may,
under favourable conditions, become 16,000,000 in 24 hours.
=Methods of Examination.=—Until Koch discovered the method of
cultivating bacteria on solid media, the science of bacteriology
remained in its infancy, as it was impracticable to obtain pure,
_i.e._ unmixed cultures of a given bacterium. Koch hit on the idea
of mixing minute portions of cultivations of bacteria which were
growing in liquid broth with liquefied gelatine, and then spreading the
mixture on glass plates, and allowing it to solidify under cover, so
that no atmospheric bacteria could contaminate the growth. When this
was done, individual bacteria formed individual “colonies” scattered
over the gelatine, and these could be identified by sub-culturing and
other methods, for the details of which books on bacteriology must be
consulted.
The _food supply_ of most bacteria is vegetable or animal refuse.
Some of them have a most useful purpose in nature, that of breaking
down complex organic substances and reducing them to a simpler form.
Thus bacteria play an essential part in purifying the soil, and in
the operations in sewage tanks and on sewage farms (pages 192, 195
and 220). A thimbleful of ordinary garden soil which has received a
periodical manurial dressing contains one to three million bacteria.
Certain bacteria have been found to be capable of exercising an
opposite effect, _i.e._ fixing the atmospheric nitrogen and building
it up into the nitrogenous tissues of plants. Thus the nodules on the
roots of leguminous plants consist of bacteria living in symbiosis
with the protoplasm of the plant and supplying it with nitrogen in an
assimilable condition. Pure cultures of these bacteria have been put on
the market as _nitragin_, for enriching land poor in nitrogen. Thus a
fairly complete cycle of nature is secured, and by rotation of plants
(legumes alternating with other seeds), manures, especially nitrogenous
manures, may be partially saved.
The _souring of milk_ is caused by the _bacillus lactis_. This souring
is an indispensable preliminary to the making of cheese, and the
bacillus can now be used in pure culture to hasten the natural process.
The peculiar aroma of good butter is due to a bacterium which has been
isolated; and it can now be supplied in pure culture for butter-making,
thus obviating bad butter.
Certain bacteria are disease-producing or =pathogenic=. The largest
of these is the _Bacillus of Anthrax_, a disease common in sheep and
oxen, and sometimes communicated to man. This bacillus is 1·2 µ thick
and 6 to 8 µ long. When an animal dies of this disease it should be
buried without cutting the skin. When exposed to the air this bacillus
forms minute spores, very difficult to destroy. They may live for
several years in pits in which animals dying from anthrax have been
buried. Butchers have died when inoculated through cracked fingers
when dressing the carcase of a cow which has had anthrax. Similarly
men handling the hides of such animals may be inoculated, either
with a form of disease in which rapid blood-poisoning is produced,
or with a malignant carbuncle, from which recovery is possible if it
be treated promptly. Wool sorters of mohair wool are very liable to
suffer from a fatal form of pneumonia due to the dust from wool derived
from animals which have died from anthrax (page 107). This disease
gives a good instance of possible _attenuation of virus_, of which
another example is seen in small pox (page 293). Pasteur grew anthrax
bacilli in broth at a temperature of 110° Fahr. At this temperature
the bacilli multiplied by division, and no spores were formed. By
repeatedly sub-culturing after the bacilli had become old (_i.e._ by
putting minute quantities of the growth into fresh broth) and exposing
to air, he obtained anthrax bacilli which were only slightly virulent,
only producing slight constitutional disturbance when inoculated,
_i.e._ injected under the skin of sheep, and yet protected them against
ordinary infection by anthrax. Other methods of attenuation of virus
have been discovered. For instance the growth of the bacillus in the
presence of a feeble antiseptic, or passing it through the circulation
of an animal which is relatively insusceptible to the particular
bacillus has this effect.
Other important pathogenic bacteria will be considered later (pages
298 to 398). It is only necessary here to mention that suppuration,
erysipelas, puerperal fever, and a number of forms of blood-poisoning
are due to the invasion of the system by =cocci=. A single round
cell (commonly not more than 1∕25000 inch in diameter) is called
a _micrococcus_. When in pairs as in the micro-organism causing
pneumonia they are called _diplococci_; when in chains, _streptococci_
(_i.e._ twisted); when in masses, _staphylococci_. When cocci and
other micro-organisms are kept out of wounds, healing occurs without
suppuration; this is the principle of the antiseptic and aseptic
methods of treating wounds (pages 106 and 110). The question of
immunity is discussed on page 288.
=Saccharomycetes= occur in fermenting substances, as in the
fermentation of saccharine solutions. The only organism belonging
to this order, which is associated with diseased conditions, is the
_Sarcina Ventriculi_. This is found occasionally in the vomit or even
in the urine of some persons.
The =Zygophyta= occur as thread-like growths, forming a mycelium. This
is composed of jointed branching tubular cells, in which minute spores
are produced. Each spore, when liberated from its tube, is capable of
producing another mycelium, and thus the growth spreads. The spores
may be carried through the atmosphere, thus producing infection at a
distance. They have an average diameter of 6 µ = about 1∕4000 inch.
The following are the chief Zygophytous parasitic diseases:—=Thrush= is
associated with the growth of a minute filamentous fungus, the _oidium
albicans_. It is common in babies, who are improperly fed, and in old
people, or in persons exhausted by any chronic disease. Small white
patches collect on the tongue and neighbouring parts, and these are
often followed by the formation of minute ulcers. When it occurs in
children, the food must be carefully attended to, and feeding bottles
frequently scalded, etc.
=Ringworm= is due to the growth of a large spored or a small spored
fungus (known under the names of _Microsporon Audouini_; _Trichophyton
megalosporon endothrix_, _Trichophyton megalosporon ectothrix_) which
attacks the skin. It is most difficult to eradicate when it occurs in
hairy parts, as the growth penetrates to the roots of the hairs, and
continues to live here long after it has been destroyed on the general
surface of the skin. The fungus spreads on the skin in gradually
enlarging circles, forming rings with a slightly raised margin. It is
extremely contagious, being especially apt to spread in schools. The
spores may be carried about by means of hats or bonnets, by gloves,
towels, razors, and other means. The disease often remains undetected
for some time; and many cases, especially where the scalp is affected,
remain contagious after they have been apparently cured.
The removal of ringworm, as of all other skin parasites, is effected by
some local parasiticide. Prolonged treatment, including the pulling out
of diseased hairs, is required for ringworm of the scalp. A special cap
should be worn, when the patient mixes with others.
=Favus=, or “scald-head,” is due to the growth in the skin of a minute
fungus called the _Achorion Schönleinii_, which invades the same parts
as those affected by ringworm, but differs in its mode of formation
of spores; yellow cupped discs from 1∕4 to 1∕3 inch in diameter being
produced. It is very rare in England, and almost confined to persons
(especially children) who are kept in a filthy condition. It is a
common and fatal disease in mice. The treatment is similar to that of
ringworm.
=Tinea versicolor= is caused by the growth in the epithelial cells of
the skin, of a fungus called the _microsporon furfur_, which, unlike
the two last, does not invade the hair or nails. It forms light brown
patches covered with a horny scurf, which gradually spread, until
nearly the whole trunk may be covered. It does not attack children, and
never affects uncovered parts of the body. It chiefly occurs in those
who do not take frequent baths, and who perspire freely. It can be
removed by daily washing with soap and water and rubbing with a rough
towel, followed by the application of a weak carbolic lotion.
=Animal Parasites.=—Animal parasites are found on the skin or in
internal organs or in the blood or lymphatic vessels. The following are
the most common:—
The _Acarus Scabiei_ is a minute animal not unlike a cheese mite, which
causes the disease known as =scabies= or the itch. It is probably
never more than 1∕77 of an inch in length. The female has eight legs,
with terminal suckers on the four front legs and hairs on the hind
legs. The male is smaller than the female, and in the adult condition
the two hindmost legs have suckers, as well as the four anterior. It
remains on the surface of the skin, while the female burrows deeply in
the substance of the epidermis. At the bottom of the oblique burrow
it deposits ten to fifteen or more eggs, which hatch in a fortnight
and then commence similar operations on their own account. Scabies
generally starts between the fingers, whence it rapidly spreads. The
disease is acquired from some patient suffering from the disease, or by
contact with his apparel. It may become very severe when suspicion as
to its parasitic character has not been entertained. Formerly it was
called “the seven years’ itch,” from the great difficulty in curing it
before its true cause was discovered.
The irritation caused by the insect produces eczema, and this may be
thought to be the only disease present, unless careful examination is
made for the burrows of the insect.
To remove this parasite, first the skin is softened, the superficial
epidermis is removed, and the burrows are laid bare, by the daily
use of hot baths with soft soap, and subsequent rubbing with flesh
towels. Then some parasiticide, such as the well known sulphur
ointment, is rubbed into all the affected parts of the skin. A few
days’ perseverance in this treatment usually suffices for a cure. The
patient’s clothes and bed clothes ought also to be thoroughly purified
by boiling or by steam disinfection or by baking in an oven; otherwise
he may become re-infected.
The =Larvæ= of several insects have been found embedded in the skin. In
the ox, the larva or bot of the gadfly produces a troublesome disease,
a large boil being formed under the skin as the larva grows. This larva
has, on rare occasions, attacked human beings. Rare cases are recorded
where other larvæ have become developed in men, in all upwards of
twenty separate kinds of insects having been recognized. The treatment
consists in removing the parasite.
The =Chigoe=, commonly known as the jigger or sand-flea, is a minute
parasitic insect, found in the West Indies and northern parts of South
America. It is so small as to be scarcely visible; but the impregnated
female possesses a proboscis, by means of which it penetrates the skin
generally near the nails and there develops a bladder the size of a
pea, which sets up severe inflammation. To get rid of the intruder, the
orifice by which it entered must be dilated with a needle, until large
enough to admit of its extraction, without rupturing the cyst.
Several species of =Fleas= infest the human frame. They are propagated
by means of eggs, the worms from which enclose themselves in a tiny
cocoon before assuming the adult form.
Three varieties of =Lice= occur on the human skin. The first
(_pediculus capitis_) infests the head, especially of children, and
multiplies with astonishing rapidity, the female laying altogether
about fifty eggs. The other two varieties are the body louse
(_pediculus corporis_) and the crab louse (_pediculus pubis_).
Strict attention to cleanliness is the best means of getting rid of
fleas and bugs. A wash made of carbolic acid and vinegar painted over
bed crevices is very efficient. Lice may be removed from the head by
cutting the hair short, and carefully cutting out any hairs to which
_nits_ are attached. The nits are cemented to the shafts of hairs.
Washing the hair with methylated spirit or paraffin is also helpful in
removing them. Afterwards the use of white precipitate ointment will
prevent their re-appearance.
The =Trematoda= or Flukes furnish two human parasites, viz. the
liver-fluke (_Distoma hepatis_), and the Bilharzia hæmatobia. The
liver-fluke occasionally produces jaundice in man. In sheep it is the
cause of the disease known as the “rot.” The _Bilharzia hœmatobia_ is
chiefly found in Egypt, and the Cape Colony. It is about a quarter of
an inch long, and infests the blood vessels, more particularly of the
kidneys; setting up severe irritation and the discharge of blood. It is
probable that the eggs of this parasite are received in drinking water
or on salads, though occasionally inoculation may occur through the
skin when bathing.
The family of =Nematoda= possesses numerous parasitic members. The
common thread worm (_Oxyuris Vermicularis_) is one of the most common
of these. The female is 1∕3 to 1∕2 inch in length, and inhabits chiefly
the lower bowel. The ova, which are from 1∕490 to 1∕1100 inch in
diameter, often gain access to drinking water, or are carried by flies,
or received on salads, etc. The injection of salt and water into the
bowel, and treatment tending to improve the general health, are the
proper remedies.
The round worm (_Ascaris Lumbricoides_) inhabits chiefly the small
intestine; hence medicines for its removal require to be given by the
mouth. The female is from 10 to 14 inches long; the ova, of which each
female discharges on an average 160,000 daily, are from 1∕340 to 1∕440
inch in diameter.
The whip-worm (_Trichocephalus Dispar_) is a smaller nematode, which
is rarely met with in this country. The _Dochmius Duodenalis_ is met
with chiefly in Italy and Egypt. It sucks the blood in the intestine,
causing dangerous anæmia. The =Strongylus Gigas= is chiefly found in
the kidneys of the ox, dog, etc., and is very rare in man. It resembles
a very large round worm. In the kidney it produces severe disorders.
How it gets there is not known.
The _Trichina Spiralis_ has been already described (page 23).
The _Filaria Dracunculus_ (Guinea Worm) seems to gain access into the
stomach along with water, or possibly in some cases, by perforating the
skin. It burrows among the tissues, especially of the legs, and attains
a length of several feet. It causes large boils and sores, and through
these the eggs escape and pass into water. Here the embryo which has
escaped from the egg meets with a fresh water crustacean (_cyclops_),
enters its body, undergoes larval growth, and is swallowed with its
host by a man, in whom it burrows and undergoes its next stage of life.
The embryos of three species of =Filaria= infest the blood of man,
chiefly in the tropics. One embryonic species is found in the blood
of infested patients by day, one by night, and one during both day
and night. The length of the embryos varies from 1∕75 to 1∕125 inch,
and its width from 1∕3000 to 1∕3500 inch. The night embryo, which is
the most common, is produced by the _Filaria Bancrofti_. This adult
worm infests the lymphatic system of man, sometimes reaching a length
of three to four inches. Its embryos may obstruct lymphatic vessels,
causing obstruction of the flow of chyle (hence originates _chyluria_),
and _elephantiasis_, in which enormous swelling of the legs and other
parts ensues.
The nocturnal migration into the lymphatic vessels, and thence into
the blood of the embryo of the _F. Bancrofti_, is an adaptation to
the nocturnal habits of a particular mosquito (_culex pipiens_ or
_ciliaris_). When the mosquito bites an infested person, his proboscis
removes some embryo filariæ, which are quickly transferred to its
stomach. Some of these escape digestion, develop within the mosquito,
and when the mosquito dies in water they bore their way out, and are
subsequently swallowed by man.
It is essential, therefore, in order to prevent this disease to boil or
efficiently filter all drinking water, and to prevent the access of
mosquitoes to water. Persons infested with filariæ should sleep inside
mosquito nets, in order that they may not, when bitten by mosquitoes,
spread the disease.
=Tape-worms= are found infesting the alimentary canal of man. Each has
a double phase of existence. In the first, the characteristic head, or
_scolex_, along with a bladder-like body, lies embedded in the solid
tissues of an animal; in the second, the _strobilus_ or tape-worm,
occupies the alimentary canal of another animal. The tape-worm consists
of a number of flat segments, each of which is capable of producing
a large number of eggs, from each of which a six-hooked embryo is
developed. The segments escape from the alimentary canal, and their
ova are discharged and scattered broad-cast. These eggs are swallowed
by another animal, the hooked embryo escapes from its case, migrates
into the solid tissues, and there produces a scolex. When the host is
eaten by another animal or by man, the scolex enters the alimentary
canal, loses its bladder-like body, and developes a chain of segments.
It follows from the above that two distinct hosts are necessary to
complete the cycle of existence of these creatures, one being commonly
a herbivorous, and the other a carnivorous animal. Thus:—
= Cystic Form.= =Tape-worm Form.=
_Cysticercus Cellulosæ in _becomes_ _Tænia Soluim in the alimentary
the muscles of the pig_ canal of man._
_Cysticercus Bovis in the „ _Tænia Mediocanellata in the
muscles of the ox_ alimentary canal of man._
_Cænurus Cerebralis of the „ _Tænia Cænurus in the alimentary
sheep’s brain_ canal of the dog._
_Echinococcus of man, etc._ „ _Tænia Echinococcus in the
alimentary canal of the dog._
The _cysticercus cellulosæ_ has been already described (page 23). The
cystic form of the dog’s tape-worm (_echinococcus_) is a most dangerous
parasite for man. When the egg of the dog’s tape-worm is swallowed by
man, the embryo escaping from this egg burrows from the alimentary
canal, and forms large cysts, chiefly in the liver, but occasionally
in the lungs, brain, and other organs. For the removal of these,
surgical interference is required. This form of cyst is commonly known
as a _hydatid_. It is most frequently seen in Iceland and Australia,
though not uncommon in this country. Its frequency depends largely on
the number of dogs, and on the facility with which the ova of their
tape-worms can gain access to water.
The adult _Tape-worms_ are usually derived in man from eating meat
containing the cystic form. The _cysticercus_ of the pig produces
_Tænia Solium_; that of the ox, the _Tænia Mediocanellata_.
These are the two most common forms of tape-worm in man. The minute
head of _T. Solium_ has four suckers and a double row of hooklets,
28 in number; while the head of _T. Mediocanellata_ has four suckers
but no hooklets. The segments of _T. Solium_ are smaller than of _T.
Medioc._, and the structure of the segments of the two is somewhat
different.
=Preventive Measures.=—In avoiding the various Entozoa described, it is
important (1) to _carefully avoid all underdone meat_. The eating of
smoked sausages, or of meat which is not cooked throughout, is a common
source of tape-worm and of trichinosis.
(2) _All vegetables should be thoroughly washed_: this is especially
important in the case of water-cress, lettuce, etc., which are eaten
raw.
(3) If the purity of the _water_ is not ensured, it _should be boiled_
or filtered through a Pasteur-Chamberland filter (page 98), especially
in tropical climates, and where many dogs are kept. Dogs should be kept
out of the kitchen, lest ova accidentally gain access to articles of
food.
(4) The possibility of _flies_ and _mosquitoes_ acting as carriers of
parasitic disease must be remembered, and precautions taken.