CHAPTER XLI.

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.