Parasitic Animals

Parasitic Animals are those which live on or in other organisms, from which they derive their food. But this mode of life has many forms and degrees; the hosts may be animals or plants; the parasites may be external or internal (ectoparasitic or endoparasitic), fixed or with the power of movement; they may be parasitic temporarily, for a prolonged period, or for the whole life; restricted to one host, or requiring to pass from one kind of animal to another if the life-cycle is to be completed. For the parasitic animals which infest plants, see CORN INSECTS, GALLS, &c.; and ASCARIS, FLUKE, TAPEWORM, THREAD-WORMS, TRICHINA, &c., and other parasites are separately discussed.

Grades of Parasitism.—The grades recognised by Leuckart are: (1) Temporary Parasitism.—

'To this category belong almost exclusively ectoparasites, which differ from their free-living relations only in their diet.' (2) Ectoparasitism—an established and invariable habit during a prolonged period or during the whole of the parasite's life. This is called 'stationary' ectoparasitism in the translation of Leuckart's great work, but the term hardly suggests the idea. 'These parasites either pass through all their developmental stages on the host, or at first lead an independent existence under a form more or less different from that of the adult.' (3) Endoparasitism, in which the parasites are during a great part of their life, and almost invariably during their maturity, 'boarders' within the body of their host. Moreover, all the developmental stages are almost never passed through within one host, a transference from one kind to another being necessary for the completion of the life-cycle. But of endoparasitism there are many varieties.

The Hosts of Parasites.—Probably no animals, except some of the simplest, are free from the attacks of parasites. Yet some are more liable than others—e.g. because they offer greater inducements to those parasitically inclined, because they are more accessible, or because they eat infected food. Thus, vertebrates are more abundantly infested than invertebrates. 'Man has more than fifty distinct species of parasites, the dog and the ox some two dozen each, the frog perhaps twenty.' Some species of parasite are limited to one kind of host; thus, the adults of Bothriocephalus latus and Oxyuris vermicularis are not known except in man, while Trichina spiralis is found in man, pig, rat, dog, cat, ox, &c., and Distomum hepaticum in many ruminants and other ungulates, in rodents, in the kangaroo, and in man. The systems most infested are the most accessible—viz. the skin and the alimentary canal, by ecto- and endo-parasites respectively; but there are no organs in which parasites may not be harboured.

Origin of the Parasitic Habit.—It is probable that most cases of parasitism began gradually. Animals found temporary shelter on or in others, and the habit grew upon them. In some cases it might begin fortuitously—e.g. as the embryos or adults wandered or were swallowed; or it might be a shift saving those which adopted it from some presumed keenness in the struggle for existence; or it might simply express a sluggish constitution. In many cases, however, we can hardly doubt that the habit began with the naturally more sluggish females, prompted not by hunger, but by the impulse to seek some conveniently sheltered place for the birth of the young. In fact, there are not a few parasitic female Crustaceans whose mates live freely. Of the evolution of parasites from free-living ancestors the free stages still included in the life-history of most, the close relationships between some free and some parasitic members of the same class—e.g. Crustaceans and Nematodes—and the frequent occurrence of temporary parasitism afford sufficient evidence. It is also instructive to consider the three classes of Plathelminthes—Turbellarians (Planarians, &c.), Trematodes (Flukes, &c.), and Cestodes (Tapeworms, &c.)—of whose genetic relationship there seems little doubt; the Turbellarians are almost all free-living; the Trematodes are mostly external, but sometimes internal parasites; the Cestodes are all endoparasitic.

Life-history of Endoparasites.—Most endoparasitic animals have an eventful life-history. They are not always parasites, or they are not always parasites within the same kind of host. Most of them are at some time free; many of them have some sort of metamorphosis. But, as their life-histories are very various, they do not readily admit of being summed up in general statements.

Let us begin, however, with the adult sexual animals. In this state they are always almost parasitic, partly because rich copious diet, warmth, and relative quiescence favour reproductive maturity; partly because many probably began their parasitic career at the reproductive period, when shelter and readily attained food were specially advantageous; partly because it is not likely that animals which had become parasitic would relinquish this habit in adult and mature life. In fact, with the exception of some thread-worms (Gordius, Mermis, &c.) and some few insects (ichneumon-flies, gad-flies, &c.) which are parasitic in their youth and free as adults, it is generally true of parasitic animals that the eggs are produced, fertilised, and deposited in the parasitic stage. In regard to the reproduction it should be noted (1) that the fertility is often enormous, for a tapeworm may produce 42,000,000 eggs, and a female thread-worm 64,000,000 in a year; (2) that in those cases where the female alone is parasitic fertilisation may take place before parasitism has begun; that otherwise it occurs within the body of the host; that Trematodes and Cestodes are hermaphrodite and sometimes fertilise themselves; (3) that in tapeworms the fixed 'head' buds off a long chain of joints, each of which is sexually complete, becomes eventually distended with eggs and embryos, and is liberated singly or along with others from the intestine of the host.

The eggs or embryos of the parent endoparasite usually pass from the host along with the excreta, and 'there are no intestinal worms, at least among the typical and constant parasites, whose embryos come to maturity near the parent; or, in other words, there are none which pass their whole life-cycle in one locality.' Some of the embryos are locomotor—e.g. those of the liver-fluke and of Bothriocephalus latus, which are active migratory; others are passively carried along with food and drink into new hosts. There the embryos rarely become or remain quiescent, but wander from the food-canal through the tissues and organs of the host until a fit resting-place is found. But, to state another of Leuckart's general conclusions, 'the quiescent stage following upon the wandering embryonic stage does not conclude the life-history of the parasite, which requires, in order to complete its development, a radical change in its environment—in other words, a second migration.'

But before leaving the so-called intermediate host—which is different from that of the parent or that of the adult—we should notice that within it asexual multiplication may occur. Thus, several asexual generations characterise that part of a liver-fluke's life during which it sojourns in a water-snail (Limnæus) prior to reaching its final or 'definitive' host in the sheep. In other cases, the asexual multiplication within the intermediate host is of a simpler kind, being restricted to budding, as when the bladder-worm or proscolex of Tænia echinococcus within ox or man develops many 'heads' or scolices, each of which on being transferred to dog or wolf will grow into a tapeworm. Or there may be no true multiplication—e.g. in the numerous bladder-worms which form only one head, and remain quiescent until the host happens to be devoured by another, within which the 'head' of the bladder-worm may bud off an adult tapeworm chain.

Connected with this change of host there are two main problems: (1) How is the change effected? (2) how did this extension of the life-history to two distinct hosts arise? In regard to the modes of transference it will be enough to give two illustrations. The young liver-fluke actively migrates from a water-snail and from the water, encysts on stems of grass, and is then eaten by a sheep.

Here, and in some other cases, the migration is in part active. On the other hand, the bladder-worm of the pig lies quite passive in the muscles or connective tissue of that animal, and cannot reach its final host unless 'measly' pork be eaten by man. Here, and in most other cases, the migration is passive. The second problem is very difficult. Is the host in which the adult is found the primitive host, and has that of the immature stages been intercalated? or is the intermediate host really the primitive one in which the animals used to become mature, while the final host represents a secondary prolongation of the life-history? Leuckart expresses himself unconditionally in favour of the second theory that 'the intermediate hosts were originally the true definitive carriers, which formerly brought their intestinal worms to sexual maturity, but have since become merely intermediate, because the development of the parasites has extended itself over a greater number of stages in the course of further differentiation.'

The Environment of Parasites.—It is at present debated (see HEREDITY) whether the precise influences exercised on parasites by their hosts are transmissible or not. But it can hardly be doubted that the habits and surroundings of parasites have been somehow influential in their evolution. It is certain that individual parasites may vary in different parts of the body and in different hosts, and it is admitted by all that parasites exhibit 'adaptations' to their life and surroundings. It is therefore important to take account of the precise relations between host and parasite. Ectoparasites will experience mechanical influences due to the movements of their bearers, they will often be carried from one locality to another, they will sometimes share in the warmth of their hosts, they usually find abundant food, and they are often not only sheltered but sedentary. Endoparasites will experience pressure from the tissues in which they lie, or from the peristaltic movements of the food-canal in which they are lodged; their immediate environment usually involves confined space, scant oxygen, considerable warmth, and total darkness; they will be affected by abundant and rich nutrition, by surrounding gases and juices, and by their frequently sedentary life. Now it is at least a plausible theory that the usual absence of sense-organs in endoparasites is due to the unstimulating character of the environment, which has caused them to degenerate, and this view is partly confirmed by the occasional occurrence of sense-organs in the larvæ alone, and by the facts that locomotor appendages are absent or much reduced in the adults of many fixed ectoparasitic crustaceans, because they have gone out of use; that a food-canal is absent in many endoparasites, partly because the superficial absorption of complex surrounding juices left it functionless; that the passivity of many is increased by living in surroundings in which the respiratory processes must be very sluggish; or that the prolific reproduction—especially perhaps the budding growth of tapeworms—is in part due to the abundant and yet stimulating nutrition.

Effects of Parasites on their Hosts.—In the 17th and 18th centuries the injurious effects of parasites were much exaggerated. All sorts of diseases, including many which we now know to be associated with Bacteria, were said to be due to 'worms,' and physicians gravely discussed 'An mors naturalis sit substantia verminosa?' As accurate diagnosis began to be less unusual, a strange reaction in favour of parasites found many supporters. Intestinal worms were called 'the good angels and unfailing helpers of children,' and were said to aid digestion and even development. But since the middle of the 19th century, when the experimental study of parasites began in earnest, a knowledge of the various injuries which parasites may do, to man and to domesticated animals at least, has become more and more precise and complete. Only a few illustrations need be given. Numerous large parasites will certainly diminish the nutritive supplies of their host; large bladder-worms and the like press upon adjacent organs, cause obstructions, and give rise to many troubles; the movements and migrations of parasites within the body of their host produce pain and inflammation, and may even result in the perforation and destruction of important organs. Even external parasites may do considerable damage; witness those crustaceans which occur beneath the tails of crabs, and sometimes effect the virtual castration of their hosts. On the other hand, there are many less important parasites whose effects are very slightly if at all injurious. It is a question of much practical importance how the endoparasites which infest man find their way to their host, but as details will be given in such articles as TAPEWORM and TRICHINA, it is enough here to say that food in which parasites are known to lurk should be inspected, cleaned, and sufficiently cooked.

Historical.—Most of the ancient and mediæval naturalists and physicians who expressed any opinion on such matters believed that parasites were spontaneously generated within the bodies of their hosts. It was not till the 17th century, when Swammerdam and Redi showed how maggots, lice, &c. developed from eggs, that the belief in generatio æquivoca began to be seriously disputed. It was gradually replaced by the theory that parasites came from without, that, ceasing to be free-living, they entered the bodies of other animals and were there modified. But this conclusion was too hastily leaped at, and no care was taken to prove that the free-living forms in question did really develop into parasites. In many cases, indeed, it was soon shown that they did not, and this disappointing result helped Pallas and others in the latter part of the 18th century to recognise rightly that parasites were propagated like other animals by means of eggs. They concluded, however, that these eggs were more or less directly carried from one host to another, there to develop into the original form, while we know that the life-history of parasites is rarely so simple; nor was there more than a slight warrant for another favourite idea that young animals inherited parasites from their mothers. At the beginning of the 19th century the helminthologists, such as Rudolphi and Bremser, were very active and greatly extended the list of known parasites, but the life-histories remained a puzzle, and many naturalists relapsed into a belief in spontaneous generation. The increasing use of the microscope led to most important results: in 1831 Mehlis discovered the Infusorian-like embryo of certain flukes; Von Siebold (1832) detected the six-hooked embryo within the still unliberated ova of the tapeworm; Eschricht (1841) compared the life-history of internal parasites to that of ichneumon-flies and bot-flies; Steenstrup (1842) published his famous essay on alternation of generations; Von Siebold (1843-50) and Van Beneden (1849-50) worked out the metamorphoses of several parasitic worms; Küchenmeister (1853), Leuckart (1856), and others showed experimentally how infection with larval stages resulted in the development of adult parasites. The foundations of modern helminthology were thus laid, and we have now a vastly increased knowledge of the number of parasites, a precise acquaintance with the life-history and migrations of some of the most important, a scientific system of medical diagnosis and treatment, and some realisation of the general biology of parasitism.

See ASCARIS, BOT, BOTHRIOCEPHALUS, COMMENSALISM, CORN INSECTS, DEGENERATION, ENVIRONMENT, FISH-LOUSE, FLUKE, GALLS, GREGARINIDA, GUINEA-WORM, HAG, LEECH, LOUSE, MITE, TAPEWORMS, THREAD-WORMS, TRICHINA; also Leuckart, Parasiten des Menschen (2d ed. 1881 et seq.; trans. by Hoyle, vol. i. Edin. 1886); Küchenmeister, Parasiten des Menschen (2d ed. 1878; trans. Ray Society); Cobbold, Parasites (1879); Van Beneden, Vers Intestinaux (Paris, 1858), Animal Parasites and Messmates (Inter. Sc. Series, Lond. 1876); Von Linstow, Compendium der Helminthologie (1878); Moniez, Les Parasites de l'Homme (1888).