Parasites

PARASITES (from Greek para- around and siton- food), beings that feed on living organisms of the plant or animal world, temporarily or permanently residing on them or within them; parasites feed on juices, body tissues, or food found in the digestive canal of animals. Parasites can be animals (zooparasites) as well as plants (phytoparasites). Based on their location on hosts, parasites are divided into external (ectoparasites, Epizoa) and internal (endoparasites, Entozoa). Some ectoparasites only temporarily settle on the host's surface to feed (for example, mosquitoes, horseflies, leeches); others not only feed but also live on the host's body (lice, Mallophaga, parasitic Copepoda). Some parasites live within the thickness of the skin coverings; they are called intradermal; for example, the scabies mite burrows through and lives in the horny layer of the epidermis of mammals; in the Malpighian layer of human skin, the larva of the stomach botfly (Gastrophilus) can parasitize; the female of the sand flea (Sarcopsylla penetrans) becomes completely embedded in the epidermal coverings. In the corium of the skin and in the subcutaneous tissue live the larvae of flies, the warble fly (Hypoderma), and others. Cavity parasites inhabit body cavities that have wide communication with the external environment; such are the nasal cavity, ear cavity, oral cavity, and the conjunctiva of the eye (larvae of botflies Oestrus and Rhinoestrus). Endoparasites can live in any organ or tissue of the host. Based on details of localization, the following are distinguished: a) endoparasites of organs communicating with the external environment (lungs, intestines, urinary organs, starting from the kidney and ending with the urethra); various flagellates, rhizopods, worms, and others live here; b) blood parasites with subdivision into parasites inhabiting the blood plasma, red blood cells, and white blood cells (microfilariae, haemosporidia, trypanosomes are blood parasites); c) endoparasites of proper tissues or tissue parasites living within the thickness of various tissues of the host, for example, in striated muscles (Sarcosporidia, trichinae), in the brain (trypanosomes, fincae, cenuri), in the notochord (larva of a fluke in lungfish), in cartilage (myxosporidia Lentospora in some young fish), in connective tissue (myxosporidia), in nerve fibers (Myxobolus neurobius), and others; d) endoparasites of coelomic cavities (some helminths, gregarines). The division of parasites given above is conditional already by the fact that some parasites during different periods of their life migrate through the host's body, entering various tissues and organs. In such cases, the place of residence of the parasite is considered to be that part of the body where the parasite finally establishes itself; on this basis, the trichina is considered a parasite of muscles, although the larvae of these nematodes live here, and their reproduction occurs in the intestine; at the same time, the ascaris - another migrating nematode - establishes itself in the thick intestines and reaches sexual maturity here. The localization of tissue parasites in some cases can be further specified when they inhabit the cells of the tissue itself. Intracellular parasites live in the cell protoplasm (for example, coccidia, leishmania) or in the nucleus (Nucleophaga). The term endoglobular parasites is used to denote parasites living in red blood cells. FORMS OF PARASITES. For true, or obligatory (obligate) parasites, the parasitic mode of feeding is a species characteristic. Facultative parasites, however, are free-living animals, some individuals of which may fall into conditions of parasitic life. The food of parasites are blood, lymph, tissue juices, the tissues of the body themselves and their derivatives (horny scales, hair, feathers, epidermal cells, connective tissue and other parts of the 'host'). Some parasites have a very wide circle of hosts (for example, the tick Ixodes ricinus sucks blood from many mammals, as well as birds and lizards); such parasites are called polyphagous or euryzoic parasites. Their opposites are parasites that live at the expense of one specific species or a few species of hosts. Such stenozoic parasites are accordingly monoxenic or oligoxenic (for example, the head louse, the malarial plasmodium). The attachment of stenozoic parasites to specific hosts is so constant and definite that it takes on a group character. For example, tapeworms of chickens cannot parasitize in ducks or geese, etc. The malarial plasmodium, in addition to humans, occurs only in some monkeys. Such examples illustrate the principle of 'host specificity' put forward by Fuhrmann. In some cases, based on this principle and knowing the parasite, one can judge the systematic position of its host; for example, based on the characteristics of tapeworms of woodpeckers, Fuhrmann is inclined to agree with Fürbringer's view of the closeness of woodpeckers to passerine birds. There are parasites common to humans and anthropoid apes, for example Necator americanus. HOSTS OF PARASITES. The true host of a parasite is one that is typically used by a given species of parasite; the true host must be distinguished from accidental hosts, on which the presence of the parasite is caused by an exceptional combination of circumstances, but not by biological relationships, for example, cases of paradoxical parasitism of voles' fleas on dogs or of ground squirrels' fleas in human dwellings. In nature, cases are frequent where the parasite goes through a complex development cycle and lives at the expense of several hosts (heteroxenic parasites). Thus, Ixodes ricinus in each stage of metamorphosis attacks some host, needing in general three separate hosts: for the larva, nymph, and adult tick. Along with such a three-host tick, one can note the one-host species Boophilus calcaratus, which goes through all stages of development on the same cow. In the first case, one can speak of the phenomenon of host change. It may be connected not only with the feeding procedure but also with the reproduction of the parasite. When the parasite feeds in different stages of metamorphosis on different hosts, the latter are called first, second, etc. hosts. As different stages of the life cycle of the parasite pass, associated with different methods of reproduction, the phenomenon of host change occurs, accompanied by the alternation of generations of the parasite. The terminology here accordingly becomes more complicated. The main (final, definitive) host is called such an animal in which sexual reproduction of the parasite occurs, while in intermediate hosts the parasite reproduces asexually. For the malarial plasmodium, man is an intermediate host, because in his blood schizogony (multiple division of the plasmodium) occurs; at the same time, Anopheles should be recognized as the main host, because in its organism the plasmodium goes through the sexual cycle of development. In parasitic worms, the matter is different, because in cestodes both sexual (laying of eggs by the tapeworm form) and asexual (development of the tapeworm by budding from the finca) reproduction actually occur in the same host; however, the forms of the parasite capable of one or the other method of reproduction inhabit different hosts. The above is explained by the following scheme: Main host for unarmed tapeworm - man Intermediate host for unarmed tapeworm - cattle In the intestine of the swallowed finca, the tapeworm buds (asexual reproduction), which by sexual method gives a mass of eggs (sexual reproduction) In the muscles, from the embryo that emerged in the digestive canal from the egg, a finca grows, capable of asexual reproduction, which is carried out in the main host Consequently, the criterion for determining the category of the worm host - main or intermediate - is actually not the very fact of reproduction, but the place of development and residence of the stage of the parasite capable of a certain method of reproduction. However, some parasitologists already consider the very process of formation of the scolex of the finca from the embryo (oncosphere) of the worm as asexual reproduction. The change of generations of parasites goes not only according to the type of metagenesis (alternation of forms with sexual and asexual reproduction) but also of heterogony (alternation of different forms that reproduce sexually). DISTRIBUTION. Parasites are widely distributed in the animal and plant world. There is hardly any species of animal that would be absolutely free from any parasites, but at the same time it is noted that the quantitative indicators of infection with parasites vary very significantly among different host species; as a rule, some animals are often heavily infected with parasites, while other species are very little; for example, out of 5000 sea stars once examined, only three had parasitic infusoria Orchitophrya, and at the same time in the stomach of each cow there is invariably a multitude of infusoria of the genus Ophryoscolex. The species diversity of the parasite fauna of different hosts is different and great. Among ciliated worms, only a few protozoa parasitize, while some mammals are hosts of more than 50 species of various parasites belonging to protozoa, worms, and arthropods. The total number of species of parasites that can live at the expense of man exceeds 400. Parasites are distributed quite unevenly among the types of the animal world. Among the protozoa there are many very important parasites, and some classes, for example the sporozoa, consist exclusively of parasites. Coelenterates have only one or two parasitic species. Worms (Vermes), especially flatworms, nematodes, and acanthocephalans, are very rich in species of parasites; among annelids, well-known parasites are leeches.

Among representatives of the type Echinodermata, there are no parasitic species. Mollusca are very poor in parasitic forms; but among Arthropoda there are many Parasites, though they are distributed very unevenly among classes and orders. Among crustaceans, Parasites are found among lower forms (Entomostraca, Rhizocephala, Copepoda, etc.). Among arachnids, Parasites are ticks (Acarina). Insects are rich in ectoparasites and also have some endoparasitic species; such orders as Mallophaga, Anoplura, and Aphaniptera consist entirely of Parasites. Chordates (including vertebrates) have no Parasites at all, if we do not consider myxines and the fish Fierasier, which occurs in holothurians.

Features of the parasitic mode of life and its influence on the Parasite's organism. The parasitic mode of life is characterized by great uniqueness. Constant Parasites, living on or in their host, are provided with food, as they do not have to search for it; in connection with this, the functions of movement are weakened, and the organs of movement are reduced; examples: reduction of wings in bugs, fleas, lice, feather lice; reduction of limbs in rhizocephalan crustaceans. It is interesting to note the presence of limbs in the freely living stages of metamorphosis of some parasitic crustaceans and their subsequent reduction in the ontogeny of the Parasite (e.g. Sacculina). At the same time, for Parasites it is very important to firmly hold onto their host, because if the latter is lost, some Parasites under no circumstances can re-enter another host and are therefore doomed to obvious death. In connection with this, we see in Parasites the development of attachment organs, the role of which is played by clinging legs, powerful mouth organs, special appendages, cuticular hooks, strong muscular suckers, etc. To hold Parasites in the fur, hair, and feathers of the host serve various outgrowths of the body, long bristles and hairs, which are appendages of the skin cover.

The digestive system also undergoes great changes. In blood-sucking forms, piercing-sucking mouth organs develop. Many endoparasites have a simplified digestive system, while others are completely devoid of digestive organs, which have been completely reduced in the phylogeny of the corresponding species. This is the case, for example, with tapeworms, which, living in the intestine, "bathe in food" and absorb it with the entire surface of the body (endosmotic nutrition). Sacculina, however, feeds on the juices of the crab, which it absorbs with root-like branched attachment processes that grow into the host's body. It should be noted that the reduction of digestive organs is also observed in some free-living animals; in others, there is a complete loss of the function of nutrition in the adult stage of metamorphosis (adult bot flies, which feed abundantly in the larval stage).

In the saliva and digestive juice of various bloodsuckers there are powerful anticoagulins; other Parasites secrete a proteolytic enzyme that destroys the tissues of the host's organs "59 {the dysentery ameba, causing ulceration of the large intestine, the cercariae of schistosomes, penetrating the human coverings into the veins). The Parasites themselves, living in the intestine, possess stable antienzyme properties, thanks to which they live without being subject to the digestive power of the host's digestive juices (worms). Many Parasites live in an environment where there is almost no free oxygen (worms in the large intestines); their respiration occurs due to oxygen released during the decomposition of food substances (glycogen)-so-called intramolecular respiration. In the process of metabolism in an anaerobic environment, the chemical decomposition of nutrients does not go to completion, but stops at the stage of formation of intermediate substances that have toxic properties (fatty acids); consequently, the anaerobic mode of life of worms determines the toxicity of their excretory products; the absorption of the latter into the host's organism results in chronic poisoning as one form of the harmful influence of worms on hosts.

Many sense organs are completely reduced in endoparasites (e.g., the absence of eyes in flukes). In some Parasites, the general habitus is changed to unrecognizability, and the systematic position of such forms could only be established after a detailed study of their regressive metamorphosis, beginning with the stages of free-living young forms having a typical appearance (e.g., parasitic crustaceans-Lernea, Sacculina, etc.-have a nauplius larva, characteristic of crustaceans).

The influence of the external environment on Parasites has a certain uniqueness. For endoparasites, the external environment is primarily the host's organism and precisely that organ or tissue of it which is the biotope for the habitation of the Parasite. Consequently, the "official" external environment, i.e., the environment surrounding the host itself, does not have direct contact with endoparasites. However, the effects of factors of the external environment (in the usual understanding of it) on endoparasites can still occur. The degree and character of the effect, for example, of temperature on endoparasites is determined (besides the properties of the Parasite itself) by the peculiarities of the host's organism in which the Parasite lives. In animals without a constant body temperature, fluctuations in the external temperature cause heating or cooling of the organism and of the Parasites living in it. Thus, the malarial plasmodium in hibernating Anopheles is subjected to sharp temperature effects that can be fatal for the Parasite, especially if one takes into account the considerable cold resistance of the common malaria mosquito. At the same time, the malarial plasmodium in human blood lives at slight fluctuations in temperature; changes in the latter in the external environment can only indirectly influence the Parasite to some extent; this explains, for example, the occurrence of relapses of malaria during colds or cooling of the body, when the malarial plasmodia, which had been living hidden before, are stimulated to reproduce. Abnormal increases in the host's body temperature can have a fatal effect on Parasites. Cases have been noted of frequent spontaneous exit of ascarids from the intestine of feverish patients. Enemas with the maximum tolerable hot water or its introduction through a probe into the duodenum can affect parasitic worms and some protozoa. As for the chemical composition of the habitat of Parasites living in the host's tissues, they are also in relatively constant conditions, especially if the Parasites are surrounded by a capsule (e.g., trichina larvae in muscles); however, such constancy is relative, because over time the process of lime deposition in the capsule also covers the larva itself. The environment of the intestine is subject to great fluctuations, into which food can enter that has a specific effect on intestinal parasitic worms. It has been noted that after eating pumpkin seeds, the expulsion of worms occurs; onions as well as various medicinal substances (anthelmintics) have an unfavorable effect on these intestinal parasites. A pronounced avitaminosis reduces the body's resistance to infection by certain parasitic worms. It is remarkable that fluctuations in the chemical composition of the blood in connection with the entry of food substances into it also affect blood parasites (sugar in the blood and the intensity of development of the plasmodium of avian malaria and some trypanosomes). The introduction of drugs sharply changes the chemical composition of the Parasite's habitat, and the latter under the influence of new and direct effects of its external environment dies or is removed from the host's organism. When drugs are introduced, changes in the chemical composition can also occur in the organism in general, which affects Parasites located in distant or deeply hidden parts of the host's body. The administration of drugs per os can also affect some blood parasites (e.g., quinine and the malarial plasmodium); subcutaneous injection of santonin can expel ascarids from the intestine; injections of emetine are very effective in the treatment of amebic dysentery, etc. In general, the therapy of parasitic diseases is nothing other than an ecological effect on the Parasite-the causative agent of the corresponding disease. It can take on a specific character in the case of the use of parasitotropic agents, i.e., agents that are fixed to the Parasite (e.g., the parasitotropy of arsenic preparations for spirochetes, protozoa, etc.).

The influence of Parasites on hosts. Such circumstances as the age of the host and various states of it, in turn, affect the Parasites. If the effect of the host on the Parasite is very diverse in character and important for the Parasite, then the reverse relationship, i.e., the influence of the Parasite on the host, is in turn of extreme importance for the latter. Although in a number of cases the presence of the Parasite in the host does not noticeably affect its health, there is still a mass of evidence of the pathogenicity of Parasites, which manifests itself both locally and in general pathological changes. Sometimes such changes are only "outlined" in the host's organism; in this case, they can only be discovered by special studies on a large amount of material; some Parasites clearly affect the hosts, causing them various diseases by the fact of their parasitism, called invasions {ezh.}, or parasitoses. Their causative agents are generally nosoparasites. The special pathology of parasitoses is nothing other than the study of the general biological question of the effect of parasites on hosts (see Parasitism). Having certain parasitoses can cause a state of immunity, the causes of which are diverse. Invasions of hosts by parasites.

The parasitic way of life requires the parasite to enter the host's body. The implementation of host invasion is associated for the parasite with the influence of numerous chance factors. The susceptibility of humans to parasitic worms is a consequence of the influence of a long series of conditions, which together contribute to the parasite embryo reaching the invasive state in the egg. For helminth eggs, such conditions include: the nature of the environment, particularly the soil onto which the helminth eggs have fallen with excrement, its humidity, temperature, degree of illumination or shading, duration of the eggs' stay in the external environment, and the movements to which they may be subjected here. In all this, the egg will only produce a new parasite when it enters the host's gastrointestinal tract, the condition of which is favorable for the development of the parasite. The chain of living conditions for parasites that develop with intermediate hosts is even more complex. Undoubtedly, the chances of a parasite successfully bypassing all obstacles and completing its full life cycle are small (which is the unfavorable side of parasitism for the parasite from a biological point of view). Through natural selection, parasites have developed properties that compensate for the influence of chance; such a characteristic of parasites is in particular their exceptional fecundity. Thus, the hookworm gives up to 25,000 eggs per day, Fasciolopsis Buski up to 48,000, and the human roundworm up to 200,000 eggs. The sexual product of large tapeworms is even greater. Such quantitative predominance of the reproductive function is associated with very strong development of reproductive organs, along with which there occurs a reduction of "unnecessary" organs for the parasite. Sacculina ultimately represents something like a bag filled with eggs. The ripe uterus of Taenia solium displaces all other parts of the reproductive apparatus and others. The practical significance of parasites as pests of human health, domestic and commercial animals, and as factors regulating the number of organisms in nature is very great. In the economy of nature and in agriculture, the significance of parasites of harmful animals and parasites of parasites is particularly great. Secondary parasites and superparasites. In the nucleus of Jodamoeba buetschli, Nucleophaga intestinalis parasitizes, and in the protoplasm of Entamoeba coli, Sphaerita sp. Such parasites are called secondary. Secondary parasitism is distinguished from superparasitism or additional parasitism. This concept is put forward mainly in relation to agricultural entomology. This includes cases when the same host becomes infected twice with different species of parasites; the fate of the latter is different: one parasite may turn out to be a secondary parasite in relation to the other and destroy it; there are cases of survival of both parasites or, conversely, their death. The significance of parasites as causative agents of parasitoses brings to the forefront the fight against parasites as a method of preventing parasitic diseases. The fight against parasites is conducted in the external environment if it contains various stages of parasite development, either on the host or in it, with respect to endoparasites and stationary ectoparasites. In the latter cases, methods of fighting practically come down to therapeutic measures. Diagnosis of parasites. If the object of therapeutic interventions is the parasite, then the first prerequisite for conducting such should be the accurate diagnosis of the parasite. The establishment of the fact of the parasite's presence in the host's body is done by observation of the parasites themselves that have emerged (balantidia, amoebas, segments of worms, pinworms) or their forms of reproduction (eggs, cysts of protozoa). Examinations are made of the secretions of those organs in which parasites reside (feces, urine, sputum), or the host's tissues (blood, muscles-biopsy, X-ray examination). It must be borne in mind that the discovery of a helminth egg, a protozoan, or a fly larva in feces does not always mean that the corresponding parasite actually was and lived in the human body. Protozoan embryos may enter from outside, or coprophilic forms that pass through the human digestive tract in the inactive cyst stage may develop in the excrement. Finally, various parasites, both characteristic and not characteristic of this host, may be ingested with food. The finding of traces of their presence in the human body must be critically examined to obtain correct conclusions; without such precaution, it is possible to take pseudo-parasites for truly parasitic ones in the subject being examined. Such pseudo-parasites are "Oxyuris incognita", described from eggs in human feces. In reality, the eggs belong to the nematode Heterodera radicicola, which lives in lettuce and garden vegetables. When eaten with food, the nematodes are digested, and their eggs enter the feces. The coccidia Eimeria oxyspora, Eimeria snijdersi and Eimeria wenyoni, described from material in feces as parasites of humans; in reality they are parasites of herring, European sprat, and sardines (E. chipearum and E. sardinae), in which they live in the liver and testes, and also enter the feces with food. It is very important that helminths that actually parasitize humans can in individual cases be pseudo-parasites. This is the case with the liver and lancet flukes, whose eggs can enter the feces when parasites eaten with the liver of sheep or cattle infected with flukes are digested. Such transit eggs have a completely different diagnostic significance than idiogenic eggs, i.e., excreted by parasites living in the host's body. Modern parasitology requires the establishment of differential diagnosis between transit and idiogenic eggs or between real and pseudo-parasites. The standard technique of microscopic examination of feces for helminth eggs is insufficient in such cases. It is necessary either to determine the distinctive features of the fine structure of a living egg or to resort to indirect methods (repeated examination on a milk-vegetable diet, etc.). In connection with the fact of discovering pseudo-parasites, in parasitology they begin to distinguish real fascioliasis from pseudo-fascioliasis. Finally, it must be borne in mind that sometimes various objects simulate the appearance of parasites. Thread-like rolls of mucus may resemble roundworms in shape, fragments of mucus sheets resemble segments of worms; various elements of plant fiber may be very similar to eggs or larvae of worms, and many others. For such a category of objects, the term "omeoparasites" (E. Pavlovsky) is applied; their correct recognition is important, because a false interpretation of them as parasites can lead to unjustified and harmful therapeutic interventions for the "host" (for example, the prescription of an operation to remove a kidney due to pseudo-parasitism in its pelvis by Swainson's giant, whose eggs were taken to be leucine balls). Pathogenic parasites should be of as close interest to physicians as pathogenic bacteria. In connection with this, physicians should consider themselves obliged to make the correct zoological determination of the species of parasite according to the truly characteristic features, as they are obliged to do in relation to bacteria. On antiparasitic agents-see Disinsection, Anthelmintics, as well as separate infectious diseases, helminthiases, and individual parasites. Lit.-see literature for the articles Parasitism and Parasitology.