TOPIC: 2: Parasitic system. Concept about parasitocenoses

↑

▷ Содержание

⇐ ПредыдущаяСтр 3 из 7Следующая ⇒

SUBJECT “ECOLOGICAL PARASITOLOGY”

1. Parasitic system.

2. Influence of the parasite population on the host population: regulation of the host population.

3. Influence of abiotic, biotic and anthropogenic factors on parasitic systems.

4. Stability of parasitic systems. Specificity of parasites. Evolution of parasitic systems.

5. Distribution of parasite populations in host populations. Change of the genetic structure of the host population. Extensity and intensity of an invasion, intraspecific competition, effect of congestion.

6. The concept of parasitocenosis. Mix-invasion: examples of synergy and antagonistic relations between parasites of a single parasitocenosis.

1. Parasitic system.

Parasitic system - two or more populations of different species, one of which is a population of parasite, interacting with each other in the biocenosis. The concept of "parasitic system" was formulated by E. Ch. W. Martini in 1932 and further developed by V.N. Beklemishev. The parasitic system is formed by the population of the parasite and all populations of its hosts. In the biocenosis there are double (parasite - host), triple (parasite - vector - host) and multiple (parasite - vector - several hosts) parasitic systems.

The parasitic system can be simple when the existence of a parasite is provided by host population of a single species, and complex if it involves populations of several species - several species of arthropods carrying parasites or several species of vertebrate animals.

2. Influence of the parasite population on the host population: regulation of the host population [1, Р.189].

The model of interaction of populations in the parasite-host system corresponds to the model of interaction between a predator and its prey. A dynamic self-regulating system describes these models. As the population size of the host increases, the population size of the parasite increases also. On the other hand, increasing the number of parasites leads to a decrease in the number of hosts, and the last event again reduces the number of parasites. And there fluctuations in the population size of the host and parasite with deviations from some optimal level are periodic.

The study of predation and parasitism as factors regulating the number of individuals in populations shows that the negative impact of predators and parasites is usually not great if both species (predator and its prey, or host and parasite) have existed together for a long time. There are many vivid examples of such parasitism. For instance, the aborigines of Africa developed relative immunity to malaria, and this provides survival of both parasite and human. Trypanosome, which causes sleeping sickness in humans, lives in the blood of large herbivores without harm. Infection of a person with tapeworms does not lead to death. However, in any of these cases, a steady fine balance can be disturbed as a result of some changes in the ecosystem or other events. Serious consequences arise, in particular, when the predator and the victim (or parasite and its host) meet each other for the first time. If the ecosystem into which they come will be unbalanced, predators and parasites can cause drastic reduction in the population size of the prey or host.

3. Influence of abiotic, biotic and anthropogenic factors on parasitic systems [1, Р.186-187].

The natural environment, being the natural habitat of the parasitic system, has a direct regulatory influence on each of its components: parasite, its hosts and vectors. Parasitic organisms are closely associated with numerous components of water and ground biocenoses.

Parasites are adapted not only to specific organisms that are related by trophic connections but also to all environmental factors. It has been established that in the conditions of a permanent transformation of the environment that occurs under the influence of human, the balance of the functioning of parasitic systems is disturbed. This phenomenon is very typical for urban areas where increase in the number of vectors and hosts of parasites in conditions of environmental transformation are occurs. In the urban territories of the subtropical zone of Russia and the border countries exists favorable ecological conditions for development of mites, insects, mollusks, rodents, wild carnivores taking part in the processes of transmission of parasites to humans. Due to a change of the functioning of parasitic systems, the conditions for excessive invasion of helminths in humans and animals are created. This situation is now qualified as a parasitic pollutionof territories.

4. Stability of parasitic systems. Specificity of parasites. Evolution of parasitic systems.

Parasites in the majority of cases have a dual habitat: the environment of I order (the host organism) and the environment of II order (external factors). The parasite is "not interested" in the disappearance of the host as the environment, since in this case the parasitic system will not exist. Consequently, the process of development of the stability of the parasitic system should be considered its primary goal.

Specificity of parasites is a historically developed degree of adaptation of the parasite to the host. Forms of manifestation of specificity: a) hostal: monoxenous – the parasite needs in one species of the host (ascaris), polyxenous (heteroxenous) - the parasite needs in hosts of different species (Trichinella); b) topical (the place of localization of parasite): ascaris (intestine); c) age (enterobiosis in children) d) seasonal (amoebic dysentery infections – in late spring and summer seasons).

Host specificity of parasites is the association of certain parasite species to certain host species. Distinguish highly specific parasites associated with only one host species or a small number of host species, and low specific parasites that parasitize on a wide range of hosts.

For example, tick Ixodes simplex parasitizes only on bats, preferring one species of it. A representative of the same genus - taiga tick Ixodes persulcatus can parasitize on 177 species of birds and 100 species of mammals.

Highly specific human parasites are plasmodium, pinworm, large intestinal round worm and some others. They are specific human parasites and cause diseases called anthroponosis. A number of other parasites that occur in humans can also attack different mammalian species. These are, for example, lice, most helminths (liver fluke, cat liver fluke, fish tapeworm, trichinella and others) that parasitize on different mammals, including humans, are not very specific. The diseases caused by these parasites are called zoonosis or zooanthroponosis.

In the same species of parasites, specificity can be expressed in varying degrees at different stages of the life cycle. In some species, specificity is more pronounced in larval stages, and sexually mature organisms live in a large number of final hosts. Such, for example, are flukes, larvae of which are adapted to certain species of mollusks, and adult forms can parasitize in different species of mammals.

Many tape worms, on the other hand, are strictly specific to the final hosts and show less specificity to intermediate hosts.

Specificity manifestations also sometimes depend on the age of the host.

The study of the specificity of parasites is necessary for personal prophylaxis of infection. Indeed, to prevent invasion with leishmaniasis, trypanosomiasis or malaria, it is enough to protect against bites by a certain type of bloodsucking insects, while for personal prophylaxis, for example, toxoplasmosis requires a complex set of measures.

There are cases when, when the final host moves to a new geographical region where the intermediate hosts of parasites are absent, other suitable species of animals become specific intermediate hosts of the larvae of this parasite. This phenomenon is called the parasitological vicariate. If in new region there are species that are closely related to the final host of the parasite, the parasite can spreads to these new hosts, and is easier when the specificity of the parasites is low.

Many blood-sucking ectoparasites, being either mechanical or specific vectors of parasites, can systematically introduce them into the blood of other host species. New species of parasites can arise with the adaptation of the parasite to habitation in new hosts.

Sometimes, parasitizing in one organ in certain hosts, the parasite can adapt to the habitation in other organs. Such expansion of parasite often leads to their divergence into a number of new species adapted to a particular tissue or organ of host's body.

5. Distribution of parasite populations in host populations. Change of the genetic structure of the host population. Extensity and intensity of an invasion, intraspecific competition, effect of congestion.

Populations of both hosts and parasites are obligatory members of biocenoses, the stability of which depends on the species diversity. Parasites in ecosystems are consumers of the second and third orders, which play an important role in the biotic cycle of substances. Even the most pathogenic of them, which cause the death of a large number of hosts, act on the one hand as stabilizers of the number of hosts, periodically removing from the populations their excess.

On the other hand, the most severe course of parasitic diseases is usually observed in individuals with weakened immunity, suffering from hereditary defects, or with a congenital predisposition to allergic reactions. The death of such organisms has a beneficial effect on the genetic structure of host populations, removing alleles that reduce viability. Thus, the relationship between the populations of hosts and parasites contributes to their stability and simultaneously acts as a factor of natural selection, reducing the nonspecific genetic load of the host population.

In addition, the loss of most susceptible host individuals is accompanied by the loss of a part of the parasite population. This causes the evolutionary processes in the remaining part of the parasite population contributing to the appearance of new adaptations.

At present there is no relation between parasitism and natural selection in human populations because off the social nature of human. However fighting against parasites became an important factor in the evolution of the parasites themselves.

The study of parasite distribution in host populations shows that it depends on many factors, one of which is the age of the host. A number of parasites are more common in adults. Indeed, adult organisms are more likely to come into contact with many parasites and be invaded. For example, Trichomonas vaginalis affects only sexually mature people, because it is transmitted only by sexual intercourse. In cause of diphyllobothriasis and opisthorchiasis, adults become infected by eating poorly cooked fish. Such way of invasion is unlikely for children because it is more common among middle-aged people like hunters, travelers, participants in geological exploration parties. Invasion of adult individuals often characteristics for intermediate hosts: adult large fish have more opportunities to become invaded by metacercariae of flukes or plerocercoids of tapeworms than small fish. However, some parasites are more common in children. The reasons for this are varied. Firstly, this is due to immaturity of the immune system of children, and secondly, due to bad personal hygiene skills. Therefore, children are characterized by a specific range of parasite diseases like lambliasis, enterobiasis, hymenolepiasis, ascaridosis. Invasion is carried out simply by swallowing cysts, eggs or larvae, without the participation of intermediate hosts. More frequent invasion of children is facilitated by their close and prolonged contact with each other in children's institutions.

The probability of invasion is often related with the profession. So, workers of meat cutting plants are usually invaded with teniasis, miners and workers of agricultural enterprises – with ancylostomiasis in temperate climate and in the tropics. Diphyllobothriasis is more often in fishermen, and alveococcosis – in hunters and persons processing fur raw materials.

Genetic polymorphism of human populations is a factor from which depends invasion also. Thus, people with AB blood group are most sensitive to invasion with lymphatic filariasis. Malaria affects first of all people who do not have sickle cell mutation.

Some parasites, especially the protozoa, as well as pinworm, dwarf threadworm, dwarf tapeworm and trichinella are able to reproduce into the human body. Larvae of flukes and some tapeworms are able to reproduce in intermediate hosts. All this contributes to the fact that in host population parasites are not distributed evenly, but are scattered. This means that in a smaller part of the host population most of the parasite population is concentrated. The scattered distribution of the parasite in the host population should be taken into account when carrying out preventive measures. Indeed, to survive a parasite population needs to have only a few individuals or even one organism carrying a very large number of parasites.

The presence of a large number of endoparasites of one species into the host body causes intraspecific competition that is manifested in the congestion effect. In this case, the average size of each individual parasite and the reproductive potential are reduced. The survival rate of new parasites in the host decreases but the time of maturation and duration of the life cycle are increased.

Parasites of different species can simultaneously be present in the host organism. They interact with each other, so that different combinations of species of parasites can both strengthen and weaken negative effects.

In relation with different needs for the habitat conditions of different species of parasites in one host, their inter-species relationships are relatively weakened. Competitive interactions are found only between very close species.

At present the extensity and intensity of invasion are widely used in parasitology.

Extensity of invasion, or the occurrence of parasites (EI), is the percentage of invaded hosts by a particular species or group of parasites: EI = Np / n × 100%, where Np is the number of invaded hosts; n is the total number of hosts.

Example: in the 5 of 20 investigated miсe were found nematodes Heligmosomum mixtum, EI H. mixtum is = 25%.

Intensity of invasion (II) is the mean number of parasites per one invaded host: II = Par / Np, where Par is the number of parasites in the Np invaded by this parasite hosts.

Example: in the 5 invaded mice, 8 nematodes H. mixtum were found. Intensity of invasion by H. mixtum is 1.6 parasites.

6. The concept of parasitocenosis. Mix-invasion: examples of synergy and antagonistic relations between parasites of a single parasitocenosis [1, Р.194].

Parasitocenosisis a historically developed complex of parasites and other symbionts that live in the human body, animals, or plants. The concept was offered in 1937 by E.N. Pavlovsky. Nowadays, the term “parasitocenosis” is used exclusively in the national medical parasitology. General, veterinary and phytoparasitology use the term "parasitic system" instead of the term "parasitocenosis". The term "parasitocenosis" is not used also in foreign literature.

Parasitocenosis is formed by parasites and hosts populations. In multiple parasitic systems, parasitocenosis also includes populations of vectors. A particular case of parasitocenosis is an organism of host populated by parasites. The parasitocenosis of single host can include many species and many individuals of parasites. In humans, invasion with parasites of several species of helminths and protozoa is found in tropical countries.

The number of species of the parasites does not remain constant in time and vary even among the hosts of the same species. It is well known that parasites are distributed unevenly in the human groups affected by helminths. As a rule, almost 90% of all parasites are located in the body of only a few individuals, whereas only a few helminths are found in other members of the group. This uneven distribution is determined by the characteristics of the host and parasite. By V.D. Belyakov's definition, parasitocenosis is a dynamic self-regulating system.

The structural and functional organization of parasitocenosis affects the morphological and physiological characteristics of parasites, their reproduction, life span and pathogeny. For instance, dysenteric amoeba is pathogenic only in the presence of intestine microflora.

The presence of parasites in the host body can increases the susceptibility to other pathogens. For example, a higher incidence of acute intestinal infections is observed in case of intestinal helminthiasis. Helminthes also play an overlaid role in the infections of typhus, hepatitis, tuberculosis. All these cases refer to mix-invasions (infections). On the other hand, cases of antagonistic action of intestine microflora on the growth and development of parasites are noted. For example, some cases of enterobiosis are cured with the addition of sour-milk products containing live bifidobacterial cultures.

The interaction of the parasite with the host organism and, in particular, with its microflora, is a complex process. The study of parasitocenosis represents one direction of modern parasitology - the theory of parasitic systems as components of biocenosis. Knowledge of the relationships within the parasitocenosis is very important for the doctor, as it allows him to increase the success of treatment.

Познавательные статьи:



Алгоритмические операторы Matlab

Конструирование и порядок расчёта дорожной одежды

Исследования учёных: почему помогают молитвы?

Почему терпят неудачу многие предприниматели?