Life cycle- This is the totality of all stages of development "from egg to egg" and, in the absence of an egg, from each stage to the next similar one. The most important biological aspects of the life of an organism are adaptations that ensure the preservation of the individual and the species.
In parasites (unlike free-living animals), nutrition is provided continuously and therefore the reproductive activity of the body is increased. The increased reproduction of the parasite due to the spatial and temporal limitations of its habitat leads to rapid overpopulation of this place and the associated need to relocate the species for its conservation.
The totality of all stages of the ontogeny of the parasite and the routes of transmission from one host to another is called the life cycle.
Forms of cycles
After finding out that parasites are organisms that use other living things for their development, it is important to understand what possibilities for life development there are. According to the classification, there are simple and complex cycles. The first occurs without a change of ownership. Examples include the development of roundworms, amoebas, whipworms, etc. A complex group includes several hosts at once. These can be vertebrates, fish, shellfish, etc. An example is helminths.
The development cycle of whipworms does not require an intermediate host.
After penetrating the final host, the parasites grow and multiply. Depending on the type of pathogen, the larvae can either remain in the body or leave the body. In most cases, excretion occurs via the intestines. This makes it possible to determine the type of pathogen using simple tests.
Characteristics of cyclic phases
Each stage of development has its own characteristics. Even treatment is determined only based on this factor. This is explained by the fact that, for example, not all drugs work on larvae, while it is much easier to get rid of adult parasites.
The intermediate and final carrier of the parasite depends on the type of helminthiasis.
In this context, let's look at how the development cycle works:
- Propagation - This cycle occurs when the intermediate host, which is the source but not the final stage, is currently considered the only option, i. e. H. , there is no potential definitive host. In such a situation, the intermediate host is used for further development and nutrition.
- Active Growth – Once the parasite has reached the most suitable conditions, it stops, repairs itself if appropriate tools are available, and begins growth to sexual maturity.
- Migration to another habitat - after an adult reproduces eggs, in most cases it migrates for further development. They can be distributed in different ways. Most often, parasites travel through the digestive system with food mass. There are also those that, due to their size, easily penetrate the bloodstream and spread throughout the body.
- Asexual reproduction – some types of parasites are characterized by the fact that they do not need a second partner to reproduce. The most striking example is the tapeworm, in which each strobila has a uterus that reproduces mature eggs.
Important concepts
The first thing that should be highlighted when getting acquainted with parasites is the concept of "host". This is an organism in which parasites develop and multiply. The "intermediate host" stands out separately. In this case, the pathogen remains in the body until it has the opportunity to migrate to the most favorable environment offered by the definitive host.
The cycle can occur with a change of 1-4 hosts. In this case, the first is medium difficulty and the rest are additional. Parasites invade the final host through direct contact or via an intermediate host. This is where development and sexual reproduction take place.
The development of the parasite begins with penetration into the definitive host.
There are also concepts such as reservoir parasitism and host predation. In the first case, we are talking about a situation in which the parasite, having reached suitable conditions, can remain unchanged for a long time, waiting for a more favorable settlement option.
Provider Owneris an organism that functions exclusively as food. The easiest option is pliers. If one finds out how parasites of this species feed, one can understand that they require human blood to exist, but do not remain in or on the human body for a long period of time.
The term "parasite reservoir" or "reservoir host" is also differentiated. This is a host in whose body the pathogen can accumulate for a long time, multiply and spread throughout the environment.
Biology of parasites
The transmission of parasites is considered separately - in the case of pathogenic parasites that live in the human body, but the development of the disease does not occur. However, such a person poses a danger to others.
The parasite and its host influence each other.
Harmful effects of the parasite on the host:
- Mechanically;
- Poisonous;
- food deprivation;
- Violation of tissue integrity.
Accordingly, the host's body "reacts" to the influence of the parasite.
Infections caused by parasites can be divided according to the susceptibility of the pathogen to the host:
- Anthroponotic – humans act as hosts;
- Zoonoses – various animals act as hosts;
- Anthropozoonotic diseases are invasive and infectious diseases that commonly occur in humans and animals.
Medical parasitology includes three main sections:
- Protozoan Parasites – Protozoology.
- Parasitic worms, helminths - Helminthology.
- Arthropods - Arachnology.
Phases of the life cycle
In most cases, protozoa have specialized stages suitable for transition from one host to another. These phases are called propagative.
For intestinal parasitesReproductive stagesusually adapted to experiences in the external environment. Most intestinal protozoa form cysts covered with a dense membrane. As cysts of a number of species (Entamoeba histolytica, E. coli, Lamblia intestinalis, etc. ) mature, several successive nuclear divisions occur.
After I hit a ripe onemultinucleated cystIn the new host, the cytoplasm divides to form multiple individuals. Cysts are typically supplied with nutrients that are consumed during the maturation process and when the cyst remains in the external environment. The reproductive stage of coccidia is a membrane-covered fertilized female germ cell (oocyst).
Most parasitic protozoaVertebrate tissue and blood are transferred from one host to another using a vector. Reproductive stages in this case are localized in the blood or external skin of the vertebrate. The causative agent of Chagas disease, Trypanosoma cruzi, multiplies in the cells of internal organs during the leishmania phase. Leishmanic forms of the parasite transform into trypanosomes, which enter the bloodstream but do not multiply there.
Transmission of an infectionoccurs through a vector, a blood-sucking beetle. The causative agent of Indian visceral leishmaniasis (Kala-azar), Leishmama donovani, multiplies in histophagocytic cells of tissues that are poorly accessible to the vector. However, at a late stage of the process, late leishmanoid may form on the patient's skin - a lesion containing a large number of leishmania. In some cases, leishmania is also detected in the blood with this disease. The reproductive stages of malaria parasites are gamonts that circulate in the host's bloodstream.
Along withReproductive stagesIn the life cycle of tissue parasites there are so-calledinvasive stages, adapted for penetrating a vertebrate host. Thus, the development of representatives of the Trypanosoma genus in the vector ends with the formation of metacyclic trypanosomes, which no longer reproduce in the vector and are adapted for development in a vertebrate host.
The invasive stages of malaria parasites are sporozoites.
Groups of helminths
Each species of helminth only develops under certain conditions. Depending on the development conditions, parasitic worms are divided into two large groups:BiohelminthsAndGeohelminths.
Biohelminths
ToBiohelminthsThese include those parasites that develop with the participation of two or more organisms. The adult forms of the worm live in one organism and the larval stages live in the other.
An organism in which adult forms parasitize and sexual reproduction occurs is calledfinal(or ultimate) owner.
The organism in which the larval forms develop isin betweenthe owner. For example, the adult bovine tapeworm is a parasite in the human intestine, and the development of its larvae occurs in the body of cattle.
Therefore, for this tapeworm, the human is the definitive host and the cow is the intermediate host.
Biohelminths include most representatives of the flatworm species.
Geohelminths
Geohelminthsare parasites that do not require a change of host during their development. Their eggs are excreted from the body into the external environment along with feces, and at a certain temperature and humidity, larvae develop in them.
Such an egg containing a larva becomes infectious. In the human body (in its intestines), the larvae are freed from the eggshells, penetrate certain organs and grow into a sexually mature form. In some helminths, the larva is released from the egg in the external environment. Such a larva lives in water or soil, goes through certain stages of development and then actively penetrates the body through the skin.