Life cycle- this is the totality of all stages of development "from egg to egg", and in the absence of an egg - from any stage to the closest similar one. The main biological aspects of the life of any organism are the adaptations that ensure the preservation of the individual and the species.
In parasites (unlike free-living animals), nutrition is provided on an uninterrupted basis and therefore increases the reproductive activity of the body. The increased reproduction of the parasite due to the spatial and temporal limitation of its habitat leads to rapid overpopulation of this place and the consequent need for resettlement of the species for its conservation.
The totality of all stages of parasite ontogeny and the routes of its transmission from one host to another is called the life cycle.
Shapes of cycles
Having discovered that parasites are organisms that use other living beings for their development, it is important to understand what options exist for the development of life. According to the classification, there are simple and complex cycles. The first occurs without a change of owner. Examples include the development of roundworms, amoebas, whipworms, etc. A complex group includes several hosts at once. They can be vertebrates, fish, shellfish, etc. An example is helminths.
The development cycle of whipworms does not require an intermediate host.
After entering the final host, the parasites grow and multiply. Depending on the type of pathogen, the larvae can remain inside or exit the body. In most cases, excretion occurs through the intestines. This allows the type of pathogen to be determined using simple tests.
Characteristics of cyclic stages.
Each stage of development has its own characteristics. Even the treatment is determined solely 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 mature parasites.
The intermediate and final carrier of the parasite depends on the type of helminthiasis.
In this sense, let's see how the development cycle progresses:
- Dispersal: This cycle exists when the intermediate host, which is the source but not the final stage, is considered the only option at the moment, i. e. there is no potential final host. In such a situation, the intermediate host is used for further development and nutrition.
- Active growth: once the most suitable conditions are reached, the parasite stops, repairs itself if suitable devices are available, and begins its growth to a state of sexual maturity.
- Migration to another habitat: After a mature individual reproduces eggs, in most cases it migrates for further development. They can be distributed in different ways. Most often, parasites migrate 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 distinguished by the fact that they do not require 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 to highlight when getting acquainted with parasites is the concept of "host". This is an organism in which the development and reproduction of parasites occurs. The "intermediate host" is highlighted separately. In this case, the pathogen remains inside the body until it has the opportunity to migrate to the most favorable environment, which is provided by the final host.
The cycle can occur with a change of 1 to 4 hosts. In this case the first is intermediate and the rest are additional. Through direct contact or through an intermediate host, parasites enter the final host. This is where sexual development and reproduction take place.
Parasite development begins when it enters the final host.
There are also concepts such as reservoir and host-feeder parasitism. In the first case, we are talking about a situation when the parasite, having reached suitable conditions, can remain unchanged for a long time, waiting for a more favorable solution option.
Supplier-ownerIt is an organism that is used exclusively as food. The simplest option is pliers. By discovering how parasites of this type feed, it is possible to understand that they need human blood to exist, but they do not remain in or on the human body for a long period of time.
The concept of "parasite reservoir" or "reservoir host" is also distinguished. This is a host in whose body the pathogen can live for a long time, accumulating, multiplying and spreading throughout the surrounding territory.
Biology of parasites.
The carriage 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 represents a danger to others.
The parasite and its host influence each other.
Harmful effects of the parasite on the host:
- Mechanic;
- Toxic;
- Food removal;
- Violation of tissue integrity.
Consequently, the host's body "gives" a response 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;
- Zoonotic: various animals act as hosts;
- Anthropozoonotic diseases are invasive and infectious diseases common to humans and animals.
Medical parasitology includes 3 main sections:
- Protozoan parasites - protozoology.
- Parasitic worms, helminths - helminthology.
- Arthropods - arachnology.
Life cycle stages
In most cases, protozoa have special stages adapted to carry out the transition phase from one host to another. These stages are called propagative.
In intestinal parasitespropagative stagesgenerally adapted to experience in the external environment. Most intestinal protozoa form cysts covered by a dense membrane. When cysts of various species (Entamoeba histolytica, E. coli, Lamblia intestinalis, etc. ) mature, several successive divisions of the nucleus occur.
After hitting a mature womanmultinucleated cystIn the new host, the cytoplasm divides to form several individuals. Cysts usually receive a supply of nutrients, which are consumed during the maturation process and when the cyst remains in the external environment. The propagative stage of coccidia is a fertilized female germ cell (oocyst) covered with a membrane.
Most parasitic protozoa.Vertebrate tissue and blood are transmitted from one host to another by a vector. The propagative stages in this case are located in the blood or in the external integument of the vertebrate. The causative agent of Chagas disease, Trypanosoma cruzi, multiplies in the leishmanial phase in the cells of internal organs. Leishmanial forms of the parasite become trypanosomes, which enter the bloodstream, but do not reproduce there.
Infection transmissionIt occurs through a vector, a blood-sucking insect. 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, late in the process, late leishmanoid, a lesion containing a large amount of leishmania, may form on the patient's skin. In some cases, with this disease, leishmania is also found in the blood. The propagative stages of malaria parasites are gamonts that circulate in the host's bloodstream.
Withpropagative stagesIn the life cycle of tissue parasites there are the so-calledinvasive stages, adapted for penetration into a vertebrate host. Thus, the development of representatives of the genus Trypanosoma in the vector ends with the formation of metacyclic trypanosomes, which no longer reproduce in the vector and are adapted to develop in a vertebrate host.
The invasive stages of malaria parasites are sporozoites.
Groups of helminths
Each type of helminth develops only under certain conditions. Depending on the conditions of development, parasitic worms are divided into two large groups:biohelminthsANDgeohelminths.
Biohelminths
TObiohelminthsThese include those parasites that develop with the participation of two or more organisms. In one organism the adult forms of the worm live, in the other the larval stages.
An organism in which the adult forms parasitize and sexual reproduction occurs is calledfinal(or definitive) owner.
The organism in which the larval forms develop isintermediateowner. For example, the adult bovine tapeworm is a parasite of the human intestine and the development of its larvae occurs in the body of cattle.
Thus, for this tapeworm, humans are the definitive host and cows are the intermediate host.
Biohelminths include most representatives of the flatworm type.
Geohelminths
GeohelminthsThey are those parasites that do not require a change of host during their development. Their eggs are excreted from the body along with feces into the external environment and, at a certain temperature and humidity, larvae develop in them.
Such an egg containing a larva becomes infectious. Once in the human body (in its intestines), the larvae break free from the egg shells, penetrate certain organs and grow into a sexually mature form. In some helminths, the larva is released from the egg into the external environment. Such a larva lives in water or soil, goes through certain stages of development and subsequently actively penetrates the body through the skin.