Life cycle- it 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. The main biological aspects of the life of any organism are adaptations that ensure the preservation of the individual and the species.
In parasites (unlike free-living animals), nutrition is provided uninterruptedly and therefore the reproductive activity of the body increases. Increased reproduction of the parasite due to spatial and temporal limitation of its habitat leads to rapid overpopulation of that location and the associated need for resettlement of the species for its conservation.
The set of all stages of the ontogeny of the parasite and the ways of its transmission from one host to another is called its life cycle.
Cycle forms
Having discovered that parasites are organisms that use other living things 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 ownership. Examples include the development of roundworms, amoebas, whipworms, etc. A complex group includes multiple hosts at a time. These can be vertebrates, fish, crustaceans, 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 may stay inside or leave the body. In most cases, excretion occurs through the intestines. This makes it possible to determine the type of pathogen using simple tests.
Characteristics of cyclical steps
Each stage of development has its own characteristics. Even the treatment is determined solely on the basis of this factor. This is explained by the fact that, for example, not all drugs act 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 regard, let's look at how the development cycle takes place:
- Dispersion - 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 nopotential final host. In such a situation, the intermediate host is used for further development and nutrition.
- Active growth - after reaching the most suitable conditions, the parasite stops, attaches if suitable devices are available and begins growth to a state of sexual maturity.
- Migration to another habitat – after a mature individual has reproduced its eggs, it in most cases migrates to continue its development. They can be distributed in different ways. Most often, parasites migrate through the digestive system with the food mass. There are also those that, due to their size, easily enter 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 that of the tapeworm, in which each strobila has a uterus that reproduces mature eggs.
Important concepts
The first thing to emphasize when learning about parasites is the concept of "host". This is an organism in which the development and reproduction of parasites occurs. The "middle host" stands out separately. In this case, the pathogen remains inside the body until it has the opportunity to migrate to the most favorable environment provided by the final host.
The cycle can occur with a change of 1 to 4 hosts. In this case, the first one is intermediate and the others are supplementary. Through direct contact or via an intermediate host, parasites enter the final host. This is where development and sexual reproduction take place.
The development of the parasite begins upon entry into the final host.
There are also concepts such as reservoir parasitism and host-feeder. 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.
Supplier Owneris an organism used exclusively as food. The simplest option is pliers. By understanding how parasites of this type feed, it is possible to understand that they require human blood to exist, but do not stay in or on the human body for long.
We also distinguish the notion of "parasitic reservoir" or "host reservoir". This is a host in whose body the pathogen can live for a long time, accumulating, multiplying and spreading throughout the surrounding area.
Parasite biology
Parasitic carriage is considered separately - in the case of pathogenic parasites living 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.
Adverse effects of the parasite on the host:
- Mechanical;
- Toxic;
- Food withdrawal;
- Violation of tissue integrity.
As a result, the host 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:
- Anthroponotics – 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 particular stages adapted to carry out the transition phase from one host to another. These stages are called propagative.
In intestinal parasitesstages of propagationgenerally suited to experience in the outdoor environment. Most intestinal protozoa form cysts covered by a dense membrane. When cysts of several species (Entamoeba histolytica, E. coli, Lamblia intestinalis, etc. ) mature, several successive divisions of the nucleus occur.
After banging a maturemultinucleated 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 propagation stage of coccidia is a fertilized female germ cell (oocyst) covered by a membrane.
Most parasitic protozoaVertebrate tissues and blood are transmitted from one host to another using a vector. The propagation stages are in this case located in the blood or in the external integument of the vertebrate. The causative agent of Chagas disease, Trypanosoma cruzi, multiplies during the Leishmanian phase in the cells of internal organs. Leishmanial forms of the parasite transform into trypanosomes which enter the bloodstream but do not reproduce there.
Transmission of infectionoccurs via a vector, a blood-sucking insect. The causative agent of Indian visceral leishmaniasis (kala-azar), Leishmama donovani, multiplies in the histophagocytic cells of tissues that are poorly accessible to the vector. However, at a late stage of the process, late leishmanoids - a lesion containing a large number of leishmania - can form on the patient's skin. In some cases, with this disease, leishmania is also found in the blood. The stages of spread of malaria parasites are gamonts that circulate in the bloodstream of the host.
Withstages of propagationin the life cycle of tissue parasites there is 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.
Helminth groups
Each type of helminths develops only under certain conditions. Depending on the conditions of development, parasitic worms are divided into two large groups:biohelminthsAndgeohelminths.
Biohelminths
HASbiohelminthsThese include parasites that develop with the participation of two or more organisms. In one organism live the adult forms of the worm, in the other, the larval stages.
An organism in which adult forms parasitize and sexual reproduction occurs is calledfinalowner (or definitive).
The organism in which the larval forms develop isintermediatethe owner. For example, the adult bovine tapeworm is a parasite of the human intestine, and the development of its larvae occurs in the body of livestock.
Thus, for this tapeworm, man is the definitive host and the cow the intermediate host.
Biohelminths include most representatives of the flatworm type.
Geohelminths
Geohelminthsare 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 there.
Such an egg containing a larva becomes contagious. Once inside the human body (in its intestines), the larvae break free from the egg shells, enter certain organs and develop until they reach 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 then actively enters the body through the skin.