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 one. The main biological aspects of the life of any organism are adaptations that ensure the preservation of the individual and the species.
In the case of parasites (unlike free-living animals), nutrition is provided continuously, thus increasing the reproductive activity of the organism. 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 associated need to relocate the species for its preservation.
The totality of all stages of the parasite's ontogeny and the ways of its transmission from one host to another is called its life cycle.
Forms of cycles
Since we have learned that parasites are organisms that use other living beings for their development, it is important to understand what possibilities 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, flagellates, and so on. A complex group includes several hosts at once. These can be vertebrates, fish, shellfish and so on. An example is helminths.
The flagellar development cycle does not require an intermediate host.
After entering the definitive host, the parasites grow and multiply. Depending on the type of pathogen, the larvae can either stay inside or leave the body. In most cases, excretion takes place through the intestines. This makes it possible to determine the type of pathogen with simple tests.
Characteristics of cyclic phases
Each stage of development has its own characteristics. Even treatment is determined based on this factor alone. 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.
With that said, let's take a look at how the development cycle unfolds:
- Dispersion - this cycle exists when an 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 - having reached the most suitable conditions, the parasite stops, is fixed if there are suitable devices and begins its growth to the sexually mature state.
- Migration to another habitat - after the mature individual reproduces the eggs, they in most cases migrate for further development. They can be distributed in different ways. Most often, parasites migrate through the digestive system with food. There are also those that, due to their size, easily penetrate into the bloodstream and spread throughout the body.
- Asexual reproduction - some types of parasites are characterized by the fact that they do not need another partner for reproduction. 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 emphasized when getting acquainted with parasites is the concept of "host". This is the organism in which the development and reproduction of the parasite takes place. The "intermediate host" stands out. 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.
A cycle can occur with a change of 1-4 hosts. In this case, the first one is intermediate and the others are additional. 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 when it enters 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 the appropriate conditions, can remain unchanged for a long time, waiting for a more favorable settlement option.
Provider-Owneris 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 realize that they need human blood to exist, but they do not stay long in or on the human body.
The concept of "parasite reservoir" or "host reservoir" is also distinguished. This is the host in whose body the pathogen can live for a long time, accumulating, multiplying and spreading in the environment.
Biology of parasites
Carrying 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 affect each other.
Harmful effects of the parasite on the host:
- Mechanically;
- Toxic;
- Food withdrawal;
- Injury to tissue integrity.
Consequently, the host's body "gives" a response to the parasite's influence.
Infections caused by parasites can be divided according to the sensitivity of the pathogen to the host:
- Anthroponotic - people act like hosts;
- Zoonosis - 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 stage of transition from one host to another. These stages are called propagative.
In intestinal parasitespropagative phasesusually adapted to experience in the outdoor environment. Most intestinal protozoa form cysts covered with a thick membrane. When the cysts of numerous species (Entamoeba histolytica, E. coli, Lamblia intestinalis, etc. ) mature, several successive nuclear divisions occur.
After hitting the ripemultinucleate cystIn the new host, the cytoplasm divides to form several individuals. Cysts are usually supplied with stores of nutrients, which are used up during the maturation process and when the cyst remains in the external environment. The reproductive stage of coccidia is a fertilized female germ cell (oocyst) covered with a membrane.
Most parasitic protozoavertebrate tissue and blood is transmitted from one host to another by means of a vector. The reproductive stages in this case are localized 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. The leishmanial forms of the parasite turn into trypanosomes, which penetrate the bloodstream, but do not reproduce there.
Transmission of infectionoccurs through a vector, a blood-sucking bug. 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, a late leishmanoid - a lesion containing a large number of leishmania - can form on the patient's skin. In some cases, along with this disease, leishmania is also found in the blood. The reproductive stages of the malaria parasite are gamonts that circulate in the host's bloodstream.
Together withpropagative phasesin the life cycle of tissue parasites there are so-calledinvasive stages, adapted to penetrate the 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 for development in the 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:biohelminthsIgeohelminths.
Biohelminths
ThatbiohelminthsThis includes those parasites that develop with the participation of two or more organisms. Adult forms of worms live in one organism, larval stages in the other.
An organism in which the adult forms parasitize and reproduce sexually is calledthe final(or definitive) owner.
The organism in which the larval forms develop ismediumthe owner. For example, an adult bovine tapeworm is a parasite in the human intestine, and the development of its larvae takes place in the body of cattle.
So, for this tapeworm, humans are the definitive host and cows are the intermediate host.
Biohelminths include most representatives of the type of flatworms.
Geohelminths
Geohelminthsare those parasites that do not require a change of host during their development. Their eggs are excreted from the body together with feces into the external environment, and larvae develop in them at a certain temperature and humidity.
Such an egg containing a larva becomes infectious. Once inside the human body (in its intestines), the larvae break free from the egg shell, penetrate into 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 then actively penetrates the body through the skin.