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 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 continuously and therefore the reproductive activity of the body increases. 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 related need to relocate the species for its preservation.
The set of all stages of the ontogeny of the parasite and the ways of its transmission from one host to another is called the life cycle.
Forms of cycles
Having understood that parasites are organisms that use other living things for their development, it is important to understand what possibilities for the development of life exist. According to the classification, there are simple and complex cycles. The first occurs without a change of owner. Examples include the development of roundworm, amoeba, whipworm, etc. The complex group includes several hosts at once. These can be vertebrates, fish, shellfish, etc. An example of this is helminths.
The development cycle of worms does not require an intermediate host.
Once inside the final 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 occurs through the intestines. This makes it possible to determine the type of pathogen through simple tests.
Characteristics of cyclic stages
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.
On that note, let's take a look at how the development cycle works:
- 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, that is, there is no potential final host. In such a situation, the intermediate host is used for further development and feeding.
- Active growth - after reaching the most suitable conditions, the parasite stops, is fixed in the presence of suitable devices and begins its growth to a sexually mature state.
- Migration to another habitat - after a mature individual reproduces 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 mass. There are also those that, due to their size, easily penetrate the blood and spread throughout the body.
- Asexual reproduction - some types of parasites are distinguished by the fact that they do not need a second partner for reproduction. The most striking example is the tapeworm, where 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 parasites develop and reproduce. The "intermediate 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 when 1-4 hosts change. In this case, the first is intermediate, and the rest are additional. Through direct contact or through an intermediate host, parasites enter the definitive host. This is where development and sexual reproduction takes place.
The development of the parasite begins with its entry into the final host.
There are also such concepts as reservoir parasitism and host-feeder. In the first case, we are talking about a situation where the parasite, having reached suitable conditions, can remain unchanged for a long time, waiting for a more favorable settlement option.
Supplier Owneris an organism that is used exclusively as food. The simplest option is pliers. By understanding how parasites of this type feed, it is possible to understand that they need human blood to exist, but do not remain in or on the human body for a long period of time.
The concept of "parasitic reservoir" or "host reservoir" is also distinguished. It is a host in whose body the pathogen can live for a long time, accumulating, multiplying and spreading in the surroundings.
Biology of parasites
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 is a danger to others.
The parasite and its host influence each other.
Harmful effects of the parasite on the host:
- Mechanical;
- Toxic;
- Refusal of food;
- Violation of tissue integrity.
Accordingly, 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:
- Anthroponous - humans act as a host;
- Zoonosis - different animals act as hosts;
- Anthropozoonous 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 parasitespromotional stagesusually adapted to experience in the external environment. Most intestinal protozoa form cysts covered with a dense membrane. During the maturation of cysts of a number of species (Entamoeba histolytica, E. coli, Lamblia intestinalis, etc. ), several successive divisions of the nucleus occur.
After hitting ripemultinucleated cystIn the new host, the cytoplasm divides to form several individuals. Cysts are usually supplied with nutrients that are used up in the maturation process and when the cyst remains in the external environment. The propagative stage of coccidia is a membrane-encased fertilized female germ cell (oocyst).
Most parasitic protozoatissue and blood of vertebrates is transmitted from one host to another by means of a vector. The propagative stages in this case are localized in the blood or in the outer shell of the vertebrate. The causative agent of Chagas disease, Trypanosoma cruzi, multiplies in the leishmanial phase in the cells of the internal organs. The leishmanial forms of the parasite become trypanosomes that penetrate the blood, but do not multiply in it.
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 can 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 malarial parasites are gamonts that circulate in the host's bloodstream.
Together withpromotional stagesin the life cycle of tissue parasites there are so-calledinvasive stages, adapted to penetrate 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 for development 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 live the adult forms of the worm, in the other - the larvae.
It is called an organism in which adult forms are parasitized and sexual reproduction takes placefinal(or ultimate) owner.
The organism in which the larval forms develop isintermediateowner. 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.
Thus, 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 released from the body together with feces into the external environment and at a certain temperature and humidity, larvae develop in them.
Such an egg containing a larva becomes infective. Once inside the human body (in its intestines), the larvae are released from the egg shells, penetrate into certain organs and grow into sexually mature form. In some helminths, the larva is released from the egg into the external environment. Such a larva lives in water or in the soil, goes through certain stages of development and subsequently actively penetrates the body through the skin.