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 nearest similar one. The main biological aspects of the life of any organism are adaptations that ensure the preservation of the individual and species.
In parasites (unlike free-living animals), nutrition is provided uninterruptedly, 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 associated need for the resettlement of the species for its conservation.
The totality of all stages of the ontogeny of the parasite and the routes of its transmission from one host to another is called its life cycle.
Forms of cycles
Having found out that parasites are organisms that use other living beings for their development, it is important to understand what options for life development 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, 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 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 either remain 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.
Features of cyclic stages
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 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 proceeds:
- Dispersal - this cycle exists when the intermediate host, which is the source, but not the final stage, is considered to be 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 nutrition.
- Active growth - having reached the most suitable conditions, the parasite stops, fixes itself if there are 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 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 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 highlighted 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" stands out 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 1-4 hosts. In this case, the first one is intermediate and the rest are additional. Through direct contact or through 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 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.
Provider-owneris an organism that is used exclusively as food. The simplest option is pliers. By figuring out how parasites of this type feed, it is possible to understand that they need human blood to exist, but they do not stay 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 area.
Biology of parasites
Parasite 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.
Harmful effects of the parasite on the host:
- Mechanical;
- Toxic;
- Food withdrawal;
- Violation of tissue integrity.
Accordingly, 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 the host;
- 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 stagesusually adapted to experience in the external environment. Most intestinal protozoa form cysts covered with a dense membrane. When cysts of a number of species (Entamoeba histolytica, E. coli, Lamblia intestinalis, etc. ) mature, several successive divisions of the nucleus occur.
After hitting a maturemultinucleate cystIn the new host, the cytoplasm divides to form several individuals. Cysts are usually supplied with a supply of nutrients, which are consumed during the process of maturation and when the cyst remains in the external environment. The propagative stage of coccidia is a membrane-clad fertilized female germ cell (oocyst).
Most parasitic protozoanstissue and blood of vertebrates is transmitted from one host to another using a vector. Propagative stages in this case are localized in the blood or in the outer 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 turn into trypanosomes, which penetrate the bloodstream, but do not reproduce 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, late leishmanoid - 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 propagative stages of malaria parasites are gamonts that circulate in the host's bloodstream.
Along withpropagative stagesin the life cycle of tissue parasites there are 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 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 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. 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 calledfinal(or definitive) owner.
The organism in which the larval forms develop isintermediatethe 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.
Thus, for this tapeworm, humans are the definitive host and cows are the intermediate host.
Biohelminths include most representatives of the flatworm type.
Geohelminths
Geohelminthsare those parasites that do not require a change of hosts 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 his intestines), the larvae are freed from the egg shells, 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 in soil, undergoes certain stages of development and subsequently actively penetrates the body through the skin.