Life cycle- this is the whole of all stages of development "from egg to egg", and if there is no egg, from any stage to a similar one. The main biological aspect of the life of any organism is the adaptation that ensures the preservation of individuals and species.
In parasites (unlike free-living animals), nutrition is provided without interruption, and therefore the reproductive activity of the body increases. The increased reproduction of the parasite due to the spatial and temporal limits of its habitat leads to a rapid overpopulation in this place and the associated need for the relocation of the species for its conservation.
The whole of all stages of the parasite's ontogeny and its transmission path from one host to another is called its life cycle.
Cyclic form
Having known that parasites are organisms that use other living things for their development, it is important to understand the options for the development of life that exist. 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. These can be vertebrates, fish, shellfish, etc. An example is helminths.
The developmental cycle of whipworms does not require an intermediate host.
After entering the final host, the parasite grows and multiplies. Depending on the type of pathogen, the larvae can remain in or leave the body. In most cases, excretion occurs through the intestines. This makes it possible to determine the type of pathogen through a simple test.
Characteristics of cycle stages
Each stage of development has its own characteristics. Even the 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 easier to get rid of mature parasites.
Intermediate and late carriers of the parasite depend on the type of helminthiasis.
In this regard, let's see how the development cycle unfolds:
- Dissemination - 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 cases, intermediate hosts are used for further development and nutrition.
- Active growth - having reached the most suitable conditions, the parasite stops, repairs itself if there is a suitable device, and begins its growth to a sexually mature state.
- Migration to other habitats - after mature individuals breed eggs, they in most cases migrate for further development. They can be distributed in different ways. Often, parasites migrate through the digestive system with the food mass. There are also some that, because of 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 need a second partner for reproduction. The most notable example is the tapeworm, where each strobila has a uterus that produces mature eggs.
Important Concepts
The first thing to highlight 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. "Intermediate hosts" stand out separately. In this case, the pathogen remains in the body until it has the opportunity to migrate to the most favorable environment, provided by the last host.
Cycles can occur with 1-4 host changes. In this case, the first is intermediate and the rest are additional. Through direct contact or through an intermediate host, the parasite enters the final host. This is where sexual development and reproduction takes place.
Development of the parasite begins when it enters the final host.
There are also concepts such as reservoir parasitism and host feeders. In the first case, we are talking about a situation where the parasite, having reached a suitable state, can remain unchanged for a long time, waiting for a better solution option.
Owners-suppliersare organisms that are used exclusively as food. The easiest option is pliers. By thinking about how these types of parasites 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 time.
The concept of "parasite reservoir" or "reservoir master" is also distinguished. This is the host in which the pathogen can live for a long time, accumulate, multiply and spread throughout the surrounding area.
Parasite biology
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 people pose a danger to others.
The parasite and its host influence each other.
Harmful effects of parasites on the host:
- mechanical;
- Toxic;
- food production;
- Violation of tissue integrity.
Thus, the host's body "gives" a response to the parasite's influence.
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 commonly experienced by humans and animals.
Medical parasitology includes 3 main parts:
- Protozoan parasites - protozoology.
- Parasitic worms, helminths - helminthology.
- Arthropoda - arachnology.
Life cycle stages
In most cases, protozoa have special stages adapted to carry out the transitional phase from one host to another. This stage is called propagative.
In intestinal parasitesreproductive stageusually adapted to experience in the external environment. Most intestinal protozoa form cysts covered with a dense membrane. When the cysts of some species (Entamoeba histolytica, E. coli, Lamblia intestinalis, etc. ) mature, several successive divisions of the nucleus occur.
After hitting ripemultinucleate cystIn the new host, the cytoplasm divides to form several individuals. The cyst is usually supplied with a supply of nutrients, which are consumed during the maturation process and when the cyst remains in the external environment. The reproductive stage of the coccidia is the membrane-bound fertilized female germ cell (oocyst).
Most protozoa are parasitesvertebrate tissues and blood are transmitted from one host to another using vectors. The propagative stage in this case is localized in the blood or in the outer integument of vertebrates. The causative agent of Chagas disease, Trypanosoma cruzi, multiplies in the leishmanial phase in the cells of internal organs. The leishmanial parasite forms into trypanosomes, which penetrate the bloodstream, but do not multiply in it.
Transmission of infectionoccurs through vectors, blood-sucking bugs. The causative agent of Indian visceral leishmaniasis (kala-azar), Leishmama donovani, multiplies in the histophagocytic cells of tissues that are less accessible to the vector. However, at the final stage of the process, late leishmanoids - lesions containing large numbers of leishmania - can form on the patient's skin. In some cases, with this disease, leishmania is also found in the blood. The reproductive stage of the malaria parasite is the gamont that circulates in the host's bloodstream.
Withreproductive stagein the life cycle of tissue parasites there are so-calledinvasive stage, adapted for penetration into vertebrate hosts. Therefore, 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 stage of the malaria parasite is the sporozoite.
A group of helminths
Each type of helminth develops only in certain conditions. Depending on the state of development, parasitic worms are divided into two large groups:biohelminthsAndgeohelminths.
Biohelminths
TobiohelminthsThis includes parasites that develop with the participation of two or more organisms. In one organism lives the form of an adult worm, in another - the larval stage.
Organisms in which adults form parasites and sexual reproduction occurs are calledfinal(or definitive) owner.
The organism in which the larval form develops isintermediatethe owner. For example, the adult bovine tapeworm is a parasite in the human intestine, and its larval development takes place in the body of the cow.
Therefore, for this tapeworm, humans are the definitive host and cattle are the intermediate hosts.
Biohelminths include most representatives of the flatworm type.
Geohelminths
Geohelminthsis a parasite that does not require a change of host during its development. Their eggs are removed from the body along with feces to the external environment and, at a certain temperature and humidity, the larvae develop inside.
Such eggs containing larvae become infectious. Once in the human body (in the intestine), the larva is released from the egg shell, penetrates certain organs and develops into a sexually mature form. In some helminths, larvae are released from eggs in the external environment. Such larvae live in water or in the soil, undergo a certain stage of development and then actively penetrate the body through the skin.