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 animals), food is provided continuously and therefore the reproductive activity of the organism increases. The increased reproduction of the parasite due to the spatial and temporal limitation of its habitat leads to the rapid overpopulation of this place and the accompanying need for the restoration of the species for its preservation.
The totality of all stages of the parasite's ontogenesis and its transmission routes from one host to another is called its life cycle.
Forms of cycles
Having discovered that parasites are organisms that use other living beings for their development, it is important to understand what options 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. A complex group includes several hosts at the same time. These can be vertebrates, fish, shellfish, etc. An example is helminths.
The development cycle of whipworms does not require an intermediate host.
After entering the definitive host, the parasites grow and multiply. Depending on the type of pathogen, the larvae may 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.
Characteristics of cyclic phases
Each stage of development has its own characteristics. Even the treatment is determined only on the basis of 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 see how the development cycle continues:
- Distribution - 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 possible 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, adjusts if it has suitable equipment and begins its growth in 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 to note when learning about parasites is the concept of the "host". This is an organism in which the development and reproduction of parasites takes place. "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 is intermediate and the rest are additional. Through direct contact or via an intermediate host, parasites enter the final host. This is where sexual development and reproduction takes place.
Development of the parasite begins when it enters the definitive host.
There are also concepts such 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 solution option.
The provider-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 stay in or on the human body for a long period of time.
The concept of "reservoir parasite" 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
The transport 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 a person poses a danger to others.
The parasite and its host affect each other.
Harmful effects of the parasite on the host:
- Engineering;
- Toxic;
- Food withdrawal;
- Violation of tissue integrity.
Therefore, 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 - humans act as hosts;
- Zoonotic - different 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 transition from one host to another. These stages are called propagation.
In intestinal parasitesstages of propagationusually adapts to experience in the outdoor environment. Most intestinal protozoa form cysts covered with a dense membrane. When the cysts of a number of species (Entamoeba histolytica, E. coli, Lamblia intestinalis, etc. ) mature, several successive divisions of the nucleus occur.
After hitting a roastmultinucleate 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 maturation process and when the cyst remains in the external environment. The propagating stage of coccidia is a fertilized, membrane-encased female cell (oocyst).
Most parasitic protozoavertebrate tissue and blood is transmitted from one host to another using a vector. The stages of spread in this case are localized in the blood or in the external integrity 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 in it.
Transmission of infectionit occurs through a vector, a blood-sucking insect. The causative agent of Indian visceral leishmaniasis (kala-azar), Leishmama donovani, multiplies in histophagocytic cells of tissues that are not 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 proliferative stages of malaria parasites are gamonts that circulate in the host's bloodstream.
Together withstages of propagationin 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 conditions of development, parasitic worms are divided into two large groups:biohelminthsANDgeohelminths.
Biohelminths
for herbiohelminthsThese 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 calledfinaleowner (or final).
The organism in which the larval forms develop isintermediatethe ownerFor example, the adult tapeworm of cattle 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 flatworm.
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, at a certain temperature and humidity, the larvae develop in them.
Such an egg containing a larva becomes infective. Once in the human body (in his intestines), the larvae are released from the eggshells, 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 on land, undergoes certain stages of development and then actively penetrates the body through the skin.