Circle of life- this is the set 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 any organism's life are 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 organism increases. The increased reproduction of the parasite due to the spatial and temporal limitation of its habitat leads to a rapid overpopulation of this place and the relative need to reintroduce the species for its preservation.
The set of all the stages of the parasite's ontogenesis and its transmission routes from one host to another is called its life cycle.
Shapes of circles
Having discovered that parasites are organisms that use other living things for their growth, it is important to understand what options for the development of life exist. According to the classification, there are simple and complex circles. The first happens without a change of owner. Examples include the development of roundworm, amoeba, flagella and so on. A complex group includes many hosts at the same time. 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 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 is 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 vector of the parasite depends on the type of helminthiasis.
With that in mind, let's see how the development cycle is progressing:
- Dispersion - 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, i. e. there is no possible final host. In such a situation, the intermediate host is used for further growth and nutrition.
- Active development - having reached the most suitable conditions, the parasite stops, fixes itself if there are suitable devices and begins its development in a sexually mature state.
- Migration to another habitat - after a mature individual reproduces eggs, in most cases it migrates for further development. They can be distributed in different ways. Most often, parasites migrate through the digestive tract with the 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 point out when getting familiar 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 1-4 host changes. In this case, the first is intermediate and the rest are additional. Through direct contact or through an intermediate host, parasites enter the final host. This is where growth and sexual reproduction take place.
Development of the parasite begins when it enters the definitive host.
There are also concepts such as reservoir parasitism and feeder-host. 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 used exclusively as food. The simplest option is pliers. By discovering 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 time.
The concept of "parasite tank" or "host tank" 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 transfer 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 is a danger to others.
The parasite and its host affect each other.
Harmful effects of the parasite on the host:
- Engineer;
- Toxic;
- Food withdrawal?
- Violation of tissue integrity.
Consequently, the host's body "gives" a response to the parasite's effect.
Infections caused by parasites can be divided according to the susceptibility of the pathogen to the host:
- Anthropological - humans act as hosts.
- Zoonotic - various animals act as hosts.
- Zoonotic 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 phase transition from one host to another. These stages are called multiplicative.
In intestinal parasitesproliferative 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 ripemultinucleated cystIn the new host, the cytoplasm divides to form many 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 reproductive stage of coccidia is a membrane-fertilized female germ cell (oocyst).
Most parasitic protozoaVertebrate tissue and blood is transmitted from one host to another using a vector. The proliferative stages in this case are found 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 internal organs. The leishmanial forms of the parasite transform 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 reproductive stages of malaria parasites are gamons that circulate in the host's bloodstream.
Together withproliferative 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 growth in a vertebrate host.
The invasive stages of malaria parasites are the sporozoites.
Groups of helminths
Each type of helminth develops only under certain conditions. Depending on the conditions of growth, 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 the 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 tapeworm is a parasite in the human intestine and its larval development 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.
Geohelminthes
Geohelminthesare those parasites that do not require a change of hosts during their development. Their eggs are excreted from the body together with the faeces 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 develop 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 soil, undergoes certain stages of development, and then actively penetrates the body through the skin.