The life cycle of many parasites is amazingcomplicated. To go through all stages of development, they need to change their host several times. One of these hosts will be the main one. In his body, the parasite will be able to reproduce sexually or asexually. But we will talk about the intermediate host.
The intermediate host is an insect, an animalor a person whose organism gives the parasite the possibility of living in the larval stage. Within the intermediate host, asexual reproduction can occur. For example, the human body provides a temporary habitat for malarial plasmodium, echinococcus and other parasites.
However, not every organism in which a parasitevisited different stages of development, is considered his intermediate master. This is the name of only an animal, the passage through the organism of which is an indispensable condition for the transition to the next cycle of parasite development.
The behavior of larvae in the intermediate environment is divided into 3 types:
Consider a specific type of parasite, the main owner of which is a man, the intermediate owner - cattle (a cow, a bull). This is an unarmed bullish chain.
In the human body, this largest bandThe worm gets with poorly processed beef. Zesters are introduced into tissues and begin active development. The result of this development is a huge tapeworm living in the human intestine. The length of the worm can reach 12-14 meters!
In the body of the main host, the parasite canlive up to twenty years, during this time he produces billions of eggs that will leave the intestines with calves, some of which will continue to develop in the body of the intermediate host. As it was said, in this case the intermediate owner is a cow or bull.
In the intestine of these representatives of the large hornedcattle from eggs will appear larvae (onkosfery), equipped with special hooks. They will drill the intestinal tissues and spread throughout the body. In the muscles of cows, the larvae will move to the next stage, they form the Finns, who will expect to enter the body of the main carrier.
Consider another type of parasite with a complexa maturation cycle in which several intermediate hosts are present. This is a group called hepatic trematodes. This includes hepatic and feline fluke, giant, lanceolate and Chinese trematode.
The first intermediate host of the hepaticflukes. In his body, larvae undergo several stages of degeneration: miracidia, sporocysts, redias. And only the third generation of larvae - cecaria, leave the body of the mollusk in search of the next intermediate host.
The second intermediate host of the hepaticflukes (flukes) - fish. Most often they belong to the family of cyprinids. If the technology of salting is violated or the thermal processing is insufficient from fish tissues, the cecaria enter the organism of the final host, settling in the liver or biliary tract. For a man, the Chinese trematode and the cat's fluke are dangerous.
Another species is the lanceolate trematode, from the bodythe first intermediate mollusk enters the body of the next intermediate host, the ant, and then into the organism of the final host, the herbivore.
To the cycle of development was repeated, from the bodythe main owner of the eggs must get into the natural environment, namely into the pond. Here they are "swallowed" by the intermediate owner. This happens from generation to generation, as a person does not sufficiently monitor the cleanliness of water bodies and allows himself to discharge wastewater without adequate cleaning.
By and large, evolution, forcing parasiteschange hosts, reduces the burden on one organism, reduces intraspecific competition and eliminates dependence on one condition. A complex evolutionary path is fixed at the genetic level of parasites and allows them to derive maximum benefits at each stage of development.
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