The life cycle of many parasites is surprisingly complex. To go through all stages of development, they need to change the owner 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 an intermediate host.
What is or who is an intermediate host?
An intermediate host is an insect, animal or person whose organism provides the parasite with the opportunity to live in the larval stage. Asexual reproduction may occur within the intermediate host . For example, the human body provides a temporary habitat for malarial plasmodium, echinococcus and other parasites.
However, not every organism in which the parasite has been at different stages of development is considered its intermediate host. So they call only an animal, the passage through the body of which is a prerequisite for the transition to the next cycle of development of the parasite.
How does a parasite behave in an intermediate host?
The behavior of the larvae in the intermediate medium is divided into 3 types:
- The intermediate organism prepares the larva for transfer to the final host. In this case, its noticeable development does not occur. A striking example is trypanosomes, which develop in the body of mammals, when they get bitten by a horsefly, they enter their proboscis with blood, survive an episamide-like form in the intestines of insects and are transmitted to another mammal during the next bite.
- The second type of behavior is observed in the body of intermediate hosts, where parasites change significantly, but do not multiply. At the exit from the body, the number of parasites entering it does not increase. Several types of roundworms and tapeworms have such a development cycle.
- The third type of behavior is observed if the intermediate host is an organism in which not only development, but also asexual reproduction is carried out. In this case, one parasite will burn into the intermediate organism, and at the exit there will be several thousand, ready to infect the final hosts.
Intermediate parasite hosts
Consider a specific type of parasite, the main owner of which is a person, the intermediate owner is cattle (cow, bull). This is an unarmed bull chain.
This largest tapeworm enters the human body with poorly processed beef. Cisterka penetrate into the tissue and begin active development. The result of this development is a huge tapeworm living in the human intestines. The length of the worm can reach 12-14 meters!
In the body of the main host, the parasite can live up to twenty years, during which time it produces billions of eggs that will leave the intestines with feces, some of them will continue to develop in the body of the intermediate host. As was said, in this case the intermediate owner is a cow or a bull.
In the intestines of these representatives of cattle , larvae (oncospheres) equipped with special hooks will appear from the eggs. They will drill intestinal tissues and spread throughout the body. In the muscles of cows, the larvae will move on to the next stage, they form Finns, who will wait for the main carrier to enter the body.
Liver flukes
Consider another parasite species with a complex maturation cycle in which several intermediate hosts are present. This is a group called hepatic flukes. These include hepatic and cat fluke, giant, lanceolate and Chinese fluke.
The first intermediate host of the hepatic trematode is the mollusk. In his body, the larvae undergo several stages of degeneration: miracidia, sporocysts, redia. And only the third generation of larvae - cecaria, leave the mollusk body in search of the next intermediate host.
The second intermediate host of the hepatic trematode (fluke) is fish. Most often they belong to the family of cyprinids. In case of violation of salting technology or insufficient heat treatment from fish tissue, the cecaria enters the body of the final host, settling in the liver or biliary tract. Chinese fluke and cat fluke are dangerous for humans.
Lanceolate fluke
Another species is a lanceolate fluke, from the body of the first intermediate mollusk it enters the body of the next intermediate host - the ant, and then into the body of the final host - the herbivore.
In order for the development cycle to repeat, eggs from the body of the main host must enter the natural environment, namely, into the body of water. Here the intermediate owner "swallows" them. This happens from generation to generation, since a person does not sufficiently monitor the cleanliness of water bodies and allows himself to discharge waste water without proper treatment.
By and large, evolution, forcing parasites to change hosts, reduces the load on one organism, reduces intraspecific competition and eliminates dependence on one condition. The complex evolutionary path is fixed at the genetic level of parasites and allows them to derive maximum benefits at each stage of development.