The life cycle of many parasites is surprisingly complex. To go through all the 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'll talk about the intermediate host.
What or who is an intermediate host?
Intermediate host is an insect, animal or person whose body provides the parasite with the opportunity to live 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 the parasite has visited at different stages of development is considered its intermediate host. This is the name of only an animal whose passage through the body is a prerequisite for the transition to the next cycle of parasite development.
How does the parasite behave in the intermediate host?
The behavior of larvae in the intermediate environment is divided into 3 types:
- Intermediatethe organism prepares the larva for transfer to the final host. In this case, its noticeable development does not occur. A vivid example is trypanosomes that develop in the body of mammals, when bitten by horseflies they get on their proboscis with blood, survive the episamtigous form in the intestines of insects and are transmitted during the next bite to another mammal.
- 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 that have entered 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 takes place. In this case, one parasite will burn into the intermediate organism, and at the exit there will be several thousand of them, ready to infect the final hosts.
Intermediate hosts of parasites
Let's consider a specific type of parasites, the main host of which is a person, the intermediate host is cattle (cow, bull). We are talking about an unarmed bull tapeworm.
This largest tapeworm enters the human body with poorly processed beef. Cesters are introduced into the 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 can live up to twenty years, for whichwhile it produces billions of eggs that will pass out of the intestines in the feces, some of which will continue to develop in the body of the intermediate host. As mentioned, in this case, the intermediate host is a cow or a bull.
In the intestines of these representatives of cattle from eggs, larvae (oncospheres) equipped with special hooks will appear. They will perforate the intestinal tissue and spread throughout the body. In the muscles of cows, the larvae will move to the next stage, they form Finns, which will wait for the main carrier to enter the body.
Liver flukes
Let's consider another parasite species with a complex maturation cycle, in which there are several intermediate hosts. This is a group called liver flukes. This includes liver fluke, cat fluke, giant fluke, lanceolate fluke, and Chinese fluke.
The first intermediate host of the liver fluke is a mollusk. In his body, the larvae go through several stages of rebirth: miracidia, sporocysts, redia. And only the third generation of larvae - cecariae - leave the body of the mollusk in search of the next intermediate host.
The second intermediate host of the liver fluke (fluke) is a fish. Most often they belong to the carp family. In case of violation of the s alting technology or insufficient heat treatment from fish tissues, cecariae enter the body of the final owner, settling in the liver or biliary tract. Chinese fluke and cat fluke are dangerous for humans.
Lancet fluke
Another species, the lanceolate fluke, from the body of the first intermediate mollusk 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 cycle of development to repeat itself, eggs from the body of the main host must fall into the natural environment, namely into a reservoir. Here they are "swallowed" by the intermediate host. This happens from generation to generation, because a person does not monitor the cleanliness of water bodies enough and allows himself to discharge wastewater 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. A complex evolutionary path is fixed at the genetic level of parasites and allows them to extract maximum benefits at each stage of development.
