The parasite is the relationship between two different species, wherein the benefit ( parasite ) lives at the expense of another ( host ), damaging it.
Types of parasites
If the parasite lives inside the body of the host, it is called endoparasite . This is the case of Entamoeba histolytica and Taenia solium , respectively, protozoan and worm, which can parasitize the human intestine.
When lodged outside the body of the host, it is called ectoparasite . This is the case with lice, ticks and fleas.
The causative organism of the disease is called an etiological agent .
Penetration of the parasite usually occurs through the skin and mucous membranes or orally.
There is active penetration when the parasite has the ability to overcome the barriers and penetrate the body of the host. An example is yellowwort, whose infecting larvae, present in the soil, pierce human skin and reach the bloodstream.
If the parasite does not have this capacity, passive penetration occurs . It is the case of some parasites that are transported to the host by another organism, such as an insect. In this case, the insect, when feeding, perforates the skin and introduces parasites in the body of the host or facilitates its entrance.
The organism responsible for the transport and transmission of the parasite is called the vector . For example, Plasmodium malariae , a protozoan that causes malaria , is transmitted (transmitted) by the mosquito-nail, the vector of this disease.
After entering the body of the host, the parasite reaches the cells and organs of its preference, where it develops, usually undergoing transformations, and reproduces, originating descendants, that will carry out this same life cycle.
Some parasites have a single host in their life cycle. These are called monoxene parasites ; an example is the worm that causes ascariasis , the worm, which has man as its only host.
Heteroxene parasites have more than one host in their life cycle. These parasites, depending on the host, can reproduce asexually or sexually .
An example is the worm that causes schistosomiasis , a disease popularly known as a water-belly. This worm performs its life cycle on the snail and on the man. The snail is called an intermediate host; in it, the parasite reproduces asexually. Man is the definitive host, in which the parasite reproduces sexually.
The parasites have natural reservoirs , which are a living being (animal or plant) or a substrate, such as soil and water. In it, the parasite can survive and reproduce until it is delivered to the host.
A classic example of a natural reservoir is the armadillo. It hosts the protozoan parasite that causes Chagas’ disease , Trypanosoma cruzi , but without contracting the disease. This probably occurs because the armadillo has adaptations that allow resistance or tolerance to the parasite, which may have been selected over thousands of years.
In man, this same parasite causes various symptoms and signs. In some cases the infection by this parasite is fatal. There is treatment that minimizes or prevents the progression of the disease.
However, rather than treating the disease is to adopt a set of measures to avoid its contagion or its spread, the so-called prophylaxis .
Some adaptations of the parasites
Over time, some parasites have developed strategies that favor their reproductive success. Here are some examples.
- Great reproduction capacity. Some parasites usually produce thousands of eggs; others present hermaphroditism, that is, they have both sexes, which increases the chance of fertilization; there are still those who produce an egg that develops in an organism without fertilization.
- Mechanisms of resistance . For example, an outer cuticle that protects them from host defense, or cysts , forms of resistance that originate in unfavorable conditions and that open under appropriate conditions, releasing the parasite.
The cysts have a protective shell that allows the protozoa to live temporarily in terrestrial environment with low metabolic activity and to return to develop when the environmental conditions are favorable to the life.
- Disappearance or acquisition of structures . There are parasites with poorly developed or absent digestive organs and also no locomotor and sensory organs. Others have host attachment structures, such as hooks, suction cups, and oral plaques. These parasites are able to live in environments with little oxygen, such as the intestines of animals.