Understanding Parasite Life Cycles and Survival Strategies

Parasite Life Cycles

Direct (Monoxenic) Cycles

Direct cycles are those that do not require the presence of an intermediate host.

Indirect (Heteroxenic) Cycles

Indirect cycles require an intermediate host to complete their cycle.

Key Features of Parasites

Resistance to the Outside

To cope with climatic factors and chemical agents, eggs, cysts, or larvae are protected with coverings that make them resistant to protein.

Pathogenicity

Pathogenicity is associated with morbidity and mortality.

Auto-infection

Auto-infection is the way for the parasite to remain longer in the host.

It can be:

• Autoexoinfection, which is outside for a very short time
• Autoendoinfection, which multiplies within the host

Recontamination is done on the inside.

Prepatent Period

The prepatent period is the time between the entry of the parasite into the host and the demonstration of this, or forms of development.

Viability

It is important that the forms issued abroad by the parasite are viable through resistant structures, both in the medium and the intermediate host.

Diapause

Diapause is the state in which the larvae of the worms often stay in the host organism in a latent form, encapsulated or forming cysts, to evade the immune response.

Longevity

The longevity of a parasite supports two ways:

• True longevity, if they stay many years in an organism
• Perpetuating through auto-infection, although the parasite has a very short life

Fertility

Fertility is the ability to issue a certain amount of parasitic forms; the parasite uses this to perpetuate.

Evasion of the Immune Response

When a parasite enters a body, the body tries to eliminate it by recognizing it as a foreign agent, and the parasite sets in motion a series of elements to evade the attack, and thus be able to stay in the host.

Mechanisms of Escape

1. Production of Antigenic Variation in the Membrane

The parasite has on its surface glycoproteins that act as antigens. When entering the body, it produces a number of these antigens, and the host develops an antibody response. However, when they reach the parasite, there has been a variation in the genetic code of glycoproteins, and they cannot be attacked.

2. Seclusion: The Parasite is Located in Areas Inaccessible to the Immune System

Inside the cells, forming cysts, or in organs like the eye and the brain, which have a low immune response.

3. Fast Multiplication

Some parasites can quickly change from one stage to another, faster than the host can develop its antibodies. Consequently, when they come to attack, the parasite is not recognized because the new stage has other antigens.

4. Dynamics of Membrane or Capping

The parasite has antigens on its surface, the host generates antibodies, and antigen-antibody complexes are formed. There is a movement of the membrane, and these complexes are located at a point, forming a cap. Capping is secreted abroad or eliminated by phagocytosis.

5. Release of Blocking Factors

The host produces antibodies to eliminate the parasite, and the parasite responds by releasing blocking substances into the environment that inactivate them.

Protozoa: Evolutionary Forms

Trophozoite

The trophozoite is the vegetative form through which the parasite feeds and reproduces.

Cyst

The cyst is a form of resistance, allowing the parasite to live in harsh environments.

There are cysts (simple), coming from coated zoites, and oocysts, the product of a zygote that is in stages of reproduction.