82. Phylum Apicomplexa. Toxoplasma gondii · Biology · MU Sofia

Phylum Apicomplexa. Toxoplasma gondii.

All organisms within the Phylum Apicomplexa are intracellular parasites that infect animals, meaning they must live and reproduce within the cells of their host organisms.
A defining feature of these parasites is the presence of a unique structure called the apical complex, located at one end of the cell.
The apical complex is made up of cytoskeletal components and membrane-bound organelles. It plays a crucial role in how the parasite attaches to and enters host cells.
Once inside the host cell, the parasite forms a protective space for itself called the parasitophorous vacuole, where it can survive and multiply.
The parasite releases proteins that alter the host cell, causing organelles such as the endoplasmic reticulum, Golgi complex, and mitochondria to move closer to the vacuole to support the parasite’s survival and energy needs.
One mode of asexual reproduction seen in this group is endodyogeny, especially common in Toxoplasma gondii. In endodyogeny, two daughter cells are produced inside a parent cell.
The reproductive cycle of Apicomplexa includes both asexual and sexual stages:
Gametogony involves the formation of gametes and meiosis.
Sporogony involves the formation of sporozoites, which are the infectious stages for humans and other hosts.
Sporozoites result from multiple rounds of mitosis following fertilization.

Toxoplasma gondii is an obligate intracellular parasite, meaning it cannot survive or reproduce without invading a host cell.
It is capable of infecting virtually all warm-blooded animals, including humans.
It is one of the most widespread and common parasites in humans globally.
Cats (family Felidae) are the definitive hosts, where sexual reproduction of the parasite takes place in the cells of the cat’s intestine.
All other warm-blooded animals, including humans, serve as intermediate hosts. In these hosts, only asexual reproduction occurs.
Once infected, intermediate hosts can carry tissue cysts for life, which can remain infectious if consumed by other animals or humans.
Animals that often serve as intermediate hosts include cattle, sheep, pigs, and poultry.

Toxoplasma gondii exists in three main forms that can infect cells:

This form is responsible for acute infection.
It multiplies rapidly in the early stages after infection.
Tachyzoites can cross the placenta, infecting the fetus.
They may also be transmitted through blood transfusions or organ transplants.