How is the life cycle of Toxoplasma gondii?

Toxoplasma gondii is a kind of edge Apicomplexaa group of protists that are obligate intracellular parasites. Your infection causes toxoplasmosisone of the most common parasitic diseases in the countries of the so-called developed world.

The seroprevalence of antibodies against T. gondii, which indicates exposure to the parasite, varies by country. For example, in Europe it can vary between 20 and 80% and in the United States it is around 25%.

Most obligate parasites have a very narrow host range, but Toxoplasma is capable of parasitizing virtually any warm-blooded animal (birds and mammals), although it requires a feline as its host. definitive host to complete its life cycle, including domestic cat.


The life cycle of Toxoplasma gondii covers multiple stages and multiple hosts. It can be divided into two phases or components:

  • sexual phase: is the one that completes or starts the life cycle and is characterized by the fact that the parasite multiplies through sexual reproduction, that is, with the formation of male and female gametes that fuse to form a zygote. this phase only occurs in cats and therefore they are the definitive hosts.
  • asexual phase: occurs in a wide variety of birds and mammals, including rodents, cattle and humans, which are the intermediate hosts. In this phase, T. gondii It multiplies by means of asexual reproduction.

Another important feature is that the sexual phase occurs in the intestine of the definitive host, while the asexual phase occurs in other tissuessuch as brain, liver, or muscles.

Unlike other parasites that require intermediate hosts and definitive hosts to be transmitted to other animal species, Toxoplasma gondii can be transmitted to different species without passing through the definitive host and, therefore, without the need to go through the sexual phase, thus easily forming extensive reservoirs of the parasite in nature.

Briefly and schematically, the life cycle of T. gondii would go through these phases:

  1. In the cat, T. gondii can reproduce by sexual reproduction and form oocysts with sporozoites. The oocysts are released into the environment with the cat's feces.
  2. A intermediate hostincluding the human eat the oocysts through water, soil or food contaminated with cat feces.
  3. of the oocysts sporozoites are released in the intestine and infect mucosal epithelial cells.
  4. Sporozoites multiply and differentiate into tachyzoiteswhich can leave the intestine and infect other tissues.
  5. At 7-10 days after infection, the tachyzoites differentiate into bradyzoiteswhich multiply much more slowly and form the tissue cysts.
  6. When the cat ingests tissue cystsfor example, by eating an infected mouse, the bradyzoites contained in the cyst will be able to differentiate into gametocytes, which allows sexual reproduction and the cycle is completed.

sexual phase in the definitive host

A feline, which is the definitive host, is infected by T. gondii to the eating meat from infected animalsfor example if a cat eats an infected mouse with tissue cysts in its muscles.

In each of these cysts there are tens or hundreds of bradyzoitesone of the forms it takes T. gondii in its life cycle.

In the cat's intestine, cysts release bradyzoites and invade epithelial cells of the intestinal mucosa. Within the epithelial cells they begin to divide and can differentiate into tachyzoites or gametocytes:

  • tachyzoites: the way of T. gondii that invades animal tissues and performs the phase of asexual reproduction.
  • gametocytes (masculine and feminine): they merge and give rise to a zygote (sexual reproduction) that matures into the oocyst or oocyst full of sporozoites.

oocysts are released into the environment next to cat feces. contact with polluted land and water with infected faeces is the most important route of transmission to intermediate hosts.

Cats can also be infected by ingesting oocysts, although they are less susceptible to oocyst infection than intermediate hosts. In this case, unlike infection with tissue cysts that released bradyzoites, the oocyst releases sporozoites that differentiate into tachyzoites and follow the asexual phase of the cycle without passing the sexual phase.

Intermediate host infection and asexual phase

T. gondii It infects intermediate hosts in three ways or stages. The median oocyst that was formed in the cat, the rapidly dividing tachyzoites that infect host cells, and the more slowly dividing bradyzoites within tissue cysts.

When a bird or mammal, including humans, ingest oocyststhe cyst wall is dissolved by the proteolytic enzymes of the digestive system and sporozoites are released in the stomach and intestine.

The sporozoites invade the epithelial tissue of the intestine and, within the epithelial cells, they differentiate into tachyzoites. In the tachyzoite phase, T. gondii is motile and rapidly divides asexual reproduction.

Tachyzoites can infect almost any type of host cell and leave the intestinal epithelium to invade other tissues. Tachyzoites multiply rapidly within vacuoles in the cytoplasm of infected cells until they die and rupture, releasing more tachyzoites that spread to all organs via the bloodstream.

Birds and mammals can also ingest meat infected with tissue cysts containing bradyzoites. Like the sporozoites of oocysts, bradyzoites invade the intestinal mucosa and differentiate into tachyzoites that follow the asexual phase. Thus, T. gondii it does not need to go through the cat and the sexual phase to be transmitted to other species.

Tissue cyst formation

After the initial acute infection produced by the tachyzoites and their rapid proliferation, the host's immune system forces T. gondii to differentiate in bradyzoitesa cell phase of much slower proliferation.

The cells infected by bradyzoites form the tissue cystswhich can reach a size of 50 μm, about half the diameter of a human hair.

Cyst development usually occurs 7 to 10 days after the initial infection. Although they can appear in any tissue, they are more common in the brain, liver, eyes, and striated muscle tissue (including heart muscle).

However, the proportion of cysts per tissue may vary depending on the host being considered. For example, in pigs they form mainly in skeletal muscle, whereas in mice most cysts form in the brain.

If meat contaminated with tissue cysts is consumed by a cat or other feline, T. gondii will be able perform sexual reproduction again and form oocysts with sporozoites, thus completing the life cycle. If the tissue cysts are ingested by any other animal, the asexual phase will continue until eventually T. gondii reach a cat.


infection by Toxoplasma gondii is known as toxoplasmosis. It affects hundreds of millions of people worldwide, although most do not develop symptomatic disease.

However, in some people, such as those with weakened immune systems and in babies of mothers who contracted the disease during pregnancy, toxoplasmosis can leave serious consequences brain, eye and other organ damage.

The most common routes of infection are handling of infected cat droppingsfor example when cleaning the defecation area of ​​domestic cats, drinking contaminated water or eating undercooked or raw infected meat. For this reason it is not recommended that pregnant women consume undercooked meat, including sausages, cured meats such as Serrano ham and other meat derivatives.

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