Toxoplasma gondii is a parasitic protozoan, which belongs to the Toxoplasma genus, and shares the class Coccidia with Plasmodium that is responsible for malaria in humans. While its primary host is the cat, this parasite can infect a majority of warm-blooded animals, including birds and mammals. Upon infection, it tends to invade the cells of their brain and intestine. In humans, it causes varying degree of illnesses, depending upon the overall health of the patient.
The life cycle of T. gondii is categorized into two main stages, the sexual and asexual phase. Each of these require a specific host. The sexual part of its life cycle, for instance, occurs only in felines (wild or domesticated cats). On the other hand, the asexual part takes place in any of the prospective warm-blooded animals (like birds, dogs, pigs, cattle, humans, etc.).
The beginning of this phase is marked by ingestion of tissue cysts by the cat, probably after eating an infected rodent. The cysts make their way to the cats stomach and intestines. Once they reach the intestines, they reside in the epithelial cells of the small intestinal lining. Then, they undergo sexual reproduction, which leads to the formation of oocysts. The sexual life cycle ends when oocysts are excreted by the cats in their feces within 3 - 10 days after ingestion of the tissue cysts. These oocysts can survive in the environment for more than a year, which is when they are ingested by an intermediate host.
The T. gondii oocysts present in the excreta of cats, are ingested by other prospective hosts. Humans acquire this parasite on eating uncooked meat and contaminated foods (that contain oocysts), or through eating with unwashed hands. Once inside the host's body, they release the sporozoites, which invade the macrophages of the intestine. The sporozoites are differentiated into motile tachyzoites, which are then distributed to other parts of the body by blood circulation. The tachyzoites enter the bradyzoite stage, resulting in tissue cysts, and when these tissue cysts are ingested by a cat, it initiates the sexual cycle all over again.
The disease caused due to infection by Toxoplasma gondii is called toxoplasmosis. People who have mental health problems are at a higher risk of getting infected by T. gondii parasite compared to others. In a healthy individual, the condition is usually mild, and subsides on its own within a few days. In pregnant females, it can lead to serious medical problems, particularly in the unborn. There have been reported cases of brain abnormalities in the developing fetus, which were traced to T. gondii infection. Though the instances are rare, it can even result in miscarriage at times.
According to some studies, sexual and asexual phases in the life cycle of this parasite are independent to each other. Asexual tissue cysts can undergo several cycles in different intermediate hosts, until they enter cats (definitive host). And lastly, the infection of this parasite in cats and dogs is of less concern, as it does not cause severe medical problems in these animals.