Toxoplasma gondii
Brief facts
Toxoplasma gondii is an obligate intracellular parasite classified in the phylum Apicomplexa, which includes numerous important human and animal pathogens such as Plasmodium species, Cryptosporidium species, Neospora caninum, etc. (click here for more examples).
Toxoplasma gondii is capable of infecting a wide variety of mammals and birds. This single-celled protozoan organism might cause a disease known as toxoplasmosis and is arguably the most successful protozoan parasite on Earth. Between 15 and 85% of the world adult human population is chronically infected with Toxoplasma gondii depending on geographical location.
The invasive stages of apicomplexans are characterized by the presence of an apical complex composed of specialized cytoskeletal and secretory organelles: micronemes (involved in attachement and penetration), rhoptries (required for creation of a transient structure, moving junction, and the establishment of parasitophorous vacuole (PV)), and dense granules (secrete proteins throughout most of parasite stages). Parasitophorous vacuole (PV) is a unique compartment delimited from the host cell cytoplasm by PV membrane (PVM), which prevents T. gondii degradation by the host cell endocytic machinery and enables parasite intracellular propagation.
Cats (Felis catus, domestic cat at GeoChemBio: taxonomy, brief facts, digestive system, development) are primary hosts of Toxoplasma gondii. About sexual cycle of the parasite and its distribution by cats' feces please see section Developmental stages below.
Toxoplasmosis is a potentially fatal disease of the developing human fetus and immunocompromised (e.g., AIDS and transplant) patients and can cause severe ocular disease in otherwise healthy individuals.
Most infections remain asymptomatic and treatment is not prescribed, however, toxoplasmosis is strongly linked with development of psychosis, depression, and anxiety disorders, reckless behavior and impulsivity in youth. Unfortunately, all above mentioned disorders are quite common and their etiology often is not explored fully.
Experiments on rats and mice, natural prey of cats, had shown that infected rats and mice exhibit hyperactivity, fearlessness toward cat urine ordor, and novelty seeking behavior - types of conduct that most likely make them victims of cat predation. It is speculated that the pathogen targets brain of an intermediate host to increase its exposure and vulnerability to its natural predators. Although some hosts, such as humans, cannot be considered to become a prey of a cat, it can be postulated that all hosts can be affected by similar pathways.
A functional immune system of the host suppresses T. gondii and makes it to encyst in various tissues including brain. Therefore, healthy host is a key condition for persistence of T. gondii. In an immunodeficient host, tachyzoite growth causes tissue damage, and persistent infection is not established. Furthermore, when healthy but chronically infected host becomes immunocompromized, bradyzoites transform into tachyzoites, causing cerebral injuries. There is no available drug that can reach the parasite encysted in the brain tissue.
According to most recent studies, it seems that these pathogens can secrete protein kinases into host cells to subvert host-cell signalling pathways and that this explains many of the differences in virulence among the three dominant clonal lineages.
Recent population genetic studies have identified a remarkably limited number of Toxoplasma gondii genotypes in nature, the vast majority of which fall into one of only three distinct lineages. They can be associated with different types of toxoplasmosis as a disease (ocular, brain, muscle, necrosis, etc.).
Genetic analysis of strains indicates that Toxoplasma gondii sexual recombination between different strains of the parasite is very rare in natural populations of the host (felines).
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Infection pathways
from mother to fetus during pregnancy
inhaling oocysts (from litter boxes of domestic cats, from soil)
contamination of hands with feline feces while gardening, playing in sandboxes, etc.
ingestion of toxoplasma cysts contained in contaminated undercooked meat