Toxoplasma gondii
Encyclopedia
Toxoplasma gondii is a species of parasitic protozoa
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...

 in the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Toxoplasma. The definitive host
Host (biology)
In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...

 of T. gondii is the cat
Cat
The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...

, but the parasite can be carried by many warm-blooded animals (bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s or mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, including humans). Toxoplasmosis
Toxoplasmosis
Toxoplasmosis is a parasitic disease caused by the protozoan Toxoplasma gondii. The parasite infects most genera of warm-blooded animals, including humans, but the primary host is the felid family. Animals are infected by eating infected meat, by ingestion of feces of a cat that has itself...

, the disease of which T. gondii is the causative agent, is usually minor and self-limiting but can have serious or even fatal effects on a fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...

 whose mother first contracts the disease during pregnancy or on an immunocompromised
Immunodeficiency
Immunodeficiency is a state in which the immune system's ability to fight infectious disease is compromised or entirely absent. Immunodeficiency may also decrease cancer immunosurveillance. Most cases of immunodeficiency are acquired but some people are born with defects in their immune system,...

 human or cat.

Life cycle

The life cycle
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...

 of T. gondii has two phases. The sexual
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...

 part of the life cycle (coccidia
Coccidia
Coccidia is a subclass of microscopic, spore-forming, single-celled obligate parasites belonging to the apicomplexan class Conoidasida. Coccidian parasites infect the intestinal tracts of animals, and are the largest group of apicomplexan protozoa....

 like) takes place only in cats, both domestic and wild (family Felidae
Felidae
Felidae is the biological family of the cats; a member of this family is called a felid. Felids are the strictest carnivores of the thirteen terrestrial families in the order Carnivora, although the three families of marine mammals comprising the superfamily pinnipedia are as carnivorous as the...

), which makes cats the parasite's primary host. The second phase, the asexual
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...

 part of the life cycle, can take place in other warm-blooded animals, including cats, mice
MICE
-Fiction:*Mice , alien species in The Hitchhiker's Guide to the Galaxy*The Mice -Acronyms:* "Meetings, Incentives, Conferencing, Exhibitions", facilities terminology for events...

, human
Human
Humans are the only living species in the Homo genus...

s, and bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s. The hosts in which asexual reproduction takes place is called the intermediate host. Rodents are the typical intermediate host.
In both kinds of hosts, the Toxoplasma parasite invades cells and forms a space called a vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...

. Inside this specialized vacuole, called a parasitophorous vacuole, the parasite forms bradyzoites, which are the slowly replicating versions of the parasite. The vacuoles containing the reproductive bradyzoites form cyst
Cyst
A cyst is a closed sac, having a distinct membrane and division on the nearby tissue. It may contain air, fluids, or semi-solid material. A collection of pus is called an abscess, not a cyst. Once formed, a cyst could go away on its own or may have to be removed through surgery.- Locations :* Acne...

s mainly in the tissues of the muscles and brain. Since the parasites are inside cells, they are safe from the host's immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

, which does not respond to the cysts.

Toxoplasma's resistance to antibiotics varies, but the cysts are very difficult to eradicate entirely. Inside the vacuoles, T. gondii replicates itself (by endodyogeny) until the infected cell fills with parasites and bursts, releasing tachyzoites, the motile, asexually reproducing form of the parasite. Unlike the bradyzoites, the free tachyzoites are usually efficiently cleared by the host's immune system, although some of them manage to infect cells and form bradyzoites, thus maintaining the infection.

Tissue cysts are ingested by a cat (e.g., by feeding on an infected mouse). The cysts survive passage through the stomach of the cat and the parasites infect epithelium of the small intestine
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...

 where they undergo sexual reproduction and oocyst formation. Oocysts are shed with the feces. Animals and humans that ingest oocysts (e.g., by eating unwashed vegetables) or tissue cysts in improperly cooked meat become infected. The parasite enters macrophage
Macrophage
Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...

s in the intestinal lining and is distributed via the blood stream throughout the body.

Similar to the mechanism used in many viruses, Toxoplasma is able to dysregulate host’s cell cycle by holding cell division before mitosis (the G2/M border). This dysregulation of the host’s cell cycle is caused by a heat-sensitive secretion (with a molecular mass larger than 10 kDa). Infected cells secrete the factor which inhibits the cell cycle of neighboring cells. The reason for Toxoplasma’s dysregulation is unknown, but studies have shown that infection is preferential to host cells in the S-phase and host cell structures with which Toxoplasma interacts may not be accessible during other stages of the cell cycle.

Acute stage Toxoplasma infections can be asymptomatic, but often give flu-like symptoms in the early acute stages, and like flu can become, in very rare cases, fatal. The acute stage fades in a few days to months, leading to the latent stage. Latent infection is normally asymptomatic; however, in the case of immunocompromised patients (such as those infected with HIV
HIV
Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...

 or transplant recipients on immunosuppressive therapy), toxoplasmosis
Toxoplasmosis
Toxoplasmosis is a parasitic disease caused by the protozoan Toxoplasma gondii. The parasite infects most genera of warm-blooded animals, including humans, but the primary host is the felid family. Animals are infected by eating infected meat, by ingestion of feces of a cat that has itself...

 can develop. The most notable manifestation of toxoplasmosis in immunocompromised patients is toxoplasmic encephalitis
Encephalitis
Encephalitis is an acute inflammation of the brain. Encephalitis with meningitis is known as meningoencephalitis. Symptoms include headache, fever, confusion, drowsiness, and fatigue...

, which can be deadly. If infection with T. gondii occurs for the first time during pregnancy, the parasite can cross the placenta, possibly leading to hydrocephalus
Hydrocephalus
Hydrocephalus , also known as "water in the brain," is a medical condition in which there is an abnormal accumulation of cerebrospinal fluid in the ventricles, or cavities, of the brain. This may cause increased intracranial pressure inside the skull and progressive enlargement of the head,...

 or microcephaly
Microcephaly
Microcephaly is a neurodevelopmental disorder in which the circumference of the head is more than two standard deviations smaller than average for the person's age and sex. Microcephaly may be congenital or it may develop in the first few years of life...

, intracranial calcification, and chorioretinitis
Chorioretinitis
Chorioretinitis is an inflammation of the choroid and retina of the eye. It is also known as choroid retinitis.-Causes:...

, with the possibility of spontaneous abortion (miscarriage) or intrauterine death.

Toxoplasmosis

T. gondii infections have the ability to change the behavior of rat
Rat
Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...

s and mice, making them drawn to, rather than fearful of, the scent of cats. This effect is advantageous to the parasite, which will be able to sexually reproduce if its host is eaten by a cat. The infection is highly precise, as it does not affect a rat's other fears such as the fear of open spaces or of unfamiliar-smelling food.

Studies have also shown behavioral changes in humans, including slower reaction times and a sixfold increased risk of traffic accidents among infected males, as well as links to schizophrenia
Schizophrenia
Schizophrenia is a mental disorder characterized by a disintegration of thought processes and of emotional responsiveness. It most commonly manifests itself as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking, and it is accompanied by significant social...

 including hallucinations and reckless behavior. Recent epidemiologic studies by Stanley Medical Research Institute and Johns Hopkins University Medical Center
Johns Hopkins Hospital
The Johns Hopkins Hospital is the teaching hospital and biomedical research facility of Johns Hopkins University School of Medicine, located in Baltimore, Maryland . It was founded using money from a bequest by philanthropist Johns Hopkins...

 indicate that infectious agents may contribute to some cases of schizophrenia. A study of 191 young women in 1999 reported higher intelligence and guilt proneness in Toxoplasma-positive subjects.

The prevalence of human infection by Toxoplasma varies greatly between countries. Factors that influence infection rates include diet (prevalence is possibly higher where there is a preference for less-cooked meat) and proximity to cats.

History

The organism was first described in 1908 in Tunis
Tunis
Tunis is the capital of both the Tunisian Republic and the Tunis Governorate. It is Tunisia's largest city, with a population of 728,453 as of 2004; the greater metropolitan area holds some 2,412,500 inhabitants....

 by Charles Nicolle
Charles Nicolle
Charles Jules Henry Nicolle was a French bacteriologist who received the Nobel Prize in Medicine for his identification of lice as the transmitter of epidemic typhus.- Biography :...

 and Louis Manceaux within the tissues of the gundi (Ctenodactylus gundi). In the same year it was also described in Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

 by Alfonso Splendore in rabbits .

External links

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