Toxoplasma gondii

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iToxoplasma Gondii

Scientific classification

Kingdom:	Protista
Phylum:	Apicomplexa
Class:	Conoidasida
Subclass:	Coccidiasina
Order:	Eucoccidiorida
Family:	Sarcocystidae
Genus:	Toxoplasma
Species:	T. gondii

Binomial name

'''Toxoplasma gondii'''
(Nicolle & Manceaux), 1908

Toxoplasma is a genus of parasitic protozoa whose definitive host is cats but which can be carried by the vast majority of warm-blooded animals, including humans. Toxoplasmosis, the disease it causes, is usually minor and self-limiting but can have serious or even fatal effects on a fetus whose mother first contracts the disease during pregnancy or on an immunocompromised human or cat. It belongs to the phylum Apicomplexa and Toxoplasma gondii is the only known member species of the genus Toxoplasma.

1 Life cycle
2 Behavioral modifications of the host
- 2.1 Human personality changes
- 2.2 Toxoplasma's role in schizophrenia
3 Human prevalence
4 How to prevent infection
5 References in fiction
6 References
7 External links

[edit] Life cycle

The life cycle of Toxoplasma gondii has two phases. The sexual part of the life cycle takes place only in members of the Felidae family (domestic and wild cats), which makes these animals the parasite's primary host. The asexual part of the life cycle can take place in any warm-blooded animal, like other mammals (including felines) and birds.

T. gondii constructing daughter scaffolds within the mother cell.

In these intermediate hosts, the parasite invades cells, forming intracellular so-called parasitophorous vacuoles containing bradyzoites, the slowly replicating form of the parasite^[1]. Vacuoles form tissue cysts mainly within the muscles and brain. Since they are within cells, the host's immune system does not detect these cysts. Resistance to antibiotics varies, but the cysts are very difficult to eradicate entirely. Within these vacuoles T. gondii propagates by a series of binary fissions until the infected cell eventually bursts and tachyzoites are released. Tachyzoites are the motile, asexually reproducing form of the parasite. Unlike the bradyzoites, the free tachyzoites are usually efficiently cleared by the host's immune response, although some 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 epithelial cells of the small intestine 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 etc.) or tissue cysts in improperly cooked meat become infected. The parasite enters macrophages in the intestinal lining and is distributed via the blood stream throughout the body.

Acute stage toxoplasma infections can be asymptomatic, but often gives 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 or transplant recipients on immunosuppressive therapy), toxoplasmosis can develop. The most notable manifestation of toxoplasmosis in immunocompromised patients is toxoplasmic encephalitis, 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, intracranial calcification, and chorioretinitis, with the possibility of spontaneous abortion or intrauterine death.

[edit] Behavioral modifications of the host

Main article: Toxoplasmosis

It has been found that the parasite has the ability to change the behavior of its host: infected rats and mice are less fearful of cats; in fact, some of the infected rats seek out cat-urine-marked areas. This effect is advantageous to the parasite, which will be able to sexually reproduce if its host is eaten by a cat^[2]. The mechanism for this change is not completely understood, but there is evidence that toxoplasmosis infection raises dopamine levels in infected mice.

[edit] Human personality changes

Given the close biological similarities and common inheritance between mice and humans, there has been speculation that human behaviour may also be affected in some ways, and correlations have been found between latent Toxoplasma infections and various characteristics:^[3]

Increased risk taking behavior
Slower reactions
Feelings of insecurity and self-doubt
Neuroticism (neuroticism is one of the Big Five personality traits)

"In populations where this parasite is very common, mass personality modification could result in cultural change. [Variations in the prevalence of Toxoplasma gondii] may explain a substantial proportion of human population differences we see in cultural aspects that relate to ego, money, material possessions, work and rules." — Kevin Lafferty ^[4]

[edit] Toxoplasma's role in schizophrenia

Several independent pieces of evidence point towards a role of Toxoplasma infection in cases of schizophrenia and paranoia^[5]:

Acute Toxoplasma infection sometimes leads to psychotic symptoms not unlike schizophrenia.
Some anti-psychotic medications that are used to treat schizophrenia, such as Haloperidol, also stop the growth of Toxoplasma in cell cultures.
Several studies have found significantly higher levels of Toxoplasma antibodies in schizophrenia patients compared to the general population.^[6]
Toxoplasma infection causes damage to astrocytes in the brain, and such damage is also seen in schizophrenia.

[edit] Human prevalence

The U.S. NHANES (2004-2005) national probability sample found that 33.1% of U.S. persons above 12 years of age had Toxoplasma-specific IgG antibodies, indicating that they had been infected with the organism. This prevalence has significantly increased from the 1999-2000 data.^[7]

It is estimated that up to 65% of all people worldwide are infected with Toxoplasma gondii. The incidence of infection varies greatly between countries, with ranges of 22% in the UK to over 88% (or 45% depending on the study) in France.^[8], and South Korea's rate is only 4.3% where Brazil's rate is a high 66.9%. ^[3]

[edit] How to prevent infection

See toxoplasmosis.

[edit] References in fiction

Parasite Pig by William Sleator with an alien version of Toxoplasma gondii
In the book and film Trainspotting, a character died from having Toxoplasmosis in addition to HIV.
Peeps by Scott Westerfeld, where characters are infected by behavior-modifyng parasites.
In "Resident Evil 4" a journal of the character Luis Sera mentions "Toxoplasma gondii" in addition to other parasites known to change the behavior of the host.

[edit] References

^ Dubey JP, Lindsay DS, Speer CA (1998). Structures of Toxoplasma gondii Tachyzoites, Bradyzoites, and Sporozoites and Biology and Development of Tissue Cysts. Clin Microbiol Rev, 11(2):267–299. full text
^ Berdoy M, Webster J, Macdonald D (2000). Fatal Attraction in Rats Infected with Toxoplasma gondii. Proceedings of the Royal Society of London, B267:1591-1594. CiteULike
^ ^a ^b Carl Zimmer, The Loom. A Nation of Neurotics? Blame the Puppet Masters?, 1 Aug. 2006
^ Kevin Lafferty [1]
^ E. Fuller Torrey and Robert H. Yolken. Toxoplasma gondii and Schizophrenia, Emerging Infectious Diseases, November 2003. full text
^ Wang HL, et al. (2006-07). "Prevalence of Toxoplasma infection in first-episode schizophrenia and comparison between Toxoplasma-seropositive and Toxoplasma-seronegative schizophrenia", Acta Psychiatrica Scandinavica, 114(1):40-48.
^ Jones JL, Kruszon-Moran D, Wilson M (2003-11). Toxoplasma gondii infection in the United States, 1999–2000. Emerging Infectious Diseases. full text
^ David Adam, Guardian Unlimited. Can a parasite carried by cats change your personality?, 25 Sep. 2003