Toxoplasma gondii
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Toxoplasma gondii is a species of parasitic protozoa whose definitive host is cats (felids) but can also be carried by the vast majority of other warm-blooded animals including humans. It causes the disease toxoplasmosis which is usually minor and self-limiting but can have serious or even fatal effects, particularly in cats, or for a foetus when first contracted during pregnancy. It belongs to the Apicomplexa and is the only known member of the genus Toxoplasma.
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Life cycle
The life cycle of Toxoplasma gondii has two phases. The sexual part of the lifecycle takes place only in cats, the definitive host of Toxoplasma gondii. The asexual life cycle can take place in any warm-blooded animal, like other mammals and birds.
Image:Toxoplasma gondii.jpg In these intermediate hosts, the parasite invades cells, forming intracellular so-called parasitophorous vacuoles containing bradyzoites, the slowly replicating form of the parasite<ref>Dubey JP, Lindsay DS, Speer CA. Structures of Toxoplasma gondii Tachyzoites, Bradyzoites, and Sporozoites and Biology and Development of Tissue Cysts. Clin Microbiol Rev. 1998 April; 11(2): 267–299. full text </ref>. 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, fast 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 can be ingested by a cat, e.g. by feeding on an infected mouse. The cysts survive passage through the digestive system 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, rarely, be 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.
Behavioral modifications of the host
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 actually seek out cat urine-marked areas again and again. The parasite alters the mind, and thus the behavior, of the rat for its own benefit, leading to a propagation of the lifecycle.<ref>Berdoy M, Webster J, Macdonald D. Fatal Attraction in Rats Infected with Toxoplasma gondii. Proceedings of the Royal Society of London, B267, pp 1591 - 1594, August 7, 2000. full text </ref> The mechanism for this change is not necessarily completely understood, but there is evidence that toxoplasmosis infection raises dopamine levels in infected mice.
Given the close biological similarities and common inheritance between mice and humans, it has been suggested that human behaviour could also be affected in some way, and some epidemiological links may have been found between latent Toxoplasma infections and car crashes, slower reactions, an increase in risk-taking behaviors, and possibly some forms of schizophrenia.
Several independent pieces of evidence point towards a role of Toxoplasma infection in some cases of schizophrenia<ref>E. Fuller Torrey and Robert H. Yolken. Toxoplasma gondii and Schizophrenia, Emerging Infectious Diseases, November 2003. full text </ref>:
- 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.
- Toxoplasma infection causes damage to astrocytes in the brain, and such damage is also seen in schizophrenia.
Human prevalence
The U.S. NHANES (1999-2000) national probability sample found that 15.8% 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 had not significantly changed from the 1988-1994 data.<ref>Jones JL, Kruszon-Moran D, Wilson M. Toxoplasma gondii infection in the United States, 1999–2000. Emerging Infectious Diseases, 2003 Nov. full text</ref>
It is estimated that up to 50% of all people worldwide are infected with Toxoplasma gondii. The incidence of infection is highly specific to each nationality with ranges such as 22% infected in the UK to over 88% in France.<ref>David Adam, Guardian Unlimited. Can a parasite carried by cats change your personality?, 25 Sep. 2003</ref>
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.
References
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External links
- Toxoplasma images, from CDC's DPDx, in the public domain
- Toxoplasma in Live Science
- Cytoskeletal Components of an Invasion Machine—The Apical Complex of Toxoplasma gondii]de:Toxoplasma gondii
es:Toxoplasma gondii fr:Toxoplasma gondii it:Toxoplasma gondii sk:Toxoplasma gondii