Toxoplasma gondii

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Toxoplasma gondii
Tachyzoites

Tachyzoites

Systematics
without rank: Conoidasida
without rank: Coccidia (Coccidia)
without rank: Eimeriorina
Family : Sarcocystidae
Genre : Toxoplasma
Type : Toxoplasma gondii
Scientific name
Toxoplasma gondii
( Nicolle & Manceaux , 1908)

Toxoplasma gondii (from ancient Greek τόξον tóxon , arch 'and πλάσμα plásma , structure') is an arched protozoon with a parasitic way of life. Its final hosts are cats ,other vertebrates serveas intermediate hosts . It is the only known representative of the genus Toxoplasma . The parasite causes toxoplasmosis and is closely related to Plasmodium , the causative agent of malaria , and to Cryptosporidium .

Discovery and history of research

Toxoplasma gondii was first discovered in Tunisia in 1907 as a parasite in the gundi ( Ctenodactylus gundi ) and identified as a member of the Apicomplexa . Because of its crescent shape, the discoverers Charles Nicolle and Louis Manceaux named it as Toxoplasma (Greek toxon , arc; plasma , structure) and because of the host animal as Toxoplasma gondii . It was not until much later that it was also found as a pathogen in humans ; the disease it caused was called toxoplasmosis . In 1948, Sabin and Feldman developed a serological test based on antibodies , which they called the dye test . With the help of this method it could be established that Toxoplasma gondii is distributed worldwide and occurs very frequently in humans.

distribution

The parasite is spread worldwide, the population has a high prevalence because the infection is usually without symptoms.

Antibodies have been detected in 50% of the population in Germany. The likelihood of infection increases with age. For people over 70, it is over 70%.

features

Toxoplasma gondii differs in both shape and size depending on the stage. The cells of the free and infectious form are curved in liquid media or fresh preparations and reach sizes of two to five micrometers. If you look at them in tissue samples or fixed sections, on the other hand, they appear egg-shaped. In addition, they can occur individually or in groups in so-called pseudocysts in the tissue.

The oocysts measure up to 11 micrometers , the tissue cysts up to 300 micrometers. Two different populations of sporozoites form , the tachyzoites form after penetrating into the intermediate host and multiply there rapidly. Later come Bradyzoites , whose proliferation is greatly slowed. There is no structural difference between these two forms.

Development cycle

Post-colored image of four toxoplasmas

The oocysts are excreted by the definitive host ( feline ) with the feces and thus reach the intermediate host . They contain two sporocysts with four sporozoites each . They can remain infectious for a very long time (up to five years) and survive frost , but are not very heat-resistant. The sporocysts hatch in the intermediate host (vertebrates, e.g. birds), and these now actively penetrate into the nucleated cells of the intermediate host (primarily lymph nodes , reticuloendothelial system ). Now, reproduction through asexual division sets in, in which two daughter cells detach from the mother cell, whereby the mother cell dissolves ( endodyogeny ). This process continues until the host cell is completely filled and bursts open, so that the tachyzoites (Greek tachys = fast) are released. This process is repeated every 6 hours. After the release, the tachyzoites spread in the blood and thus also pass through the placenta into the blood of the offspring. After the host defense has started, the duration of division slows down, and one speaks now of bradyzoites (Greek bradys = slow). Tissue cysts form in the cells , which latently persist , especially in the muscles , but also in the brain or in the retina of the eye . In this form, they are in turn ingested by the cat that eats the intermediate host. The bradyzoites are released in the intestine and penetrate the epithelial cells . There is a schizogony (asexual reproduction) take and / or there are macrogamont and microgamonts formed. The macrogamonts form macrogametes (comparable to an egg cell ), while a microgamont produces microgametes (comparable to sperm ). The macrogamete is fertilized by a microgamete, and a zygote (diploid) is formed, which then matures into an unsporulated oocyst. This is excreted with the feces and matures in the outside world in 2 to 4 days under the influence of oxygen into infectious sporulated oocysts. It is infectious for up to 5 years. If the oocysts are ingested by cats, tachyzoites, bradyzoites, and tissue cysts develop. However, only a small part of these remain in the tissue and migrate into the intestinal epithelium of the cat, where they form oocysts again through schizogony and gamogony. The life cycle is divided into three phases, 1. the extraintestinal phase, 2. the external phase and 3. the enteroepithelial phase.

Harmful effect

This parasite causes toxoplasmosis in humans . It can ingest T. gondii in both forms, both as cysts in semi-raw meat and as a smear infection with cat feces. It then takes on the role of the intermediate host, i.e. the pathogens penetrate the intestinal wall in order to form cysts in the muscles, but also in other organs, which last for life. Most people go through this infection at some point and most of the time it has no symptoms . You may experience flu-like symptoms such as fever , joint and muscle pain and, for example, lymph node swelling for a few months .

Changes in behavior caused by Toxoplasma have been demonstrated in rodents. Infected animals lose their innate shyness towards the smell of cats, which is beneficial to the life cycle of Toxoplasma. In mice, the loss of shyness towards the smell of cat urine persists even after a healed infection with T. gondii . In rats infected with T. gondii , activity in the limbic system in the regions responsible for sexual attraction is increased when the animals are exposed to the odor of cat urine. This is being discussed as a possible mechanism why infected rodents lose their fear of cats.

In humans, changes in behavior that may be caused by Toxoplasma infections are repeatedly discussed.

prevention

Pregnant women should not eat meat that is not well done, if possible not come into contact with cat feces , and should not work in the garden . In an emergency, gloves or hand washing protect against meals . It makes sense to have someone else clean the litter box every day, because the oocysts only become infectious two days after they are excreted at the earliest .

Up to and including 2015 there is no vaccination against Toxoplasma gondii approved for humans .

With regard to the immunization of sheep , however, a live vaccine with the trade name Toxovax ( MSD Animal Health ) has been available for some time . This offers lifelong protection against Toxoplasma gondii .

Diagnosis

Immunofluorescence detection (IFT) of tachyzoites

An infection can usually be detected most easily by immunological test methods ( ELISA , immunofluorescence test , ISAGA ) , i.e. detection of specific antibodies . IgM antibodies speak for a fresh infection, IgM and IgG antibodies together for an infection within the last one and a half years . If both IgG and IgM antibodies are present, a so-called avidity test helps to rule out a fresh infection.

Molecular biological tests ( PCR ) are also available. They are suitable for examining amniotic fluid to demonstrate that it has already been transmitted to the unborn child. Damage to the child can be diagnosed using ultrasound . Even with immunocompromised patients, PCR or visualization of larger lesions that have already occurred using imaging methods ( CT , MRT ) is most suitable .

T. gondii is one of the infections that should be routinely tested for in pregnant women, similar to rubella , syphilis , hepatitis B , chlamydia , HIV , possibly cytomegaly . In Germany, however, the examination is not part of normal prenatal care. If an infection with T. gondii has been detected before , there is no longer any danger. The unborn child is then protected from infection during pregnancy by the maternal antibodies.

The diagnosis can be very difficult if it has to be made retrospectively in a newborn who shows signs of disease late (for example, blindness from chorioretinitis ).

therapy

An initial infection with T. gondii during pregnancy should be treated with antibiotics . Otherwise, treatment may be useful if the patient shows symptoms. The combination of pyrimethamine together with a sulfonamide or clindamycin has proven itself . Before the 18th week of pregnancy, a macrolide antibiotic , e.g. B. spiramycin , because this - in contrast to the former - probably does not cause any malformations in the unborn child. However, one is not sure how likely it is to be transmitted during early pregnancy. The child may need to be treated for a while after the birth. All in all, harm to the child from this very common parasite can usually be prevented.

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literature

  • Marianne Abele-Horn: Antimicrobial Therapy. Decision support for the treatment and prophylaxis of infectious diseases. With the collaboration of Werner Heinz, Hartwig Klinker, Johann Schurz and August Stich, 2nd, revised and expanded edition. Peter Wiehl, Marburg 2009, ISBN 978-3-927219-14-4 , pp. 234-237 ( toxoplasmosis ) and 294.

Individual evidence

  1. ↑ Nicolle, Manceaux: Sur un protozoaire nouveau du gondi (Toxoplasma n. Sp.) . Arch Inst Pasteur, Tunis 1909; 2:97.
  2. ^ A. Sabin & Feldman: Dyes as microchemical indicators of a new immunity phenomenon affecting a protozoon parasite ( Toxoplasma ) . In: Science , 1948, 108, p. 660.
  3. ^ A b Gottfried Schatz : Beyond the Genes - Zurich: Verlag Neue Zürcher Zeitung, 2008, 1st chapter: Unheimliche guests. Can parasites change our personality? NZZ-Online
  4. ^ A b Toxoplasmosis - RKI Guide for Doctors Robert Koch Institute, December 12, 2016; accessed on July 31, 2017.
  5. ↑ A. Vyas, SK Kim et al. a .: Behavioral changes induced by Toxoplasma infection of rodents are highly specific to aversion of cat odors. In: PNAS , Volume 104, Number 15, April 2007, pp. 6442-6447, ISSN  0027-8424 . doi: 10.1073 / pnas.0608310104 . PMID 17404235 . PMC 1851063 (free full text).
  6. ↑ WM Ingram, LM Goodrich u. a .: Mice Infected with Low-Virulence Strains of Toxoplasma gondii Lose Their Innate Aversion to Cat Urine, Even after Extensive Parasite Clearance. In: PloS one , Volume 8, Number 9, 2013, p. E75246, ISSN  1932-6203 . doi: 10.1371 / journal.pone.0075246 . PMID 24058668 . PMC 3776761 (free full text).
  7. ↑ PK House, A. Vyas, R. Sapolsky: Predator cat odors activate sexual arousal pathways in brains of Toxoplasma gondii infected rats. In: PloS one , Volume 6, Number 8, 2011, p. E23277, ISSN  1932-6203 . doi: 10.1371 / journal.pone.0023277 . PMID 21858053 . PMC 3157360 (free full text).
  8. ↑ KD Lafferty: Can the common brain parasite, Toxoplasma gondii, influence human culture? In: Proceedings. Biological sciences / The Royal Society , Volume 273, Number 1602, November 2006, pp. 2749-2755, ISSN  0962-8452 . doi: 10.1098 / rspb.2006.3641 . PMID 17015323 . PMC 1635495 (free full text).
  9. ↑ Parasites: remotely controlled . Zeit Online , February 2009; Retrieved July 3, 2010.
  10. ↑ Florian Rötzer : The parasite that could also influence human behavior .
  11. ↑ Can toxoplasmosis affect the psyche? W for knowledge , broadcast on April 8, 2017; accessed on November 16, 2018
  12. ^ R. Verma, P. Khanna: Development of Toxoplasma gondii vaccine: A global challenge. In: Human vaccines & immunotherapeutics , Volume 9, Number 2, February 2013, pp. 291-293, PMID 23111123 , PMC 3859749 (free full text) (review).
  13. ↑ TOXOVAX® . MSD Animal Health. Retrieved November 11, 2015.

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