Difference between revisions of "Toxoplasma gondii"

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[[Image:Toxoplasma gondii.jpg|thumb|right|150px|''Toxoplasma gondii'' - Ke Hu and John Murray]]
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[[Image:Toxoplasma gondii.jpg|thumb|right|150px|''Toxoplasma gondii'' <br> Ke Hu and John Murray, WikiMedia Commons]]
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[[Image:Toxoplasma sporulated oocyst.jpg|thumb|right|150px|''Toxoplasma'' Sporulated Oocyst - Wikimedia Commons]]
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[[Image:Toxoplasma Tacchyzoites.jpg|thumb|right|150px|''Toxoplasma'' Tacchyzoites - Wikimedia Commons]]
 
[[Image:Toxoplasma Life Cycle.jpg|thumb|right|150px|''Toxoplasma'' Life Cycle Diagram - Dennis Jacobs & Mark Fox RVC]]
 
[[Image:Toxoplasma Life Cycle.jpg|thumb|right|150px|''Toxoplasma'' Life Cycle Diagram - Dennis Jacobs & Mark Fox RVC]]
 
[[Image:Toxoplasma gondii 2.jpg|thumb|right|150px|''Toxoplasma gondii'' - Courtesy of the Laboratory of Parasitology, University of Pennsylvania School of Veterinary Medicine]]
 
[[Image:Toxoplasma gondii 2.jpg|thumb|right|150px|''Toxoplasma gondii'' - Courtesy of the Laboratory of Parasitology, University of Pennsylvania School of Veterinary Medicine]]
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[[Image:Toxoplasmosis Tissue Cyst.jpg|thumb|right|150px| Toxoplasma tissue cyst. Source: Wikimedia Commons; Author: Marvin 101 (2008)]]
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*Major pathogenic species called ''Toxoplasma gondii''
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==Introduction==
 
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''Toxoplasma gondii'' is an obligate, intracellular coccidian parasite that is capable of infecting most mammals including man. Cats and other Felidae are the definitive host for ''T. gondii'', and all other mammals are intermediate hosts. ''Toxoplasma gondii'' has three infectious stages:
 
  
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1) sporozoites;
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*Causes disease in a wide range of animal species including humans
  
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2) an actively reproducing stage called tachyzoites; and
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*Important cause of abortion in sheep
  
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3) slowly multiplying bradyzoites.
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*Zoonotic
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**Can cause abortion
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**Can cause congenitally aquired defects
  
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Tachyzoites and bradyzoites are found in tissue cysts, whereas sporozoites are contained within oocysts, which are excreted in the faeces. This means that the protozoa can be transmitted by ingestion of oocyst-contaminated food or water, or by consumption of infected tissue. Transplacental infection is also possible.
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*Forms a sporulated oocyst which is only 10μm
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**Contains 2 sporocysts with 4 sporozoites
  
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==Life Cycle==
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*Transmission through ingesting the intermediate host or via the faecal-oral route
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The life cycle of ''T. gondii'' is complex, usually indirect and often referred to as '''facultatively heteroxenous'''. Intermediate host is not essential for completion of the life cycle. Gametogony (sexual stage) is host specific for felids.
 
  
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===Cats===
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'''Life Cycle'''
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In the naive definitive host (cats), ''Toxoplasma gondii'' undergoes an '''enteroepithelial life cycle'''. Cats become infected by ingesting intermediate hosts containing tissue cysts, which release their '''bradyzoites''' in the gastrointestinal tract when the wall is digested. Bradyzoites then penetrate the small intestinal epithelium and start sexual reproduction: produce five types of '''schizonts''', which then give rise to '''merozoites'''. Male and female '''gamonts''' are formed from merozoites, which fertilise to form a '''macrogamont'''. A wall forms around the macrogamont to produce an '''oocyst''', which is passed in the faeces approximately three days after ingestion of the tissue cyst. Initially, these oocysts are unsporulated and are therefore not infectious, but after 1 to 5 days '''sporulation''' occurs to produce two sporocysts, each with four infectious '''sporozoites'''. The sporulated oocyst is only 10μm. This sporulation is dependent on temperature and aeration, and sporocyts can remain viable in the environment for up to 18 months even if exposed to high or freezing temperatures and low humidity. These can then be ingested by other mammals, including sheep and humans.
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*Complex
  
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Infection continues until the animal develops immunity (around 2 weeks) at which point the infection enters the chronic phase, the host's immune response has become effective. Groups of slow growing intracellular '''bradyzoites''' become walled off forming infective '''cysts'''. Bradyzoites inside cysts are protected from the host immune response whereas extracellular tachyzoites are killed. Cysts remain viable for months to years and are particularly numerous in muscle and nervous tissue. If immunity is suppressed the infection can revert to the acute form.
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*Usually indirect
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**Referred to as '''facultatively heteroxenous'''
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**Intermediate host is not essential for completion of the life cycle
  
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As cats generally develop immunity to ''T. gondii'' after the initial infection, they will only shed oocysts once in their lifetime.
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*Gametogony (sexual stage) is host specific for felids
  
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===Other species===
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*Any warm blooded animal can act as a facultative intermediate host
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When other, ‘’’non-feline, carnivores’’’ (such as dogs) or sheep consume tissue cysts or oocysts from cat faeces, ''Toxoplasma gondii'' initiates '''extraintestinal replication'''. This process is the same for all hosts, and does not vary with the form of the parasite ingested. '''Bradyzoites''' and '''sporozoites''', from cysts and oocysts respectively, are released in the intestine and infect the intestinal epithelium where they replicate - asexual reproduction by '''endodyogeny''' (budding) producing 8-16 '''tachyzoites'''. Tachyzoites are lunate in shape, about 6 microns in diameter and possess the ability to multiply in almost any cell type. The infected cell ruptures to release tachyzoites which then disseminate via blood and lymph to infect other tissues. Tachyzoites then replicate intracellularly and, if the cell does not burst, they eventually encyst and persist for the life of the host. Tissue cysts readily form in the CNS, muscles and visceral organs.
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**Asexual reproduction occurs in the intermediate host forming tissue cysts
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[[Image:Toxoplasmosis Life Cycle.jpg|thumb|right|300px| Life cycle of ''Toxoplasma gondii''. Source: Wikimedia Commons; Author: LadyofHats (2010)]]
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**Intermediate host swallows sporulated oocysts or tissue cysts
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**Can be transferred between intermediate hosts by carnivorism
  
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==Transmission==
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*Cats
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Although any of the three life stages described above can infect warm-blooded vertebrates, most infections are acquired following the ingestion of sporozoites or bradyzoites, as tachyzoites are easily inactivated in the gastric environment.
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**Sporulation occurs in 2-3 days
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**Cats either swallow infective (sporulated) oocysts where ''Toxoplasma gondii'' has a prepatent period of 3 weeks
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**Or eat the tissues of an infected intermediate host where ''Toxoplasma gondii'' has a prepatent period of 3-10 days
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**Self-limiting infection
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**Oocysts are shed for 1-2 weeks
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***Shedding can occur later if immunity wanes or cat is immuno-compromised
  
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Carnivores consume bardyzoites via tissue cysts in meat, and other animals ingest sporozoites via items '''contaminated with cat faeces'''.
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*Intermediate host
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As cats rarely practice coprophagy, infection is usually acquired through the ingestion of infected intermediate hosts such as rodents. Dogs tend to consume food or water contaminated with oocysts from cat faeces.
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**3 sources of infection
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***Oocysts from environment contaminated by cat faeces
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***Eating cysts in tissues of other infected hosts through carnivorism or undercooked meat
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***Transplacental transmission in some host species during the acute phase of infection
  
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If a pregnant queen or human is naive to ''Toxoplasma gondii'' at the time of ingestion, '''transplacental infection''' can occur. The outcome of this depends on the stage of gestation. Infection during the first trimester usually has severe consequences, such as stillbirth or abortion; infections during the second or third trimesters are more likely to give rise to an infected foetus.
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*Acute phase of infection
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**After infection of the intermediate host the organism undergoes a phase of rapid division and dissemination throughout the body
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***Parasite enters cell and asexual reproduction occurs by '''endodyogeny''' (budding) producing 8-16 '''tachyzoites'''
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***Tachyzoites are released when host cell bursts
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***Haematogenous spread as more cells are infected
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***Infection continues until the animal develops immunity (around 2 weeks) at which point the infection enters the chronic phase
  
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*Chronic phase of infection
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**Occurs once the host's immune response has become effective
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***Groups of slow growing intracellular '''bradyzoites''' become walled off forming infective '''cysts'''
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***Bradyzoites inside cysts are protected from the host immune response whereas extracellular tachyzoites are killed
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***Cysts remain viable for months to years and are particularly numerous in muscle and nervous tissue
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***If immunity is suppressed the infection can revert to the acute form
  
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For more information see:
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*Meat animals
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**Significant proportion of cattle, sheep, pigs and rabbits can tissue cysts
  
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<big>'''[[Toxoplasmosis - Cat and Dog|Cat Toxoplasmosis]]
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'''Pathogenesis'''
  
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'''[[Toxoplasmosis - Sheep|Sheep Toxoplasmosis]]
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*Cattle and horses
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**Sometimes infectious causing opthalmitis
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*Dogs
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**Complication of canine distemper
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**Causes [[Respiratory Parasitic Infections - Pathology#Toxoplasmosis|pneumonia]] and encephalitis
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*Toxoplasma can cause [[Pancreas Inflammatory - Pathology#Acute haemorrhagic pancreatitis|acute interstitial pancreatitis]] in systemic toxoplasmosis
  
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'''[[Toxoplasmosis - Human|Human Toxoplasmosis]]</big>
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*''Toxoplasma gondii'' causes [[Muscles Inflammatory - Pathology#Protozoa|myositis]]
  
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{{Learning
 
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|literature search = [http://www.cabdirect.org/search.html?rowId=1&options1=AND&q1=%22toxoplasma+gondii%22&occuring1=title&rowId=2&options2=AND&q2=&occuring2=freetext&rowId=3&options3=AND&q3=&occuring3=freetext&publishedstart=2000&publishedend=yyyy&calendarInput=yyyy-mm-dd&la=any&it=any&show=all&x=47&y=7 ''Toxoplasma gondii'' publications since 2000]
 
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|flashcards = [[Tissue_Cyst_Forming_Coccidian_Flashcards|Tissue Cyst Forming Coccidian Flashcards]]
 
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|Vetstream = [https://www.vetstream.com/canis/search?s=protozoa Protozoa]
 
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}}
 
  
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==References==
 
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#Merck & Co (2008) '''The Merck Veterinary Manual (Eighth Edition)''' ''Merial''
 
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#'''OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animals 2008''', Chapter 2.9.10, 1284-1293.
 
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#The Center for Food Security and Public Health, Iowa State University, College of Veterinary Medicine (2005). ''Toxoplasmosis Factsheet'', available at [http://www.cfsph.iastate.edu www.cfsph.iastate.edu]
 
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#Lappin, M R (2006) Toxoplasmosis. In '''North American Veterinary Conference Proceeeding 2006''', ''NAVC''.
 
  
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<big>'''[[Toxoplasmosis - Cat|Cat Toxoplasmosis]]
  
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{{review}}
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'''[[Toxoplasmosis - Sheep|Sheep Toxoplasmosis]]
  
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{{OpenPages}}
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'''[[Toxoplasmosis - Human|Human Toxoplasmosis]]
  
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[[Category:Liver Protozoa]]
 
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[[Category:Respiratory Parasitic Infections]]
 
 
[[Category:Tissue_Cyst_Forming_Coccidia]]
 
[[Category:Tissue_Cyst_Forming_Coccidia]]
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[[Category:Expert Review]]
 

Revision as of 23:13, 9 April 2010

Toxoplasma gondii - Ke Hu and John Murray
Toxoplasma Sporulated Oocyst - Wikimedia Commons
Toxoplasma Tacchyzoites - Wikimedia Commons
Toxoplasma Life Cycle Diagram - Dennis Jacobs & Mark Fox RVC
Toxoplasma gondii - Courtesy of the Laboratory of Parasitology, University of Pennsylvania School of Veterinary Medicine
  • Major pathogenic species called Toxoplasma gondii
  • Causes disease in a wide range of animal species including humans
  • Important cause of abortion in sheep
  • Zoonotic
    • Can cause abortion
    • Can cause congenitally aquired defects
  • Forms a sporulated oocyst which is only 10μm
    • Contains 2 sporocysts with 4 sporozoites
  • Transmission through ingesting the intermediate host or via the faecal-oral route

Life Cycle

  • Complex
  • Usually indirect
    • Referred to as facultatively heteroxenous
    • Intermediate host is not essential for completion of the life cycle
  • Gametogony (sexual stage) is host specific for felids
  • Any warm blooded animal can act as a facultative intermediate host
    • Asexual reproduction occurs in the intermediate host forming tissue cysts
    • Intermediate host swallows sporulated oocysts or tissue cysts
    • Can be transferred between intermediate hosts by carnivorism
  • Cats
    • Sporulation occurs in 2-3 days
    • Cats either swallow infective (sporulated) oocysts where Toxoplasma gondii has a prepatent period of 3 weeks
    • Or eat the tissues of an infected intermediate host where Toxoplasma gondii has a prepatent period of 3-10 days
    • Self-limiting infection
    • Oocysts are shed for 1-2 weeks
      • Shedding can occur later if immunity wanes or cat is immuno-compromised
  • Intermediate host
    • 3 sources of infection
      • Oocysts from environment contaminated by cat faeces
      • Eating cysts in tissues of other infected hosts through carnivorism or undercooked meat
      • Transplacental transmission in some host species during the acute phase of infection
  • Acute phase of infection
    • After infection of the intermediate host the organism undergoes a phase of rapid division and dissemination throughout the body
      • Parasite enters cell and asexual reproduction occurs by endodyogeny (budding) producing 8-16 tachyzoites
      • Tachyzoites are released when host cell bursts
      • Haematogenous spread as more cells are infected
      • Infection continues until the animal develops immunity (around 2 weeks) at which point the infection enters the chronic phase
  • Chronic phase of infection
    • Occurs once the host's immune response has become effective
      • Groups of slow growing intracellular bradyzoites become walled off forming infective cysts
      • Bradyzoites inside cysts are protected from the host immune response whereas extracellular tachyzoites are killed
      • Cysts remain viable for months to years and are particularly numerous in muscle and nervous tissue
      • If immunity is suppressed the infection can revert to the acute form
  • Meat animals
    • Significant proportion of cattle, sheep, pigs and rabbits can tissue cysts

Pathogenesis

  • Cattle and horses
    • Sometimes infectious causing opthalmitis
  • Dogs
    • Complication of canine distemper
    • Causes pneumonia and encephalitis


Cat Toxoplasmosis

Sheep Toxoplasmosis

Human Toxoplasmosis

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