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Pre-erythrocytic (liver) stage: Difference between revisions

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{| class="wikitable" style="border-style: solid; border-width: 5px; border-color: #023020; color:black"
|colspan="1" style = "font-size:100%; color:black; background: #afbddb |'''The schizont pathway'''
|colspan="1" style = "font-size:100%; color:black; background: #afbddb |'''The initial infection of the host'''
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The stage begins with the first cycle of '''asexual replication''' forming a recognisable “schizont” then concludes when the individual “merozoites” are released to infect new erythrocytes.
<gallery mode="nolines" widths=300px heights=300px>
File:Liver 0.jpg|<span style="font-size:90%">''Infection of the mosquito''</span>|link={{filepath:Liver 0.jpg}}
</gallery>


<gallery mode="nolines" widths=300px heights=600px>
The initial mosquito bite (Image A): blood from the host is taken through as mosquito mouthparts (m) from small vessels (v). At the same time fluid from mosquito salivary glands passes into the vessel. If a mosquitos is host to the malaria organism then parasites enter the blood with this saliva (in the form of '''"sporozoites"''' (sp)).
File:Blood_stages_schizonts_only.jpg|<span style="font-size:80%">''Basic schizont development (coloured images)''</span>|link={{filepath:Blood_stages_schizonts_only.jpg}}
</gallery>


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{| class="wikitable" style="border-style: solid; border-width: 5px; border-color: #023020; color:black"
{| class="wikitable" style="border-style: solid; border-width: 5px; border-color: #023020; color:black"
|colspan="1" style = "font-size:100%; color:black; background: #afbddb |'''Merozoite release'''
|colspan="1" style = "font-size:100%; color:black; background: #afbddb |'''Infection and replication in liver'''
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<gallery mode="nolines" widths=300px heights=300px>
<gallery mode="nolines" widths=300px heights=300px>
File:the schizont.jpg|<span style="font-size:80%">''Formation and release of merozoites''</span>|link={{filepath:the schizont.jpg}}
File:Liver 1.jpg|<span style="font-size:90%">''Formation and release of merozoites in liver''</span>|link={{filepath:Liver 1.jpg}}
</gallery>
</gallery>




(1) The stage begins with the first cycle of asexual division producing two chromatin masses</br>
The sporozoites that have entered the blood then pass through the blood vessel and into the cells of the liver (B). They may pass through multiple cells but then remain in a single liver cell (C).
(2) This is followed by further cycles of replication </br>
(3) In this case this results in the formation of 8 daughter parasites </br>
(4) The daughter parasites mature and the red cell ruptures to release the “merozoites” </br>
(5) The released merozoites very rapidly infect new red cells (so rapid that free merozoites will not usually be seen in blood).


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{| class="wikitable" style="border-style: solid; border-width: 5px; border-color: #023020; color:black"
|colspan="1" style = "font-size:100%; color:black; background: #afbddb |'''Morphological features and relevance'''
|colspan="1" style = "font-size:100%; color:black; background: #afbddb |'''From liver to blood'''
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(1) '''The number of replication cycles differs between species:''' the typical number of merozoites formed differs between species with as few as 8 (in P.malariae) up to a possible 32 (in P.vivax)</br>
The parasites in liver cells (D) then undergo repeated cycles of asexual replication (E) to form schizonts which are similar to the schizont stage in blood. At the end of the process the '''“merozoites”''' that are formed are released into blood (F).
(2) '''This stage may not always occur in blood:''' schizonts of ''P.falciparum'' adhere within the small vessels so is not seen in blood unless infection is very severe
</br></br>


<gallery mode="nolines" widths="200px" heights="220px" >
<gallery mode="nolines" widths="300px" heights="300px" >
File:Schizontreal4.jpg|Mature schizont releasing merozoites|link={{filepath:Schizontreal4.jpg}}
File:Liver 2.jpg|<span style="font-size:90%">''Mature schizont releasing merozoites''|link={{filepath:Liver 2.jpg}}
</gallery>
 
 
They then infect red cells and now have the typical appearance of early trophozoite (G).
 
 
<gallery mode="nolines" widths="200px" heights="200px" >
File:Liver 3.jpg|<span style="font-size:90%">''Trophozoites now appear in blood''|link={{filepath:Liver 3.jpg}}
</gallery>
</gallery>


<span style="font-size:200%">&#x2192;</span> [[Click for images of schizont morphology|Click for more images of schizont morphology]]


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|colspan="1" style = "font-size:100%; color:black; background: #afbddb |'''Relevance of schizonts to clinical biology'''
|colspan="1" style = "font-size:100%; color:black; background: #afbddb |'''Relevance of hepatic stage to clinical biology'''
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The release of merozoites from schizonts exposes the body to large amounts of free parasite antigens no longer contained within the erythrocytes - the result is an immune response causing high fever and illness symptoms. In some cases the development of parasites is synchronous so that all schizonts mature and release their merozoites at the same time - although rarely seen now, this pattern of development may produce a pattern of remitting fever with a distinct periodicity depending on species: underlying the older descriptive terms tertian or quartan malaria.
The hepatic stage is the period where parasites begin to replicate and cause infection - this corresponds to an incubation period before symptoms begin; this will typically last between 1-4 weeks, and during this time parasites will not be detected in blood. The number of bites required for infection may be only 1 if the mosquito is heavily infected (hence cases of airport malaria where the infection is acquired from a mosquito passenger in luggage).
 
 
For two malaria species (''P.ovale'' and ''P.vivax'') the hepatic stage may lie dormant for a period of time - this is the hynozoite (“sleeping animal”). This hyponozoite may reactivate - typically in less than a year although occasionally longer, causing clinical malaria symptoms long after the initial infection.
 


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Latest revision as of 11:01, 22 April 2024

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>Basic malaria biology
>>This page: Pre-erythrocytic (liver) stage


The initial infection of the host


  • Infection of the mosquito

    Infection of the mosquito

The initial mosquito bite (Image A): blood from the host is taken through as mosquito mouthparts (m) from small vessels (v). At the same time fluid from mosquito salivary glands passes into the vessel. If a mosquitos is host to the malaria organism then parasites enter the blood with this saliva (in the form of "sporozoites" (sp)).

Infection and replication in liver
  • Formation and release of merozoites in liver

    Formation and release of merozoites in liver


The sporozoites that have entered the blood then pass through the blood vessel and into the cells of the liver (B). They may pass through multiple cells but then remain in a single liver cell (C).

From liver to blood


The parasites in liver cells (D) then undergo repeated cycles of asexual replication (E) to form schizonts which are similar to the schizont stage in blood. At the end of the process the “merozoites” that are formed are released into blood (F).

  • Mature schizont releasing merozoites

    Mature schizont releasing merozoites


They then infect red cells and now have the typical appearance of early trophozoite (G).


  • Trophozoites now appear in blood

    Trophozoites now appear in blood


Relevance of hepatic stage to clinical biology


The hepatic stage is the period where parasites begin to replicate and cause infection - this corresponds to an incubation period before symptoms begin; this will typically last between 1-4 weeks, and during this time parasites will not be detected in blood. The number of bites required for infection may be only 1 if the mosquito is heavily infected (hence cases of airport malaria where the infection is acquired from a mosquito passenger in luggage).


For two malaria species (P.ovale and P.vivax) the hepatic stage may lie dormant for a period of time - this is the hynozoite (“sleeping animal”). This hyponozoite may reactivate - typically in less than a year although occasionally longer, causing clinical malaria symptoms long after the initial infection.


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