Pre-erythrocytic (liver) stage

From MalariaETC

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The initial infection of the host

The initial mosquito bite (Image A): blood from the host enters through the mosquito mouthparts (m) from small vessels (v). At the same time fluid from mosquito salivary glands passes in the opposite direction into the host blood allowing parasites to enter the blood with this saliva (called "sporozoites" (sp)).

Infection and replication in liver

The sporozoites that have entered the blood pass through the blood vessel wall and enter cells of the liver (B). They may pass through multiple cells, but finally settle in a single liver cell (C).

From liver to blood

In the liver cells (D) the parasites undergo successive cycles of asexual replication (E). This forms schizonts (similar to the schizont stage in blood). At the end of the process mature individual parasites (“merozoites”) are formed and released into blood (F).

The merozoites then infect red cells gaining the typical appearance of early trophozoite (G).

Relevance of hepatic stage to clinical biology

The hepatic stage is the period of parasites replication - this is the incubation period before symptoms begin that typically lasts between 1 and 4 weeks. During this time parasites are not detected in blood. Only a single bite from a heavily infected mosquito my be enough to cause infection (hence cases of airport malaria where the infection is acquired from a mosquito passenger in luggage).

In two malaria species (P.ovale and P.vivax) the hepatic stage may lie dormant for an extended period of time - this is the hynozoite (“sleeping animal”). This hyponozoite may the reactivate to cause a delayed infection. Typically this happens within the first few months and generally is less than a year (although occasionally longer).