Why do mosquitoes cause malaria

Malaria: transmission

Malaria is caused by single-cell pathogens (plasmodia) that are transmitted from person to person by female Anopheles mosquitoes. The Anopheles mosquito is primarily crepuscular and nocturnal. Occasionally, the pathogen is transmitted from the mother to the unborn child during pregnancy. Transmission through infected blood (e.g. blood supplies) or through contaminated injection needles is also possible.

During their development, the plasmodia take on different forms that can reproduce either sexually or asexually. The development cycle is divided into two parts: In humans, the pathogens multiply asexually, while in mosquitoes sexually.

When an infected mosquito bites, the infectious forms of the pathogen (sporozoites) enter the human bloodstream with the mosquito's saliva. They are flushed into the liver with the blood and penetrate the liver cells. There the pathogens transform into a form of multiplication and begin to divide (hepatic schizogony). At the end of this multiplication phase, a so-called tissue schizont can contain up to 30,000 pathogens (merozoites). The merozoites are released into the bloodstream and actively penetrate the red blood cells (erythrocytes). They multiply there until the red blood cells burst and the newly formed merozoites emptied into the bloodstream (blood schizogony). These now attack new blood cells. After several of these cycles of reproduction, the Malaria tropica and in Knowlesi malaria a large number of red blood cells are attacked, which leads to the serious clinical picture of these forms of malaria. With the milder one Malaria tertiana and quartana only a maximum of 1-3% of the red blood cells are affected.

There at P. falciparum (and at P. knowlesi) the development cycles in the blood do not run synchronously, the fever attacks caused by the bursting of the red blood cells usually do not occur regularly. At P. vivax, P. oval and P. malariae on the other hand, the development cycles in the blood usually run synchronously, which is why the fever attacks occur at time intervals typical for the pathogen.

A few pathogens transform into male and female gender forms (gametocytes). If the gametocytes in the blood are ingested by an Anopheles mosquito when it bites, they enter the intestine of the insect. There the sex forms unite to form a fertilized egg cell, from which new infectious sporozoites eventually develop. The sporozoites migrate into the mosquito's salivary gland and can be transferred from there again during a bite. The cycle begins again.

At Plasmodium vivax and Plasmodium ovale (Malaria tertiana) forms resting forms (hypnozoites) in the liver. After a variable resting phase lasting from a few weeks to several years, they can develop further and attack red blood cells. As a result, these pathogens can trigger relapses years after infection. The triggers for these relapses are little known, but stress and other infections make them worse. Plasmodium malariae (Quartana malariaIf left untreated, it can lead to a long-lasting infection of the red blood cells at a very low level, with symptoms of the disease only occurring in phases of greater proliferation. Symptoms can appear for the first time even after a very long period of time (> 10 years after infection).