Why do people have no immunity to malaria
The malaria pathogen
It is not the mosquito that causes the disease, but a tiny single-celled parasite, the so-called plasmodium. There are different types of malaria caused by different plasmodia.
The most dangerous of them is that Malaria tropicacaused by Plasmodium falciparum. Anyone who is infected with malaria tropica initially suffers from headache or body aches and gets irregular attacks of fever.
Once the Plasmodium falciparum has spread in the human body, it can attack the red blood cells. Anemia and severe organ damage can result. About ten percent of those infected die if they are not treated.
Almost as widespread as tropica malaria is Malaria tertianacaused by the Plasmodium vivax or the Plasmodium ovale. It was named after the frequency of fever attacks that occur every third day.
The regular attacks of fever are usually accompanied by severe chills. Unlike tropica malaria, tertiana malaria is rarely fatal. Both types of malaria together account for 90 percent of all malaria diseases.
A life in two hosts
The malaria parasites need two hosts in order to survive: the mosquito as the main host and humans as the intermediate host. In the course of this development, they constantly change their shape. Each of the hosts allows them a new stage of life. In order to be able to infect a person, the mosquito must first have been infected. This in turn happens through the bite on an infected person.
An eternal cycle - if it is not interrupted, the disease will continue to multiply. There are ideal conditions for the parasites where there are many mosquitoes and where many people do not receive malaria therapy. But it is a complicated cycle that does not make it easy for researchers to develop an antidote to malaria.
The Anopheles mosquito
Not every mosquito can transmit the malaria pathogen. Only certain Anopheles species are able to do this. Anopheles is the Greek word for "useless" and "harmful". The Anopheles gambiae is a particularly harmful mosquito because it transmits the deadly Malaria tropica in Africa.
Anopheles mosquitoes don't live long, but in their short lifespan they can lay hundreds to thousands of eggs, depending on the species. To do this, they primarily need stagnant water. They lay their eggs in lakes, swamps, puddles or other small bodies of water, which develop into new mosquitoes within a few days.
Female mosquitoes need blood to feed their eggs. Only females feed on human blood and therefore only they are dangerous.
If an infected female Anopheles stings a person, the pathogens, the plasmodia, enter the human bloodstream in the form of single-core sickle germs (sporozoites). Humans only feel the itching of a mosquito bite, but the asexual multiplication of the pathogen soon takes place in their bodies.
The development of the pathogen in humans
The sporozoites found their way to the liver in just 20 minutes. There they first implant themselves in the liver cells of humans. Here they change their shape for the first time: The single-core sickle germs, the sporozoites, grow into multi-core structures (schizonts).
After just a few days, these structures split into many single-grain partial sprouts. The pathogen in this form of the stage is called merozoites.
Asexual reproduction has begun. Many merozoites now penetrate the blood and from there into the red blood cells. Here the pathogen is constantly changing.
The merozoites develop again into sporozoites, which now do not grow in the liver cells but in the red blood cells to form polynuclear structures, which in turn disintegrate into many merozoites. This causes the red blood cells to burst. The new merozoites get back into the bloodstream, from there back into the red blood cells and so on. A chain reaction.
Only now do the first symptoms of the disease appear. The infected person gets attacks of fever - depending on which pathogen he was infected with, the time it takes for the pathogen to develop in the red blood cells varies. In the case of Plasmodium vivax and Plasmodium ovale, the pathogens need 48 hours to mature. Accordingly, the patient will have a fever attack every 48 hours.
These two pathogens have another peculiarity. With them, not all schizonts get from the liver into the bloodstream. Sometimes it takes years for them to set off. As a result, the infected people experience the relapses typical of tertian malaria.
If the patient is not treated after the first bouts of fever, the development of the plasmodia continues. Most of them remain in the bloodstream as merozoites and continue to multiply. The result: the organs can no longer be properly supplied with oxygen, they fail or the patient falls into a coma. One in ten dies.
However, not all merozoites remain in the bloodstream. A few transform again. They form the first forms of sexual development: sex cells, the male and female gametocytes.
The development of the pathogen in the mosquito
If an uninfected Anopheles mosquito bites an infected person, it ingests the pathogen in the form of male and female gametocytes with its blood meal.
In the mosquito intestine, the gametocytes develop into immobile gametes. The male gamete fertilizes the female gamete. The parasite stretches like a worm and transforms into a mobile cell that is now called an ookinet.
After many cell divisions, this ookinet develops into an egg ball, the oocyst. In this egg ball, new sporozoites now arise again. The cycle can start all over again: If the mosquito gets hungry for blood again and stings a person, then it transmits the pathogen in the form of the sporozoites back to the person with its saliva.
In order for the parasite to develop in the mosquito without any problems, it needs relatively high temperatures. The warmer, the faster it develops. For Plasmodium vivax, for example, the following applies: At temperatures below 15 degrees Celsius, the malaria pathogen does not multiply at all, at 20 degrees it develops within 16 days, at 28 degrees it only takes a week to reproduce.
The immune system cannot keep up
The transformation tactics of the plasmodia make it difficult for the human immune system to react in time. No sooner have antibodies formed against a form of the parasite than it changes its appearance.
If the immune system has adjusted to the changed variant after a while, the game starts all over again. So if you do not live constantly in malaria areas and do not come into contact with the parasites more often, you have little chance of building up adequate defenses.
This is especially true for tourists, who are highly endangered in risk areas, and for children, because up to the age of five they are not yet able to build up antibodies.
Anyone who lives as an adult in a malaria area in the tropics has in most cases built up antibodies. In order for these to work, however, it has to be bitten again by an infected mosquito at certain intervals. After about a year, the acquired immunity expires.
There is still no vaccination against malaria, but research has been intensifying for a number of years. The individual approaches are directed against different stages of the parasites in humans.
One tries to develop vaccines against the sporozoites so that they cannot penetrate into the liver cells or multiply there. Research is also being carried out into vaccines that are supposed to alleviate the course of malaria by containing the merozoites.
Since 2009 several African clinics have been investigating the effectiveness of the malaria vaccine RTS, S - with success. With young children in particular, the risk of developing a severe form of malaria can be significantly reduced by vaccinating them early.
Although the vaccine does not yet offer 100% protection, it could save the lives of thousands of people. While there are some promising approaches, vaccination may never be a panacea. The adaptability of the parasites also causes problems for medical professionals.
The scientists are therefore also researching other methods of containing malaria. One of the targets is the vector, the mosquito itself. Destruction with insecticides would be a method that should, however, also be viewed critically.
Others start with the parasites: In the USA, researchers are trying to use genetic engineering methods to modify the mosquitoes so that the parasites can no longer develop in the mosquito. A lengthy project - because such genetically modified mosquitoes would first have to be released and the other mosquitoes driven away.
Until this research shows success, it is above all important to develop new inexpensive drugs, reduce the breeding places for mosquitoes and make the use of mosquito nets a matter of course.
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