Malaria Host Range

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QUESTION

What are the Host range?

ANSWER

Malaria is caused by a parasite,  of the genus Plasmodium, which requires two organisms for the completion of its life cycle—an insect, which is usually referred to as the “vector,” in which sexual reproduction occurs, and a vertebrate “host,” in which asexual multiplication occurs.

Different species of Plasmodium infects a large range of vertebrate hosts, including many birds, reptiles, and dozens of different mammals, specifically primates (including humans) and rodents. These different types of Plasmodium are found all over the world; Plasmodium species that infect birds, for example, are found in much colder climates than human malaria.

The five species of Plasmodium that infect humans are found mainly in the tropics, and seasonally in the sub-tropics. Four of the species mainly infect humans only, but can also be found occasionally in other primates, such as chimpanzees and gorillas; the fifth species, P. knowlesi, is mainly considered a malaria of macaque monkeys in south-east Asia, but can also infect humans. As such, the host range of these malaria species extends wherever humans live throughout the world. However, in reality, the distribution of malaria is constrained to warmer, wetter climates based on the developmental needs of the parasite and also the vector.

For mammalian malaria, these vectors are all mosquitoes of the genus Anopheles, which breed in stagnant freshwater and also have temperature requirements, above or below which they will not develop. However, these mosquitoes are still found in many parts of the world, from far northern latitudes of Canada and Siberia in the summer to similar latitudes in the southern hemisphere and also throughout the tropics.

Structure and Size of Malaria

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QUESTION

What is the structure and size of malaria?

ANSWER

Malaria is caused by a single celled protist of the genus Plasmodium. It undergoes a complex life cycle, involving two separate asexual reproductive stages in the vertebrate host (including humans) and sexual reproduction as well as multiplication in the insect vector (all human malaria is transmitted by mosquitoes). As such, the parasite undergoes much structural and cellular change during its lifecycle, including to its size.

In terms of structure, most of the infectious stages of Plasmodium consist of a nucleus (which contains DNA, the cell’s genetic material), a mitochondrion (used for respiration and thus producing the energy for the cell), microtubules (for moving proteins and other molecules) and an apicoplast, which is a organ unique to this group of single-celled parasites which is thought to be implicated in the invasion of host cells. The surface of the organism is covered in different proteins, some of which are used for binding to target host cells.

Feeling Ill After Malaria Infection

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QUESTION

How soon after infection, will a person usually begin to feel ill?

ANSWER

There is a latent period, during which time the malaria parasites (which had entered the bloodstream via the bite of an infected Anopheles mosquito) migrate to the liver and undergo replication. The sporozoites (the life stage that enters the blood from the mosquito) differentiate in many merozoites, which are contained within a schizont in the liver cells. When the schizont ruptures, the merozoites are released and re-enter the blood, where they set about infecting red blood cells. Once in the red blood cells, the infected person will begin to experience symptoms—this is usually between 6-16 days after the initial infection, during which time the parasite is reproducing in the liver.

Pathophysiology of Malaria

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QUESTION

What is the pathophysiology of malaria?

ANSWER

Malaria causes disease through a number of pathways, which depend to a certain extent on the species. Malaria is caused by a single-celled parasite of the genus Plasmodium; there are five species which infect humans, beingPlasmodium falciparum, P. vivax, P. ovale, P. malariae and P. knowlesi.

All these species are introduced into the human blood stream through the bite of an infected mosquito; the life stage of malaria at this point is called a “sporozoite”, and they pass first to the liver, where they undergo an initial stage of replication (called “exo-erythrocytic replication”), before passing back into the blood and invading red blood cells (called “erythrocytes”, hence this is the “erythrocytic” part of the cycle). The malaria parasites that invade red blood cells are known as merozoites, and within the cell they replicate again, bursting out once they have completed a set number of divisions. It is this periodic rupturing of the red blood cells that causes most of the symptoms associated with malaria, as the host’s immune system responds to the waste products produced by the malaria parasites and the debris from the destroyed red blood cells. Different species of malaria rupture the red blood cells at different intervals, which leads to the diagnostic cycles of fever which characterise malaria; P. vivax, for example, tends to produce cycles of fever every two days, whereas P. malaria produces fever every three.

In addition, Plasmodium falciparum produces unique pathological effects, due to its manipulation of the host’s physiology. When it infects red blood cells, it makes them stick to the walls of tiny blood vessels deep within major organs, such as the kidneys, lungs, heart and brain. This is called “sequestration”, and results in reduced blood flow to these organs, causing the severe clinical symptoms associated with this infection, such as cerebral malaria.

More details on the exact biochemical mechanisms for sequestration and its effect on the pathology of the infection can be found on the Tulane University website.

Malaria Transmission

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QUESTION

Can one get malaria through contact with the infected person or is it airborne?

ANSWER

Malaria cannot be transmitted through direct contact with an infected person, nor is it airborne! It is actually transmitted directly via the bite of an infected mosquito. Only certain female mosquitoes, of the genus Anopheles, can carry malaria. The mosquito picks up the malaria parasite (there are five different types of malaria that infect humans, though all are transmitted in exactly the same way) when it feeds on the blood of an infected person. The parasite then undergoes a cycle of reproduction in the mosquito, before new parasites migrate once again to the mosquitoes salivary glands. From here, they are able to escape into the blood of a new human host when the mosquito takes another blood meal by biting the person.

Since malaria is transmitted by blood, there have been a some reports of malaria transmission via organ donor or blood transfusion, though most countries now screen for malaria before using donated blood or organs. Additionally, if a pregnant woman gets malaria, the parasite can be passed to her baby either across the placenta or during delivery; this is called “congenital malaria”, and can be quite harmful to the baby. As such, and also because pregnant women themselves are especially vulnerable to malaria, many campaigns have dedicated themselves to providing pregnant women with long-lasting insecticide treated bednets and other measures to prevent and treat malaria.

Number of Malaria Infections Per Mosquito

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QUESTION

How many people get malaria by one mosquito?

ANSWER

That is a very interesting question—the answer is that is varies between regions, and also depends on the type of mosquito. It was often believed that female mosquitoes (which feed on blood—males only feed on nectar) required a single blood meal in order to lay eggs and reproduce, and so would feed approximately once per week, infecting a single host each blood meal. However, recent evidence has shown that in many locations, Anopheles mosquitoes will actually take multiple blood meals on a single night, thus potentially being able to infect several hosts per gonotrophic (reproductive) cycle. Even more interestingly, in some studies it appears that mosquitoes infected with Plasmodium are more likely then uninfected mosquitoes to take multiple blood meals, thus suggesting that the parasite is in some way manipulating the feeding behaviour of the vector mosquito in order to facilitate its own transmission!

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