What Is Plasmodium? Definitions, Life Cycle, Characteristics, And Adaptations

Definition

Plasmodium is more commonly known as a malaria parasite, which is sometimes described as a genus of intracellular parasitic protozoa.

They are also a form of the obligate parasite, which means that they depend entirely upon their host to survive, with their hosts typically coming in the form of insects, with the most common one being mosquitos.

However, they will also use vertebrates as a host, when they are then known as a digenetic parasite, as they require two different hosts to complete their life cycle.

Within vertebrates, their multiplication occurs in the liver cells as well as the red blood cells where they gain nourishment whilst simultaneously damaging the cells. 

There are various types of species of plasmodium that can cause malaria, including:

  • P. falciparum
  • P. malariae
  • P. vivax
  • P. ovale
  • P. knowlesi

Insects like mosquitoes aren’t affected by malaria when carrying it, as they don’t possess any red blood cells for the malaria to attack, feed off, and thrive.

What Is Plasmodium? Definitions, Life Cycle, Characteristics, And Adaptations

Classification Of Plasmodium

Kingdom

Protista, otherwise known as protoctista, consists of differing single-celled eukaryotes, often found in both aquatic and terrestrial environments.

Sub-Kingdom

Protozoa – consists of free-living as well as parasitic single-celled organisms.

Phylum

Apicomplexa – it is composed of parasitic alveolates, which contain a plastid named apicoplast, which is then used to invade the cells of the parasite’s host. 

Family

Plasmodiidae – which is a family consisting of apicomplexan parasites. 

Genus

Plasmodium.

Species

Malariae, falciparum, vivax, ovale.

Characteristics And The Morphologies Of Plasmodium

Characteristics And The Morphologies Of Plasmodium

Plasmodium falciparum

This species is the most severe in comparison to the other species of Plasmodium, especially as it’s the one responsible for severe malaria (also known as malignant malaria), and is characterized by both a high fever and irregular paroxysms and can sometimes be fatal if the patient isn’t treated quickly and effectively.

However, the morphological characteristics are prone to change based upon the form or stage of the diagnosis. 

Ring Form

This form of P. falciparum is often found inside the red cells, and its name stems from the shape it is found in, a ring. It consists of a nucleus, cytoplasm, and a central vacuole.

This ring form is very small and thin and is therefore extremely fragile.

Trophozoites

These are incredibly small and can range from 1.25 to 1.5um in their size. They also feature a thin cytoplasm and have a somewhat signet ring shape to them, especially in their early stages.

Furthermore, they are also vacuolated as well as containing a singular nucleus. 

Schizonts

These are slightly larger than trophozoites, measuring in at 4.5 to 5um in their diameter. They are often found occupying two-thirds of the red cells.

They are typically characterized by either 2 or 4 merozoites, and they can even have up to 30 merozoites upon fully maturing.

They also have pigments that can look dark once stained. The Schizonts of Plasmodium Falciparum tend to be small and immobile and represent the dividing form of the parasite. 

Gametocytes

These are the sexual form of the parasite, which are characterized by their crescent shaped appearence.

As the sexual form, there both male and female forms, and are around one and a half times the size of a normal red cell. These are also infectious to mosquitos.

Sporozoites

This is the form that is infectious to humans, characterized by their thick pellicle, as well as mitochondrion, and a single nucleus. The pellicle consists of a double-membrane in addition to a layer formed of subpellicular microtubules.

The Sporozoites are also shaped like a sickle, and measure between 10 and 15um, and are the form that is capable of moving from its initial host to its second host. 

Plasmodium vivax

Rings

The plasmodium vivax has a signet ring shape, and is characterized by the large cytoplasm that contains a large chromatin. Upon development they begin to resemble an amoeboid in terms of shape.

The ring form of this parasite is around a third of the length in diameter compared to red cells.

Trophozoites

Characterized by a few specimens of large chromatin, as well as an amoeboid cytoplasm and pigments that are a mix of yellow and brown in appearance. 

Gametocytes

These are much more round and ovular in shape in comparison to Trophozoites, and appear more defined. One of the leading characteristics is the large amount of brown pigment that spreads through a red cell once infected.

Schizonts

These are characterized by having 12 to 24 merozoites, and are often big enough to fill the entire red cell. They’re also characterized by a similarly colored staining that trophozoites have when staining. 

Plasmodium ovale

Ring Form

The ring form of the P. ovale features either one or two larger chromatin dots as well as a very sturdy and thick cytoplasm. As they mature further, the Schuffner’s dots may develop further. 

Trophozoites

Similarly to the ring forms, the trophozoites are also sturdy, and contain some chromatin dots too. These appear irregularly and may appear to be compact too.

Gametocytes

These have a much more well defined shape, and tend to be round and ovular in shape. And are also large enough to fill the entire red cell.

Their brown pigment is their main characterization, and tends to appear more coarse in comparison to the P. vivax.

Schizonts

These contain 6 to 14 merozoites, and the schizonts are characterized by the large amount of dark-brown pigment that is found surrounded by the nuclei. 

Hosts

Humans aren’t the only hosts that plasmodium targets, with other vertebrates such as monkeys, pigeons, chickens, canaries, and snakes all being at risk from the various species of plasmodium. 

Life Cycle

The members of genus plasmodium are digenetic as mentioned above, therefore, the life cycle is completed in two stages across two hosts.

This cycle includes both asexual and sexual cycles, one of which happens in the insect (typically mosquitos) and the occurs in the vertebrae that the plasmodium infects.

Plasmodium relies on these processes which is only made possible by over 5,000 genes and associated proteins.

The various reproductive processes in both hosts not only ensure that the Plasmodium completes its cycle, but also that it evades any sort of response that the host’s body might have.

At the beginning of its life cycle, the mosquito is typically the host, it helps to support the adult form of the parasite, which means that it is able to sexually reproduce.

Then gametocytes form and fuse in the mosquito’s gut, and together form zygote. 

This zygote then undergoes molecular cellular changes, and further develops into ookinites that are then active and able to move.

This means that the parasite is able to move itself inside of the mosquito in the midgut area, from which it can further migrate towards the salivary gland of the mosquito.

Inside of the midgut, the oocysts undergo further division, producing a large number of sporozoites, all of which contain a singular set of chromosomes (which are haploids).

The oocyst then opens after a period of 8 to 15 days and releases these sporozoites which then invade the salivary glands.

Then once the mosquito begins to feed, it will use its proboscis to draw blood from the vertebrae, simultaneously injecting the skin with the sporozoites and infecting the person. 

As the secondary host, the vertebrae play host to an immature variant of the parasite, one that is unable to reproduce.

Once they have entered the host’s body, the sporozoites make their way to either the lymphatic system or to the blood vessels.

For the sporozoites in the bloodstream, they are then able to access the liver, although in some cases they may instead go to the spleen, endothelial cells, or the macrophages.

Inside of the liver, they begin to invade some of the hepatocyte cells and begin to proliferate in order to produce merozoites.

This phase is known as the pre-erythrocytic phase.

Inside of the hepatocytes, the sporozoites begin to proliferate and develop, residing in the parasitophorous vacuoles which then produce schizonts, which often contain over 30,000 merozoites.

This phase typically lasts between 5 to 6 days and is helped by the protein within the parasite named circumsporozoite protein.

Then, from the hepatocytes, the merozoites are then transported into the lungs, (through the lung capillaries), and are then released into the bloodstream.

By utilizing receptor-ligand interactions, the merozoites that have been released from the liver then begin to invade the red cells, all of this can happen in under a minute and is done quickly to prevent any immune cells from being able to form a response against the parasite. 

Upon attaching itself to the red blood cell, the membrane of the red cell is then deformed which allows the parasite to enter the cell by utilizing some specialized protein structures.

Inside of the cell, the parasite then starts forming a ring shape, creating a vacuole that helps to separate it from the rest of the intracellular environment that is in the red blood cell.

Haemoglobin, which is the main source of nutrition, which is then broken down, and the amino acids are then used for biosynthesis, allowing the parasite to grow and develop further, and increase in numbers too.

It’s common for the red blood cells to increase in size rather dramatically as the parasite undergoes this process. 

As the parasite divides further within the cell, it goes through several different stages, which result in the production of the trophozoites and then the schizonts.

The number will increase further and further until ultimately the cell bursts, thus releasing the new merozoites that will then begin to attack and invade other red blood cells.

Plasmodium Adaptations

Plasmodium has developed numerous adaptations over time that ensure its survival and high rate of infection. These are perhaps some of plasmodium’s most significant adaptations that have helped it survive and spread throughout the world.

Plasmodium Adaptations

High Number Of Sporozoites

Over ten to several hundred sporozoites are released into the skin as a mosquito begins feeding on the host, which means that even if some of the cells are engulfed by phagocytes, then enough cells survive to continue to infect the host. 

Complex Life Cycle

Plasmodium can face numerous threats throughout its life cycle, however these adaptations ensure that the parasite is able to develop and thrive:

  • Plasmodium is able to switch on and off the expressions of certain proteins, allowing the parasite to enter the red blood cells of the host and adapt to the new environment rapidly with little to no effect on the parasite in any way.
  • Plasmodium also changes its surface proteins as well as its metabolic pathway means that the parasite is able to avoid immune cells in the host created as a response to the infection, and has also shown to help increase drug resistance in some cases. 
  • The reproductive phases of Plasmodium are another adaptation in its life cycle that aids in its development. Because Plasmodium depends on two hosts throughout its life cycle, it switches been sexual and asexual reproductive phases. Insects like mosquitos are where Plasmodium is in its sexual reproduction phase, and then when it infects a vertebrae, it then enters its asexual reproductive phase, utilizing the host’s red blood cells. Combined, these two phases help the parasite complete its life cycle.
  • The plasmodium utilize a protein that is known as TRAP, or thrombospondin-related anonymous protein, as well as an actin-myosin motor. This allows the sporozoites to mobilize themselves when inside of the intermediate host and reach the hepatocytes, which is vital for them to continue their development.
  • The merozoites produced from the hepatocytes are full of proteins that allow the parasite to rapidly enter the red blood cells, but these proteins also put the parasite at risk from phagocytes engulfing them upon identification. However, the apical secretory organelles produce a special structure which ensures that the parasite is able to invade the red blood cell almost instantly, and prevents them from being consumed. They also produce a protective vacuole once they’ve entered the cell, which helps to separate itself from the cell’s cytoplasm.
  • The Plasmodium maintains osmotic stability in order to prevent the red blood cell from bursting too prematurely. As the parasite invades the cell, it increases in size by a considerable amount before eventually bursting, however, if the cell bursts too soon it can affect the development of the parasite. To help maintain osmotic stability, there is an increased rate of ingestion, digestion, and detoxification of the haemoglobin inside the cell. Allowing the parasite to develop as required without the risk of a premature cell burst. 

Frequently Asked Questions

There are often many questions asked surrounding Plasmodium and the nature of the parasite, so we’ve summed up some of the most common questions asked in order to help resolve any possible queries.

Who Discovered Plasmodium?

Plasmodium, and therefore Malaria, was discovered and studied by Alphonse Laveran, who was a professor of military diseases and epidemics, and was posted in Algeria in 1878 where Malaria was running rife amongst the army.

He then took it upon himself to study both the clinical aspects of the disease, as well as anatomic pathology. 

How Widespread Is Malaria?

According to the 2021 World Malaria Report, over 87 countries and territories are at risk from the transmission of Malaria, which equates to almost half the world’s population.

How Is Malaria Prevented?

There are two forms of prevention for Malaria: Mosquito control, and chemoprophylaxis.

To prevent transmissions from mosquitos, bed nets are used to prevent any mosquitos from landing on top of people sleeping (These are typically insecticide-treated nets).

Wearing clothes that covers most if not all of your exposed skin, as well as insect repellent used on exposed skin. 

Malaria chemoprophylaxis is reserved for travellers who are visiting countries where exposure to malaria is possible.

The choice of the drugs required is dependent on the travel destination, the length of time that exposure is possible, the level of transmissibility in the season, as well as the traveller’s age and whether they are pregnant or not.

How Is Malaria Diagnosed And Treated?

In order to diagnose malaria, a doctor will typically ask you questions about both your travelling history as well as your medical records, as well as scheduling a blood test and conducting a physical exam.

These blood tests are used to determine whether malaria is present in your blood sample, the type of malaria that has infected the bloodstream, if the strain of malaria you have is particularly resistant to certain drugs used for treatment, and whether the infection is set to cause any complications which could become serious. 

Blood test results can take anywhere from 15 minutes to several days depending on the test done, and if a doctor believes that your symptoms may lead to further complications, it is likely that they will conduct additional diagnostic tests.

Treatment for malaria is done with the use of drugs, however, the drugs used will depend on a number of varying factors that will be established by the doctors before they prescribe anything. These factors include:

  • The type of malaria parasite you’re infected by
  • How severe your symptoms are
  • Age
  • Whether you’re currently pregnant or not

The two most common antimalarial drugs are Chloroquine Phosphate, and Artemisinin-Based Combination Therapy – otherwise known as ACTs. 

Chloroquine Phosphate is the most preferred treatment for any possible parasite that is sensitive to the drug, however the level of drug resistance towards Chloroquine is increasing amongst parasites worldwide which renders Chloroquine useless.

ACT is a combination of multiple drugs that are effective against the malaria parasite in different ways, and is typically the second-choice for treatment should the strain of malaria be Chloroquine resistant.

The most common examples of ACT are artemether-lumefantrine (otherwise known as Coartem), and artesunate-mefloquine. 

How Many People Does Malaria Affect?

In 2020, according to the WHO, there were an estimated amount of 241 million cases of malaria worldwide, and the estimated number of deaths was 627,000.

The WHO African Region is the region with the highest number of cases and fatalities, with a disproportionate share in comparison to the world.

In 2020, 95% of the cases of malaria came from this region, as well as 96% of all malaria deaths that year. 

What Are The Symptoms Of Malaria?

Malaria can be somewhat hard to spot at first, however there are a number of symptoms to look out for that together can all culminate to indicate a Malaria infection.

  • Sweats, chills, and a high temperature.
  • Headaches and a feeling of confusion.
  • Extreme fatigue and tiredness, which is an especially common system in children.
  • Both feeling and being sick, diarrhoea and stomach pain.
  • Complete loss of appetite
  • Pain in muscles throughout the body
  • Whites of the eyes and the skin yellowing.
  • Sore throat, coughing, and overall difficulty breathing.

As a general rule, symptoms tend to arise from 7 to 18 days after the initial contact with an infected mosquito.

However, sometimes it may take a few months for symptoms to arise, which is what can make malaria so dangerous, as a rapid detection and diagnosis is necessary for a good chance at successful treatment.

Although rare, it’s even possible for it to take years for symptoms to begin to show. 

Where Does The Word Malaria Come From?

Malaria is actually an ancient disease and has been around for quite some time, with the early Greeks having recorded a disease with similar symptoms to Malaria.

For a long period of time, it was thought that malaria stemmed from the miasmas rising up from swamps. Which is actually where the name comes from, as it stems from the Italian for “bad air”. 

Conclusion

In conclusion, Plasmodium, and malaria, are incredibly dangerous parasites and hopefully with this guide you will gain a greater understanding of what Plasmodium is, its life cycle, characteristics, as well as its adaptations. 

Jennifer Dawkins

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