Malaria: Parasite or virus? Part 1
Malaria: parasite or virus?
The first historical reports of symptoms that match those of malaria date back to the ancient Egyptians (around 1550 B.C.) and the ancient Greeks (around 413 B.C.). These early descriptions noted the association between fevers and wet ground. The first records of the word malaria come from around 1730. It is a contraction of the Italian “mala aria”, meaning “bad air.” People once thought malaria was caused by poisonous air, the mal'aria associated with marshes and swamps.
Alphonse Laveran, a French army doctor, described the malarial parasite in 1880 and proposed that it caused malaria. But it was a British physician, Sir Ronald Ross, who was working in India in 1897 when he observed the development of oocysts in mosquitoes that had been fed on infected individuals. Ross's description of the complete life cycle of the malarial parasite won him the Nobel Prize for Medicine in 1902.
How does a mosquito become infected?
When a mosquito bites an infected individual, the sporozoan's male and female sexual stages, or gametocytes, are taken up in the blood meal. Fertilization ensues in the mosquito's gut, and an "ookinete" forms. The ookinete then bores through the mosquito's stomach wall, becoming an oocyst, which subsequently divides to produce about a thousand infective sporozoites. In P. falciparum this process takes five to seven days, after which the sporozoites are released. They then migrate to the insect's salivary glands. Because mosquitoes inject their saliva when they bite (it contains anticoagulants and local anesthetic substances that facilitate blood sucking), the malaria sporozoites will be passed along to the mosquito's next victim.
There are more than 125 different species of malaria that infect mammals, birds and reptiles. Malaria is defined in dictionaries as an intermittent and remittent fever caused by a single-celled protozoan parasite, known as a sporozoan, that invades the red blood cells. The parasite is transmitted by female mosquitoes in many tropical and subtropical regions who have been infected during a blood meal of a parasite-infected organism.
What exactly is a parasite?
Parasitism is generally defined as a relationship between two living species in which one organism benefits at the expense of the other. The organism that benefits is called the parasite, while the one that is harmed is called the host. A few examples of parasites are tapeworms, fleas, and barnacles.
The word, parasite, comes from the Medieval French parasite, from the Latin parasitus, the latinisation of the Greek παράσιτος (parasitos), "one who eats at the table of another". The original meaning of parasite is "a person who exploits the hospitality of the rich and earns welcome by flattery." Our earliest evidence of that use is from 1539. Evidence of the meaning referring to an organism that lives in, with, or on another organism dates to 1728.
Are there different types of parasites?
As it turns out, there are many types of parasites, other than the “human” ones.
Permanent parasites spend their entire life in the host organism such as Ascaris lumbricoides which targets the small intestine, causing Ascariasis in humans.
Accidental parasites may accidentally affect a host it isn’t usually found in such as Toxocara canis or dog roundworm, which may accidentally infect a human.
Hyper parasitism occurs when one parasite affecting a host cell is affected by another parasite, such as when Pieris rapae, a pest of the brassica plant, is parasitized by the larva of the Costesia glomerata wasp.
Ectoparasites take their nutrition from the outside of the host, such as the female Anopheles mosquito.
Endoparasites enter the host and take nutrition either parasitically or symbiotically.
Obligate parasites cannot survive (grow, develop, reproduce) without a host, such as the HIV virus which requires a Macrophage cell as host.
Facultative parasites can live with or without a host; in other words they can live in either a parasitic or non-parasitic manner.
Which type of parasite is involved with malaria?
Malaria is believed to be caused by single-celled microorganisms from the Plasmodium group. Four kinds of malaria parasites that infect humans are Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale and Plasmodium malariae.
But wait a minute…before moving on with this line of thinking, let us consider whether Malaria could be a virus…
What is a virus?
The name “virus” is from a Latin word meaning “slimy liquid” or “poison.”The earliest indications of the biological nature of viruses came from studies in 1892 by the Russian scientist Dmitry I. A virus is a small obligate intracellular parasite, which by definition, contains either an RNA or DNA genome (based on the same genetic code that's used in the cells of all living creatures) surrounded by a protective, virus-coded protein coat or shell, called a capsid. They may also contain an “envelope”, a sphere of membrane made of lipid. A virus cannot replicate alone; instead, it must infect or “take over” cells and use components of the host cell to make copies of itself.
Viruses can multiply only in living cells of animals, plants, or bacteria. Because they can’t reproduce themselves without a host, they are not considered to be living. Viruses do not have cells: they're very small, much smaller than the cells of living things. However, like cell-based life, viruses have genetic variation and can evolve. Even though they don't meet the definition of life, viruses have their own separate category. They come in many different shapes, such as the two pictured below.
The two viruses are not drawn to scale. (An Ebola particle is much larger than a chikungunya particle). Image modified from "Ebolavirus" and "Alphavirus," by ViralZone/Swiss Institute of Bioinformatics, CC BY-NC 4.0.
So, are viruses parasites?
Viruses are quintessential parasites; they depend on the host cell for almost all of their life-sustaining functions. Unlike true organisms, viruses cannot synthesize proteins, because they lack cell organelles (ribosomes) for the translation of viral messenger RNA (mRNA; a complementary copy of the nucleic acid of the nucleus that associates with ribosomes and directs protein synthesis) into proteins. Viruses must use the ribosomes of their host cells to translate viral mRNA into viral proteins.
Viruses are also energy parasites; unlike cells, they cannot generate or store energy in the form of adenosine triphosphate (ATP). The virus derives energy, as well as all other metabolic functions, from the host cell. The invading virus uses the nucleotides and amino acids of the host cell to synthesize its nucleic acids and proteins, respectively.
Are viruses all bad?
Viruses are ever-present and circulating in our bodies. This kingdom is called the “virome”; it is the sum of all the viruses within our body, found in every tissue from our blood to our brains, and even interwoven into the genetic code within our cells. Viruses are the most numerous organisms on earth. While we are thought to have roughly the same number of bacterial cells as human cells in our body (around 37 trillion), we could possibly have 10 times as many virus particles! Many of these viruses are involved in essential bodily processes, forming part of our inner ecosystem. If they all disappeared, we would too!
Does the mosquito also have a virome?
Yes it does! A recent review found reports of at least 51 viruses naturally associated with Anopheles and Anopheles mosquitoes harbor a diverse natural virome of RNA viruses. These viruses are mainly insect specific viruses, thought to multiply only in insects, but also include relatives of arboviruses that can replicate in both insects and vertebrate cells. So far 115 viruses are associated with the mosquito virome and this is expanding as research continues.
Do mosquitoes only bite during the rainy season?
The New York Times reported on a study in Mali which suggests that malaria parasites hide out during the dry season by altering the properties of red blood cells. A new study in Nature Medicine by researchers from Germany and Mali has discovered that the Plasmodium parasite enacts a genetic change that enables it to hide in an infected person’s bloodstream for months, undetected.
Blood tests in the 600 children and young adults in the study revealed that, even when samples had too few parasites to be seen under a microscope, about 20 percent of the study participants still had very low levels of parasites hiding inside some of their red blood cells. In some infected red blood cells, the malaria parasite produces sticky proteins that rise to the surfaces of the cells. These cells adhere to the walls of the veins and arteries, instead of being swept downstream into the spleen, to be destroyed. The new study found that this activity varies with the season.
When malaria parasites are abundant, they cause red blood cells to pump out sticky proteins; these cells can jam the tiny capillaries in the brain. “Cerebral malaria” is often fatal. Each parasite has a long menu of proteins written into its genes, from which it can order selectively. It can produce up to 60 variants of the proteins that migrate to the surface of the cell. Normally, a parasite will shift to a new protein every few days, to escape the antibodies produced by the host’s immune response.
During the dry season, the researchers found that the parasites in most red blood cells stopped making the sticky versions of that protein. They slipped away into the spleen to their destruction. But a few clingy survivors hung on, and appeared to slow down their metabolism, to protect themselves:
Firstly, the reclusive parasites had somehow become too “quiet” to trigger the immune counterattack that might destroy them.
Secondly, too few sticky cells remained to clog brain capillaries, so even infected children survived.
What do you think?
Is it the Plasmodium parasite that causes all the trouble? Or, is it a malaria virus that came with the Plasmodium parasite that is the problem? Or, could it be the Anopheles mosquito itself?
Check out Part 2 for more information.
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