Parasitism is a symbiotic relationship in which one organism benefits (the parasite) and the other organism is harmed (the host).

Often parasitism involves the parasite obtaining nutrition directly from the host organism. Prominent biologist, E.O. Wilson, has likened parasitism to predation in which, "...the predator consumes the prey in units of less than one." For instance, the fungi, Tinea pedis, causes the symptoms of athlete's foot in humans because it is actually consuming the tissue around the foot. However, parasitism does not always involve one organism directly consuming the other. In many cases, such as with tapeworms, the parasite simply "steals" some of the food that is being digested by the host, and, among parasitic plant species, the strangler fig of the topical rainforest doesn't consume any nutrition obtained by its host: it simply out-competes its host for light. 

Like many symbioses, parasitic relationships are typically very host-parasite specific: in some cases even to the level of the species.  Malaria is a wonderful example of the incredibly complex and specific nature of many host-parasite relationships.  While there are similar diseases that affecting other species, the cause of the disease in humans, several species of protists from the genus Plasmodium, are, with perhaps one exception, entirely specific to their human and the Anopheles mosquito hosts. 

Malaria has evolved along with the human population and the Anopheles mosquito.  Although malaria is predominantly a tropical disease today, it's range once extended into Western Europe and as far north as the Ohio River Valley in North America.  More than 300 million people are affected with malaria, and as many as 1.5 million people die of malaria each year.

WHO map showing area affected by malaria

Of the more than 300 species of mosquito in the genus Anopheles, only around 60 are known to transmit the Plasmodium protists that cause malaria.  It is important to note that the protist is a parasite to both the mosquito and humans.  Note also that in this relationship both the Plasmodium and the Anopheles mosquito could be considered parasites to the human.

Malaria is transmitted from one human to another by Anopheles mosquitos.

The Plasmodium protists pass through at least four distinct developmental stages during their life-cycle.  When an infected mosquito bites a human, the parasite passes from cells in the salivary glands of the mosquito into the human's blood stream via the mosquito's saliva.  At this point the protist is called a sporozoite.

Click for large image.


Plasmodium sporozoites

Sporozoites travel through the host's blood stream, eventually reaching the liver, where the sporozoites enter liver cells and begin to reproduce asexually, forming a mass of merozoite cells called a schizont.  This process can take days to months depending on the species of Plasmodium involved.  The merozoites are then released into the human host's blood stream where they infect red blood cells.

Schizont in human liver cell


A Plasmodium merozoite

Upon infection of a red blood cell, the merozoites reproduce asexually until the red blood cell is full of merozoites.  The cell ruptures and the merozoites are released to infect more red blood cells. At certain times, some of the merozoites actually go through meiosis to produce male and female haploid gametocytes.  These gametocytes can not unite within the human host (the environment is not right).  In order for fertilization to take place, the gametocytes must first be taken into the digestive tract of the correct species of Anopheles mosquito.

Merozoites rupturing  red blood cells

When the correct species of Anopheles mosquito feeds on an infected human with gametocytes circulating through their blood stream, some of the gametocytes will most likely end up being transmitted to the mosquito.  In this way, we could think of humans as giving a parasite to the mosquito!  The conditions within the mosquito digestive tract turns out to be just right for Plasmodium gametocyte fusion (fertilization).  Upon fertilization, the zygote (Ookinete) moves through the intestinal epithelium and into the hemocoel (body cavity) of the mosquito, forming an oocyst.  The oocyst produces sporozoites, which are released at maturation and travel to the cells of the mosquito's salivary glands.  From this point the process continues as it has for millions of years.

Plasmodium oocysts on epithelium of hemocoel.


At this point, it would be appropriate to consider a few aspects of parasitism.  In some parasitic relationships, the host organism is minimally harmed.  How many times can a person be bitten by mosquitos before they die?  Similarly, Plasmodium does not consume its host immediately, although, if left untreated, malaria can kill a human in a relatively short period of time.  For what reason do most parasites only weaken or very slowly kill their host?  The answer should be apparent by thinking of the complexities of the Plasmodium life cycle outlined above.  If infected humans were to die before other mosquitos had taken in a blood meal loaded with gametocytes, the parasite would quickly become extinct.  In this case, the parasite has adapted to killing its host relatively slowly and only incidentally.  After all, wouldn't it be better for the parasite if the host could live indefinitely? 

Parasitism, like all symbiotic interactions, demonstrates the complex nature of the biosphere.  As humans we tend to think of ecosystems as existing outside of the bodies of the organisms that fill them, but the fact is that the ecosystem extends deeply within the large and small biotic units of which it is composed.  At every level where there is energy to be obtained, organisms, most of them microscopic, are found.  The extent of biodiversity is such that even the parasites have parasites! 



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**Map obtained from:

**Anopholes mosquito:

**Plasmodium vivax life-cycle:

**Plasmodium sporozoites:

**Plasmodium schizont and oocysts:

**Plasmodium merozoite:

**Plasmodium merozoites rupturing red blood cells: