Friday, March 11, 2011

Malaria Part 1: The parasite


This week begins a 4 part series on one of the most significant public health burdens in modern times, as well as throughout the vast collective experience of human history: malaria. The name comes from old Italian, meaning "bad air", as the disease was very prevalent in that part of the world even going back as far as the Roman empire. The disease name is derived from words meaning "bad air" because it was believed that the illness was associated with sickening "miasmas" that carried illness and death in the air. Still, the disease is documented far earlier in human history throughout South America and Asia. In fact, there are records of malaria going back as far as 2700 BCE in China. So we are talking about a disease that has afflicted human beings for a very long time.  In fact, we are talking about a critter that we have probably lived with for most of our hominid existence: the Plasmodium parasite.

Malaria is a very complex disease that encompasses four important domains, which I will discuss during the course of this series. The first is the infectious agent itself, which is a parasite, and involves very complex life cycles for each of the species that infect humans. The second domain is that of the vector, which is again a mosquito, albeit of a different genus, and also involves complex ecologies depending on the particular species of Anopheles. The third is that of the disease itself and how it clinically manifests, which can vary by the species of Plasmodium parasite and by other specific epidemiologic indicators. The fourth domain, is that of geography, i.e. the physical landscape, which exerts tremendous influence on each of the first three domains particularly with respect to the ecology and epidemiology involved.

So, on to part 1 of the series and its focus: the Plasmodium parasite. Here are a couple of images of one particular species, Plasmodium falciparum:

Plasmodium falciparum

Plasmodium falciparum in red blood cells 

Malaria is caused by a protozoa, which means that the organism is single-celled and eukaryotic, unlike viruses and bacteria (and also unlike helminths, which are indeed both parasites and eukaryotes, but worms are multicellular and protozoa are unicellular). The malaria parasite is of the genus, Plasmodium, which is comprised of many species. There are four species that are most relevant for human infection and these are Plasmodium falciparum, P. vivax, P. ovale, and P. malariae. P. falciparum is associated with the greatest morbidity and mortality, and is mostly responsible for the large number of deaths in young children in countries in sub-Saharan Africa. We will see how and why this is the case when we talk about how malaria manifests as a disease. P. vivax is the most globally widespread species of malaria parasite, and therefore accounts for the greatest distribution of malaria in the world. The other two species are of intermediate status, though they can also cause severe disease and death. The species of parasite share important similarities in their life-cycles, but there are also important differences among them, which affects their virulence and other clinical aspects of disease such as drug resistance.

The life cycle of this parasite is extraordinarily complex and involves multiple stages within each of its definitive (i.e. mosquitoes) and intermediate (i.e. humans) hosts. Let's stop for a second here. What do I mean by intermediate and definitive hosts? In parasitology, there are many parasites that have evolved complex life cycles that require more than one host animal species for completion. When this is the case, the definitive host refers to that animal host in which the sexual reproductive stage of the parasite occurs. Any other animal host corresponding to other intermediate non-sexual stages of the parasite is known as an intermediate host. In the life cycle of Plasmodium, mosquitoes are the definitive hosts and humans (among other vertebrate animals) are the intermediate hosts.

So let's examine this life cycle in some detail. Here is a nice depiction of the complete life cycle published by the Centers for Disease Control and Prevention (CDC):


When the mosquito takes a blood meal, she injects her saliva, which is a cocktail of chemicals to aid in her collection of blood. As she does this, if she is infected with the malaria parasite, she will also inject a particular stage of Plasmodium that collects in her salivary glands: the sporozoites. The sporozoites are the form of the parasite that cause the initial infection in humans (or any other intermediate host). The first picture in this posting was a close-up of a sporozoite. Here it is again:



Here is another nice picture depicting the sporozoite next to two other stages of the parasite that we'll be discussing shortly:



The sporozoites are completely motile, slender protozoan cells that, when introduced into the subcutaneous tissues and capillary beds of the host, make their way to the liver by way of the venous circulation. They can make their way from the point of inoculation by the mosquito to target cells in the liver in minutes if they are successful in evading the immune system. There will be many sporozoites introduced into the host with a single mosquito bite, and each seeks out a hepatic cell (liver cell) in the host where it will begin the next stage of its development. Once inside a hepatic cell, the sporozoite will begin to replicate itself thousands of times over the next 5 to 15 days. As it does so, it forms a hepatic schizont in the hepatic cell, which is essentially a small parasite factory. When the schizont ultimately ruptures, it releases tens of thousands of the new stage of the Plasmodium parasites: the merozoites. This legion of merozoites will emerge from each hepatic cell that was infected with a sporozoite (except in the case of those Plasmodium species that harbor an additional stage, called the hypnozoite, which we'll discuss shortly). So, between roughly one to two weeks after the initial mosquito bite, the host is under attack by a tremendous army of parasites.

And, yet, up to and including this point in the infection, we feel nothing....
No signs. No symptoms.

OK. The merozoites are released into the circulating blood from the hepatic schizonts, and they are in search of something: red blood cells. The merozoites seek out these red blood cells, or erythrocytes as they are also known, and attach and enter the cells by way of specific receptors on the eryrthrocyte surface.Once it gains entry, this stage of the parasite begins to change again, differentiating into a trophozoite, which is the feeding stage of the parasite. At this point the trophozoite is feeding on the hemoglobin of the erythrocyte that it has hijacked. It will then begin replication creating new intraerythrocytic merozoites, enlarging the cell and forming another schizont.

Still we feel nothing from the infection...

When the schizont ruptures it will release approximately 20 merozoites back into the extracellular fluid. Here is an excellent photo of infected and rupturing erythrocytes, releasing the newly formed merozoites:



Now we begin to feel something....

As the new batch of merozoites are released from the burst erythrocytes, other pyrogenic (fever inducing) toxins are also released from the ruptured cell, which elicits an immune response. Indeed, on a population level, the malaria parasite is probably the most powerful stimulator of the human immune system known. The release of these merozoites and toxins from the lysed red blood cells correspond to the characteristic paroxysms of fever and chills. But we are saving a detailed discussion of the actual malarial disease for a later post, so let's return to the life cycle.

This second stage of asexual reproduction (the first being the in the hepatic schizont), which leads to the rupturing of the erythrocytic schizont and the release of about 20 merozoites takes about 48 hours for P. falciparum, P. vivax, and P. ovale, and about 72 hours for P. malariae. Once the newly formed merozoites are released from the ruptured erythrocytes, each then seeks out a new host erythrocyte and the process repeats again and again. The fecundity of this parasite is incredible. A single P. falciparum merozoite, for example, can potentially lead to 10 billion new parasites through these recurrent cycles!

After a number of the replicative cycles of the merozoites across multiple erythrocyte infection episodes, some gametocytes will begin to form along with the merozoites. The gametocytes will perform no further function in the intermediate human host.  These gametocytes are, in fact, the sexual stage of the parasite and must be transfered to the mosquito since this is the definite host. And you will remember that the definitive host is where sexual reproduction takes place. The gametocytes can be taken up by any subsequent mosquito seeking a blood meal. Here's a picture of this stage:



Once the gametocytes are inside the mosquito gut, the next stage of development can begin, which is known as the sporogonic cycle. As the erythrocytes are ingested by the mosquito, the gametocytes are released into the gut and they begin sexual reproduction. Any merozoites that are also ingested are simply digested with all the other material and play no further role in the parasite's life cycle. But the macrogametocytes (female) and microgametocytes (male) survive and fuse in the mosquito gut, thus forming the zygote. This zygote will change form over the next 12-14 hours by elongating and developing into an ookinete, which then actively seeks out the wall of the mosquito gut, penetrates it, and finally develops into an oocyst. Over the next several days, the oocyst swells as it forms upwards of around 10,000 sporozoites within. When the oocyst ruptures it releases the sporozoites, which then migrate to the salivary glands of the mosquito where they are ready to be introduced into a new human host with the mosquito's next blood meal.

This completes this the general Plasmodium life cycle. And you thought your life was complicated! Please. Here is the CDC chart one more time to bring it all together:



The whole of the sporogonic life cycle in the mosquito takes approximately 7 to 12 days. But the time required can vary markedly, which depends on several things. First, it depends on the species of Plasmodium that is involved. Second, it depends on temperature, with higher temperatures corresponding to shorter duration needed for development. Third, it depends on humidity, with increased humidity also corresponding to less time to complete this phase of the parasite's life cycle. As an example, an outside temperature of 20 degrees C will extend the sporogonic cycle of P. falciparum to 23 days, which is longer than the average lifespan of many anopheline mosquitoes. As we will see later in this series, geography is a major influence on these three important determinants of the Plasmodium life cycle in the mosquito.

Below I am posting a very nice short video that displays the whole life cycle in an animation. It provides a good summary to everything discussed up to this point:



Now let's examine some important biologic differences between the Plasmodium species. P. vivax and P. ovale are unique in that not all sporozoites entering the hepatocytes proceed immediately to forming a schizont for the development of merozoites. A few will, instead, lie dormant in their host hepatocytes forming what are known as hypnozoites, i.e. a "sleeping" stage. The parasite can lie dormant in this stage for months to years. Later the hypnozoites will differentiate and form schizonts, which will give rise to a new wave of merozoites. This variant in P. vivax and P. ovale is the cause of relapse malaria. It also requires a specific treatment regimen that targets the hypnozoites so that relapses will not occur. While P. falciparum does not have a hypnozoite stage, and thus does not relapse, it is important to keep in mind that ineffective treatment or holoendemic geography (both of which will be discussed later in the series) can lead to low persistent parasitemia, which can cause recrudescent clinical infection. So it is important to distinguishing between recrudescent and relapse malaria by distinguishing between Plasmodium species.

Gametocyte production is an important biologic variant across Plasmodium species. Following infection with P. falciparum, gametocytes don't begin to appear until after several rounds of intraeryrthrocytic cycles of the merozoite form of the parasite (typically not less than 10 days). In P. vivax, however, gametocytes begin to appear in peripheral blood almost as soon as the intraerythrocytic merozoites begin their cycling. As a result gametocytes can be present and available for transmission to new mosquitoes before symptoms occur and treatment is sought. This means that the gametocytes that are transferred prior to treatment are not subject to selective pressures that would select for drug-resistant mutants. Therefore, drug-sensitive parasites are not at a selective disadvantage compared to dug-resistant parasites. On the other hand, P. falciparum transmission from humans to mosquitoes can be blocked since early treatment with an effective drug can stop erythrocytic schizont production before gametocytes are formed. However, inevitably treatment will not be effective enough to prevent all gametocytes from developing in all cases. So those that do develop will be derived from parasites that survived treatment and, thus, may be drug resistant. This staggering of gametocyte production assists the selection of drug resistant parasites, and indeed we do see much greater drug resistance in P. falciparum than in P. vivax.

Another important distinction between the Plasmodium species is their affinities for different erythrocyte types. This distinction also contributes to differences in their virulence.  P. vivax and P. ovale only invade the young reticulocytes, which means that peripheral parasite density is typically low in these infections because the reticulocytes only comprise about 1% of the total erythrocytes in humans. P. malariae prefers older erythrocytes and so typically results in infections that are also limited. Only P. falciparum infects all types of erythrocytes. Thus, P. falciparum is able to produce high density parasitemias that result in high morbidity and mortality relative to the other three species.

This will conclude Part 1 of the series and our discussion of the parasite that causes malaria. There is much yet to cover. More nuance of the parasite will become apparent as we discuss the other facets of the disease. Next time I'll describe the malaria vector: anopheline mosquitoes.

19 comments:

  1. This description of the life cycle of the Plasmodium parasite was very clear and interesting. It’s really fascinating how a unicellular organism can have such a complex life cycle and such a sophisticated mode of transmission throughout its hosts. One aspect that I found most interesting was how after infection of a human the parasite then can go back to the mosquito. This cycle is worked out so well for such a small parasite. My question is what makes P. vivax the most globally widespread? It seems that P. falciparum has all the qualities to be most widespread including it having greater drug resistance and not being specific as to which erythrocyte it infects.

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    1. I too found the description of the life cycle to be quite interesting and very complex. I was surprised to learn that P. vivax is the most frequent species and accounts for the greatest distribution of malaria around the world. I also found it interesting that despite it being the most widespread it is not the deadliest. I think part of the reason for the widespread occurrence of P. vivax is because of its resistance to malaria drugs. In addition to being more resistant to antimalarial drugs there is some evidence that frequent international travel and environmental conditions that favor transmission have caused the frequency of p. vivax in non-endemic areas.

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    2. Alexandr PinkhasovJuly 25, 2014 at 5:29 PM

      Jane its true the life cycle is very interesting. I think whats most fascinating is its ability to potentiate to such large numbers (10 billion) from one single organism. It underlines the pathogenic capabilities of this organism. In regards to your question about P. vivax being so globally widespread, I would supposed that there is a genetic factor that affect geographic adaptation. Another idea, and I'm not exactly sure if this is could be a reason, but its global spread could be dependent on the mosquito (vector) species and the vector's geographic adaptability.

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  2. I did a little research about P. vivax. It seems that the reason it is so widespread is that it is able to hide dormant in the hosts liver cells where it is less vulnerable to the effects of antimalarial drugs. P. falciparum does not have a dormant stage and is always susceptible to drug treatment. P. vivax is also less deadly. I guess this are the reasons P. Vivax is more widespread. My brother had malaria when he was in Ghana;I wonder which strain did he have since he was required to take so many drugs before and during his trip.

    Jamal Burke

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  3. P.vivax is mostly common in the temperate zones (including North and South America, Asia and North Africa). According to the section on Landscapes, it seems that P. vivax is much more widespread because of its ability to complete the sporogonic life cycle at lower temperatures. Mosquito density and contact with humans are affected by climate, and may contribute to the prevalence seen globally.
    Can it also have something to do with the relapse seen in vivax, as opposed to falciparum?

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  4. Another fascinating aspect of this disease is how it has selected for the recessive allele that causes sickle cell anemia. The sickled shaped red blood cells are more resistant to infection by the Plasmodium parasite, resulting in a high prevalence of the disease in areas where malaria is endemic.

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  5. Nice discussion you all...For the most part on the money as far as the more widespread distribution of P. vivax. First, it has a dormant hepatic stage, such that the hypnozoites allow the parasite to wait out temperate climate winters, and second, the sporogonic cycle is more robust to lower temperatures. Just 2 points of clarification. One, P. vivax is not only common in temperate zones. In fact, in modern times there are very few temperate regions where you would find significant P. vivax. Though historically, P. vivax has had a wide global distribution, today it occurs as mostly a tropical and sub-tropical infection. Two, the sickle cell genetic variant is an adaptation that was driven by P. falciparum rather than P. vivax.

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    1. Dr Walsh its interesting to know that the sickle cell trait malaria protection effect was due the an adaptation that was driven by P.falciparum rather than P. vivax , I remember when I was in medical school in Nigeria one of my friends was doing his Master thesis in sickle cell disease biochemistry and he was discussing that one of the possible explanation why the trait provide protection against malaria was because the mutation in the RBCs render their membrane rigid and thus impenetrable by the malaria parasites, however current bio-medicine evidences demonstrated that the life cycle of the parasite was not altered in sickle cell trait, rather this protective effect is due to heme oxygenase-1 (HO-1), an enzyme whose expression is induced by sickle cell trait hemoglobin, the enzyme produce carbon monoxide that provide the protection against falciparum malaria, please correct me if I'm wrong

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  6. Another excellent post. This is really amazing. The video was a good touch as it clarified and simplified the intricate life cycle of the parasite.

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  7. This was a helpful review for me of the life cycle of Plasmodium, something that I have learned numerous times and promptly forgotten into a fog of different kinds of "zoites." Every time I re-learn a life cycle like this, I am reminded how difficult it is for eukaryotic cells to evade the human immune system, and how they must develop these elaborate lifestyles over the course of evolution to persist. The potency of natural selection in the context of sexually reproducing organisms, in contrast to largely fission-based reproduction of prokaryotes, is really remarkable.

    The dark side, of course, is that if eukaryotes are able to devise these kinds of life cycles to evade host immune systems, they are just as able to use them to evade pharmaceuticals developed in the lab. It makes me wonder personally to what extent pharmaceutical based treatment will ever be able to significantly reduce the burden of malaria beyond it's current level. I wonder if the development and implementation of genetic modifications of the definitive host would not have greater promise.

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  8. Allison GrossmanJuly 3, 2013 at 4:53 PM

    I found this post very interesting and informative. One idea that struck me was the redundancy and convolution built into the system of the Plasmodium life cycle. It's amazing to me that the liver cycle seems to exist only to get to the blood cycle, and that in the blood cycle, so many merozoites are created to produce a few gametocytes. What is the reason for the redundancy, particularly all of the extra merozoites that are produced? I imagine their production consumes a great deal of energy. Are they decoys for the immune system so that more gametocytes survive, or are they precursors of the gametocytes themselves?

    And why go through so many different stages? Is it because the parasitic cells would be more vulnerable in the blood and need to multiply before they enter a more hazardous zone?

    I also wonder how the time lags, which seem to result in more or less redundancy before the cycle moves to the next stage, vary so much between species. How does something like this evolve?

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    1. Racquel BreretonJuly 25, 2014 at 6:25 PM

      I understand that the sporozoites target hepatocytes because these are responsible for blood filtration. What I'd like to know is the mechanism behind it. What specific biological features unique to hepatocytes attract sporozoites?

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    2. Indeed the Plasmodium life cycle is a both complex and fascinating. Plasmodium ensures its continued existence by getting through the liver cycle, producing merozoites and gametocytes. Interesting point that the production probably consumes a great amount of energy. I would tend to agree with that perspective, I wouldn’t think however that they are precursors to gametocytes.

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  9. In reply to the previous post:
    Merozoite: The form of the malaria parasite that invades red blood cells. Merozoites can develop into immature gametocytes and it is more like merozoites are precursors of gametocytes.
    Gametocytes: Precursors of the sexual forms of the malaria parasite, which release either male or female gametes within the stomach of the mosquito.
    The size and genetic complexity of the parasite mean that each infection presents thousands of antigens (proteins) to the human immune system. The parasite also changes through several life stages even while in the human host, presenting different antigens at different stages of its life cycle. Thus the parasite has developed a series of strategies that allow it to confuse, hide, and misdirect the human immune system.
    Falciparum parasite we see today arose about 3200–7000 years ago: an era that coincides with the dawn of agriculture in Africa. This was a time of massive ecological change, when humans began living in large communities and the rainforest was being cut down for slash-and-burn agriculture. Other findings also support the timeframe for the birth of the modern falciparum: there was also a major change in the mosquito vector at that time, when it began biting humans instead of animals; and a human red blood cell polymorphism that protects against falciparum dates to less than 10 000 years ago. "It all fits together to about 6000 years ago," says Professor Day at the University of Oxford.

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  10. This was a very comprehensive post on malaria the parasite itself. I think one of the most interesting fascinating aspects of this disease is how blood disorders overlap with geography to confer resistance to malaria. The major and most important one is of course sickle cell trait. This protects against P. falciparum since RBCs are too weak to support the parasite and also the sickled shape of the RBCs protect against invasion, growth and development due to decreased O2. Moreover those with the infected RBCs with HbAS are selectively targeted and destroyed by the spleen. Another blood disorder that offers protection is the lack of Duffy A and B antigen on RBCs. This confers protection against P. vivax since it binds onto those Duffy antigens. Other blood disorders also include thalassemia and Glucose-6-phosphate dehydrogenase deficiency

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    1. Great highlights on the Duffy antigens Matthew! We talked about this blood group in class but not in greater detail. Aside from geography, there are also racial variations in the distribution of Duffy antigens. Two-thirds of the black population have the duffy phenotype that confers immunity to P. valvax infections, whereas such antigens are rare among Caucasian and Asian population. Taking into account these multi-factorial domains of malaria, we can better understand the dynamics of the definitive host cycle as it navigates through the human environment.

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  11. This blog is rather interesting it expounds on the intricacies of malaria parasite. Explaining the details about the life cycle of the malaria parasite which is rather fascinating, how the parasite is so complex that it needs both a human and a mosquito to fully develop. It also talks about the different species of the parasite, and the process by which the parasite causes infection. It is astonishing the process by which it is able to be infectious and at such an alarming rate. In doing some research I found out that there is a shocking rate at which the world is at risk for malaria (approximately half the world). With the majority of the deaths which is about 80% occurs in just 14 countries mostly located in Africa, Asia, and India. Unfortunately this happens to be in most of the poor countries.

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  12. The life cycle of this malaria parasite is extremely complex; I am glad that the stages were explained in a clear fashion. The delay in signs and symptoms during infection is interesting. I was surprised that approximately one to two weeks after the initial mosquito bite, although the host is under attack by numerous parasites and the trophozoite is feeding on the hemoglobin of the erythrocyte, no symptoms or signs are present. Symptoms and signs become present when the schizont ruptures and releases merozoites back into the extracellular fluid and toxins that cause fever are also released. This causes an immune response to occur.

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  13. In a biological anthropology course during my undergrad, I remember learning about the sickle cell trait and how it conferred a sort of resistance towards developing malaria. After learning about how the merozoites seek out the erythrocytes and attach/enter the cells by way of specific receptors on their surface, I can't help but wonder if the abnormal red blood cell in people who have sickle cell trait/anemia (heterozygous/homozygous genotype respectively) is lacking the receptor needed by the merozoites to complete the infection? Someone had mentioned the role of carbon monoxide in the resistance, which I had never heard before-- very interesting. Then Dr. Walsh mentioned in a comment that the resistance was driven by a single species, P. falciparum. I am curious to see if different rates of malaria are present among people who have the sickle cell trait and are infected by differing species of Plasmodium.

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