Lecture 19 - Malaria Epidemiology

Question 1

What is life expectancy?

Answer 1

Life expectancy is the defined as the number of years a newborn child would live if current mortality patterns were to stay the same

Question 2

What is special about HIV and Malaria?

Answer 2

HIV and Malaria are important because they are caused by a single pathogen.

Question 3

Taxonomic classification of Malaria

Answer 3

Phylum: Apicomplexa

Genus: Plasmodium

5 species infect Humans: P.falciparum (main one), and P. vivax

Many other species infect animals
single celled protozoan parasite, all apixomplexa invade cells like viruses

Question 4

Malaria is more common just ___ and ___ of the Equator

Answer 4

North and south

where it becomes more and more tropical, malaria more common because their life cycle extends all year round, winters stop them breeding

Question 5

What is another factor that raises the malaria prevalence in countries just around the equator?

Answer 5

The high incidence of malaria in the tropics can also be attributed to factors such as poverty, lack of access to healthcare, and limited resources for disease prevention and control. These factors are often more prevalent in tropical regions, where there may be limited infrastructure and resources to support public health interventions.

Question 6

Why is developing immunity to malaria a problem for disease transmission?

Answer 6

In areas where malaria is endemic, people may develop partial immunity to the disease over time. This can contribute to a higher prevalence of the disease in certain regions, as individuals with partial immunity may experience milder symptoms or may be asymptomatic carriers of the disease, making it more difficult to control its spread.

Question 7

What are the four species of Plasmodium that infect humans?

Answer 7

P. falciparum
P. vivax
P. ovale
P. malariae

Question 8

Which three tissues can Plasmodium infect?

What is different about the pathogen at this time?

Answer 8

The insect
The liver
Erythrocytes
It is the same pathogen, i.e. the DNA is the same, however there are morphological differences in the parasite.

Question 9

Why is malaria a big obstacle to further human development ?

Answer 9

• Close to the #1 infectious cause of child mortality
• Access to available interventions remains poor
• Drug and insecticide resistance rife
• Poor predictors of severe malaria
• No widely available vaccine and need new drugs

Question 10

What is seen in South America in terms of malaria?

Answer 10

High levels of disease, but less burden of death.
This is because P. vivax is the main species in this area
This species is less fatal

Question 11

Describe the economic impact of Malaria

Answer 11

Financial burden:
* Bed nets
* Work and School absenteeism
* Represents 10% of annual spending in Africa

Question 12

What is the vector of the malaria parasite?

Answer 12

Anopholes mosquito

Question 13

What is the relationship between malaria and sickle cell anemia?

Answer 13

• Individuals with sickle cell trait or sickle cell disease have a reduced risk of severe malaria compared to individuals without the sickle cell gene.
• The protective effect of sickle cell against malaria is thought to be related to the abnormal shape of sickle cells. These misshapen cells may make it more difficult for the malaria parasite to invade and multiply within red blood cells..

Question 14

List the stages of the life cycle of the malaria parasite

What type of reproduction is occurring at each stage?

Answer 14

1. Mosquito stage – sexual reproduction
2. Liver stage – asexual reproduction
3. Blood-stage – asexual reproduction, major amplification stage

Question 15

What is the lifecycle of P.falciparum in malaria?

Answer 15

1. Infection: The lifecycle begins when a female Anopheles mosquito infected with P. falciparum bites a human host and injects the sporozoites (infective form) of the parasite into the bloodstream. Injected sporozoites enter hepatocytes via the skin
2. Liver stage: The sporozoites travel to the liver, where they infect liver cells and multiply to form thousands of merozoites. ASEXUALLY
3. Blood stage: The merozoites are released into the bloodstream, where they infect red blood cells and multiply rapidly. ASEXUALLY. During this stage, the infected red blood cells may stick to the walls of blood vessels and cause damage to the host’s tissues.
4. Ring stage: The first visible stage of the parasite in the red blood cells is the ring stage, where the parasite is surrounded by a ring-shaped structure.
5. Trophozoite stage: As the parasite matures, it progresses to the trophozoite stage, where it continues to multiply and begins to consume hemoglobin from the host’s red blood cells.
6. Schizont stage: The parasite then progresses to the schizont stage, where it forms many daughter merozoites inside the infected red blood cell.
7. Rupture and reinfection: Eventually, the infected red blood cell ruptures and releases the daughter merozoites into the bloodstream, where they can infect new red blood cells and continue the cycle of infection.

Gametocytes form in the blood and are taken up by a feeding mosquito

Question 16

What are sporozoites?

Answer 16

Haploid
Generated by meiosis (sexual reproduction) in the mosquito
This is what infects the human

Question 17

Merozoites enter the:

Answer 17

RBCs where they start to replicate and keep amplifying

Question 18

Malaria genome has 5200 genomes encoding for a lot of proteins, but the one we are interested in is:

Answer 18

PfEMP1
(Plasmodium falciparum Erythrocyte Membrane Protein 1)

Confers virulence and immune escape

Question 19

Disease only occurs in the ______

Answer 19

Blood-stage

Fever, chills, anaemia
(hard to predict which individuals move on to develop more serious complications)
Most deaths (95%) caused by P.falciparum
>Cerebral malaria, coma
>Severe anaemia
>Placental malaria
P.vivax also results in significant morbidity but low mortality
>relapsing malaria (hypnozoites in liver)

Question 20

P. vivax has a _____ form

Answer 20

Latent

can live as a form called a hypnozoite in the liver for life
for reasons not understood, these hypnozoites can reactivate later in life and cause disease

Question 21

How does the malaria parasite avoid splenic clearance?

Answer 21

Cytoadherence/sequestration

Infected RBC is stuck to BV wall, taken itself out of circulation
Parasites are maturing inside, and across the course of 48h, parasites grown within there and burst out

Question 22

Differentiate between a merozoite and a sporozoite

Answer 22

Sporozoite: haploid parasite from sexual reproduction in the mosquito

Merozoite: emerges from replication in the liver cells

Question 23

Cytoadherence is caused by:

Answer 23

PfEMP1 IS THE ACTUAL BINDING PROTEIN

responsible for binding a receptor on the endothelial cell wall
can bind different receptors
Parasite encode different PfEMP1s, 60 different copies of the gene encoding PfEMP1, and depending on which of those genes is expressed, will determine which receptor the parasite will bind to on the microvascular endothelium of BVs

ALTHOUGH: KHARP is part of the ‘knob’ protein cluster which directs PfEMP1 to the RBC membrane and anchors it there to allow binding to the endothelial cells.

Question 24

What is splenic clearance?

Answer 24

Spleen recognises RBCs that are not normal anymore

has cross-hatched fenestrations like a little filter, RBCs go through spleen and are squeezing through these fenestrations
Thought that the spleen is testing those cells (looking for cells that are getting too old and wants to get rid of them e.g. if a cell is getting too rigid)
Clearance mechanism
- Macrophage-like system will be activated and clear those red cells

A plasmodium infected RBC wants to stay away from the spleen»_space; cytoadherence

Question 25

Cytoadherence is sometimes associated with ______ pathologies

Answer 25

Severe pathologies

depending on which BVs they sequester to (not all BVs are the same, not all RBCs are the same)
e. g. Cerebral malaria - infected brain capillary

Question 26

What microscopic feature is diagnostic of malaria parasites?

Answer 26

BV full of cells with a gold particle in them - digested heme (haemozoan)

The parasite inside the RBC is digesting the haemoglobin, eating the protein partly because it is a nutrient source and partly because it needs to create space inside this RBC to grow

Heme - iron calated structure, very toxic for all cells including parasites

Malaria parasite polymerises that digested heme into this gold crystal called haemozoan

Question 27

What does PfEMP1 bind to?

Answer 27

Receptors on the microvascular endothelium

e.g. CD36, some tissues rich in this receptor, other receptors expressed on other tissues
ICAM1 is thought to be enriched in the brain so if you had a parasite expressing PfEMP1 that liked ICAM1, it would sequester in the brain BVs (can cause cerebral malaria) and CD36 causes sequestering in the muscle BVs

Question 28

What is a Knob structure?

Answer 28

PfEMP1 is exported all the way to the RBC cell surface for cytoadherence

Question 29

What causes antigenic distinct waves of parasitaemia seen in a single person infected with a single parasite?

Answer 29

With P.falci, after a week or so, you get another wave oi parasitaemia, that then gets cleared, then another wave, then clearance - why?

each of these parasites, even though 100% genetically identical, are switching around the gene encoding for PfEMP1, using a different one each time
What you get is immunity against the parasite expressing the PfEMP1 that causes antigenic wave A, then it switches to express B, then you develop immunity against B, then it switches to C and so on

Question 30

___ genes encode PfEMPs

Answer 30

Var genes

~60 copies per genome

only one is expressed at any one time
var genes are scattered around the 14 chromosomes, particularly clustered at the ends

Question 31

Immune response against sequestered RBCs involve:

Answer 31

Antibodies directed against the PfEMP1 which stops them binding to BV walls (generated within a week or two)

Question 32

How are most var genes ‘silenced’ while allowing just one to be expressed?

Answer 32

Chromosome ends cluster at the nuclear periphery and are in a ‘silent’ heterochromatic state

if you look in the nucleus of a single parasite, and use a fluorescent probe for the ends of chromosomes, see just 3 or 4 spots
14 chromosomes so there should be 28 chromosome ends, but only see 3 or 4 spots
Research showed that if you use stronger and stronger detergents, you start to see more of those spots
Turns out that the ends of chromosomes are tightly clustered together so it looks like only have 3-4 ends
Not only clustered, but at the outer edge periphery of the nucleus > wrapped up in heterochromatin > way to physically wrap up genes
tight bundle of DNA
limits accessibility of the DNA to be transcribed into RNA (hide RNA transcription binding sites)

Question 33

Describe how the var2CSA gene gets activated

Answer 33

Silent var2CSA on Chr12 is bound up with other chromosomes in heterochromatin

If there is CSA selection, Chr12 gets separated from the heterochromatin bundle and move into an active zone to get transcribed
>active var2CSA

Question 34

Var gene silencing is caused by:

Answer 34

Epigenetic control of transcription (driven by physical histone modifications to DNA)

Histone modification at the nuclear periphery generally has genes inaccessible to transcription factors

A particular localisation is associated with a new histone modification that permits transcription

*stochastically (randomly), one of the 60 genes will move into active site for transcription, but selection allows a particular population to grow up

Question 35

Summarise parasitic PfEMP1 and var gene expression

Answer 35

Parasites response to the development of immunity to PfEMP1

Clonal antigenic variation, parasites switches to use different var genes (which encode PfEMP1s)

Var genes are epigenetically-regulated, both histone modification and nuclear location important

Results in recrudescence - parasite persistence
(antigenic variation is about persistence, giving it max time for a mosquito to come and pick it up)

Question 36

Explain Ectopic recombination

Answer 36

Ectopic recombination is a type of genetic recombination that occurs between heterologous chromosomes
This evolves very rapid diversity

The ends of the chromosomes cluster together
The ends are quite similar, despite being different chromosomes
There is recombination between these aligned genes at the ends

Question 37

How does P. falciparum undergo ectopic recombination?

Answer 37

The mechanism of ectopic recombination in P. falciparum involves the switching of the active var gene from one of the multiple var gene copies to another. This process is thought to occur through a mechanism called gene conversion, in which a region of the inactive var gene is replaced by a homologous sequence from the active var gene.

Ectopic recombination in the var gene family of P. falciparum is thought to be driven by immune selection pressure, as the parasite tries to evade the host immune response by changing the surface proteins displayed on infected red blood cells.

Question 38

What is recrudescence?

Answer 38

In the context of malaria, recrudescence refers to the reappearance of symptoms of the disease after an initial episode of illness has resolved.

Question 39

Do all PfEMP1 proteins bind to vascular endothelial cells?

Answer 39

All but 1 - The exception is the PfEMP1 that binds to CSA (chondroitin-4-sulphate) on syncitiotrophoblasts in the placenta.

Question 40

How did the presence of P. vivax in West Africa select for Duffy antigen-negative individuals?

Answer 40

The Duffy antigen is a protein that serves as a receptor for the P. vivax parasite, which causes malaria. Individuals who are Duffy antigen-negative do not express this receptor on their red blood cells and are resistant to infection by P. vivax.

It is believed that the presence of P. vivax in West Africa selected for Duffy antigen-negative individuals due to evolutionary pressure.

Individuals who lacked the Duffy antigen were less likely to be infected with P. vivax and therefore had a survival advantage in areas where the parasite was present.

Question 41

How did the presence of P. vivax in West Africa select for Duffy antigen-negative individuals?

Answer 41

The Duffy antigen is a protein that serves as a receptor for the P. vivax parasite, which causes malaria. Individuals who are Duffy antigen-negative do not express this receptor on their red blood cells and are resistant to infection by P. vivax.

It is believed that the presence of P. vivax in West Africa selected for Duffy antigen-negative individuals due to evolutionary pressure.

Individuals who lacked the Duffy antigen were less likely to be infected with P. vivax and therefore had a survival advantage in areas where the parasite was present.

Question 42

What is the main difference between the Plasmodium sporozoite and merozoite?

Answer 42

The sporozoites make their way to the liver to undergo asexual reproduction. One vaccine targets the sporozoites in this “pre-erythrocytic stage” to prevent liver infection.

The merozoites are released from the infected red blood cells in the erythrocytic phase of the infection cycle. The vaccine that could work at this stage is called the “blood-stage” (anti-merozoite) vaccine. However the merozoite is only ‘outside’ for a very short period of time after rupture of the infected red blood cell before going on to infect another red blood cell, so the time available for antibodies to work against the merozoites is limited.