Pathophysiology Of Falciparum Malaria

Front 1

What percent of Pf cases are severe and what is the mortality rate?

Back 1

~10% of falciparum malaria cases → severe disease

mortality rate of 10-50%

complications can develop rapidly

several complications can occur simultaneously

Front 2

What are the complications of Pf malaria?

Back 2

*cerebral malaria
*severe anemia
*respiratory distress (pulmonary edema)
hyperpyrexia
hypoglycemia
acidosis/hyperlactemia
GI and liver syndromes (jaundice)
renal failure (black water fever)
circulatory collapse (shock)

*most common

The high parasitemia
and sequestration can result in various complications and severe malaria encompasses a complex syndrome affecting many organs and resulting in
biochemical and hematological abnormalities

Front 3

Regarding Pf malaria respiratory distress, why is it so important and what causes it?

Back 3

respiratory distress due to metabolic acidosis

characterized by difficult and rapid breathing

can progress to respiratory failure

best predictor of death

Front 4

Identify the major complications of Pf malaria and describe the % mortality (numbers)

Back 4

severe anemia>impared consciousness>severe respiratory distress

Front 5

Why does Pf malaria have such high parasitemias?

Back 5

all erythrocytes invaded
- Pv/Po = reticulocytes
- Pm = senescent RBC

up to 36 merozoites

sequestration of infected erythrocytes
- trophozoite and schizont stages
- primarily in brain, heart, lungs, and gut
- immune evasion (spleen avoidance)

Front 6

Why does Pf malaria have such severe anemia?

Back 6

The severe anemia is due in part to the destruction of erythrocytes during
parasite maturation and merozoite release. In addition, noninfected
erythrocytes are destroyed at higher rates in infected persons as a result of
complement-mediated lysis and phagocytosis.

This increase in complement activation and phagocytosis may be due to the high levels of antibodies observed during infection and the deposition of antibody complexes on the uninfected erythrocytes.

Accordingly, severe anemia is more common in children from highly endemic areas because of repeated or chronic infections.

Furthermore, there is a decreased production of erythrocytes during
infection.

This decrease in erythropoiesis may be due to the higher levels
of cytokines and other mediators of inflammation which are known to
affect the maturation of erythroid cells.

Front 7

What is a better predictor of death in Pf malaria, cerebral edema, severe anemia or respiratory distress? Why?

Back 7

More recently it has been recognized that metabolic acidosis (i.e., low blood pH) as manifested by respiratory distress has emerged as a central
feature of severe malaria and is a better predictor of death than cerebral
malaria or severe anemia.

The first signs of lung injury are rapid and difficult
breathing and are indicative of pulmonary edema (i.e., fluid in the
lungs).

This pulmonary edema can progress to an acute respiratory distress
syndrome and even respiratory failure.

Front 8

Describe Pf malaria induced cerebral malaria and what causes it.

Back 8

Cerebral malaria is characterized by an impaired consciousness and other neurological symptoms.

Patients typically present with fever for several days followed by a loss of consciousness. The onset of cerebral malaria can be gradual or rapid.

The presenting symptoms are usually
severe headache followed by drowsiness, confusion, and ultimately an
unrousable coma.

Convulsions, vomiting, and respiratory distress are also frequently associated with cerebral malaria, especially in children.

neurological manifestations are believed to be due to the sequestration of the infected erythrocytes in the cerebral microvasculature.

Front 9

Pathology of all species of malaria is related to what?

Back 9

Pathology associated with all malarial species is related to the rupture of
infected erythrocytes and the release of parasite material and metabolites,
hemozoin (i.e., malaria pigment), and cellular debris.

In addition, there
is an increased activity of the reticuloendothelial system as evidenced by
macrophages containing ingested infected and normal erythrocytes and
hemozoin.

Furthermore, the liver and spleen are often enlarged during
malaria.

Front 10

What pathologic features contribute to severe Pf malaria?

Back 10

Cytoadherence and sequestration are also believed to contribute to the
pathology of severe falciparum malaria.

Sequestration refers to the cytoadherence
of trophozoite- and schizont-infected erythrocytes to endothelial
cells of deep vascular beds in vital organs, especially brain, lung, gut, heart, and placenta.

As discussed above, this sequestration provides several advantages
for the parasite and contributes to the high parasitemias.

In addition,
this cytoadherence and sequestration can have pathological effects on the
specific organs, most notably the brain.

Front 11

What are "knobs"?

Back 11

The parasite ligands are focused in electron-dense protuberances,
or “knobs,” on the surface of the infected erythrocyte.

Receptor–ligand interactions between protein ligands expressed on
the surface of trophozoite and schizont-infected erythrocytes and various
receptors found on endothelial cells mediate cytoadherence

knobs are restricted to P. falciparum and this suggests
that the knobs play a role in cytoadherence. In addition, there is also a good

correlation between animal Plasmodium species which express knobs and exhibit sequestration.

Front 12

How does Pf contribute to severe malaria? name labeled areas

Back 12

avoidance of spleen
low oxygen tensions
better invasion
complications

sequestration in the tissues minimizes removal of infected erythrocytes by the spleen and can be viewed as an avoidance of the immune system.

In addition, the cytoadherence to the endothelial cells of the capillaries allows for a more efficient erythrocyte invasion because the blood flow is much slower.

The low oxygen tension found in the deep tissues is also metabolically favorable for parasite growth.

Front 13

What are the parasite proteins associated with knobs?

Back 13

KAHRP and PfEMP2 are believed to interact with the submembrane cytoskeleton of the host erythrocyte

reorganization of the membrane skeleton may result in knob formation

PfEMP1 crosses the erythrocyte membrane and is exposed on the surface

Parasite proteins are
associated with the knobs found on the surface of infected erythrocytes.

These proteins probably cause a reorganization of the submembrane cytoskeleton of the erythrocyte and are important in the cytoadherence of the
infected erythrocyte to host endothelial cells.

Front 14

What parasite proteins do are not exposed on the outer surface of the outer surface of an erythrocye?

Back 14

Two proteins which probably participate in
knob formation are the knob-associated histidine rich protein (KAHRP)
and erythrocyte membrane protein-2 (PfEMP2).

Neither KAHRP nor
PfEMP2 are exposed on the outer surface of the erythrocyte, but are localized to the cytoplasmic face of the host membrane

Front 15

What parasite protein is exposed to the outer surface of the erythrocyte membrane?

Back 15

A polymorphic protein, called PfEMP1, has also been localized to the knobs and is exposed on the host erythrocyte
surface and probably functions as a ligand which binds to receptors on host endothelial cells.

Front 16

PfEMP1 is related to what gene family?

Back 16

PfEMP1 corresponds to members of the var gene family.

family of ~60 var genes

Front 17

What are the three main structures in PfEMP1?

Back 17

PfEMP1 has a large extracellular N-terminal domain,

transmembrane region

C-terminal intracellular domain.

Front 18

What does the C-terminal region do?

Back 18

conserved intracellular C-terminus
- acidic terminal segment (ATS)
- binds cytoskeleton + KAHRP

The C-terminal region is
conserved between members of the var family and is believed to anchor
PfEMP1 to the erythrocyte submembrane cytoskeleton.

In particular, this acidic C-terminal domain may interact with the basic KAHRP of the knob
as well as spectrin and actin of the submembrane cytoskeleton.

Front 19

What does the extra-cellular domain do?

Back 19

variable extracellular domain composed of modules
- 2-7 copies of Duffy-binding like domains
- - - 5 sequence types (a, b, g, d, e)
- 1-2 cys-rich interdoma
in regions
- all have DBL1a + CIDR

The extracellular domain is characterized by one to five copies of Duffy-binding-like (DBL) domains.

These DBL domains are similar to the receptor-binding region of the ligands involved in merozoite invasion

The DBL domains exhibit a conserved spacing of cysteine and hydrophobic residues, but otherwise show little homology.
Phylogenetic analysis indicates that there are five distinct classes (designated as a, b, g, d, and e) of DBL domains.

The first DBL is usually a DBLa type and this is followed by a cysteine-rich
interdomain region (CIDR).

A variable number and types of DBL domains
in various orders make up the rest of the extracellular domain of PfEMP-1.

Front 20

How does Pf avoid the spleen?

Back 20

Infected parasites (IRBC) can't filter through

Front 21

What are the possible host receptors allowing PfEMP1 to bind to different domains?

Back 21

CD36

Ig super-family
- ICAM-1
- VCAM-1
- PECAM-1

E-selectin

thrombospondin

chondroitin sulfate A

hyaluronic acid

Rosetting Receptors

Several possible endothelial receptors have been identified by testing the ability of infected erythrocytes to bind in static adherence
assays.Detailed analysis of the binding activity of PfEMP1 to the receptors
indicates that different domains of PfEMP1 are responsible for binding
to different receptors.

For example, CD36, an 88 kDa integral membrane protein found on monocytes, platelets, and endothelial cells binds to the CIDR domain.

Similarly, the different classes of DBL domains bind to different endothelial cell receptors.

Front 22

What PfEMP1 domain does CD36 host receptor bind to?

What disease process is it known for?

Back 22

cysteine-rich
interdomain regions (CIDR)

Front 23

What PfEMP1 domain does Intracellular adhesion
molecule-1 (ICAM1) host receptor bind to?

Back 23

DBLb Implicated in cerebral malaria

ICAM1 is a member of the immunoglobulin
superfamily and functions in cell–cell adhesion. As discussed above,
binding of infected erythrocytes to ICAM1 is speculated to play a role in the development of cerebral malaria.

Front 24

What PfEMP1 domain does Complement receptor-1host receptor bind to?

This is part of what special receptor?

Back 24

Duffy-binding-like
(DBL) domain - DBLa

Rosetting receptor

Front 25

What PfEMP1 domain does Chondroitin sulfate A host receptor bind to?

Where in the host is this receptor found?

Back 25

DBLy

Receptor in the placenta

Front 26

What are the Ig super-family host receptors?

Back 26

ICAM-1
VCAM-1
PECAM-1

Of importance, ICAM1 is a member of the immunoglobulin
superfamily and functions in cell–cell adhesion.

As discussed above,
binding of infected erythrocytes to ICAM1 is speculated to play a role in the development of cerebral malaria.

Front 27

What are the rosetting receptors in host receptors?

Back 27

Complement receptor-1 (CR-1)
glycosaminoglycan
blood group A

Front 28

How does the parasite to avoid clearance by
the host immune system but maintain the cytoadherent phenotype?

Back 28

Antigenic variation.

Changes in the expression of PfEMP1 alleles will result in an infected erythrocyte with a different binding phenotype as well as a different antigenic type.

the expression of antigens on the surface of the infected erythrocyte provides a target for the host immune system.

The parasite counters the host immune
response by expressing antigenically distinct PfEMP1 molecules on the
erythrocyte surface, thus allowing the parasite to evade the immune system.

Several studies have indicated that the expression of different PfEMP1 genes correlates with different receptor-binding phenotypes.

Thus, the cytoadherence function is preserved through its ability to recognize multiple receptors

This allows the parasite to avoid clearance by the host immune system but maintain the cytoadherent phenotype.

Front 29

What is the Sequestration Hypothesis?

Back 29

cytoadherence --> cerebral ischemia --> hypoxia, metabolic effects --> coma --> death

cytoadherence would lead to a mechanical blockage
(i.e., cerebral ischemia) and subsequently hypoxia.

In addition, the parasite exhibits a high level of glycolysis which could cause localized metabolic effects such as hypoglycemia and lactic acidosis.

The hypoxia and metabolic effects would then disrupt normal brain function and lead to coma and subsequent death.

Front 30

What are the problems with the Sequestration Hypothesis?

Back 30

rapid reversibility
lack of ischemic damage
low levels of permanent neurological damage
sequestration occurs in non-cerebral malaria cases

long-term neurological sequelae among survivors of cerebral malaria are lower than would be expected for ischemia and the coma associated with cerebral malaria is rapidly reversible upon treatment.

Front 31

What unexpected findings do you find with cerebral malaria survivors?

Back 31

long-term neurological sequelae among survivors of cerebral
malaria are lower than would be expected for ischemia and the coma associated
with cerebral malaria is rapidly reversible upon treatment.

Front 32

What is rosetting?

Back 32

Rosetting is another adhesive phenomenon exhibited by P. falciparuminfected
erythrocytes which may play a role in pathogenesis.

Some infected erythrocytes bind to multiple uninfected erythrocytes resulting in a large clump of erythrocytes, or a rosette.

Some studies have shown an association between the rosetting phenotype and severe malaria.

These clumps could restrict microvascular flow in a similar fashion as cytoadherence to endothelial cells and thus contribute to the pathology.

Front 33

What is the cytokine theory?

Back 33

cytokines, such as
TNF-a, or short-lived intermediates, such as nitric oxide, are speculated to be responsible for the pathology. In this cytokine theory, malarial antigens
stimulate the production of TNF-a and other cytokines which could then induce nitric oxide or have other pathological effects. Nitric oxide is known to affect neuronal function and it could also lead to intracranial hypertension
through its vasodilator activity.

Front 34

What are the problems with the cytokine theory?

Back 34

minimal lymphocyte infiltration or inflammation

A capillary is filled with infected erythrocytes which are adhered to the brain endothelial cells with relatively little inflammation.

It is unlikely, though, that the systemic release of cytokines would cause coma and there is minimal lymphocyte infiltration or inflammation associated with the blocked capillaries

Front 35

Pathological events that contribute to severe Pf Malaria

Back 35

potentially high parasitemias
sequestration
complex (and not fully understood) host-parasite interactions
differential var gene expression?

Front 36

Are there differences in expression of var alleles in different organs?

Back 36

Frequency of PfEMP1 types
in organs of malaria patients.

The organs of patients dying from severe malaria were analyzed for transcripts of var genes.

The various types of PfEMP1 were grouped according to their sequences at positions of limited variability. Different types of PfEMP1 predominated in the various organs.

Brain and lung have high amounts of group 4; heart group 5