36. Parasitology (HT) Flashcards
Malaria, trypanosomiasis and leishmaniasis are caused by…
Unicellular parasites
Do infectious diseases have a greater impact on global deaths or on global disability adjusted life years (DALYs)?
DALYs, because they frequently affect young people, unlike, for example, ischaemic heart disease.
What parasite is responsible for malaria?
Plasmodium
What are the 5 Plasmodium species known to infect humans and cause malaria?
- P. falciparum
- P. vivax
- P. ovale
- P. malariae
- P. knowlesi
Describe the symptoms of malaria.
Common symptoms:
- Fever and chills -> Often cyclic, every 2-3 days.
- Headache
- Myalgias
- Arthralgias
- Weakness
- Vomiting, and diarrhoea
Other clinical features:
- Splenomegaly
- Anaemia
- Thrombocytopenia
- Hypoglycaemia
- Pulmonary
- Renal dysfunction
- Neurologic changes
The clinical presentation can vary substantially depending on the infecting species, the level of parasitaemia, and the immune status of the patient.
What are some species-specific symptoms of malaria?
P. falciparum:
- Can progress to severe potentially fatal cerebral malaria (central nervous system involvement), acute renal failure, severe anaemia and adult respiratory distress syndrome.
P. vivax:
- Complications tend to include splenomegaly and (rarely) splenic rupture.
P. malariae:
- Complications tend to include nephrotic syndrome.
Which of the Plasmodium parasites is the most common cause of malaria?
Plasmodium falciparum and Plasmodium vivax
How does age affect clinical presentation of malaria?
- Younger patients tend to have more severe symptoms
- Large majority of severe malaria and deaths are in young children (<5 years)
- Not just a function of age, also pre-exposure/accumulated resistance from living in endemic areas
Describe the life cycle of malaria (using Plasmodium falciparum as an example).
[IMPORTANT]
Human phase:
- Female mosquito releases sporozoites into human
- In the liver:
- Sporozoites enter liver hepatocytes and form schizonts (multinucleate cells)
- The schizonts undergo schizogony (several rounds of fission) and the hepatocytes rupture, releasing merozoites into the blood
- In RBCs:
- Asexual reproduction:
- Merozoites infect RBCs and become trophozoites and then schizonts.
- RBC rupture leads to the release of merozoites into the blood.
- Gametogenesis:
- Some of the infecting merozoites are programmed for gametogenesis.
- They become trophozoites and then gametocytes.
- Some are male (microgametes) and some are female (macrogametes).
- Asexual reproduction:
Mosquito phase:
- Male (microgamete) and female (macrogamete) gametocytes are taken up by a female mosquito.
- In the mosquito midgut, microgametes (male) penetrate the macrogametes (female), generating zygotes.
- The zygotes becomes an ookinete that is motile and invades the midgut wall, where it develops into an oocyst.
- The oocyts ruptures and releases sporozoites that enter the salivary gland of the mosquito, ready for transfer to the next human host.
What explains the waves of symptoms of malaria?
Asexual reproduction within erythrocytes takes around 48 hours and the RBCs rupture in waves every 2 days.
Summarise the cell types involved in the different stages of the malaria life cycle.
Human phase:
- Receive sporozoites
- In the liver:
- Sporozoites -> Schizonts -> Merozoites
- In RBCs:
- Asexual reproduction:
- Merozoites -> Trophozoites -> Schizonts -> Merozoites
- Gametogenesis:
- Merozoites -> Trophozoites -> Gametocytes
- Asexual reproduction:
Mosquito phase:
- Receive gametocytes
- In the midgut:
- 2 x Gametocytes -> Zygotes -> Ookinete -> Oocyst -> Sporozoites
- Sporozoites released from salivary gland.
What are some features of the malaria life cycle that make it an effective pathogen?
- Initial site of infection is liver cells -> Good site for maintaining a long-term asymptomatic infection
- Replication occurs as schizonts inside host cells -> Only short extracellular phases which are exposed to the immune system
- Sexual life cycle stage -> Allows effective genetic exchange
Describe the mechanism of Plasmodium invading erythrocytes.
- Plasmodium uses non-specific methods to attach to the erythrocyte, which leads to the rotation of the Plasmodium such that the apical complex is pointing to the erythrocyte
- Specific proteins on the Plasmodium (EBA and PfRH) interact with receptors on the erythrocyte, triggering a series of signalling events that lead to the secretion of the invasion machinery
- RON4 is secreted by the rhoptries and embeds in the erythrocyte membrane
- AMA1 is secreted by the micronemes and binds to the RON4 -> This is known as the moving complex
- The moving complex able to move outwards (using actin and myosin)
- This pulls the Plasmodium into the erythrocyte in a vacuole
What two parts of the invasion mechanism are highly conserved between Plasmodium species?
- Microneme
- Rhoptry
What is the role of the microneme in Plasmodium?
- Produces adhesins:
- Apical membrane antigen (AMA-1)
- Thrombospondin-related anonymous protein (TRAP)
- Produces proteases -> Involved in microneme protein processing
Stores and discharges adhesins in response to cell-cell contact.
What is the role of the rhoptry in Plasmodium?
- Produces rhoptry neck proteins (RONs) -> Act in complex with AMA-1 to form ring at moving junction. Can be inserted into host cell membranes.
- Rhoptry bulb proteins (ROPs) -> Proteases, phosphatases, kinases. More Plasmodium species-specific.
How do Plasmodium species enter hepatocytes?
It is probably similar to invasion of erythrocytes, but the receptors responsible for the initial strong binding are likely to be different.
Summarise the stages of Plasmodium falciparium life cycle in erythrocytes. Include the various organelles.
- Rapid invasion, entering a parasitophorous vacuole
- Plasmodium starts producing proteins, essentially forming new organelles
- This includes Maurer’s clefts, which are involved in forming knobs on the erythrocyte surface
- The knobs contain parasite adhesin PfEMP1
- There is also a waste organelle for sequestering the haem iron that is harmful to the parasites due to the formation of ROS
- The Plasmodium undergoes many rounds of division without cytokinesis, forming a schizont
Actual stages of development: Ring, Trophozoite, Schizont
What are surface knobs in malaria and what important protein do they contain?
- They are modifications on the surface of erythrocytes during a malaria infection.
- They make the erythrocytes more adherent and less mechanically deformable.
- The key protein to this is PfEMP1 surface receptor inserted there by the Plasmodium.
How is antigenic variation possible in Plasmodium species?
[IMPORTANT]
- Many variants of PfEMP1 (the protein involved in erythrocyte knobs) are encoded by the parasite var multigene family.
- Specific adhesion properties depends on the particular var gene expressed -> Differenet PfEMP1 variants bind to different targets
- There is a library of around 60 var genes -> The Plasmodium can change which is expressed during a chronic infection, making antigenic variation possible
Describe how Plasmodium is able to transport proteins to the erythrocyte surface.
- Parasite protein signal peptide recognition, translation into the ER and secretion are normal
- Export from the parasitophorous vacuole is done by a protein translocon (PTEX)
- Parasites drive formation of multiple new structures in the erythrocyte cytoplasm, including specialized vesicles (electron-dense vesicles (EDVs), J-dots) and Maurer’s clefts -> These are involved in export
What are Maurer’s clefts and what is their function?
- They are proteins secreted by Plasmodium parasites that have infected erythrocytes
- Maurer’s clefts play a key role in transporting parasite proteins to the erythrocyte plasma membrane
- The key proteins are REX1, PfSBP1 and MAHRP1
- Maurer’s clefts are somewhat analogous to the ER or export vesicles
Describe how Plasmodium infecting erythrocytes leads to pathology.
- The infection leads to the expression of adhesion molecules, such as PfEMP1, on the surface of the erythrocyte
- These PfEMP1 molecules can allow the erythrocyte to adhese to various targets on the endothelium wall (e.g. ICAM1) and other erythrocytes (e.g. CR1)
- This can lead to blockage of blood vessels, particularly in the brain, placenta, etc.
- Adhesion leads to:
- Increased coagulation and platelet adhesion
- Increased inflammation and oxidative stress
- Increased endothelial leakage
- Reduced microvascular flow
What is the apicoplast?
[EXTRA]
- It is an organelle in Plasmodium with 4 membranes
- This suggests that it is a plastid, which is a red or green alga that became symbiotic with the Plasmodium and was eventually incorporated into the cell
- It plays a vital role in synthesis of:
- Isoprenoid
- Fatty acids
- Iron sulfur clusters
- Heme?
- It is also a drug target for drugs such as doxycycline and other tetracyclines
What are malaria vaccines difficult to make?
- Antigenic variation
- Limited natural immunity
Are there any existing vaccines for malaria?
One approved vaccine:
- RTS,S -> Low efficacy, not recommended by WHO for routine use
What are some targets for malaria vaccines?
Describe the diagnosis of malaria.
- Gold standard is parasite identification in a blood smear
- Manual process, requires skilled microscopist
- Relatively slow (hours), less suitable for rural regions
- Antigen detection (fast):
- Rapid Diagnostic Tests (RDTs)
- Laminar flow/dipstick tests
- PCR
- Involves detection of parasite DNA or RNA
- But is slow, although good confirmation test
What are some treatments for malaria?
Most drugs target the parasite in the blood:
- Chloroquine [IMPORTANT]
- Atovaquone-proguanil (Malarone®)
- Artemether-lumefantrine (Coartem®)
- Mefloquine (Lariam®)
- Quinine
- Quinidine
- Doxycycline (combinatorial therapy with quinine)
- Clindamycin (combinatorial therapy with quinine)
But can also target the parasite when it is dormant in the liver:
- Primaquine
Drug resistance is a major problem.
