lecture 18

Question 1

What are the main causes of malaria globally?

Answer 1

• P. vivax

- P. falciparum

Question 2

Is malaria a modern pandemic?

Answer 2

• no
• malaria is an ancient disease
• in 400 BC, Hippocrates described the symptoms of malaria
• presence of Plasmodium DNA in the remains of King Tut (1324 BC)
• has evolved with us

Question 3

Who is at greatest risk of malaria? What is the burden of malaria?

Answer 3

• young children and pregnant women
• up to 1 millions deaths per year
• 300 - 500 million cases per year
• a leading cause of childhood deaths globally
• malaria in pregnancy:
• • low birth weight
• • miscarriages and stillbirths
• impedes economic development
• impacts on learning and education
• compounds poverty
• malaria most affects resource-poor communities

Question 4

What are obstacles to combating malaria ?

Answer 4

• no highly effective control measures available: partially effective measures, poorly applied
• no vaccine available (yet)
• drug resistance widespread and increasing
• insecticide resistance
• economic, political, and social factors

Question 5

What are the Plasmodium species and the differences in disease they cause?

Answer 5

P. falciparum

• causes majority of severe malaria disease and death
• population at risk: 2.2 billion

P. vivax

• increasingly recognised as a cause of severe illness
• population at risk: 2.6 billion
• dormant liver stage

P. ovale and P. malariae

• limited distribution, mild disease
• proposed that P. ovale is actually two species

P. knowlesi

• zoonotic infection, can be severe
• present in macaques throughout SE asia
• human to human transmission for the first four
• if he had malaria and they filled the room with mosquitoes, no one would get it
• there’s an incubation period before they become infective

Question 6

What causes the bulk of Malaria disease?

Answer 6

P. falciparum

Question 7

What is P. falciparum transmitted by?

Answer 7

female Anopheles mosquitoes

Question 8

Is there an animal reservoir for P. falciparum?

Answer 8

no

Question 9

At what stage do we see malaria?

Answer 9

disease only during blood stage

Question 10

What are the immune responses primarily against?

Answer 10

• blood stage parasites

- involve both humoral and cellular responses

Question 11

What are the clinical features of malaria?

Answer 11

Uncomplicated (mild) malaria:

• ‘flu-like’ illness
• fever, headaches, malaise
• 90-95% of cases

severe malaria:

• severe anaemia
• cerebral complications (cerebral malaria)
• • coma, convulsions
• • long-term neurological deficits
• respiratory distress and metabolic acidosis
• • reduced tissue perfusion
• • lung damage
• other: hypoglycemia, kidney failure, blood clotting problems
• deathrate of 15-20%

Question 12

What is the treatment of malaria?

Answer 12

mild malaria

• short course of effective anti-malarial tablets
• aremisinin combination therapy (ACT):
• • e.g. artemether-lumefantrine (AL) (older drugs - chloroquine, sulfadoxine-pyrimethamine)
• clearance of P. vivax liver stage - primaquine

severe malaria

• anti-malarials: intavenous artemisinin or quinine (7-10 days)
• IV fluids, blood transfusion if required
• supportive treatment (intensive care)
• anticonvulsant, anticoagulant, anti-inflammatory drugs?
• little improvement in mortality rate for decades
• urgent need for new treatments

Question 13

What is the epidemiology and immunity of malaria?

Answer 13

• immunity eventually develops after many episodes
  three main types of immunity
  1. immunity that prevents severe malaria
  2. immunity that prevents any malaria
  3. immunity to malaria in pregnancy

Question 14

What are reasons for slow development of immunity?

Answer 14

parasite factors:

• multiple antigenic targets (~5000 genes)
• antigenic diversity: major targets show substantial polymorphism
• antigenic variation: gene families allow switching to evade responses

host factors:

• inadequate response (especially young children)
• non-functional/irrelevant responses
• poor development of memory responses?

Question 15

How does the malaria illness develop?

Answer 15

Unrestricted replication in the blood stream

1. malaria parasites accumulate in vital organs
2. inflammatory responses
3. destruction of red blood cells

severe illness: multi-system involvement

• coma (cerebral malaria)
• severe anaemia
• acidosis and respiratory distress

Question 16

What antigens are on the surface of infected red blood cells?

Answer 16

• produced by P. falciparum developing inside RBC - they are transported (‘trafficked’) to the RBC
• enable infected red blood cells to adhere in the blood vessels and avoid clearance by spleen
• infected cells accumulate in large numbers in vascular beds and can cause severe disease (e.g. brain, placenta)
• antigens are highly variant to evade immune recognition
• important in virulence and immune evasion
• adhesion to endothelial cell
• forming clumps with platelets

Question 17

What does the antigenic variation and diversity of Plasmodium enable?

Answer 17

• antigenically distinct waves of parasitemia enable
• chronic and recrudescent infections
• repeat infections

Question 18

How do we get antigenic diversity and variation?

Answer 18

antigenic diversity and polymorphisms

• most infectious organisms have evolved polymorphisms in targets of immune responses
• • enables reinfection with different ‘strains’
• each infection is caused by genetically different parasites that have different polymorphisms in key antigens

antigenic variation

• an ability to switch the expression of different variant antigens
• a feature of Plasmodium and trypanosomes

Question 19

What is PfEMP1?

Answer 19

• the major antigen on the surface of parasitised red blood cells
• serum samples from kenyan adults have antibodies that recognise malaria antigens
• PfEMP1 is both polymorphic and encoded by a large gene family
• encoded by var genes – multigene family
• each genome has around 60 different var genes – can express around 60 different PfEMP1 variants
• between genomes, PfEMP1 variants differ
• extensive polymorphism, and clonal variation in expression of different PfEMP1 types mediates immune evasion
• • thousands of different var genes exist in nature
• actual antigenic diversity is not as extensive as suggested by sequence analysis

Question 20

What happens when we grow a parasite in the lab?

Answer 20

• single parasite grown in the lab
• spontaneous variation
• different phenotypes
• if you look at the different phenotypes at a molecular level they are expressing different variants of PfEMP1
• this gives different adhesive properties

Question 21

What does the expression of different PfEMP1 variants enable?

Answer 21

• enables infected cells to adhere in different organs
• each variant is antigenically different: not recognised by the same antibodies
• expression of new variant form of PfEMP1 allows escape from the developing immune response - waves of parasitemia

Question 22

What are immune responses to malaria?

Answer 22

blood stage:
- passive transfer of antibodies is protective

antibodies to merozoites

• inhibit RBC invasion and growth
• • direct inhibition by antibodies
• • antibody-dependent cell mediated inhibition of parasite growth

antibodies to infected RBCs

• parasite antigens expressed on the surface of RBCs
• opsonization for phagocytosis

not very good at targeting the liver stage

Question 23

What is antibody-mediated killing of malaria blood-stage parasites?

Answer 23

• e.g. opsonise merozoites and parasitised RBCs for killing by immune cells (monocytes, macrophages, neutrophils)

Question 24

What are pregnant women at a high risk of malaria infection?

Answer 24

• more frequent infections, higher density of parasitemia
• occurs despite immunity that may have developed prior to pregnancy
• risk of malaria decreasing with each pregnancy:
• • women develop immunity to malaria in pregnancy
• serious consequences for mother and baby
• pregnancy studies in Blantyre, Malawi

Question 25

How do we identify key properties of malaria infections?

Answer 25

- parasites from the placenta mostly adhere to carbohydrate receptors (esp. CSA) - from children mostly adhere to other receptors e.g. CD36, ICAM-1 - can use this to determine key receptors and malaria proteins for infection and disease

Question 26

What specific variant of PfEMP1 is important for placental infection?

Answer 26

- var2csa - mediates adhesion of parasitised RBCs to receptors in the placenta: Primary receptor is a carbohydrate (CSA, chondroitin sulfate A) - poorly recognised by children and adults who have not been pregnant: little immunity to it is acquired before pregnancy - enables parasitised RBCs to accumulate in large numbers in the placenta - evades existing immunity

Question 27

Is it possible to have a vaccine against PfEMP1?

Answer 27

- PfEMP1 is a highly diverse antigen - a major challenge to vaccine development - effective vaccines would require the inclusion of many different variants - may be possible for malaria in pregnancy: - - one major PfEMP1 variant? - - this would not protect children and non-pregnant adults - - use in combination with other antigens? - - clinical trial planned

Question 28

What are the economic and health benefits of vaccines?

Answer 28

- immunising children is one of public health's "best buys" - vaccines boost development through: - - direct medical savings - - indirect economic benefits such as cognitive development, educational attainment, labour productivity, income, savings and investment - vaccines boost economic growth in the poorest countries - expanding childhood immunisation rates in the world's 72 poorest countries over the next decade would result in an estimated $151 billion in treatment and productivity savings

Question 29

What are the research objectives at Burnet?

Answer 29

Malaria in early childhood - development of immunity - specific targets of protective antibodies malaria in pregnancy - mechanisms of placental infection - immunity to malaria in pregnancy vaccines and interventions - identification and prioritisation of vaccine candidates - clinical trials: vaccines and preventive approaches - new therapeutics

Question 30

What is a challenge faced by people researching malaria?

Answer 30

- access and resources in rural and remote communities