What causes Malaria?
single called intracellular eukaryotic parasite
human parasites:
plasmodium falciparum
- most virulent species, 90% of cases
plasmodium vivax
- major cause outside of africa
plasmodium malariae
plasmodium ovale
What is the problem with a malaria animal model
human parasites don‘t develop in mice
-> model organism needed
primate: plasmodium knowlesi
- not used often
rodent: plasmodium berghei, yoelii, vinckei, chabaudi
- knock out mice, susceptibility varies -> disease outcome relies on combination of rodent and parasite
- non pathogenic for human
Infection routes (two ways)
- infection with fresh parasitised blood (pRBCs):
- can be cryopreserved
- parasite density must be determined
- infection gives rise to blood-stage malaria
- immune mechanism and pathology - infection with sporozoites:
- can‘t be cryopreserved
- maintenance of whole infectious cycle needed (host and vector!!)- mosquitoes feed on infected mice & are kept for 3 weeks to develope sporozoites
- infection via mosquito bite or injection of isolates from mosquitoes
- give rise to liver stage malaria
- mosquitoes feed on infected mice & are kept for 3 weeks to develope sporozoites
How to monitor the malaria infection?
- clinical signs of illness (usually after 5 dpi)
- peripheral blood parasitemia
- whole-body parasite burden
What are aspects of the clinical examination?
clinical score of:
1. activity
2. body weight
3. general condition
4. unprovoked behaviour
other score:
Rapid murine coma and behaviour scale (RMCBS)
Determination of blood parasitemia
thin blood film on OT
% parasitemia = (# infected RBCs / # total RBC) x100
Whole-Body Parasite burden
- circulating and tissue sequestered parasites
- more accurate than blood parasitemia
- visualisation via transgenic plasmodium strains expressing luciferase
- injection of luciferin => bioluminescence
name mouse models for malaria
plasmodium berghei und p. yoelii YM/17XL -> severe malaria
plasmodium chabaudi: chronic infection,
plasmodium yoelii 17X(NL): self-resolving in immunocompetent mice
What are the limitations of the mouse model?
- not the natural host
- infection often via injection (not natural)
- naive mice vs. infection in endemic area
- mice can tolerate higher parasitemia
pathophysiology of cerebral malaria
- mortality with treatment is 20%
mechanisms:
1. mechanical obstruction of blood flow by cytoadherence if infected RBC
2. immune-mediated: pRBC-induced cerebral inflammation, leukocyte infiltration, release of IL1, NO, TNFα
-> tissue damage and vascular leakage
What is tissue sequestration and its consequences
- pRBCs are removed by spleen
- p. falciparum counters with cytoadherence -> sequestration
sequestration is mediated by virulence factor PfEMP1 -> enables binding to CD36 and ICAM
it ultimately leads to clogged blood vessels
Infection of C57BL/6 mice with P. berghei ANKA leads to…
- fatal cerebral pathology
- as in humans, rapid deterioration
- death within 4-5h after onset of neurological symptoms
How is the integrity of the BBB measured?
staining with evans blue
dye has high affinity to serum albumin
- BBB intact -> no leakage of albumin into brain tissue -> no color
- BBB damaged -> leakage of albumin into brain tissue -> brain is blue
Mouse model for severe malaria
P. berghei NK65 in C57BL/6 mice
associated with MA-ARDS -> Malaria-associated acute respiratory distress syndrome
symptoms of MA-ARDS
- sequestration of RBCs, monocytes and neutrophils
- pulmonary edema
- high degree of inflammation
What is hemozoin
- end product of heamoglobin digestion
- serves as malaria pigment
- produced when toxic heme is detoxified into hemozoin
What is hepatosplenomegaly
- bigger liver and spleen
- organs get bigger in response of pRBC removal
Malaria whole-organism vaccines
- heat killed sporozoites: no invasion & development
- irradiated sporozoites: invasion, but no proper development in liver; attenuation via DNA strand breaks
- genetically arrested sporozoites: invasion, but no proper development in liver
- sporozoites + chloroquine cover: invasion & development in liver but no blood stage malaria; attenuation via suppression
