Malaria

Malaria

1. The problem of malaria (epidemiology)

   • Endemic in parts of;
     • Asia
     • Africa (malaria belt - subsaharan)
     • Latin america
     • Oceania
     • 41% of the worlds population live in regions where malaria is transmitted
       • 200-500 million cases each year
         • 1-2 million deaths each year
           • 75% of these are kids in africa
           • = 2,700 deaths each day (1 death every 30 seconds)
             • 4th leading cause of death after perinatal coditions, pneumonias and diarrhoeal diseases

2. Causitive agent

   • Transmission
     • Causitive agent is transmitted in the saliva of the female Anopheles mosquito
       • During blood meal
       • 30-40 different species of Anopheles mosquito known to transmit plasmodium (A. gambiae is the most well known)
     • Associated with living close to still water (mosquito breeding area)
       • Puddles to swamps
   • Life cycle
     • In insect salivary gland: sporozoites (transmissable form)
       • Enter host blood stream during blood meal
         • Travel to liver and invade hepatocytes (30mins after infection)
           • In hepatocyte: sporozoite nucleus divides rapidly and asynchronously - producing a schizont (one membrane, many nuclei)
             • The schizont differentiates to becomes many mononucleated merozoites
               • Hepatocyte ruptures - releasing merozoites into the blood
                 • Infect other hepatocytes
                 • Infect RBCs
                   • Asexual cycle
                     • In RBC merozoite differentiates to become a large mononucleated trophozoite (ring stage - seen in a thin blood slide under a microscope)
                       • Trophozoite divides asexually to produce a second schizont (multinucleated cell)
                         • The schizont differentiates to become many mononucleated merozoites
                           • RBC ruptures releasing merozoites into the blood stream
                             • Merozoites invade other RBCs
                   • Sexual cycle
                     • Some merozoites differentiate into male (micro-) and female (macro-) gametocytes
                       • RBCs containing gametocytes are taken up by a new vector mosquito during a second blood meal
                         • RBC breaks down in mosquito gut
                           • Gametocytes become their respctive micro- (male) and macro- (female) gametes (sex cells)
                             • The microgamete
                               • Undergoes three nuclear divisions and develops flagella
                                 • Nucleated flagella separate and fuse with the macrogamete -> zygote (diploid)
                                   • Zygote becomes a mobile ookinete
                                     • Ookinete crosses the insect gut epithelium into the basal side
                                       • Ookinete undergoes meiosis in the insect's gut wall
                                         • This forms oocysts (haploid)
                                           • Oocysts undergo repetitive cycles of mitosis (all daughter cells still haploid)
                                             • Daughter cells = sporozoites
                                               • Oocyst ruptures releasing the sporozoites into the haemocoel
                                                 • Sporozoites migrate from the gut wall to the salivary gland (start of cycle)
   • Apicomplexan (class of protozoan) protozoal disease caused by....
     • Pasmodium spp.
       • Four species are pathogenic to humans
         • P. falciparum
           • The most clinically important
             • 15% of all malarial infections...
               • .... But 90% of malaria deaths
         • P. vivax
           • Actually the most common species
         • P. ovale
         • P. malariae

3. Pathology and bioscience

   • 1) onset
     • 6-18 days after mosquito bite parasites appear in the blood
       • This is the prepatent time (i.e. time to complete the liver stage)
         • It varies with species; falciparum (6-9 days), vivax (8-12 days), ovale (10-14 days) and malariae (15-18 days)
     • Incubation time is the time between RBC infection and the start of symptoms
       • This also varies between species (from 7 days [falciparum] up to 40 days [malariae])
     • Classical symptoms (last 4-8 hrs)
       • Chills/rigor
         • Feels cold (excessive shivering) despite an elevated temperature
       • Fever + headache, nausea, vomiting, malaise etc
       • Sweats and becomes tired
       • Exhaustion -> sleep -> wake up apparently fine
       • Repeated every 2/3 days (ovale and vivax = 2 days, malariae = 3 days; falciparum = almost continuous fever)
         • Cyclic fever coinsides with RBC infection and rupture (rupture causes fever)
           • Why?
             • RBC infection and development of parasites is synchronous (all in RBCs at the same time and all released at he same time)
             • Lysis of RBC -> parasite antigens in blood -> stimulates immune cells to produce TNF-alpha and other cytokines -> fever
               • Fever becomes less severe with age (immunity difference with age)
     • 2) development
       • Symptoms intensify
       • Irregular high fever
       • Anxiety, delerium and other mental manifestations
       • Sweating, increased pulse rate, exhaustion
       • GI symptoms
       • Hepatoslenomegaly!!!
       • 3) severe malaria
         • Occurs in 10% of P. falciparum cases
         • 50% mortality
         • Several manifestation (can appear simultaneously or sequentially)
           • Nn specific fever with loss of conciousness
           • Severe headache, drowsiness, neurological abormalities
           • Convulsions, vomiting and coma -> death
           • Drop in haematocrit (infected and non-infected RNCs destroyed, with reduced RBC production (due to cytokines))
             • Reduction in O2 supply to tissues
           • Other abnormalities (renal, respiratory, glucose levels, jaundice etc.)
   • Sequestration/cytoadherence
     • Sequestration
       • Infections with P. falciparum - only the ring trophozoite can be seen (in RBCs) in ther peripheral blood
         • The schizonts cause the RBCs to become 'sticky' (cytoadherence) this causes them to attach to the endothelium of venules (= sequestration)
           • P. falciparum infected RBCs are sequestered in many organs; heart, lungs, kidneys, brain, adipose, intestines, liver, spleen and placenta
     • Cytoadherence
       • RBCs containing schizonts become sticky
         • Stick to other RBCs (and endothelial cells)
           • Infected cell binding to non-infected cells = rosetting (organised rosettes of healthy RBCs arranged around a central infected RBC)
           • Infected cells binding to other infected cells = clumping (irregular clumps of infected cells which have lost their structure [shperocytes])
           • Seen in 50% of cases (correlates with disease severity)
             • Sticky cells clog-up blood vessels and cause haemorrhage (especially seen in cerebral malaria)
         • Rosetting/clumping is caused by PfEMP1 (P.falciparum erythrocyte membrane protein)
           • Multidomain protein produced by the parasite that are transferred to the RBC cell surface
             • Forms structures called 'knobs' on the membrane of the RBC
           • Allows infected cells to bind other infected/non-infected cells and endothelial cells (which are expressing cell adhesion molecules)
             • Cell adhesion molecules expressed by venule endothelium due to inflammatory IFN-gamma and TNF-alpha
               • Cell adhesion molecules expressed (can bind PfEMPs) = CD36 and ICAM1
           • PfEMP1 structure
             • NH2-multiple cysteine-rich domains (feature duffy binding-like (DBL) and cysteine-rich interdomain regions (CIDR))-transmembrane domain-COOH
               • Number and arrangement of DBLs and CIDRs vary between different PfEMPs
             • Extremely antigenic (elicits a strong immune response)
               • However, PfEMPs are encoded by a large multi-gene family (VAR genes)
                 • Parasites are able to switch between PfEMP gene expression (antigenic variation)
                   • One one gene is expressed at any one time (like Trypanosoma antigenic variation [VSG]) - allelic exclusion
                     • Deopends on the VAR promoter
                       • Swithching VAR gene may result in a new adhesion phenotype
                         • 2% of the parasite population switch VAR genes per cycle
                 • About 60 VAR egens

4. Acquired immunity

   • People living in endemic areas acquire immunity through natural exposure to the parasite
     • Acquired (or natural) immunity occurs only after copntinued exposure from multiple infections over time
       • Acquired immunity limits high-density parasitaemia; however it does not lead to sterile protection
         • Clinical immunity gives protection gives prtoection against severe effects of malaria but fails to provide strong protection against infection