Malaria

Question 1

What are the four species of the apicomplexan genus Plasmodium that cause malaria?

Answer 1

1. Plasmodium malariae – results in quartan malaria (fever every 4th day), relatively mild infection and can persist in blood at very low levels for decades.
2. Plasmodium ovale – results in tertian malaria, mainly in tropical Africa. It is rarely fatal but can persist in livers.
3. Plasmodium vivax – results in tertian malaria.
   - It is the most prevalent and it is rarely fatal.
   - Needs ambient temperatures of 16 degrees, not found in tropical Africa since the duffy antigen which is needed for the disease to occur is absent in black Africans.
   - Persists in the liver.
   - Originated from zoonosis.
4. Plasmodium falciparum – results in tertian malaria but the fever episodes can be masked as non-synchronised stages.
   - Causes cerebral malaria and is responsible for most malaria deaths.
   - It needs temperatures above 20 degrees.
   - Usually occurs in local children or in foreign/tourist adults.

Question 2

What two organisms does the malarial life cycle occur in? How long does it take?

Answer 2

Occurs in the female anopheles mosquito.

Takes roughly a week.

Question 3

Describe, in detail, the malarial life cycle.

Answer 3

• During a blood meal, saliva is injected into wound containing sporozoites.
• They are taken to the liver via the blood and infect a liver cell, facilitated by the apical organelles.

In the liver:-

• The sporozoite then develops into a schizont which will undergo hepatic schizogony (merogony) and produce progeny called merozoites.
• The merozoites multiply until they cause the hepatocyte to burst, releasing the merozoites into the circulatory system.

In the RBC:-

• The merozoites will then infect an erythrocyte by recognising specific proteins on its surface and actively invading. (e.g. P.vivax recognises duffy antigen)
• The merozoite will increase in size to form trophozoites. - - During this stage the parasite will digest haemoglobin into amino acids, and the malarial pigment hemozoin.
• The trophozoites will then mature into schizonts which will produce merozoites in erythrocytic schizogony until cell bursts, allowing more merozoites to infect RBCs.
• The release of antigens and waste products with the rupture of infected RBCs is responsible for the fever observed.

In the mosquito:-

• As an alternative, some parasites differentiate into sexual erythrocytic stages (gametocytes) which will circulate in blood until they are taken up an anopheles mosquito during a blood meal.
• Once ingested, the gametocytes will escape from the erythrocyte and travel to midgut of the mosquito where they undergo gametogenesis to form micro and macrogametes.
• While becoming micro/macrogametes, the parasite will also associate with the flagellum (exflagellation)
• The highly mobile microgametes will then fuse with macrogametes generating a zygote.
• The zygote transform into the motile and elongated ookinete. It will invade the epithelial midgut wall of the mosquito where they develop into oocysts.
• The oocyst will undergo asexual reproduction to form the sporozoites – sporogony.
• Upon maturation, the oocyst will rupture, releasing sporozoites which make their way to the mosquito’s salivary glands – they are highly motile. The sporozoites will then be expelled into the human host as the vector takes a blood meal.

Question 4

What are the methods of vector control against malaria?

Answer 4

1. Treat larval breeding sites (physical, chemical, biocontrol)
   - You need to limit oxygen and water. Spread thin film of beads, use oils, introduce fish (biocontrol)
2. Attack adult mosquitoes: spray houses with insecticides
   - Most people are bitten at their houses.
   - DDT is the best compound to spray; only needed to be sprayed once a year. It was replaced however, due to its overuse as insecticide in agriculture led to its accumulation in the food chain and it caused a thinning in egg shells and lowered the number of birds.
   - DDT was also very persistent so it promoted selection pressure and resistance was imminent.
   - DDT is now permitted again (WHO), some use in southern Africa.
3. Deter adult females
   - Bed nets, especially insecticide-treated (with permethrin –insecticide originated from plants). Permethrin will prevent mosquito going through holes in net.
   - Repellents, most commonly N,N-Diethyl-meta-toluamide (‘Deet’) – needs to be applied very regularly.
4. Transgenic methods
   - e.g. adapt mobile genetic elements (transposons) to spread through mosquito populations to either deplete populations or make them unable to transmit malaria.
   - It’s also being tested for its use in dengue fever which is also transmitted by mosquitoes. E.g. releasing insects containing dominant lethal (RIDL) gene which causes their offspring to die.

Question 5

How is vaccine development going on for malaria?

Answer 5

 The fact that malaria is difficult to control means there’s an interest in developing a vaccine.
 There are currently no approved vaccines for malaria but there are a number that are currently in development.

 There are 63 candidates in trials, with only the GSK RTS,S vaccine being the only one to reach the phase 3 clinical stage.
In October 2013 GSK announced that it was seeking approval for the RTS,S vaccine after trial data showed it almost halved malaria cases in children.

 The RTS,S vaccine is engineered using genes from the sporozoite epitope from P. falciparum.

 There are aims by WHO to license two vaccines:-
o 2015 a vaccine that has 50% protective efficacy against severe disease/death, with efficacy for at least one year [evidence suggests the RTS,S vaccine is on track to achieve this]
o 2025 to license a a vaccine with >80% protective efficacy, with protection lasting many years

Question 6

What are the adaptive strategies of the malarial parasite to ensure successful transmission?

Answer 6

1. Evade immune response
   - RBC’s do not carry class I MHC proteins so there is no immune surveillance by cytotoxic T cells
   - But infected RBCs are cleared by the spleen after 120 days.
   - Plasmodium has evolved the pfEMP1 protein which functions as a ligand to bind to receptors on the host endothelial cells. This allows it to delay its passage through the spleen and binding to capillaries is a major factor for causing cerebral malaria.
   - PfEMP1 is however susceptible to the antibody response – it poses as a good target.
   - The plasmodium conquers this by making the PfEMP1 have a great deal of antigenic variation, so that one antibody will only recognise one of the variants and the others will be safe. A single infection will express different antigens.
   - There are 40-50 genes in PfEMP1 (var gene family) that enable antigenic variation – called the var gene family. Antigenic switching occurs in around 2% of each generation in the absence of immune pressure.
2. Transmission to mosquito
   - an experiment to see if the presence of malaria parasites in humans would make them more attractive to the vector (anopheles mosquito) found that children infected with gametocytes would attract twice as many mosquitoes as children uninfected and than those infected with asexual stage of plasmodium e.g. merozoites.
3. Transmission to human
   - Biting rate is increased in vectors that have sporozoites vs uninfected vectors and they feed for longer than uninfected.
   - The sporozoites will interfere with aspyrase secretions of the salivary gland. They will promote platelet formation, reducing the mosquito’s ability to take a normal blood meal so that it will have to feed more frequently.
   - However increased biting is risky to the mosquito and sprozoite containing mosquitoes will be more likely to die than uninfected.
   - This creates evolutionary conflict of interest – the parasite will be interested in more frequent feeding but this creates cost to host in increased mortality. The balance is towards the parasite.
4. Survival of developmental period
   - If mosquito takes in gametocytes it isn’t infective so that premature feeding isn’t useful to parasite. However sometimes mosquito will need to feed again, as it didn’t feed enough to first time.
   - Not surprisingly, oocysts will reduce motivation to bite as the main focus is on survival while sporozoites (infective stage) will increase motivation to bite.
5. Resistance against mosquito’s immune system
   - Mosquitoes do not have adaptive immune system but they have an innate immune system:
   - They can kill ookinetes by nitric oxide, melanise or internalise and then kill by intracellular processes in the gut wall.
   - The plasmodium can supress the immune system in the mosquito: using sephadex beads as foreign object in experiments show that the parasite can supress the immune response, as there is reduced melanisation.