Malaria Site: Pathophysiology Of Malaria

Some patients are at special risk for developing malaria. These include patients with no immunity against malaria - infants and unexposed individuals and patients who have lost the acquired immunity like pregnant ladies, elderly and people who have left the area of transmission. Also it has been observed that certain inborn defects in the blood hemoglobin offer some sort of a protection against either clinical attacks of malaria or its complications. The exact mechanism of this protection is not well understood. However, many possibilities have been suggested.

Pre-erythrocytic schizogony: Sporozoites are injected by the mosquito into the subcutaneous tissue (less frequently directly into the bloodstream) and travel to the liver either directly or through lymphatic channels. They reach the liver in 30-40 minutes by brisk motility conferred by Circum Sporozoite Protein (CSP). Approximately 8-15 (up to 100) sporozoites are injected and therefore only a few hepatocytes are infected, therefore this stage of the infection causes no symptoms (rarely however a prodromal illness characterised by vague aches and pains, headache, nausea etc. may be present). Recent evidence indicates that sporozoites pass through several hepatocytes before invasion. The co-receptor on sporozoites for invasion involves, in part, the thrombospondin domains on the circumsporozoite protein and on thrombospondin-related adhesive protein (TRAP). These domains bind specifically to heparin sulfate proteoglycans on hepatocytes in the region in apposition to sinusoidal endothelium and Kuppfer cells. Within the hepatocyte, each sporozoite divides into 10000-30000 merozoites. This phase is called pre-erythrocytic schizogony, meaning development of schizoid forms of the parasite before reaching the red blood cells. This phase takes about 10 - 15 days in P. vivax malaria and about 7-10 days in P. falciparum malaria.

Erythrocytic schizogony: At the completion of the pre-erythrocytic schizogony, the mature schizonts rupture the liver cells and escape into the blood, wherein they infect the red blood cells. These infective forms are called merozoites and they continue their growth and multiplication within the red blood cells. In P. vivax malaria, the young red blood cells are predominantly infected, while in P. falciparum malaria, red blood cells of all ages are affected. Thus the infective load and severity of infection are more in case of P. falciparum malaria.

The sequence of invasion into red cells

Engage receptors on RBC for binding: Duffy for P. vivax, many for P. falciparum

Apical re-orientation

Junction formation

Signaling

Vacuole formed from the RBC plasma membrane

Enters the vacuole by a moving junction

?Cleavage of a RBC surface protein by a parasite serine protease

The sequence of invasion is probably similar for all Plasmodium spp. The merozoite first attaches to red cells. In P. falciparum, Erythrocyte Binding Antigen 175 and Merozoite Surface Protein 1, 2 with sialoglycoproteins have been identified as the ligands and in P. vivax, Duffy antigen on RBC is the site of binding. After the attachment to the red cell, the merozoite reorientates itself so that apposition of apical end occurs. This is followed by localized invagination and interiorization of the merozoite. The entire process is completed in 30 seconds.

The growth and multiplication cycle within the RBCs (Erythrocytic schizogony) takes about 48 hours for one cycle (72 hours in case of P. malariae). Each merozoite divides into 8-32 (average 10) fresh merozoites. The merozoites grow in stages into rings - trophozoites (trophos = nourish) and divide in a Schizont(=split) to release more merozoites (mero = separate). At the end of this cycle, the mature schizonts rupture the RBCs and release the new merozoites into the blood, which in turn infect more RBCs.

The parasite has complex metabolic processes: It utilises amino acids from hemoglobin and detoxifies heme; pLDH, Plasmodium aldolase have been identified as enzymes of anerobic glycolysis. It has been found that the parasites increase the permeability of RBC to get nutrients, yet maintain the RBC structure for 48 hours. strengthened by RESA, allowing tthe immature parasite to survive. At the end, RBC ruptures and each schizont releases 6-36 merozoites

Merozoite’s Nutrition in RBC: The parasite ingests hemoglobin from RBC to form a food vacuole where it is degraded and heme is released. The toxic heme is in turn detoxified by heme polymerase and sequestrated as hemozoin (malaria pigment). (Many of the antimalarial drugs act by inhibiting heme polymerase thereby causing accumulation of toxic heme).

All the clinical features of malaria are caused by these developments in the blood. The growing parasite progressively consumes and degrades intracellular proteins, principally hemoglobin, resulting in formation of the 'malarial pigment' and hemolysis of the infected red cell. This also alters the transport properties of the red cell membrane, and the red cell becomes more spherical and less deformable. The rupture of red blood cells by merozoites releases certain factors and toxins (such as lycosylphosphotidylinositol anchor of a parasite membrane protein, phospholipoprotein, RBC membrane products, protease sensitive components with hemozoin, ? malarial toxins etc.), which could directly induce the release of cytokines such as TNF and interleukin-1 from macrophages, resulting in chills and high grade fever. This occurs once in 48 hours, corresponding to the erythrocytic cycle. In the initial stages of the illness, this classical pattern may not be seen because there could be multiple groups (broods) of the parasite developing at different times, and as the disease progresses, these broods join and the synchronous development cycle results in the classical pattern of alternate day fever. It has been observed that in primary attack of malaria, the symptoms may appear with lesser degree of parasitemia or even with submicroscopic parasitemia. However, in subsequent attacks and relapses, a much higher degree of parasitemia is needed for onset of symptoms. Further, there may be great individual variations with regard to the degree of parasitemia required to induce the symptoms.

		The progeny from a single parasite in the liver could destroy all the host’s RBCs within 12-14 days! In case of P. vivax infection, the parasitemia is limited by exhaustion of suitable red cells, specific and non-specific immune response, destruction of meronts by high fevers and splenic clearance.
Limited parasitemia in P. vivax
In P. falciparum infection, the available red cell numbers are higher as red cells of all ages are infected and the parasitised red cells escape from destruction by sequestration. The P. falciparum has better skills for attack and can enter most RBCs, has plenty of redundant receptors and pathways and hence higher parasitemia. It also has better skills for survival: the parasite constantly changes itself with 2% antigenic variation, it changes the RBC structure by producing sticky Knobs on the surface, it changes the immune response by numbing the dendritic cells and hides in deeper tissues by sequestration.
		Higher parasitemia in P. falciparum

P. falciparum malaria is characterised by development of sticky knobs on the surface of red cells, adhesion of red cells to the endothelial cells of post-capillary venules and formation of rosettes with uninfected cells.

Cytoadherence is mediated by strain specific, high MW P. f Erythrocyte Membrane Protein 1 (PfEMP 1) that is exported to the surface of infected erythrocyte and anchored through the membrane to a sub-membranous accretion of parasite derived histidine rich protein (Pf HRP). These accretions cause humps or knobs on the surface of the red cell and these are the points of attachment to vascular endothelium. Some parasites are knob negative and yet show cytoadehrence. A protein called sequestrin has also been identified recently. Altered red cell membrane components may also play a role. Cytoadherence may be modulated by the spleen and cytoadherence does not occur after splenectomy.

On the endothelium, Leukocyte differentaiting antigen CD36, intercellular adhesion molecule 1 (ICAM1), Thrombospondin, VCAM and ELAM have been idenbtified as binding proteins. TNF upregulates binding to ICAM 1 (not CD36) and binding is higher at low pH and high calcium levels. ICAM 1 is abundant in brain microcirculation and CD36 elsewhere.

Rosetting is adherence of parasitised red cells with uninfected red cells. It is independent of venular cytoadherence and exhibits 5 times stronger adhesion than cytoadherence. Rosetting causes higher microvascular obstruction than cytoadherence and is associated with cerebral malaria (cytoadherence with other vital organ damage). Rosetting reduces blood flow, encourages cytoadherence to endothelium, enhances anerobic glycolysis and reduces the pH.

As the parasite matures, flexible biconcave disc becomes progressively more spherical and rigid. Reduced membrane fluidity, increasing sphericity, enlarging and relatively rigid intra-erythrocytic parasites make the red cells less filtrable and cause obstruction at mid capillary level itself.

Unbridled cytoadherence-rosetting-sequestration results in poor tissue perfusion, organ dysfunction, anerobic glycolysis in tissues and lactic acidosis, malfunctioning of dendritic cells and T cells due to CD36 binding. While low levels of cytokines may be beneficial, high levels are found to be harmful, contributing to placental dysfunction, suppression of erythropoeisis, inhibition of gluconeogenesis and increased cytoadherence.

Exo-erythrocytic schizogony: In P. vivax and P. ovale, some exo-erythrocytic forms remain as single celled dormant forms called hypnozoites. This helps them to survive in temperate countries. These hypnozoites can get re-activated once in 3-6 months to cause ‘relapses’. This phase of the infection is called as exo-erythrocytic schizogony. In P. falciparum and P. malariae infections, relapses from the liver do not occur; however, the blood infection may remain chronic and, if untreated, may remain for years in case of P. falciparum and decades in case of P. malariae.

Some of the merozoites in the blood transform into sexual forms, called as gametocytes. These appear in the peripheral blood after 7-10 days of the infection in P. vivax and 10-20 days in P. falciparum infection. When anopheles mosquito bites an infected individual, these gametocytes enter the mosquito and continue their sexual phase of development within the gut wall of the mosquito. This completes the asexual - sexual cycle of the malarial parasite.