|MicrobiologyBytes: Microbiology Notes: Malaria: Biology||Updated: October 19, 2004||Search|
The sporozoites from the mosquito salivary gland are injected into the human as the mosquito must inject anticoagulant saliva to ensure an even flowing meal. Once in the human bloodstream, the sporozoites arrive in the liver and penetrate hepatocytes, where they remain for 9-16 days, multiplying within the cells. On release, they return to the blood and penetrate red blood cells in which they produce either merozoites or micro and macrogametocytes, which have no further activity within the human host. Another mosquito arriving to feed on the blood may suck up these gametocytes into its gut, where exflagellation of microgametocytes occurs, and the macrogametocytes are fertilized. The resulting ookinete penetrates the wall of a cell in the midgut, where it develops into an oocyst. Sporogeny within the oocyst produces many sporozoites and, when the oocyst ruptures, the sporozoites migrate to the salivary gland, for injection into another host.
Animation: Lifecycle of a malaria parasite from mosquito to blood stages
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This highly specialised life cycle requires specialised biology on the part of the Plasmodium species. The reason that not all mosquitos are vectors for Plasmodium parasites is that refractory mosquitos posses substances toxic to Plasmodium within their cells (7). A higher trypsin-like activity was also found in the midgut of resistant species (8), possibly inhibiting ookinete development. Plasmodium parasites seem capable of adapting to any suitable anopheline mosquito, given sufficient time and contact (1). Sporogeny within the mosquito is governed by environmental temperature as Anopheline mosquitos are poikilotherms.
Once injected into the human host, all Plasmodium species will penetrate hepatocytes. However, P.falciparum and P.malariae sporozoites trigger immediate schizogony whereas P.ovale and P.vivax sporozoites may either trigger immediate schizogony or have a delayed trigger, resulting in dormant hypnozoites. Some strains, such as the North Korean strain, seem to consist of sporozoites with universally delayed triggers, so they all form long lasting hypnozoites. P.vivax may have an incubation period of up to 10 months. Gametocytes produced in the primary attack seem to contain all the genetic information required to create sporozoites of several different activation times. The same seems true for gametocytes produced in relapses where the hypnozoites become activated.
Sexual development of Plasmodium begins as the merozoites invade the erythrocytes after their release from the liver. Within the erythrocyte, shizogony occurs to produce either more merozoites (taking 22 1/2 hours in the case of P.berghei), or the sexual micro and macrogametocytes (taking 26 hours) (9). In P.falciparum, erythrocytic schizogony takes 48 hours and gametocytosis takes 10-12 days. Normally a variable number of cycles of asexual erythrocytiic shizogony occurs before any gametocytes are produced (10). The immune system may produce antibodies to the gametocytes at this stage.
Once drawn into the mosquito, the gametocytes increase in volume and escape the erythrocyte. Microgametes are formed by 3 mitotic divisions within the microgametocyte, and are expelled explosively. No further changes affect the female macrogametocyte until fertilisation where the plasmalemmas of male and female gametes fuse and the nucleus of the microgamete enters the female cytoplasm (1). After fertilisation, the zygote is a motionless globular cell, but after 18 to 24 hours it becomes elongated and motile, containing micronemes and a pellicle. The cell invades the microvillus border, passes through the midgut cells, and lies beneath the basement membrane (1). The ookinete then becomes a static oocyst, between the basal lamina and the basement cell membrane, and bounded by a thick plasmalemma. The chief source of nutrients is the haemolymph in which the oocyst develops. Sporoblasts form, and sporozoites bud off.
After the cyst ruptures, the sporozoites escape into the haemocoele and migrate to and penetrate salivary gland cells where they lie in vacuoles for up to 59 days. These sporozoites develop and become up to 1000 times more infective than when in the oocyst (1). They are more antigenic, and bear circumsporozoite polypeptide on their plasmalemma. Sporozoite motility is involved in their invasion of cells and escape from the salivary gland. The sporozoites are about 12µm long and 1µm across, with a single nucleus, anterior to which lie micronemes, and posterior to which lies ER and mitochondria (1). They posses a complex pellicle, which is responsible for motility, and contains circumsporozoite protein. The apical penetrating region contains extensions of the microneme ducts which release an agent which interacts with host cell plasma membrane during penetration.
A biting mosquito transfers about 10% of its sporozoite load into the capillaries or perivascular tissue. Now the sporozoites must begin their evasion of the host defences, possibly by binding serum proteins for 'camouflage' (13). Some are destroyed by macrophages, or by antigen specific antibodies in immune individuals, but in non immune individuals, they reach the hepatocytes and initiate schizogeny or become hypnozoites depending on their delay trigger. All sporozoites have left peripheral circulation within 45 minutes (1).
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