Parasitology Yr1- Plasmodia spp.

sporozoite

infective stage of plasmodium

The mosquito phase (sporogony)

1) When a female anopheles mosquito ingests parasitised erythrocytes along with its blood meal, the asexual forms of malaria parasites are digested, but the gametocytes are set free in the stomach and undergo further development. 2) Within 15 mins of entry into the stomach of the mosquito, the male gametocyte divides into 8 nuclei from each of which protrudes a long, actively motile whip like filament. These flagella which are the male gametes lash about for sometime and then break free. 3) The female gametocyte doesn't divide, but undergoes a process of maturation to become the female gamete. It is fertilised by one of the microgametes to produce the zygote. 4) Fertilisation occurs in half to 2 hours after the blood meal. 5) The zygote which is initially a motionless round body, elongates and within 18 to 24 hours becomes a vermicular motile form. This is called the ookinete 6) The ookinete penetrates the epithelial lining of the mosquito stomach wall and comes to lie beneath its basement membrane. 7) The ookinete becomes rounded into a sphere with an elastic membrane. This stage is called the oocyst. There may be several hundred pigmented oocysts in the stomach of a mosquito. 8) The oocyst matures, increasing in size with the nucleus undergoing multiple divisions. This sporogony leads to the development within the oocysts of about a thousand sporozoites. 9) When it ruptures, the sporozoites enter the haemocoele and reach the salivary glands. 10) The mosquito is now infective and when it feed on humans, the sporozoites are injected into the skin capillaries to initiate human infection

How many species of plasmodia cause malaria in humans?

4 species

Malaria

A disease caused by mosquitoes implanting parasites in the blood.

What happens after a few cycles of erythrocytic schizogony?

After a few cycles of erythrocytic schizogony some merozoites that infect RBC don't proceed to become schizonts, but instead develop into sexually differentiated forms, the gametocytes. They grow in size till they almost fill the red cell, but the nucleus remains undivided.

ring form malaria

As the ring form develops, it enlarges in size becoming irregular in shape and shows amoeboid motility. This is called the amoeboid form. When the amoeboid reaches a certain stage of development its nucleus starts dividing. The parasite within the erythrocyte till the time its nucleus starts dividing is called the trophozoite. The ring form is called the early trophozoite and the amoeboid from the late trophozoite.

From the time the nucleus starts dividing, the parasite within the erythrocyte is called what?

From the time the nucleus starts dividing, the parasite within the erythrocyte is called the schizont. At first only the nucleus divides into a variable number of small nuclei, the cytoplasm remaining entire and undivided. This stage is called early schizont. This continues to the late schizont when each daughter nucleus becomes surrounded by cytoplasm. The mature schizont is the fully grown form, in which a number of small merozoites are seen, each having a nucleus with surrounding cytoplasm

The human phase (malaria)

Human infection comes through the bite of the infective female anopheles mosquito. The sporozoites which are the infective forms of the parasite are present in the salivary gland of the mosquito. They are injected into the blood capillaries when the mosquito feeds on blood after piercing the skin. Usually 10 to 15 sporozoites are injected at a time, but occasionally many hundreds may be introduced. The sporozoites pass into the blood stream where many are destroyed but some reach the liver and enter the parenchymal cells (hepatocytes)

How is human malaria transmitted?

Human malaria is transmitted by the vector female anopheles mosquito.

In P.vivax and P.ovale how many types of sporozoites are seen?

In P.vivax and P.ovale, there are 2 kinds of sporozoites: some which multiply inside hepatocytes promptly to form schizonts (tachyzoites) and others which remain dormant. These latter forms are called hypnozoites. Hypnozoites remain inside the hepatocytes as uninucleated forms for long periods. From time to time, some are activated to become schizonts and release merozoites

which 2 species of plasmodium account for 95% of all malaria worldwide?

P.vivax and P.falciparum

Plasmodia spp.

Plasmodia spp. are the causative agents of malaria, a disease known for antiquity. It is is the most important parasitic disease of mankind.

4 species of Plasmodium that cause malaria

Plasmodium malariae Plasmodium vivax Plasmodium ovale Plasmodium falciparum

Pre-erythrocytic schizogony involves what?

Pre-erythrocytic schizogony involves only a very small proportion of liver cells and causes no significant damage or clinical illness

lifecycle of Plasmodia spp.

The lifecycle of malaria parasites comprises of 2 stages: an asexual phase occurring in humans and the sexual phase occurring in mosquitoes. As the sexual phase occurs in the mosquito, it is considered the definitive host of malaria parasites. Humans are the intermediate host as the human phase consists of asexual multiplication

Difference between male and female anopheles mosquitoes?

The male mosquito feeds exclusively on fruit juices, but the female needs at least 2 blood meals before the first batch of eggs can be laid

What happens to the mature schizont?

The mature schizont bursts releasing the merozoites into the circulation. The residual mass of unutilised cytoplasm containing all the accumulated malarial pigment is also released at the same time into the circulation. The merozoites invade fresh erythrocytes in which they go through the same process of development.

Endoerythrocytic stage (so far)

The merozoites released by pre-erythrocytic schizonts invade the red blood cells. The merozoites then enter the erythrocyte by endocytosis and the red blood cell membrane seals itself to form a vacuole enclosing the merozoite. The process of entry into the RBC takes about 30 seconds. Once inside the RBC, the merozoite rounds up and loses its internal organelles. In the erythrocyte, the merozoite appears as a rounded body having a vacuole in the centre with the cytoplasm pushed to the periphery and the nucleus situation at one pole. These young parasites are therefore called ring forms. The parasite feeds on the haemoglobin of the erythrocyte. It doesn't metabolise haemoglobin completely and so leaves behind a residue a haematin- globin pigment called the malaria pigment. These iron containing pigments accumulate as dark granules which become more prominent as the parasite grows

exoerythrocytic (tissue) stage (process)

The sporozoite which have elongated spindle shaped bodies become rounded inside the liver cells. They enlarge in size and undergo repeated nuclear division to form several daughter nuclei, each of which is surrounded by cytoplasm. The hepatocyte is distended by the enlarging schizont and the liver cell nucleus is pushed to the periphery. In 5.5 to 15 days the schizont becomes mature and bursts releasing thousands of merozoites. They enter the blood stream and infect the erythrocytes by the process of invagination.

exoerythrocytic stage (so far)

Within an hour of being injected into the body by the mosquito, the sporozoites reach the the liver and enter the hepatocytes to initiate the stage of pre-erythrocytic schizogony or merogony

Schizont

a cell that divides by schizogony to form daughter cells. A collection of the merozoites in the liver is called a schizont.

Which animal may act as a reservoir host for P.malariae in Africa?

chimpanzees

Merozoites

new cells that emerge from the liver and infect red blood cells

mature gametocyte

the mature gametocytes are round in shape except in P.falciparum in which they're crescent shaped. In all species, the female gametocyte is larger (macrogametocyte) and the male is smaller (microgametocyte). A person with gametocytes in circulation is a carrier or reservoir. Gametocytes don't cause any clinical illness in the host, but are essential for transmission of the infection. They don't develop further or divide in the vertebrate host and unless taken up by the vector mosquito, they die in a few days.