MALARIAPlasmodium Life cycle

MALARIA
Plasmodium Life cycle
Plasmodium species that infect humans
Until recently, there were four plasmodium
species that were considered responsible for malaria disease in humans: P. vivax, P. falciparum, P. ovale and P. malariae. In 2008,
P. knowlesi , a species that used to infect exclusively apes of the genous Macaque , was recognised by WHO as the fifth plasmodium
species that infect humans.
Transmission routes
The main mode of transmission of the disease is by bites from infected Anopheles mosquitoes that have previously had a blood meal from an individual with parasitemia. Less common routes of transmission are via infected blood transfusion, transplantation, infected needles, and from a mother to her fetus during pregnancy.
Plasmodium life cycle
The life cycle (Figure 1) is almost the same for all the five species that infect humans and follows three stages:

(I) ifection of a human with sporozoites

(II) asexual reproduction

(III) sexual reproduction

The two first stages take place exclusively into the human body, while the third one starts in the human body and is completed into the mosquito organism.

The human infection begins when an infected female anopheles mosquito bites a person and injects infected with sporozoites saliva into the
blood circulation. That is the first life stage of plasmodium ( stage of infection).
The next stage in malaria life cycle is the one of asexual reproduction that is divided into different phases: the pre- erythrocytic (or better,
exoerythrocytic ) and the erythrocytic phase .
Within only 30- 60 minutes after the parasites inoculation, sporozoites find their way through blood circulation to their first target, the liver . The sporozoites enter the liver cells and start dividing leading to schizonts creation in 6- 7 days. Each schizont gives birth to thousands of
merozoites ( exoerythrocytic schizogony) that are then released into the blood stream marking the end of the exoerythrocytic phase of the asexual reproductive stage.
It is worth mentioning that, concerning P. vivax
and P. ovale , sporozoites may not follow the reproduction step and stay dormant ( hypnozoites) in the liver; they may be activated after a long time leading to relapses entering the blood stream (as merozoites) after weeks, months or even years. The exoerythrocytic phase is not pathogenic and does not produce symptoms or signs of the disease. Its duration is not the same for all parasite species.
Merozoites released into the blood stream, are directed towards their second target, the red blood cells (RBCs). As they invade into the cells, they mark the beginning of the erythrocytic phase. The first stage after invasion is a ring stage that evolves into a trophozoite . The trophozoites are not able to digest the haem so they convert it in haemozoine and digest the globin that is used as a source of aminoacids for their reproduction. The next cellular stage is the erythrocytic schizont (initially immature and then mature schizont). Each mature schizont gives birth to new generation merozoites ( erythrocytic schizogony) that, after RBCs rupture, are released in the blood stream in order to invade other RBCs. This is when parasitaemia occurs and cinical manifestations appear. The liver phase occurs only once while the erythrocytic phase undergoes multiple cycles; the merozoites release after each cycle creates the febrile waves.
A second scenario into the RBCs is the parasite differentiation into male and female
gametocytes that is a non pathogenic form of parasite. When a female anopheles mosquito bites an infected person, it takes up these gametocytes with the blood meal (mosquitoes can be infected only if they have a meal during the period that gametocytes circulate in the human’s blood). The gametocytes, then, mature and become microgametes (male) and
macrogametes (female) during a process known as gametogenesis. The time needed for the gametocytes to mature differs for each plasmodium species: 3- 4 days for P. vivax and
P. ovale , 6- 8 days for P. malariae and 8- 10 days for P. falciparum .
In the mosquito gut, the microgamete nucleus divides three times producing eight nuclei; each nucleus fertilizes a macrogamete forming a
zygote . The zygote, after the fusion of nuclei and the fertilization, becomes the so- called
ookinete . The ookinete, then, penetrates the midgut wall of the mosquito, where it encysts into a formation called oocyst. Inside the oocyst, the ookinete nucleus divides to produce thousands of sporozoites ( sporogony ). That is the end of the third stage (stage of sexual reproduction/ sporogony). Sporogony lasts 8- 15 days .
The oocyst ruptures and the sporozoites are released inside the mosquito cavity and find their way to its salivary glands but only few hundreds of sporozoites manage to enter. Thus, when the above mentioned infected mosquito takes a blood meal, it injects its infected saliva into the next victim marking the beginning of a new cycle.
The duration of each above desribed phase is different for each of the plasmodia as shown in Table 1 that follows.
Plasmodium species

The Structure and Life Cycle of Entamoeba (With Diagram)

The Structure and Life Cycle of Entamoeba (With Diagram)
Article Shared by Ajit Medhekar
Read this article to learn about the Structure and Life Cycle of Entamoeba !

Systematic Position:

Phylum: Protozoa

Subphylum: Class: Rhizopodea

Entamoeba histolytica is a pathogenic parasite in the intestine of human beings and many other primates. It inhabits the mucous and sub-mucous layers of the large intestine. It feeds mainly on the tissues of the intestinal wall and often produces severe ulcers and abscesses. In chronic cases, it may enter the blood circulation to reach the liver, lung, brain and other organs. It causes a serious and often fatal disease known as amoebic dysentery or amoebiasis. E. histolytica exists in two distict forms: the magna Trophozoite or form and the minuta or precystic form, (fig.9.1).

Trophozoites:

The adult trophic form of Entamoeba is known as Trophozoite or Magna. It inhabits anterior part of large intestine, i.e. colon of human beings. It resembles amoeba in structure but differs in parasitic mode of life. Its body is covered by plasma lemma and cytoplasm is differentiated into ectoplasm and endoplasm. There is a single large, broad and blunt pseudopodium formed of ectoplasm. Endoplasm contains single spherical nucleus and food vacuoles. Nucleus has peripheral crown of chromatin blocks and a centrally located nucleolus.
The trophozoites multiply by repeated binary fission in the intestinal wall of host. Some of the daughter entamoebae grow into normal adults while others stop growing. These are distinctly smaller than the normal trophozoites and are called Minuta forms.

Precystic (minuta Form):

It is smaller, spherical and non- pathogenic stage.

Normally, it lives in the lumen of the intestine and rarely found in tissues. It undergoes encystations and helps in transmission of parasites from one host to other.

Life cycle:

Entamoeba histolytica is monogenetic, i.e., its life cycle is completed on one host only; the man.

Its life cycle is completed as follows:

Encystment:

In the precystic forms, entamoeba remains only in the intestinal lumen. They undergo encystment but before encystment, the parasites round up, eliminate food vacuoles and accumulate considerable amount of food materials in the form of glycogen and black rod-like chromatoid granules. Each parasite secretes a thin, rounded, resistant, colourless and transparent cyst wall around it.
The cysts of Entamoeba histolytica vary in size. Its cytoplasm is clear and each cyst is mononucleate at this stage. Presence of chromatoid bodies is the characteristic of the cysts of Entamoeba histolytica. They occur either singly or in the multiples of two or more. The nucleus of the cysts divides twice so that each cyst now becomes tetra nucleate (fig. 9.2). At this stage, the cyst is infective to a new host. Encysted forms pass out with the faecal matter of the host.

Transfer to new host:

The infective cysts remain viable for a considerable length of time outside the human intestine, if environmental conditions are favourable. Infection of fresh human host takes place by swallowing the infective cysts with contaminated food and drinks.

Excystment:

The metacystic trophozites feed on the contents of the intestine and grow in size to form the trophozites of the next generation. The trophozoites stay in the lumen of the intestine for a particular period when they may attack the wall of the intestine and start the life cycle again. Entamoeba histolytica causes amoebic dysentery, abscesses in liver, lungs and brain and non-dysenteric infections.

Following measures may be helpful in protecting ourselves against the disease:

1. Sanitary disposal of faecal matter

2. Perfect sanitation and protection of water and vegetables from pollution.

3. Washing of hands with antiseptic soap and water before touching the food.

4. Cleanliness in preparing the food.

5. Protection of foods and drinks from houseflies, cockroaches, etc.

6. Raw and improperly washed and cooked vegetables should be avoided.