Phylum Apicomplexa

Phylum Apicomplexa

E.g. Plasmodium malaria.

Characteristics

1. Most of them possess a unique type of plastid called an apicoplast, used for piercing host cells.
2. They are unicellular and spore forming organisms. Almost all species are obligate endoparasites of animals, except nephromyces which live symbiotically in marine animals.
3. They have an infectious stage known as sporozoite.
4. They reproduce asexually by schizogony and sexually by sporogony.
5. The parasite changes its shape depending on the host it inhabits. For instance, it is sickle shaped in salivary glands of mosquito, while it is spherical or amoeboid in liver cells of humans.
6. They are non-motile.
7. They form resistant spores after fertilisation.

Class: protozoa Genus: plasmodium Species: P. vivax, P. malaria, P. falciparum.

All the species cause malaria (meaning bad air in)

Structure of Plasmodium

They are oval or spindle in shape or can even be called sickle with the small amount of cytoplasm with the nucleus occupying a large area in the center. The body shape is maintained by the presence of pellicle.

N.B; P. falciparum is most dangerous of all species that it attacks the liver.

1. Sickle cell sporozoites 2um long enter the blood stream of man with saliva of female anopheles mosquito when it pierces the skin.
2. They remain in the blood stream for an hour and then enter in the liver.
3. They feed on liver cells (glucagon and fat) and form meront (schizont)
4. They then divide mitotically and repeatedly to form nuclei which are surrounded in cytoplasm to form merozoites / schizozoites.
5. Liver cells burst and release the merozoites/schizozoites.
6. The merozoites enter the blood, others re-enter the liver cells (pre-erythrocytes cycle) which takes 8-10 days.
7. Inside the RBC's the schizozoites feed and grow to become amoeboid in shape.
8. A vacuole appears on the central region and pushes the nucleus to the sides forming ring structure (detected in blood when examined under microscope) growth and multiple division form the schizozoites, some excretory granules are formed from cytoplasm of RBC; as it bursts and open the schizozoites and excretory granules/ organelle are released to the plasma some schizozoites re- enter the RBC (erythrocytes cycles).
9. The toxic effect takes about 48 hours (process) which causes a characteristic malaria fever.
10. Some schizozoites stop dividing and grow to form gametocytes.
11. Female gametocytes have a small nucleus and cytoplasm is rich in food materials and granulated.
12. The male gametocyte has a large nucleus with clear cytoplasm.
13. If the gametocytes are ingested by a female anopheles mosquito they develop into male and female gametes which unite and form a zygote (i.e. male gamete penetrates through the female gamete through papillae).
14. From the zygote, multi nuclei structures called sporocysts develop which within a few days divide into thousands of tiny spindle shaped cells called sporozoites which migrate to the mosquito's salivary gland.

When the mosquito bites another victim the cycle begins

Life Cycle of Plasmodium

The parasite in the form of sporozoites enters the blood stream of the human being after a mosquito bite. It then travels to the liver and invades it. The sporozoites grow, divide and produce many haploid forms called merozoites or schizozoites in liver cells. The merozoites in the liver cells exit and re-enter blood stream, where red blood cells are invaded.

Adaptation to Modes of Life

Structural Adaptation

i. Having a spindle/ sickle structure, they can easily penetrate the liver cells and RBC thus making it easier for them to re infect both liver cells and RBC's thus increasing chance of survival. ii. The zygote may develop a cyst.

Physiological Adaptation

i. Have well developed chemo tactic responses which enable it to detect different tissues/organs in the host's body e.g. sporozoites remain in the general circulation for one hour and migrate to the liver. ii. In the mosquito's body mature sporozoites reside in the mosquitoes salivary gland iii. They are resistant (highly) to hosts (i.e. humans and mosquitoes) anti bodies and toxins and gametocytes are not digested in the salivary gland by mosquitoes. iv. The plasmodium is an obligate parasite confined to the hosts thus increases their chances of survival. v. They can live / tolerate low oxygen content as they are parasites they need very little energy because all processes are performed by the host except reproduction. vi. They are adjusted to different osmotic conditions in the hosts i.e. mosquito and human beings and thus increase chance of survival.

Reproductive Adaptation

i. They have a high reproductive rate and this increases their chance of survival.

Effects on the Host

i. Plasmodium vivax causes tertian fever, recurring at 3days intervals and is at least with survival. ii. Plasmodium malariae causes quartan fever recurring at 4days interval iii. Plasmodium falciprum causes maligent quotidian malaria being most lethal with a continuous fever. iv. The incubation period is 8 – 10 days. This is when the parasites are in the liver, over 8- 10 days malaria fever begins characterized by cold, hot and sweating increases i.e. shivering occurs and the patient feels cold, temperature rises followed by sweating. The patient then shows recovery signs except for some weaknesses. But each fresh attack has an increasing debilitating effect.