Microbiology Lecture 7

HUMAN PARASITOLOGY

‘parasitism is a close relationship in which one organism, the parasite, is dependent on another organism, the host, feeding at its expense during the whole or part of its life (- +)’. It is frequently a highly specific relationship that always involves a degree of metabolic dependence of the parasite upon its host and often, though not always, results in measurable harm to the host. The association is usually prolonged and although it may ultimately result in death of the host, this is not usually the case. It is therefore distinct from predation in which predator usually kills and consumes its prey within a short period of time. 

The Importance of Parasites in Human Medicine

Parasitic diseases afflict large numbers of us humans: in 2004 the WHO estimated that on a global scale infectious and parasitic diseases were responsible for 16.7% of male and 15.6% of female deaths. 

Ten percent (10%) of the world’s population suffers from one or more of the eight major tropical diseases – many of which are protozoan or helminth parasites

• Malaria
• Schistosomiasis (Bilharzia)
• Lymphatic filariasis (leading to elephantiasis)
• Onchocerciasis (leading to river blindness)
• Leprosy
• African Trypanosomiasis (African sleeping sickness)
• Chagas Disease (American trypanosomiasis)
• Leishmaniasis (including Kala Azar) 

MALARIA

Although there are over 150 named species of malaria, only four of them infect humans of which Plasmodium falciparum is the cause of most fatalities. 

The four main species of Plasmodium infecting humans are:

• Plasmodium falciparum
• Plasmodium vivax
• Plasmodium ovale
• Plasmodium malariae.

They exhibit marked differences in their biology and molecular evidence suggests that they evolved from separate lineages. That is, they are more closely related to Plasmodium species that parasitise other animals than they are to one another 

Malaria Life Cycle

The Plasmodium species that infect humans have a complex life cycle that involves both asexual and sexual reproductive stages. They are all transmitted by female anopheline mosquitoes and multiplication takes place in both humans and the mosquito vector. Only female mosquitoes feed on blood and therefore they are the ones that transmit the disease. 

1) Humans become infected when they are bitten by a female mosquito harbouring the sporozoite stage of the parasite. Once they have gained access to the blood stream the sporozoites are transported to the liver where they penetrate the liver cells.

2) Within the liver cells the sporozoites undergo morphological change and multiply asexually to form thousands of merozoites.

3) After the merozoites are produced they leave their host cell and invade red blood cells in which they transform into another morphological form called the trophozoite stage and these also reproduce asexually.

4) The growth of the parasites destroys the red blood cells releasing the merozoites in the process. These merozoites then infect other red blood cells and the process of infection, replication, and destruction is repeated many times.

5) At some point in this cycle, certain merozoites transform into male and female gametocytes. These gametocytes do not develop any further or fuse but remain within their host red blood cell until it is ingested by a suitable female anopheline mosquito.

6) Shortly after they are ingested by the mosquito the male and female gametocytes leave their host…