Question Corner

Malaria vaccine

QUESTION: Why is vaccination against Malaria difficult?

V.V.Balasubramanyam, Anantapur, A.P

ANSWER 1:Malaria is caused by 4 protozoan species classified as Plasmodium falciparum. It is transmitted by female anopheline mosquitoes. Malaria may also be transmitted by blood transfusion or inoculation.

Malarial parasites need at least two hosts to complete their life-cycle. When the parasite enters inside human beings it spends its first stage in the liver and then starts to multiply in the erythrocytes (RBC's).

When a person is infected with a particular type of malarial parasite, for example P.falciparum, the immune system produces selected antibody against P.falciparum thus preventing the disease. But he is vulnerable to other plasmodium species.

For years researchers are trying to find a vaccine against malarial parasites.Induction of protective immunity to many infectious diseases is the most successful and most widely applied example of immunotherapy. For active immunisation it is necessary to administer several doses of material which will induce a host-protective immune response in the form of serum antibodies, secretory antibodies or T cell mediated reactivity. In developing further strategies for immunisation against diseases knowledge of the natural history of the disease is essential.

R.Muhuntan, Coimbatore

ANSWER 2: Malaria is caused by the unicollulor parasite, plasmodium. There are four such species causing the disease, plasmodium vivax, plasmodium falciporum, plasmodium ovale and Plazmodium malaria.

Vaccines for any disease are usually prepared from or microbial agents responsible for the disease. The lifecycle of the parasite consists of three separate stages within the vertebrate hosts, and the organism at each stage has a completely distinct antigenic make-up. The organism that enters the bloodstream by the bite of Anopheles mosquito is in the sporozoite stage. Sporozoites enter the liver cells and multiply there. After 1-2 weeks, the organisms are released into the bloodstream again, or as merozoites. Many of the clinical symptoms of malaria are due to morozoites infecting redblood cells, multiplying and being released after 2 or 3 days to reinfect fresh red blood cells.

A small fraction of merozoites eventually develop into sexual forms called macro and micro gametocytes. When these enter the gut of a mosquito, they develop and breed producing ookinetes that eventually produce sporozoites again. In addition to the problems presented by the complexity of the lifecycle of the malarial parasite, some of the malarial antigens have apparently evolved structures that do not stimulate the Immune system to produce specific immunocompetent cells or antibodies.Inspite of these problems posed, vaccine development against malaria is well on its way. Vaccine development has targeted each of the stages that occur in the human host. Sporozoite vaccine, is effective in animal models, but do not confer enough protection on humans. It needs stronger immunogenicity, can be obtained with the whole sporozoites, but it is dangerous because even a single sporozoite escaping and entering a livercell will eventually cause the disease.

The initial step in the preparation of subunit vaccine is the identification of the particular antigen, to which most of the resulting antibodies are effective. In this manner, the circumsporozoite protein was identified as the predominant antigen.

The gene coding for this circumsporozoite protein is cloned, produced and use for immunization. The last one is Merozoite vaccine where again the killed preparation of merozoites is used to induce the production of antibodies. However, these antibodies unable to cross cell membrane have to wait for the release of merozoites from the cell. The subunit vaccine produced by recombinant DNA technology is more promising and effective in the control of malaria.

M. Kavitha, Arni, T.N

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