Current Status Of Malaria Vaccinology Essay Research

Current Status Of Malaria Vaccinology Essay, Research Paper

In order to assess the current status of malaria vaccinology one must

first take an overview of the whole of the whole disease. One must

understand the disease and its enormity on a global basis.

Malaria is a protozoan disease of which over 150 million cases are

reported per annum. In tropical Africa alone more than 1 million

children under the age of fourteen die each year from Malaria. From

these figures it is easy to see that eradication of this disease is of

the utmost importance.

The disease is caused by one of four species of Plasmodium These four

are P. falciparium, P .malariae, P .vivax and P .ovale. Malaria does not

only effect humans, but can also infect a variety of hosts ranging from

reptiles to monkeys. It is therefore necessary to look at all the

aspects in order to assess the possibility of a vaccine.

The disease has a long and complex life cycle which creates problems for

immunologists. The vector for Malaria is the Anophels Mosquito in which

the life cycle of Malaria both begins and ends. The parasitic protozoan

enters the bloodstream via the bite of an infected female mosquito.

During her feeding she transmits a small amount of anticoagulant and

haploid sporozoites along with saliva. The sporozoites head directly for

the hepatic cells of the liver where they multiply by asexual fission to

produce merozoites. These merozoites can now travel one of two paths.

They can go to infect more hepatic liver cells or they can attach to and

penetrate erytherocytes. When inside the erythrocytes the plasmodium

enlarges into uninucleated cells called trophozites The nucleus of this

newly formed cell then divides asexually to produce a schizont, which

has 6-24 nuclei.

Now the multinucleated schizont then divides to produce mononucleated

merozoites . Eventually the erythrocytes reaches lysis and as result the

merozoites enter the bloodstream and infect more erythrocytes. This

cycle repeats itself every 48-72 hours (depending on the species of

plasmodium involved in the original infection) The sudden release of

merozoites toxins and erythrocytes debris is what causes the fever and

chills associated with Malaria.

Of course the disease must be able to transmit itself for survival. This

is done at the erythrocytic stage of the life cycle. Occasionally

merozoites differentiate into macrogametocytes and microgametocytes.

This process does not cause lysis and there fore the erythrocyte remains

stable and when the infected host is bitten by a mosquito the

gametocytes can enter its digestive system where they mature in to

sporozoites, thus the life cycle of the plasmodium is begun again

waiting to infect its next host.

At present people infected with Malaria are treated with drugs such as

Chloroquine, Amodiaquine or Mefloquine. These drugs are effective at

eradicating the exoethrocytic stages but resistance to them is becoming

increasing common. Therefore a vaccine looks like the only viable

option.

The wiping out of the vector i.e. Anophels mosquito would also prove as

an effective way of stopping disease transmission but the mosquito are

also becoming resistant to insecticides and so again we must look to a

vaccine as a solution

Having read certain attempts at creating a malaria vaccine several

points become clear. The first is that is the theory of Malaria

vaccinology a viable concept. I found the answer to this in an article

published in Nature from July 1994 by Christopher Dye and Geoffrey

Targett. They used the MMR (Measles Mumps and Rubella) vaccine as an

example to which they could compare a possible Malaria vaccine Their

article said that "simple epidemiological theory states that the

critical fraction (p) of all people to be immunised with a combined

vaccine (MMR) to ensure eradication of all three pathogens is determined

by the infection that spreads most quickly through the population; that

is by the age of one with the largest basic case reproduction number Ro.

In case the of MMR this is measles with Ro of around 15 which implies

that p> 1-1/Ro 0.93 Gupta et al points out that if a population

of malaria parasite consists of a collection of pathogens or strains

that have the same properties as common childhood viruses, the vaccine

coverage would be determined by the strain with the largest Ro rather

than the Ro of the whole parasite population. While estimates of the

latter have been as high as 100, the former could be much lower.

The above shows us that if a vaccine can be made against the strain with

the highest Ro it could provide immunity to all malaria plasmodium "

Another problem faced by immunologists is the difficulty in identifying

the exact antigens which are targeted by a protective immune response.