ASPECTS OF MALARIA
The disease malaria is caused by the protozoan parasites belonging to genus Plasmodium.
There are four species of malaria that infect humans: P. falciparum, P. malariae, P. ovale
and P. vivax; and all are intracellular parasites. Apart from the initial multiplication
stages in the hepatocytes, these parasites are entirely confined to infecting the erythro-
cytes in the circulating blood. To survive they replicate within the erythrocyte, the
progeny escape from the red blood cell and each new individual invades a new erythro-
cyte. Each time this event occurs, parasite antigens in the form of metabolic waste prod-
ucts or secretions etc are released into the circulation. The major aspects of the
epidemiology of malaria are:
n Children up to the age of six living in the rural areas in the tropics may have had
malaria fever more than once. Older children often have the parasites in their system
but do not have fever or any of the other symptoms of malaria: a condition referred
to as ‘clinical tolerance’ or ‘antitoxic immunity’.
n Many of the adults who live in endemic areas also have parasites in their blood but gen-
erally at a much lower level than observed in children.
n Adults rarely develop clinical malaria, although they continue to be infected from
time to time. If immunity to malaria develops in such individuals it is acquired slowly
despite continuous exposure to the infection, ie being continuously immunologically
challenged. The immunity, if it occurs, is rarely complete.n 10.1 OBSTACLES TO THE DEVELOPMENT OF A VACCINE
The main obstacles to the development of anti-malaria vaccine are:
The parasite has a unique habitat within the non-nucleated red blood cells (mature ery-
throcytes do not express HLA molecules). The parasite antigen may not be consistent
from one generation to another, that is the antigen molecules are variable and may be
presented in more than one shape (polymorphic).
Red blood cells (RBCs) infected with P. falciparumexpress the parasite antigens on their
cell surface membrane, known as ‘knob proteins’. These antigens expressed on the sur-
face of infected RBCs bind to receptors on the endothelium and are considered to be a
potential target for a vaccine against P. falciparum.