Page 54 - Medical Parasitology_ A Textbook ( PDFDrive )

46 6 Malaria Parasites and Babesia

from the body. It can prevent superinfection, but not reinfection. This type of
immunity associated with asymptomatic infection is called premunition and
disappears once the infection is eliminated. Acquired antibody-mediated
immunity is transferred from mother to foetus across the placenta and is evi-
dent in endemic areas where infants below the age of 3 months are protected
by passive maternal antibodies. Young children are highly susceptible to
malaria. As they grow up, they acquire immunity by subclinical or clinical
infections. Incidence of malaria is low in older children and adults.

Recrudescence

Recrudescence occurs when parasites persist although the level of parasitaemia is
below the fever or microscopic threshold. Erythrocytic schizogony continues in the
body at low levels and parasitaemia gradually increase to cross the fever threshold.
New malarial attacks then occur. These malaria attacks appear within 8 weeks after
the primary attack. Recrudescence may be due to waning immunity of the host or to
antigenic variation and is seen in all human malaria.

Relapse

It is seen in P. vivax and P. ovale infections, caused by the reactivation of hypnozoite
stage in the liver. This leads to initiation of erythrocytic cycles and new attacks of
malarial fever. Reactivation of hypnozoite stage usually occurs from 24 weeks to 5
years after the primary attack.

Diagnosis

1. Microscopic examination (Gold standard)
Demonstration of malarial parasite in the peripheral blood in thin and thick
smears. Thin smear is used for detecting the parasites and determining the spe-
cies by studying its morphological details. The thick smear is more sensitive and
is used for detection of malarial parasite when there is low parasitaemia. Species
identification is not easy in thick smear. Both thin and thick smears can be used
to determine the parasitaemia level.
2. Rapid diagnostic tests (RDT)
The tests aid in the diagnosis of malaria by detecting malaria parasite antigens
in human blood.
3. Molecular diagnosis
PCR on blood.
4. Serodiagnosis
It is used mainly for seroepidemiological survey and to identify the infected
donors in transfusion malaria.