Biology Reference
In-Depth Information
temperate phenotype, then it requires a greater proportional reduction in
activatable hypnozoites to prevent all relapses.
8. VIVAX MALARIA FOLLOWING FALCIPARUM
MALARIA
In East Asia, a remarkably high proportion (20-50%) of symptom-
atic infections with
P. falciparum
is followed by an infection with
P. vivax
(
Looareesuwan et al., 1987
;
Mayxay et al., 2004
;
Douglas et al., 2011
)
.
The
intervals between the onset of treatment for the acute
P. falciparum
infec-
tion and the subsequent
P. vivax
infection are very similar to the intervals
between acute vivax malaria and the first relapse. As the treatments given
for falciparum malaria are highly effective, and therefore, should be cura-
tive against the blood-stage infections of
P. vivax
, these recurrent malaria
infections are highly likely to be relapses. Furthermore, as with relapses
after vivax malaria, the probability of vivax recurrence after falciparum
malaria is also age related (
Douglas et al., 2011
). Although some mixed
infections may be acquired from a single doubly infected mosquito, studies
of wild-anopheline vectors suggest that this is very unlikely, and the weight
of evidence points to these vivax episodes being relapses of latent hypnozo-
ites acquired
before
the
P. falciparum
inoculation.
9. THE PERIODICITY OF RELAPSE
Various theories to explain the remarkable periodicity of
P. vivax
infections have been proposed (
Lysenko et al., 1977
). These include reinfec-
tion of liver cells from released merozoites, intrinsic differences in latency
periods of the inoculated sporozoites (tachyzoites, bradyzoites), and activa-
tion of dormant parasites by external stresses or seasonal stimuli (
Shute,
1946
;
Cogswell, 1992
). In bird malarias, there is reinfection of tissues from
blood-stage parasites but there is no convincing evidence of this in the pri-
mate malarias.
The temperate-zone
P. vivax
usually had an incubation period of
8-9 months, so emergence of an infection acquired in late summer or
autumn could coincide with vector emergence in the following late spring
or summer. Further south in temperate climes,
P. vivax
infections had a
primary illness 2-3 weeks after mosquito inoculation but the first relapse
occurred 8-9 months later. Although the interval (latency) between the
primary infection and first relapse for this 'Madagascar' phenotype was long
