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but this is potentially misleading.The absolute prevalence of
P. vivax
in heavily
endemic zones may also reach or exceed 70%. Relatively low parasite den-
sities in blood (arising from its strict preference for reticulocytes) may lead
to high rates of false-negative diagnoses by microscopy or rapid diagnos-
tic tests (RDTs) (
Mueller et al., 2009a
). Microscopic diagnoses very often
underestimate the true prevalence of
P. vivax
in blood in both high- and
low-transmission settings (
Mueller et al., 2009b
;
da Silva et al., 2010
;
Harris
et al., 2010
;
Katsuragawa et al., 2010
;
Steenkeste et al., 2010
). Further details
regarding the diagnosis of
P. vivax
may be found in a review elsewhere in
this thematic volume of
Advances in Parasitology
(Chapter 4, Volume 80).
Missed diagnosis is important in mixed species infections where
P. vivax
may
be a minor contributor to parasitaemia and often overlooked (
Mayxay et al.,
2004
). There is no proven evidence that this is due to cross-species immu-
nity, rather that plasmodia species are mutually suppressive in mixed infec-
tions (
Richie, 1988
). While
P. falciparum
tends to dominate
P. vivax
, infection
with
P. vivax
reduces the intensity of falciparum infections (
Snounou and
White, 2004
). Mixed infections are complex and often under-diagnosed
(
Mayxay et al., 2004
) and it is difficult to interpret the effect they may
have on prevalence estimates. The reader is referred to a review of acquired
immunity to
P. vivax
provided in this volume (Chapter 3, Volume 81).
The difference in PR spectrums between
P. vivax
and
P. falciparum
could
also be a reflection of the age range used as the input data for the endemic-
ity model. To model endemicity,
P. vivax
predictions were standardised across
the 1-99 age range, rather than the 2-10 years of age range, which was
used for
P. falciparum
(
Gething et al., 2011a
). Malaria transmission peaks in
2-10-year olds and, therefore, the use of the 1-99-year age range 'dilutes' the
prevalence estimates. Regardless, it is evident that the association between
the risk of disease and parasite prevalence is markedly different for
P. vivax
than
P. falciparum
, with significant risk of
P. vivax
disease at lower parasite
densities.
The maps reviewed here represent progress towards an improved under-
standing of the epidemiology of this unique malaria parasite. Included
in this work is the first ever
P. vivax
-specific global endemicity map and
updated limits of transmission. These maps are intended to aid control strat-
egy formulation and operational decision-making. Stratification by trans-
mission intensity has been supported by mathematical modelling in the
context of
P. falciparum
management (
Smith et al., 2006
,
2008
;
Okell et al.,
2008
;
Smith and Hay, 2009
;
Chitnis et al., 2010a
,
2010b
;
Griffin et al., 2010
;
Ross et al., 2011
), but
P. vivax
is rarely differentiated by endemicity level.
