Biology Reference
In-Depth Information
Duffy antigen negativity (
Miller et al., 1976
;
Ryan et al., 2006
), however, the
role of Duffy antigen in protecting against vivax anaemia and other severe
disease is less well defined (Chapter 2, Part B). Alpha thalassaemia has been
associated with reduced risk of severe anaemia from
P. falciparum
(
Fowkes
et al., 2008
), attributed to a lesser reduction in haemoglobin from the loss
of microcytic cells, and has been hypothesised to similarly protect against
vivax anaemia (
Fowkes et al., 2008
). The role of ovalocytosis and other red-
cell polymorphisms is reviewed by Zimmerman et al. in Chapter 2, Part B.
10.4. Parasite Factors
10.4.1. Differential virulence
Data from the malariotherapy era showed clear differences in risk of mor-
bidity and mortality from different strains of
P. vivax
, with the greatest risk
reported in patients infected with the Madagascar strain (case-fatality rate
of 10-14%) (
James, 1933
). Whether naturally acquired
P. vivax
strains vary
in virulence and risk of severe vivax malaria is unknown.
10.4.2. Chloroquine resistance
The high prevalence of chloroquine-resistant
P. vivax
found in Indonesian
Papua (
Sumawinata et al., 2003
;
Ratcliff et al., 2007
) may be an important
contributor to the high risk of severe vivax disease, particularly, severe anae-
mia reported in this region (
Tjitra et al., 2008
) compared to areas with less
chloroquine resistance (
Genton et al., 2008
). In Africa, the rise of chloro-
quine resistance in
P. falciparum
coincided with an increase in severe malarial
anaemia and mortality (
Trape et al., 1998
). The prevalence of chloroquine-
resistant
P. vivax
is rising with reports of varying degrees of confidence and
severity apparently across the vivax endemic world (
Douglas et al., 2010
).
The role of emerging chloroquine resistance and vivax severe disease war-
rants further investigation (
Price et al., 2009
).
10.4.3. Mixed
Plasmodium
infections
Studies in areas of low malaria endemicity, such as Thailand, have shown
that mixed infections with
P. vivax
appear to attenuate
P. falciparum
disease
severity (
Luxemburger et al., 1997
;
Snounou and White, 2004
;
Price et al.,
2001
;
Mayxay et al., 2004
). Conversely, in areas with higher endemicity
of both species, mixed infections are associated with an increased risk of
severe malaria (
Tjitra et al., 2008
;
Genton et al., 2008
), including severe
anaemia (
Tjitra et al., 2008
;
Genton et al., 2008
) and coma (
Manning
et al., 2011
). In some series from these regions, mixed
P. falciparum-P. vivax
