Biology Reference
In-Depth Information
infections have a higher reported case-fatality rate among patients with
severe malaria than seen with either species alone (
Yadav et al., 2012
;
Manning et al., 2011
).
11. PATHOPHYSIOLOGY OF DISEASE IN VIVAX
MALARIA
11.1. Comparative Pathobiology of
P. vivax
There are a number of differences in pathobiology between
P. vivax
and
P.
falciparum
that are important in understanding the pathophysiology of vivax
malaria (see
Kitchen, 1949b
;
Anstey et al., 2009
for reviews).
11.1.1. Parasite Biomass
Plasmodium falciparum
invades RBC of all ages, and progresses to high para-
site burdens if uninhibited by treatment or host immunity, with parasite
biomass a major independent determinant of the risk of death (
Dondorp
et al., 2005
;
Yeo et al., 2008
). In contrast,
P. vivax
has a very strong predilec-
tion for infecting RBCs that have emerged from the bone marrow within
the last 14 days (
Kitchen, 1949b
;
Simpson et al., 1999
), particularly early
in the course of infection (
Kitchen, 1938
). This property contributes to
the lower parasite biomass seen in
P. vivax
infections. Unlike
P. falciparum
infections, parasitaemias in vivax malaria rarely exceed 2% of circulating
RBCs (
Ross and Thomson, 1911
;
Kitchen, 1949b
;
Field and Shute, 1956
).
Although a high
P. vivax
parasite burden (140,000/µl) compared to that
usually observed has been reported in a fatal case of vivax ARDS (
Valecha
et al., 2009
), this appears to be exceptional. Although the series reporting
parasite counts in severe vivax malaria have documented relatively high par-
asitaemias for
P. vivax
, all have been <100,000/µl (about 2% parasitaemia),
ranging from 6000 to 90,400 (
n
= 11) (
Kochar et al., 2005
), 680 to 28,847/
µl (
n
= 17) (
Alexandre et al., 2010
), 8400 to 60,000/µl (
n
= 40) (
Kochar
et al., 2009
) and 7600 to 60,000/µl (
Kochar et al., 2010
). None of these
studies presented comparative data of parasitaemia between patients with
uncomplicated and severe vivax malaria. The correlation of
P. vivax
parasi-
taemia and disease severity has been limited by the use of semi-quantitative
measures (grading 1+-4+) frequently used in endemic areas, extensive pre-
treatment, lack of reporting and lack of comparison with uncomplicated
vivax malaria. Nevertheless, two recent studies have demonstrated an asso-
ciation between disease severity and semi-quantitative parasitaemia in
P.
vivax
infection (
Nurleila et al., 2012
;
Lanca et al., 2012
).