Biology Reference
In-Depth Information
the true prevalence of
P. vivax
parasitaemia and, therefore, suggest that sub-
patent infections are an important source of
P. vivax
transmission (
Harris
et al., 2010
;
Gunasekera et al., 2011
;
Poschl et al., 2010
;
Steenkeste et al.,
2010
). Active case detection based on light microscopy alone will miss this
important group of potentially infectious individuals, and thus have limited
efficacy. InYanomami people living in the Amazon rainforest, a microscopy-
based active case detection and treatment campaign during the late 1960s
resulted in a 46% reduction in the incidence of malaria over a 3-year period.
However, during this time, the ratio of
P. falciparum
to
P. vivax
infections fell
from 42% to 24% indicating that the campaign had been much less effective
against
P. vivax
than
P. falciparum
(
Macauley, 2005
)
.
Newer, more sensitive, means of detecting low-density
Plasmodium
infec-
tions may increase the yield of active detection and treatment efforts. Rapid
diagnostic tests are simple and relatively cheap. Some have been shown to
be highly sensitive for low-density
P. falciparum
infections but their perfor-
mance at detecting low-density
P. vivax
infections remains poor (
World
Health Organization et al., 2009
). Loop-mediated isothermal amplification
(LAMP) has greater sensitivity for
P. vivax
infections than microscopy and
is cheaper, simpler and faster than PCR-based methods (
Poschl et al., 2010
;
Chen et al., 2010
). LAMP may have a role both in clinics and as a tool
for detection of asymptomatic infections in endemic regions (
Poschl et al.,
2010
;
Chen et al., 2010
;
Sirichaisinthop et al., 2011
). Nevertheless, LAMP is
not yet widely available and although cheaper than PCR, is likely to remain
prohibitively expensive and too slow for widespread use, at least in the near
future. For highly resourced regions, PCR remains the most sensitive means
of detecting
P. vivax
malaria and, therefore, hypothetically, the best means
of maximising the yield of active case detection and treatment campaigns.
Again, given logistical and financial constraints, PCR is unlikely to be used
in clinical practice.
Our inability to determine whether an individual harbours
P. vivax
hypnozoites is the single greatest limitation of the MSAT strategy for
reducing transmission of vivax malaria. Dormant hypnozoites can seed
multiple future relapses and, given their longevity, will be highly preva-
lent in endemic populations, even if the entomological inoculation rate
is relatively low. This claim is indirectly supported by the very high rate
of heterologous recurrence following falciparum malaria seen in many
regions with
Plasmodium
co-endemicity (
Ratcliff et al., 2007
;
Hasugian
et al., 2007
;
Douglas et al., 2011
;
Smithuis et al., 2010
;
Karunajeewa et al.,
2008
). In cross-sectional treatment campaigns, the only way of ensuring that
