Biology Reference
In-Depth Information
Defining the limits of transmission
.
Plasmodium vivax
annual clinical inci-
dence data were available from all 19 countries that are considered to be
endemic in the Americas. The most recent year of reporting was 2008 for
the majority of the countries. Guyana, Nicaragua and Panama reported data
up to 2007; Belize, El Salvador, French Guiana and Guatemala provided
data up to 2006 and Colombia's last year of reporting was 2005.
Endemic areas in this region were estimated to span 9.46 million km
2
,
most of which were found in the Amazon basin. The majority of the areas at
risk in the Americas (85%; 8.08 million km
2
) were at stable transmission (
Fig.
1.3
C1). This indicated that, for the most part, the disease was either endemic
or wholly absent in this region with limited areas at risk of unstable transmis-
sion. The transmission level was generally found to be inversely proportional
to the population density: areas experiencing stable transmission were those
of lower population density. For example, the largest area at risk of stable
transmission in a single country was 4.40 million km
2
in Brazil, which was
54% of the stable transmission area for the Americas region. However, the
population at stable risk in Brazil was 26% (13.02 million) of the region's total
PAR of stable transmission. The Americas comprised 53% of the global land
area at stable transmission but only 5% of the population at that level of risk.
Estimating endemicity
. There were relatively few PR records and surveys
for the Americas, with 388 unique records available for modelling. The
three most data-rich countries in this region were Brazil (
n
= 175; 45%),
Venezuela (
n
= 65; 17%) and Peru (
n
= 51; 13%), as shown in
Fig. 1.3
C1.
The predicted prevalence, or
Pv
PR
1-99
, was highly variable throughout
the Americas. Much of the areas of stable transmission had PRs between 3
and 5%, with isolated areas nearing 0% and others exceeding 7%. Regions of
high prevalence (>7%) were found in Amazonia (Northeast Brazil) and Cen-
tral America (Nicaragua and Honduras). The uncertainty in the
Pv
PR
1-99
predictions was relatively high overall due to the relatively sparse PR sur-
vey data in this region (
Fig. 1.4
C1). However, when population density
was incorporated into the weighted uncertainty calculations, the resulting
uncertainty index was greatly lowered because the areas with stable risk and
high uncertainty predictions had low population densities (
Fig. 1.4
C2). This
implies that the observed uncertainty has relatively little operational impor-
tance at the global scale, although highlights the current lack of precision for
defining high-risk foci in remote Amazonian settings.
Population at risk
. The PAR in the Americas was relatively low given the
geographic limits of
P. vivax
due to low population densities in areas of high
transmission. It was estimated that there were 137.45 million people living
