Biology Reference
In-Depth Information
Heterogeneity in prevalence within districts was estimated empirically
for 64 districts, representing a total of 969 prevalence surveys ranging
from 10 to 99 surveys per district. These data showed that within-district
distributions were highly skewed and best described by a logit-normal
distribution, and standard deviation increased with increasing preva-
lence. On this basis, a normal distribution was applied around the logit-
transformed mean prevalence and subsequently back-transformed to
estimate the proportion of the total district population in each five-
percentile prevalence class, and summation of these estimates provide
an overall estimate of numbers infected.
Third, for countries within Africa where detailed data were lacking,
Bayesian geo-statistical modeling was used to predict the prevalence of
infection, using available data and environmental information.
33
e
34
This
approach is predicated on the role of environmental factors in influencing
the large-scale geographic distributions of A. lumbricoides, in the absence
of substantive control measures.
33,35
e
37
Fourth, prevalence estimates were
adjusted for those countries
*
which have recently implemented large-
scale treatment campaigns, through either school-based de-worming
programs or community-based lymphatic filariasis elimination
programs. Information about the coverage of these campaigns was
assembled from relevant sources and adjustments were made that
reflected treatment coverage levels, using a mathematical model of
transmission dynamics.
38
Based on these analyses, it is estimated that 0.762 billion people are
infected with A. lumbricoides in 2010 (
Table 13.2
), representing 14.6% of the
world's population. This estimate is markedly lower than previous esti-
mates provided by Crompton
17
and Bundy et al.,
15
and reflect the dramatic
reductions in prevalence in China and parts of Latin America and the
Caribbean. Prevalence remains around 20% for countries in south and
Southeast Asia, central and western Africa, and Andean Latin America
(
Figure 13.1
B). In numerical terms, however, the greatest numbers of
A. lumbricoides infection occur in China and India (
Figure 13.1
C).
GLOBAL DISEASE BURDEN
As noted above, quantifying the disease burden caused by ascariasis is
difficult and there are therefore no direct estimates of morbidity. Instead,
*
Bangladesh, Belize, Burkina Faso, Burundi, Cambodia, Cameroon, Cape Verde, C
ˆ
te
d'Ivoire, Democratic People's Republic of Korea, Dominican Republic, Ecuador, El
Salvador, Guatemala, Guinea Bissau, Haiti, Honduras, Kenya, Lao People's Demo-
cratic Republic, Madagascar, Malawi, Mali, Myanmar, Nepal, Nicaragua, Niger, Peru,
Philippines, Sierra Leone, Uganda, Venezuela, Vietnam.
