Biology Reference
In-Depth Information
to
define targets for control or to inform the most appropriate control
strategy
In general, egg count-based threshold levels of
infection
e
should be decided locally, principally because the fecundity of
female A. lumbricoides appears to be markedly geographically heteroge-
neous.
46
For example, the World Health Organization indicates that when
>
e
10% of a target population have high intensity infections with
A. lumbricoides
e
defined by the threshold of 50,000 eggs per gram of
feces
intensive control efforts (two or three rounds of MDA per year)
should be undertaken.
239
Yet, because of geographically variable fecun-
dity, this threshold is associated with markedly different worm burdens
in different locations.
The observed variability in fecundity also has profound implications
for the generality of transmission models. Female worm fecundity, in
conjunction with the severity of density dependence, is critical in
relating an individual's worm burden to his or her contribution to
transmission. It is also a component of the basic reproductive number,
R
0
, and the theoretical transmission breakpoint density. Consequently,
both of these quantities will be locale specific. Fecundity can only be
estimated from paired egg count
e
worm count data, the latter obtained
by chemo-expulsion techniques. Thus, for transmission models to be
most useful in informing local control policy and strategy, epidemio-
logical surveys would ideally include limited and judiciously planned
chemo-expulsions.
In most settings, elimination of A. lumbricoides by MDA alone is
unrealistic. Reasons for this include the high reproductive capacity of
female worms; the overdispersed distribution of worms among hosts
which enhances the parasite population's resilience to perturbation and
facilitates persistence at very low densities, and the increasing degree of
overdispersion that occurs with decreasing parasite density. Conse-
quently, morbidity reduction is often a more achievable aim. It is thus
rather conspicuous that the relationship between infection and disease is
poorly understood. This is because the effects of chronic infection,
mediated chiefly by malnutrition, are rather covert and not associated
with readily measurable biomarkers; such as hemoglobin as an indicator
of iron-deficiency anemia caused by hookworm infection. Attempts to
quantify population levels of morbidity associated with A. lumbricoides
have applied
e
a threshold worm
burden above which morbidity is observed.
56,57,206
More research is
clearly needed in this area so that populations can be targeted for control
on the basis of disease rather than infection; to facilitate the development
of disease models; and to enable cost-effective analysis of morbidity
control strategies.
In future, the development of spatially explicit population dynamics
transmission models which could be coupled to geo-statistical models
e
without empirical
justification
e
