Biology Reference
In-Depth Information
hybrids perhaps represents a dynamic phase of population differentiation
at the tips of the evolutionary tree.
IMPLICATIONS FOR CONTROL: A FOCUS
ON ZANZIBAR
Despite these subtle ambiguities and conjectures on the taxonomy of
Ascaris, what relevance does this have for disease control? In Africa, for
example, there are nowmany interventions delivering anthelmintic drugs
to people across large geographical swathes within the endemic area, in
a general attempt to control neglected tropical diseases. Monitoring and
surveillance of these programs is often challenging and has been perhaps
best recorded on the islands off East Africa, in particular Zanzibar (Pemba
and Unguja). Here there is an interesting history of surveillance and
control of ascariasis;
88,89
over the last decade there have been several
campaigns distributing albendazole and/or mebendazole, often along-
side praziquantel (for schistosomiasis) or ivermectin (for lymphatic fila-
riasis).
88
During this time there have been some clear declines in the
prevalence of ascariasis in children with the spatial structure of infections
by village revealing “hot spots” of transmission. However, rather
surprisingly there was a general increase in the prevalence of ascariasis in
school children, albeit small, in the 2004
2006 period.
90
e
93
Cure rates
with albendazole and mebendazole (and in combination with iver-
mectin
94
) have been documented, confirming concerns on the diminish-
ing performance of these drugs.
95
In an attempt to explain this putative “drug resistance,” adult Ascaris
retrieved from Zanzibar by chemo-expulsion have been subjected to
molecular DNA analysis. Inspection of the genetic variation in beta-
tubulin genes associated with resistance did not provide evidence to
support a genetic basis of diminished drug efficacy with these loci,
96
although a broader screen of the genetic diversity of Ascaris at mitochon-
drial and microsatellite loci
42
pointed towards some interesting alterna-
tives. Foremost was that the Ascaris population on Zanzibar was
heterogeneous with some spatial structuring across villages as revealed by
mitochondrial DNA (
Figure 10.2
). This might therefore point towards the
supposition that the known heterogeneities in parasitological cure by
village (JRS, unpublished data) may have an underlying heritable
component, the extent and mechanism of which is presently unknown.
Present hot spots of infection at Kandwi village, for example, may also
suggest different behavioral practices (e.g. geophagy) and local environ-
mental heterogeneities (e.g. soil type) which themselves might encourage
the transmission of certain genotypes of Ascaris. It is enigmatic why
“pig-like” worms on Zanzibar are still found despite the cessation of
e
