The Simulium and S.Metallicum species complexes in Mexico: identification, distribution and relation to onchocerciasis

Abstract

Both S.ochraceum and S.metallicum, which are vectors of
onchocerciasis, are now known to be species complexes
(Hirai et in preparation; Conn, 1988).

Larval collections were made in a variety of streams in
the three onchocerciasis foci in Mexico and identified by
cytotaxonomic criteria. S.ochraceum cytotype A was found in
the Soconusco focus, cytotype B in the Oaxaca focus and
cytotype C in the Chamula focus. Cytotype C was recorded in
Mexico for the first time. S.metallicum cytotypes A, B, H
and I were found in all three of the foci. Cytotypes B, H
and I were recorded in Mexico for the first time. In
addition a new cytotype, named X, was discovered in the
Oaxaca focus. Attempts to analyse chromosomes of
S.ochraceum adults were unsuccessful.

The larval head patterns and body colouration of
different cytotypes of S.metallicum were examined and
variations were found between them, Cytotypes A, B, H and I
could be distinguished in the majority of cases on these
morphological differences. Cytotype X however resembled
cytotype B.

Cuticular hydrocarbon analysis by gas liquid
chromatography, a technique to distinguish adult members of
species complexes, was attempted on S.ochraceum and
S.metallicum specimens. Some separation was achieved
amongst S.ochraceum cytotypes A, B and C.

The distributions and abundances of the larvae of
different members of the S.ochraceum and S.metallicum
species complexes and other Simulium species were
investigated in relation to selected environmental
variables, by multivariate analyses. Different cytotypes of
each vector complex were found to be associated with
characteristic stream conditions.

The distribution of the different cytotypes of
S.ochraceum and S.metallicum in relation to onchocerciasis
is discussed.

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