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Abstract
The 20 or so species of Leishmania which have been recorded as human infections are
all either zoonotic, or have recent zoonotic origins. Their distribution is determined
by that of their vector, their reservoir host, or both, so is dependent on precise
environmental features. This concatenation of limiting factors leads to specific environmental
requirements and focal distribution of zoonotic or anthroponotic sources. Human infection
is dependent on the ecological relationship between human activity and reservoir systems.
Examples are available of the emergence of leishmaniasis from the distant past to
the present, and can be postulated for the future. These emergences have been provoked
by the adoption of new, secondary reservoir hosts, the adoption of new vector species,
transport of infection in humans or domestic animals, invasion by humans of zoonotic
foci, and irruption of reservoir hosts beyond their normal range. The leishmaniases
therefore present an excellent model for emerging disease in general, and for the
generation of the principles governing emergence. The model is, however, limited by
gaps in our knowledge, usually quantitative, sometimes qualitative, of the structure
of reservoir systems.