The six diseases of WHO. Leishmaniasis.

Leishmaniasis is not a single disease; rather, it is a collection of diseases caused by several different species of Leishmania, each of which has its own potential to cause a characteristic set of symptoms in man. No reliable estimates of the incidence and prevalence of leishmaniasis exist. It has been estimated that there are 400 000 new cases of leishmaniasis every year,' but the importance to public health is perhaps better gauged from the estimates of the number of cases in the recent epidemic of visceral leishmaniasis in Bihar, when it was conservatively estimated that there were 18 000 cases in 1977 and 40 000 in 1978 (R W Ashford, personal communication). Within the vertebrate host the protozoan parasite is restricted almost exclusively to cells of the mononuclear phagocyte (reticuloendothelial) system, in which they are present as amastigotes. Two main clinical manifestations of leish-maniasis exist-cutaneous and vis-ceral-the distribution of which is 0 shown in fig 1. In the cutaneous disease the parasites are restricted to macro-phages of the skin, whereas in the visceral manifestation, though amas-tigotes may be found in many tissues, the major centres of infection are within the phagocytic cells of the liver, spleen, and bone marrow. No convincing explanation of these different tissue tropisms has been presented. Arguments based on differences in temperature sensitivity or metabolic control are difficult to reconcile with the observation that the same parasite may cause visceral leishmaniasis in one individual. but a cutaneous manifestation in another.3 FIG 1-Geographical distribi In both the cutaneous and visceral infections the organism is transmitted to man by an insect vector, phlebotomine sandflies. Within the alimentary canal of the sandfly the amastigotes that are ingested together with the blood meal transform to a spindle-shaped flagellated form-the promastigote. These are the infective form and are the stage of the life cycle that may be easily grown in vitro on blood-agar slopes. When an infective sandfly takes its next blood meal promastigotes are delivered from the anterior portion of the alimentary canal into the skin and thus a new infection begins. For many years progress on several aspects of leishmaniasis was handicapped by the difficulties of differentiating morphologically among the various species of Leishmania. Recent ultra-structural studies4 have shown differences in the size of amastigotes, but the main advance in identifying parasites has been the introduction of the techniques of biochemical taxonomy.56 The subdivision of the genus is complicated …