Genus Leishmania Urns
Members of this genus occur primarily in mammals. They cause disease in man, dogs and various rodents including gerbils and guinea pigs. Leishmania is heteroxenous, being transmitted by sandflies of the genus Phlebotomus. It is found in the leishmanial stage in the cells of its vertebrate hosts and in the leptomonad stage in the intestine of the sandfly and in culture.
Morphology: All species of Leishmania look alike, altho there are size differences between different strains. The leishmanial stage is ovoid or round, usually 2.5 to 5.0 by 1.5 to 2.0 u, altho smaller forms occur. Only the nucleus and kinetoplast are ordinarily visible in stained preparations, but a trace of an internal fibril representing the flagellum can sometimes be seen. This flagellum and the basal granule from which it arises can also be seen in electron micrographs (Chang, 1956; Pyne and Chakraborty, 1958). The leptomonad forms in culture and in the invertebrate host are spindle-shaped, 14 to 20 u long and 1.5 to 3.5 u wide.
Life Cycle: In the vertebrate host, Leishmania is found in the macrophages and other cells of the reticulo-endothelial system in the skin, spleen, liver, bone marrow, lymph nodes, mucosa, etc. It may also be found in the leucocytes, especially the large mononuclears, in the blood stream. It multiplies by binary fission in the leishmanial form.
The invertebrate hosts of Leishmania are sandflies of the genus Phlebotomus. When the sandflies suck blood they ingest the leishmanial forms. These pass to the midgut, where they assume the leptomonad form and multiply by binary fission. They may be either free in the lumen or attached to the walls.
Their further development varies with the particular species of Phlebotomus and strain of Leishmania. In good vectors like P. argentipes, P. papatasii and P. sergenti, they begin to extend their range forward to the esophagus and pharynx by the fourth or fifth day. They continue to multiply to such an extent that they plug up the esophagus and interfere with blood-sucking. When an infected sandfly bites, it clears the passage by injecting some of the leishmanial forms into its victim and thus transmits the parasite. Leishmania may also be transmitted when sandflies are crushed on the skin.
In other cases, the parasites remain in the sandfly midgut and do not pass forward into the pharynx. These can then be transmitted only by crushing the sandflies. A third type of development was described by Shoshina (1953), who found leptomonads in the hindgut of P. minutus var. arparklensis in Russia and suggested that feces containing them might be rubbed into the bite while scratching it.
In addition to transmission by sandflies, it has been suggested that direct infection by means of excretions of infected individuals might occur in kala azar.
Species of Leishmania: The speciation of Leishmania has been discussed by Hoare (1949), Kirk (1949, 1950) and Biaga (1953) among others. While some 22 different specific or subspecific names have been given to mammalian leishmanias, and while different strains are associated with different types of disease, neither morphologic, cultural nor immunologic characters can be used to differentiate the species of Leishmania. In practice, the species are separated on the basis of pathologic and epidemiologic differences and, since most studies have been made by parasitologists oriented toward human disease, the pathologic characters used for each strain have been those seen in man. In the earlier days of our knowledge, when relatively few types were known, it was quite easy to delineate their characteristics and set up separate species, but as more studies were made, intermediate types were found and the boundaries between species tended to disappear.
Some parasitologists consider that all the leishmanias of man and dogs should be assigned to a single species. Others prefer to assign them to two species, and still others to three. One can justify each of these schemes, but in all of them each species is still composed of a number of strains or demes.
In this book, two species of Leishmania are recognized: L. donovani, causing various visceral forms of disease, and L. tropica, causing various cutaneous and mucocutaneous forms. The third species recognized by some authorities is L. brasiliensis, which causes a mucocutaneous form of the disease.
Maps of the world distribution of leishmanioses together with climatologic and other information have been published by Piekarski (1952), Piekarski and Sibbing (1954), Piekarski, Hennig and Sibbing (1956, 1958a), the American Geographical Society (1954) and May (1954).