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Leishmaniosis

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Pathogens

The genus Leishmania is divided into two subgenera on the basis of the development in sand flies. Growth of species of the subgenus Leishmania is restricted to parts of the alimentary tract of the natural vector, anterior to the pylorus at the junction of the midgut and hindgut, whereas that of species of the subgenus Viannia occurs in both the midgut and the hindgut.

Most of the Leishmania species annotated to date were originally described on the basis of clinical, epidemiological and biological features.The subgenus Leishmania includes medically important species of the Old World species (L. tropica, L. aethiopica, L. major, L. infantum and L. donovani), two Old World species of no medical importance (found only in gerbils), medically important species of the mexicana group restricted to the New World (L. mexicana, L. amazonensis, L. venezuelensis, L. pifanoi) and several other Neotropical species of no medical importance. The subgenus Viannia is found only in Central and South America. Most important species are L. braziliensis, L. guyanensis, L. panamensis and L. peruviana all of which cause human disease.

The veterinary important species are Leishmania infantum in the Mediterranean area and L. chagasi, meanwhile found identical with L. infantum, in Latin America.

Taxonomy

Kingdom / Regnum

Animalia

Subkingdom / Subregnum

Protozoa

Phylum

Sarcomastigophora

Subphylum

Flagellata / Mastigophora

Class

Zoomastigophora

Order

Kinetoplastida

Suborder

Trypanosomatina (with a single flagellum)

Family

Trypanosomatidae

Genus

Leishmania

Figure 1: Leishmania amastigotes within the cytoplasm in a smear stained with Giemsa.

Developmental cycle

Leishmania infantum, the infectious agent of canine leishmaniosis (CanL), is an obligate heteroxeneous parasite, i.e. it needs two hosts to develop, one vertebrate host and one terminal insect host. In the vertebrate host the parasite is found intracellular as so-called amastigote. In the insect host L. infantum is found extracellular in the gut as promastigote.

Amastigotes are found in reticuloendothelial cells. They multiply by asexual binary fission forming 'cell nests'. A rupture of the host cell is following and the progeny is taken up by other local or circulating macrophages. Depending on the parasite species the amastigotes either remain in the superficial tissues, continuing their reproductive cycle or settle in macrophages in the deep organs of the reticuloendothelial system such as lymph nodes, bone marrow, spleen and liver. One single bite might be enough to cause an infection of the vertebrate host.

Amastigotes in macrophages from the skin or peripheral blood circulation are taken up when the female sand fly feeds on an infected host. Inside the insect host a conversion to promastigotes takes place in the midgut. Multiplication and translocation to the esophagus follow. From there the promastigotes are transmitted to a vertebrate host during feeding of the sand fly.

Temperature and the susceptibility of the invertebrate host determine the cycle within the sand fly. A minimum of 10°C is necessary for initiation of the invertebrate cycle and rising temperature is continuously shortening the time needed for completion of the insect phase, with at least 6 days for L. infantum.

Morphology

The amastigote shows a round to oval body, about 1.5 to 3 x 3 to 6.5 µm in size, depending on the species. It possesses a single nucleus and a rod-shaped kinetoplast. There is no free flagellum, but a rudimentary one is present. The reproduction of this form is by longitudinal binary fission. Amastigotes can be cultured in appropriate cell cultures.

The promastigote varies from 16 to 40 µm length and 1.5 to 3 µm width. Promastigotes are longer than amastigotes, with a central nucleus and anterior kinetoplast and a well-developed flagellum, which is used either for propulsion or for attachment.
Promastigotes may be cultured in various media, mostly containing defibrinated blood and inactivated serum.

   

References

  • Bañuls, A.L., M. Hide and M. Tibayrenc: Molecular epidemiology and evolutionary genetics of Leishmania parasites. Int. J. Parasitol. 29, 1999, 1137-1147
  • Killick-Kendrick, R.: The life-cycles of Leishmania in the sand fly and transmission of leishmaniasis by bite. In: Killick-Kendrick, R. (ed.): Canine leishmaniasis: moving towards a solution. Proc. 2nd Int. Can. Leishm. Forum, Sevilla, Spain, Intervet Int., Boxmeer, The Netherlands, 2002, 57-68
  • WHO: Control of the leishmaniases. Report of a WHO Expert Committee, Tech. Rep. Ser. No. 793, WHO, Geneva, 1990

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