How does infection occur in leishmaniasis?

Updated: Feb 18, 2020
  • Author: Craig G Stark, MD, FACP, FFTM, RCPS(Glasg), FISTM; Chief Editor: Pranatharthi Haran Chandrasekar, MBBS, MD  more...
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Answer

Only the female sandfly transmits the protozoan, infecting itself with the Leishmania parasites contained in the blood it sucks from its human or mammalian host. Over 4-25 days, the parasite continues its development inside the sandfly, where it undergoes a major transformation into the promastigote form. A large number of flagellate forms (promastigotes) are produced by binary fission. Multiplication proceeds in the mid gut of the sandfly, and the flagellates tend to migrate to the pharynx and buccal cavity of the sandfly. A heavy pharyngeal infection is observed between days 6 and 9 of an infected blood meal. The promastigotes are regurgitated via a bite during this period, resulting in the spread of leishmaniasis.

Following the bite, some of the flagellates that enter the new host’s circulation are destroyed, whereas others enter the intracellular lysosomal organelles of macrophages of the reticuloendothelial system, where they lose their flagella and change into the amastigote form (see the first image below). The amastigote forms also multiply by binary fission, with multiplication continuing until the host cell is packed with the parasites and ruptures, liberating the amastigotes into the circulation (see the second image below). The free amastigotes then invade fresh cells, thus repeating the cycle and, in the process, infecting the entire reticuloendothelial system. Some of the free amastigotes are drawn by the sandfly during its blood meal, thus completing the cycle.

Post–kala-azar dermal leishmaniasis. Courtesy of R Post–kala-azar dermal leishmaniasis. Courtesy of R. E. Kuntz and R. H. Watten, Naval Medical Research Unit, Taipei, Taiwan.
Mucocutaneous leishmaniasis. Courtesy of Kenneth F Mucocutaneous leishmaniasis. Courtesy of Kenneth F. Wagner, MD.

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