Leishmania tropica complex - Life Cycle, Pathogenesis, Pathology, Host Immunity, Epidemiology, Reservoir

Life Cycle of Leishmania tropica complex

The life cycle of the Leishmania tropica complex is completed in two different hosts:

• Man or other mammals- the amastigote form

• Sandfly of genus Phlebotomus and Lutzomyia – the promastigote form

These mammals include dogs, rodents, foxes, etc.

• Leishmania tropica Complex is transmitted to man and other vertebrates by the bite of a blood-sucking female sandfly

• During the blood meal, the promastigotes are transmitted from the saliva of the vector to the host’s skin

• the promastigotes are then immediately phagocytosed by the host macrophages where they transform into amastigotes

• the newly formed Leishmania tropica Complex amastigotes undergo binary fission to produce a large number of amastigotes which continue to divide inside the enlarged macrophages

• the infected macrophages rupture to release a large number of amastigotes (around 200 in number) into the host body circulation

• the parasites then reside in monocytes, polymorphonuclear leucocytes as well as in the endothelial cells of the capillaries of the skin but do not invade the internal organs or peripheral blood

• if the infected mammal host (including man) is bitten by a female sand-fly during a blood meal, the free amastigotes are also taken up with the host blood

• in the midgut of the sand-fly, within 72 hours, the amastigotes are transformed into promastigotes

• the morphological transformation takes place through a series of flagellated intermediate promastigote forms to flagellated promastigotes

• the promastigotes undergo binary fission, produce a large number of parasites, and completely fill the lumen of the gut

• after 6-9 days, the promastigotes travel to the sandfly's pharynx and buccal cavity from the mid-gut

• bite from the infected sand-fly to mammals (including humans) during its blood meal transmits the infection and the life cycle of Leishmania tropica Complex is continued

Pathogenesis, Pathology of Leishmania tropica complex

A bite from a female sandfly during a blood meal deposits the promastigotes, present in the vector’s saliva/mouthparts, in the skin. The newly introduced promastigotes are phagocytosed by host macrophages where they transform into amastigotes.

The key pathological lesions of Leishmania tropica Complex infection are papules and ulcers, which are the common forms of cutaneous leishmaniasis.

Papule

• papule occurs in the early stage of the disease

• caused by the multiplication of macrophages that harbor a large number of amastigotes (up to 200 amastigotes in each infected macrophage)

• in the lesion, host immune cells such as inflammatory cells, plasma cells, and lymphocytes can also be found

• in untreated cases, the papules get enlarged, necrotize, and eventually ulcerate

Ulcer

• ulcer may be single or multiple in number

• occurs in mainly exposed parts of the body such as the face and extremities but is mainly absent from the areas where the epidermis is thick such as the palm, sole, or the scalp

• the ulcer may persist from weeks to months

• with the healing of the ulcer, a flat atrophic scar is formed

• these ulcers leave life-long scars, serious disability, or social stigma

• number of macrophages, lymphocytes, and plasma cells also decreases as the lesion heal

Host Immunity of Leishmania tropica complex

• the host immunity in response to Leishmania tropica Complex includes profound development of cell-mediated hypersensitivity reaction

• humoral immunity is not involved in defense against cutaneous leishmaniasis

• similar to leprosy, cutaneous leishmaniasis is immunologically considered a polar disease

• the two polar forms are

  diffuse cutaneous leishmaniasis

  eishmaniasis recidivans

• the cell-mediated immunity (CMI) in response to cutaneous leishmaniasis is similar to that of lepromatous leprosy

• the tuberculoid leprosy is analogous to leishmaniasis recidivans

• cases of re-infection by the same species of Leishmania tropica Complex are low as after recovery, the resistance to the disease is very high

Epidemiology of Leishmania tropica complex

• three forms of Old World cutaneous leishmaniasis are recognized

• epidemilogically, cutaneous leishmaniasis occurs in Iran, Afghanistan, Brazil, Syria, India, Peru, Saudi Arabia

Reservoir, Source of Leishmania tropica complex

• transmission of Leishmania tropica Complex occurs due to the bite of infected female sand-fly

• the reservoir, source of Leishmania tropica Complex is the infected man or canines like the dog

• cutaneous leishmaniasis affects all age groups, especially young adults, infants, and adolescents