Leishmania braziliensis, Leishmania mexicana - Life Cycle, Pathogenesis, Pathology, Host Immunity

Life Cycle of Leishmania braziliensis, Leishmania mexicana

The life cycle of Leishmania braziliensis Complex and Leishmania mexicana 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 braziliensis Complex and Leishmania mexicana Complex are 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 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 the reticuloendothelial cells and lymphatic tissues 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 braziliensis Complex and Leishmania mexicana Complex is continued

Pathogenesis of Leishmania braziliensis, Leishmania mexicana

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

The key pathological lesions of Leishmania braziliensis Complex and Leishmania mexicana Complex infection are papules and ulcers, which are the common forms of mucocutaneous leishmaniasis.

Pathology of Leishmania braziliensis, Leishmania mexicana

The pathology of Leishmania braziliensis, Leishmania mexicana are as follows:

Espundia

• the incubation period for oriental sore may range from 2 months to 4 months

• espundia is a typical ulcerative lesion that occurs on the skin

• the syndrome begins as a single, red, pruritic papule at the site of the bite i.e. at the site of inoculation

• lesions may expand rapidly and forms large, long-lasting ulcers with weeping surfaces and distinct margins

• as the size of the papule increases, it becomes crusted and eventually ulcerates and this ulcer is called an oriental sore

• the oriental sore is circular, shallow, and may measure around 2 cm with a base formed by granulation tissue and erythematous elevated margin

• the base of the ulcer may contain the amastigote forms of the parasite

• as the disease progresses, the ulcer produces serous or seropurulent discharges which may result in the formation of scales or crusts

• the ulcer may also cause secondary infections such as streptococcal and staphylococcal infections

• regional lymphadenopathy is mostly unusual

• in some cases, the ulcer has lasted for 1 year to 2 years, and healing leaves behind small, flat, and depigmented scar

• in 5% of espundia cases, mucosal metastasis may occur by direct extension or through blood and invade mucous membranes in the mouth, nose, pharynx, and larynx either

Ulcer

• primary ulcers occur mostly in the hand and foot

• are round in shape with raised bodies and a granulating base covered by the exudate

• a chain of nodules may appear along the line of lymphatics due to lymphatics draining these ulcers

• mucosal leishmaniasis ulcers are fungative, indurated, invade and destroy superficial tissues and cartilages of the nose and larynx

• under a microscope, the edge of the ulcer and the serum exudate reveals the presence of amastigotes

Host Immunity of Leishmania braziliensis, Leishmania mexicana

Host Immunity of Leishmania braziliensis, Leishmania mexicana involves cell-mediated immunity (CMI) while the role of humoral immunity is unclear.

• mucocutaneous leishmaniasis involves the development of marked cell-mediated hypersensitivity reaction

• as the cell-mediated immunity (CMI) is suppressed, amastigotes are freely able to multiply and spread in the skin

• circulating antibodies are seen in the patient's serum