Leishmania tropica

Leishmania tropica: Pathogenesis, Clinical Signs, Diagnosis & Treatment

Leishmania tropica is a protozoan hemoflagellate parasite belonging to the family Trypanosomatidae and is one of the principal causative agents of Old World cutaneous leishmaniasis. The parasite is transmitted by the bite of infected Phlebotomus sand flies and primarily affects the skin, producing localized lesions that may heal spontaneously but often leave permanent scars.

Leishmania tropica infects humans as well as several animal species, including dogs, cats, rodents, and other mammals, with rodents serving as important reservoir hosts in many endemic regions. The disease is distributed across parts of South Asia, the Middle East, and North Africa, where it remains a significant public health concern due to its zoonotic potential and impact on both human and animal health.

Parasite Overview

• Disease: Cutaneous leishmaniasis, Oriental sore, Delhi boil, and Aleppo button
• Host: Mainly dogs and humans, but cats, bullocks, horses, monkeys, and bears may also be affected.
• Reservoir host: Gerbils, rodents, cats, and dogs
• Predilection Site: Skin, endothelial cells of capillaries, lymph nodes, mononuclear cells, leukocytes, and, very rarely, the viscera
• Vector: Phlebotomus sergenti in India and P. papatasi in other countries
• Distribution: India (common in northern India), South Asia, and Iran
• Zoonotic: Yes (Since L. tropica is transmitted from animals to humans, it is classified as an anthropozoonosis.)

Taxonomical Classification

• Kingdom: Protista
• Phylum: Euglenozoa
• Class: Kinetoplastea
• Order: Trypanosomatida
• Family: Trypanosomatidae
• Genus: Leishmania
• Subgenus: Leishmania
• Species: Leishmania tropica
• Common Name: Old World cutaneous leishmaniasis parasite

Pathogenesis

Following inoculation of promastigotes by a sand fly during feeding, they are engulfed by macrophages in the skin, where they multiply and transform into amastigotes. The infected macrophages eventually rupture, releasing the amastigotes.

These amastigotes are then transported to other areas, where they infect new cells and continue multiplying. At the site of multiplication, infiltration of lymphocytes and plasma cells may occur.

Within 2–5 days after the sand fly bite, reddish papules initially appear, followed by epithelial acanthosis, hyperkeratosis, necrosis, crust formation, and shallow ulcers that develop over the next few days. These ulcers may reach several centimeters in diameter and involve larger areas.

The ulcers may begin to heal within 2–12 months after infection, leaving a deep, pigmented, depressed scar covered with granulation tissue, with an indurated periphery. Metastasis to secondary foci and secondary bacterial infections are common, but anemia does not occur.

Clinical Signs

• Reddish papules at the site of the sand fly bite.
• Papules gradually develop into nodules and shallow ulcers.
• Ulcers are often covered with crusts and have indurated margins.
• Lesions commonly occur on exposed areas of the skin.
• Healing usually occurs spontaneously over several months, leaving permanent depressed scars.
• Secondary bacterial infection may complicate the lesions.

Cutaneous Leishmaniasis

Cutaneous Leishmaniasis is manifested in two forms:

1. Dry/Urban Cutaneous Leishmaniasis is caused by L. tropica minor and is more prevalent in tropical countries such as India. It has a long incubation period of approximately 6 months with a prolonged clinical phase. The lesions appear as papules measuring 2–10 mm in diameter, which persist for several months and contain large numbers of amastigotes. This condition is very common in dogs.
2. Wet/Rural Cutaneous Leishmaniasis is caused by L. tropica major and has a short incubation period with a short clinical phase. In this form, ulcers develop rapidly and exude serous or serosanguineous fluid. These lesions contain few amastigotes and heal spontaneously.

Immunity usually develops following spontaneous recovery from L. t. major infection and may persist for up to 20 years.

Diagnosis

• Examination of smears prepared from material obtained by scraping the edges of the lesion. Amastigote stages can be observed.
• Biopsy of lymphoid tissue beneath the lesion.
• Culture the material in NNN medium.
• Leishmania test, IFAT, and DAT.

Treatment

• Cryosurgery
• Quinacrine HCl: 200 mg
• Berberine sulfate 2%: 2 mL twice weekly for 8 weeks.
• For Dogs:
  • Anthiomaline: 1 mL IM for 4–6 months
  • Levamisole: 23 mg/kg, 2 days/week for 7 weeks.