Introduction

Overview of leishmaniasis

According to the WHO, Leishmaniasis is a parasitic disease or infection caused by a protozoan from the genus Leishmania and is transmitted by a vector commonly known as sandfly from the Psychodidae family and Diptera order.⁴ Leishmania are diploid organisms with 25, 3, or 37 chromosomes.⁷ The sandfly is from the genus Lutzomyia and the genus Phlebotomus.¹ Species of phlebotomus include: argentipes, papatasi, longipes, serganti, perniciosus, martini, orientalis and chinensis.³ Leishmaniasis can occur in three forms: Visceral Leishmaniasis, Mucocutaneous Leishmaniasis, and Cutaneous.² Where visceral is the most severe form and musculocutaneous brings about disability. It is transmitted by the bite of the sandfly. Countries that have reported Leishmaniasis include: Brazil, East Africa, India, America, the Mediterranean basin, the Middle East, Central Asia, Bolivia, Ethiopia, and Peru, Ethiopia. Different forms of Leishmaniasis are found in different countries.

Causative agents (leishmania species)

The causative organism of Leishmaniasis is Leishmania. There are different species of this organism distributed across different countries. These species include: 

• Leishmania donovani complex; It includes L. donovani, L. infantum, and L. chagasi
• Leishmania mexicana complex; It includes L. mexicana, L. amazonensis, L. venezuelensis
• Leishmania tropica
• Leishmania major
• Leishmania aethiopica
• Sungenus viannia; L.(V.) braziliensis, L.(V.) guyanensis, L.(V.) panamensis and L. (V.) periviana³

Transmission and epidemiology

According to the WHO, about 700,000 to 1 billion cases are reported annually, with 20,000 to 30,000 distributed around the world in different forms of leishmaniasis.⁵ According to the Pan American Health Organisation (PAHO) from 2001 to 2023, there have been 1,178,436 new cases of cutaneous leishmaniasis, with an average of 51,236 new cases annually or Musculocutaneous, and 51,236 average cases of both cutaneous and visceral leishmaniasis. Visceral leishmaniasis has a case fatality of 8% in about 22 countries.⁶ Visceral leishmaniasis is reported mostly in Brazil, East Africa and India. It is also known as the “Kala Azar”. Cutaneous leishmaniasis reports about 66,941 cases every year in the Americas. It is highly prevalent in Brazil, Peru and Colombia.

Clinical forms

It exists in various forms, depending on the physical and pathological manifestations:

1. Cutaneous Leishmaniasis: Presents as the most common form of leishmaniasis with a clinical manifestation of ulceration on the skin, hence its name.  It does not visceralize²⁴
2. Mucocutaneous Leishmaniasis: It is characterised by disfiguring ulcerations on organs like the nasal and oropharyngeal mucosa.^25,26
3. Visceral (Kala-azar): It is characterised by visceral lesions and manifestations affecting the reticuloendothelial system ( liver, spleen, bone marrow)²⁸

Importance of histopathology in diagnosis

Histopathologic analysis is the use of histological techniques to collect tissues for visualisation under a microscope.³⁰ Histopathology helps identify parasites and their effect on tissue, the extent of damage, and helps diagnose the form of the disease. It increases accuracy and is reliable when done properly.

Pathogenesis and tissue tropism

Life cycle of leishmania

The life cycle of Leishmania has two stages namely; the infective stage and the diagnostic stage. The diagnostic stage is the stage of the parasite that occurs and can be identified in the intermediate host, while the infectious stage occurs in the definitive or intermediate host. In the case of Leishmania, the infectious stage occurs in the sandfly, and the diagnostic stage occurs in humans.

The sandfly takes a blood meal from an infected person. It ingests the promastigote stage into the person, which is engulfed and phagocytised by mononuclear cells and macrophages. The promastigotes then transform into Amastigotes and replicate in cells and tissues, spreading across and affecting other tissues.³¹ The sandfly takes a blood meal, this time ingesting the Amastigote form of the virus. In the gut of the sandfly, the amastigote stage is transformed into the promastigote form of the parasite, replicating in the guts and migrating to the proboscis of the sandfly. The cycle starts all over again.³

Gross and clinical presentation

In cutaneous and musculocutaneous forms of the disease, sores or ulcerated wounds are seen. In visceral leishmaniasis, the spleen and liver enlarge (splenomegaly and hepatomegaly), the bone marrow becomes infected and reduces or loses function, leading to common symptoms such as anaemia, leukopenia and blood-related symptoms, enlargement of the lymph nodes and other symptoms related to the reticuloendothelial system. Although visceral leishmaniasis is known to affect internal organs, it also affects the skin and is able to penetrate deep into these internal organs. It thrives in the skin, and the keratinocytes get exposed to Leishmania.²⁸ This exposure activates proinflammatory cytokines.⁸

Histologic stages

The early stages of the disease elicit an acute inflammatory reaction, especially in cutaneous leishmaniasis. Histology reveals infiltration of leukocytes and plasma cells. Promastigotes are phagocytised by macrophages and transform into Amastigotes, proliferating and becoming abundant in the cytoplasm of cells. In the intermediate stage, chronic inflammation commences. Chronic inflammation includes granulomatous reactions, fibrosis and giant cell proliferation with increased parasitic overload. In the last stage, Extended fibrosis and scarring are seen in damaged and disfigured tissues. At this stage, the host immune response has become massive and therefore, parasites are absent or few. In visceral leishmaniasis, there is infiltration of tissue-resident macrophages in the reticuloendothelial system, leading to Amastigote overload in organs and pathologic insult to organs.⁹

Histopathologic features in skin biopsy (Cutaneous leishmaniasis)

Cutaneous leishmaniasis arises from L. aethiopica, L. mexicana, and L. amazonensis. It is seen as a papule in the early stages and advances further into an erythematous plaque or nodule. It then advances further into a plaque and finally leaves a scar at exposed areas where the sandfly takes the blood meal. It is the most common form and may become chronic over time, which induces symptoms such as lupoid and eczematous lesions.¹⁰

Histopathology of skin infected with cutaneous leishmaniasis, the parasite is seen abundantly in H and E stains of the skin biopsy. This inference is confirmed by amastigote's presence using CDa immunostain, such as MTB1, Novocastra, Buffalo Grove, Illinois.¹¹ Studies have revealed transient to total ulceration, granulomatous inflammation, infiltration of lymphocytes, pseudoepitheliomatous hyperplasia, infiltration of plasma cells, giant multinucleated cells, sarcoidal patterns, and histiocytic infiltration^11,12

Histopathologic features in mucocutaneous leishmaniasis

Mucocutaneous leishmaniasis is present as severe and causes destruction and disfiguration of affected areas. It is caused by braziliensis, L. panamensis, and  L. guyanensis. It is known to affect the mucosa, causing ulceration at exposed places like the nose, mouth or through. It does not affect the bones and does not disfigure the septum. Mucocutaneous leishmaniasis can intertwine with cutaneous leishmaniasis; therefore, it is important that differential diagnosis be made in the early stage. There are 3 stages of mucocutaneous leishmaniasis. They are:

1. Edematous
2. Granulomatous
3. Proliferative
4. Granulomatous necrotizing¹³

Histopathology reveals few parasites at the affected site with epithelioid cells, giant cells, lymphocytes, ulceration and necrosis.¹⁴

Histopathologic features in bone marrow smear (visceral leishmaniasis)

Indications for bone marrow aspiration

Visceral leishmaniasis affects the reticuloendothelial system, which consists of the liver, lungs, and bone marrow, caused by Leishmania donovani and Leishmania infantum.^17,18 To examine the bone marrow, a small liquid sample of the bone marrow must be taken and examined. After aspiration, a smear is made on a glass slide and examined under a microscope.¹⁷ Under the microscope, Leishmania Donovan bodies (LD) are seen in the Amastigote form(small) in the macrophages and histiocytes.¹⁵ After the macrophages burst open, LD can be seen coming out of the macrophages. Common stains used include Giemsa stain and Leishman stain. With these stains, the slide appears as pale-blue cytoplasm with red red-stained large nucleus with a red or violet kinetoplast. LDs are small(1-5µm) spherical bodies centrally or eccentrically located within the parasite.¹⁶
Symptoms erupting from visceral leishmaniasis include: anaemia, leukopenia, thrombocytopenia, pancytopenia, hypercellularity, hemophagocytosis, granulomas, fibrosis, and focal hypercellularity.^19,20

Ancillary techniques

• A. Giemsa and Wright Stains: Giemsa stain reveals Amastigotes usually in macrophages that have a round and oval shape and have a pink coloured cytoplasm with a dark purple nucleus, which has a kinetoplast that looks like a dot or is rod-liked⁶
• B. PCR and Molecular Diagnostics: This test is used to detect Leishmania parasite DNA, providing high specificity and sensitivity. These texts include: Polymerase Chain Reaction(PCR), Loop Mediated Isothermal Amplification (LAMP)²²
• C. Immunohistochemistry: These are tests done to measure the immune system's response to the parasite. The intensity or amount of immune cells and response or chemicals is a measure of the disease and its intensity. Immunohistochemical tests commonly done for leishmaniasis include: Serological tests, which detect and measure antibody production. They are; rk39 Diagnostic test (RDK), Direct Agglutination test (DAT), Enzyme-linked Immunosorbent Assay(ELISA) and Leishmania Skin test²³
• D. Culture Methods: There are several culture methods for Leishmania parasites. Leishmania can be cultured based on the stage and what exactly it is being cultured for. The promastigotes stage in the sandfly may be cultured. It is known to be flagellated and motile. They can be cultured in Novy MacNeal Nicholle media and Schneider’s medium, while the Amastigote form of the parasite can be cultured in macrophage culture²⁴

Differential diagnosis 

Differential diagnosis for cutaneous and musculocutaneous leishmaniasis includes: Fungal infections such as Blastomycosis, sporotrichosis, and other diseases such as Myiasis, leprosy, syphilis eczema.²⁵

Differential diagnosis for cutaneous leishmaniasis includes: Malaria, brucellosis, schistosomiasis, hepatosplenomegaly, leukaemia, lymphoma, and hemolytic anaemia.²⁶

Treatment

One parenteral drug, called liposomal amphotericin B by intravenous administered intravenously for visceral leishmaniasis. Miltefosine is taken orally for cutaneous and mucocutaneous leishmaniasis in adults and adolescents. Pentamidine isethionate is also used for cutaneous leishmaniasis.²⁷

Conclusion

In conclusion, Leishmaniasis is a parasitic disease or infection caused by a protozoan from the genus Leishmania and is transmitted by a vector commonly known as a sandfly. It is characterised by sores on the skin, ulceration of mucous membranes and a gradual decline in work by the reticuloendothelial system. Leishmaniasis is a neglected disease; therefore, more attention should be given to it. Those at risk include those with high exposure to sandflies; therefore, vector control is important in such areas. More regimen is necessary to increase treatment options. Education is also necessary to prevent further spread and help raise public awareness, so as to enable affected individuals to receive immediate treatments. More health facilities and health workers are necessary for areas with a high level of poverty.

Summary

Leishmaniasis is a parasitic infection caused by Leishmania and carried by the vector, named sandfly. There are 3 types of leishmaniasis, namely: cutaneous, mucocutaneous and visceral leishmania. 

Cutaneous leishmaniasis is known to cause sores and ulcers on exposed areas of the skin where the sandfly takes the blood meal.

Mucocutaneous is known to cause disfiguration and affects the nasal passage and oropharyngeal layers.

The visceral leishmaniasis occurs in organs like the liver, spleen and bone marrow, disrupting their functions.

Countries that have reported Leishmaniasis include: Brazil, East Africa, India, America, the Mediterranean basin, the Middle East, Central Asia, Bolivia, Ethiopia, and Peru, Ethiopia.

Species of Leishmania known to cause leishmaniasis are: Leishmania donovani complex; It includes L. donovani, L. infantum, and L. chagasi. Leishmania mexicana complex; It includes L. mexicana, L. amazonensis, and L. venezuelensis

• Leishmania tropica
• Leishmania major
• Leishmania aethiopica
• Sungenus viannia; L.(V.) braziliensis, L.(V.) guyanensis, L.(V.) panamensis and L. (V.) periviana³

The life cycle occurs in both the intermediate and definitive host after the sandfly takes a blood meal and undergoes several generations, going back into the guts of the sandfly for another blood meal.

It is diagnosed by histopathology examination, bone marrow smear and other auxiliary techniques such as PCR, Culture methods, immunohistochemical methods and staining methods.