Leishmaniasis and Haemoflagellates — Kala-azar, LD Bodies, African and American Trypanosomiasis

Haemoflagellates are flagellated protozoa that live in blood and tissues. In Microbiology they belong to the family Trypanosomatidae: the genus Leishmania (leishmaniasis) and the genus Trypanosoma (African and American trypanosomiasis). Both genera are vector-borne, so the vector, the parasite stage found in humans and the geography are the three things the exam keeps asking about.

More than 20 Leishmania species infect humans. Species linked to visceral disease are L. donovani and L. infantum; species linked to cutaneous disease include L. major, L. tropica, L. mexicana and L. aethiopica. L. donovani occurs in Central Africa, South Asia, the Middle East, India and Bangladesh. The parasite has two forms: the promastigote (extracellular, in the sandfly) and the amastigote (intracellular, in vertebrate phagocytes).

1. An infected female sandfly feeds on a human or reservoir host and injects promastigotes.
2. Promastigotes are taken up by macrophages and settle in phagolysosomes.
3. Inside the macrophage they become amastigotes and multiply by simple division until the cell ruptures and infects neighbouring macrophages.
4. A sandfly feeding on an infected host ingests macrophages with amastigotes; in the gut they transform back to promastigotes and migrate forward to the pharyngeal valve, ready for the next bite.

On histology the hallmark is numerous small round or oval amastigotes, 2 to 4 µm, inside macrophages. Each shows a distinct nucleus and a rod-shaped kinetoplast, and the organisms often line up at the edge of the vacuole in a carousel or marquee pattern. These are the Leishman–Donovan (LD) bodies, named after the pathologist and the clinician who described ovoid bodies in spleen smears from India. The type of phagocyte infected steers the disease: skin macrophages give cutaneous disease, while amastigotes spreading through the blood to the mononuclear cells of the liver, spleen, bone marrow and intestinal lymph nodes give visceral disease.

WHO describes visceral leishmaniasis as irregular bouts of fever, weight loss, enlargement of the spleen and liver, and anaemia. More than 90% of visceral cases occur in Brazil, Ethiopia, India, Somalia, South Sudan and Sudan, and kala-azar was first described in India in the 1800s. Cutaneous disease is the most common form worldwide.

HIV co-infection matters. People with HIV who are infected with Leishmania have a high chance of developing full-blown disease, with high relapse and mortality rates. Antiretroviral treatment reduces progression, delays relapse and improves survival. Risk factors include malnutrition, poor housing, population displacement and immunosuppression.

For visceral disease the diagnosis combines clinical signs with parasitological or serological tests, including rapid diagnostic tests (WHO). The key parasitological step is to demonstrate amastigotes by aspiration or biopsy. Bone marrow is the preferred sample because splenic aspiration carries a rare but potentially fatal risk of haemorrhage. Smears are stained with Giemsa, and material can also be inoculated into culture.

Culture uses Novy–MacNeal–Nicolle (NNN) medium, a blood agar slant overlaid with liquid medium. PCR has high sensitivity: a systematic review found a pooled sensitivity of 93% and specificity of 95% for visceral disease. For cutaneous disease, tissue from the edge of the ulcer has the highest yield, and a press-imprint smear stained with Giemsa is a simple method. Serology has low sensitivity in cutaneous and mucocutaneous disease, so clinical features plus parasitology confirm the diagnosis.

Treatment depends on the form of disease, the parasite species, co-morbidity and geography. Visceral leishmaniasis always needs treatment. Limited cutaneous disease in an immunocompetent patient often heals without treatment.

Miltefosine is the oral option in this list. WHO adds that medicines do not clear the parasite from an immunocompromised body, hence the risk of relapse when immunity is poor. For drug mechanisms and adverse effects, see antiprotozoal and anthelmintic drugs.

Human African trypanosomiasis (HAT) is caused by Trypanosoma brucei subspecies and spread by the bite of the tsetse fly (Glossina) in sub-Saharan Africa. Without treatment it is generally fatal. Two forms exist, and WHO's table of distinctions is a favourite exam item.

Stage 1 (haemo-lymphatic) brings bouts of fever, headache, enlarged lymph nodes, joint pains and itching as the parasites multiply in subcutaneous tissue, blood and lymph. Stage 2 (meningo-encephalitic) begins when the parasites cross the blood–brain barrier: behaviour change, confusion, sensory disturbance, poor coordination and the sleep-cycle disturbance that gives the disease its name. Diagnosis has three steps: screening (serology exists only for gambiense), confirmation by finding the parasite in body fluids under the microscope, and staging with clinical examination and CSF analysis, because the choice of drug depends on the stage.

Chagas disease is caused by *Trypanosoma cruzi*, found mainly in Latin America. The vector is the triatomine (reduviid, "kissing") bug; the bug passes infective trypomastigotes in its faeces, which enter through the bite wound or mucous membranes (for example when the eyes are rubbed). Congenital transmission, blood transfusion, organ transplantation, food-borne spread and laboratory accidents also occur. WHO estimates about 8 million people are infected worldwide.

In the host the trypomastigote enters cells near the site of infection and becomes an amastigote, multiplying in pseudocysts in muscle and nerve cells. The parasite has an undulating membrane, a flagellum, a nucleus and a prominent kinetoplast.

Treatment is with benznidazole or nifurtimox. WHO states that both are fully effective if given early in the acute phase, including congenital infection, and that efficacy falls the longer the person has been infected. In the StatPearls review benznidazole is given at 5 to 10 mg/kg/day divided every 12 hours for 30 to 60 days; the common adverse effect is dermatitis, and myelosuppression or peripheral neuropathy is a reason to stop it immediately. Chronic cardiac and digestive complications are managed on their own merits, sometimes with pacemakers, defibrillators or surgery.

• Amastigote vs promastigote: the form in humans and in tissue smears is the amastigote; the form in the sandfly and in NNN culture is the promastigote.
• Vector mix-ups: sandfly = Leishmania; tsetse = T. brucei; triatomine bug = T. cruzi. Only female sandflies transmit.
• Bone marrow is the preferred diagnostic sample for kala-azar because splenic aspiration can cause fatal bleeding, although splenic aspirates have the highest sensitivity.
• Pancytopenia with hypergammaglobulinaemia and splenomegaly is the visceral leishmaniasis pattern; remember malaria and lymphoma as differentials.
• Chagas disease in the chronic phase is diagnosed by serology, not blood smear.
• Melarsoprol vs suramin vs pentamidine: match the drug to the subspecies and the stage (stage 2 needs CNS penetration).
• PKDL is a rash after kala-azar and a potential infection source.

Related pages: practise previous papers at NEET PG Microbiology PYQs and NEET PG Medicine PYQs, and see the most repeated topics.