Describe leishmaniasis with its life cycle, clinical forms, laboratory diagnosis and control.

The Parasite and its Transmission

• Leishmania is an obligate intracellular protozoan of the mononuclear phagocyte system, transmitted by the bite of the female sandfly (Phlebotomus argentipes in India)
• Two forms — the amastigote (Leishman-Donovan or LD body), a small round aflagellate form found inside macrophages in man, with a nucleus and a rod-shaped kinetoplast; and the promastigote, an elongated flagellated form in the sandfly gut and in culture
• The kinetoplast is the diagnostic feature — it distinguishes an amastigote from Histoplasma and from other small intracellular organisms
• The sandfly is small, silent and a weak flier, breeding in cracks in mud walls, cattle sheds and organic debris, and biting at night. It passes through ordinary mosquito nets, so fine-mesh or insecticide-treated nets are required
• Culture in NNN (Novy-MacNeal-Nicolle) medium yields promastigotes

Pathogenesis and Immunology

The sandfly injects promastigotes into the skin → They are phagocytosed by macrophages, which would normally kill them → Inside the phagolysosome they transform into amastigotes and resist killing — they tolerate the acid pH and inhibit the respiratory burst → They multiply until the macrophage ruptures, releasing amastigotes that infect further macrophages → In visceral disease they disseminate to the spleen, liver, bone marrow and lymph nodes → Massive reticuloendothelial hyperplasia gives splenomegaly, and marrow infiltration with hypersplenism gives pancytopenia → Polyclonal B-cell activation produces a marked hypergammaglobulinaemia with a reversed albumin:globulin ratio

• The outcome depends on the T-cell response, exactly as in leprosy — a Th1 response with interferon-gamma activates macrophages and controls the infection, whereas a Th2 pattern permits unchecked multiplication and visceral disease
• The leishmanin (Montenegro) skin test reflects this: it is negative in active visceral disease (anergy) and becomes positive after cure, so it is an epidemiological and prognostic tool rather than a diagnostic one
• HIV co-infection markedly increases the risk of visceral disease, of relapse and of treatment failure, and reduces the sensitivity of serology

Laboratory Diagnosis

• Demonstration of amastigotes (LD bodies) in a Giemsa-stained smear is definitive. splenic aspirate has the highest sensitivity (over 95%) but carries a risk of haemorrhage; bone marrow aspirate is safer and is usually preferred, and lymph node aspirate is an alternative
• Culture in NNN medium showing promastigotes; and PCR, which is the most sensitive and allows species identification
• RK39 rapid immunochromatographic test — a strip test on finger-prick blood with high sensitivity and specificity for Indian L. Donovani. It requires no laboratory and has transformed field diagnosis, being the mainstay of the Indian elimination programme. Its limitation is that antibody persists for years after cure, so it cannot diagnose relapse
• Direct agglutination test and ELISA; the aldehyde (formol-gel) test is an old, cheap and non-specific test that detects the hypergammaglobulinaemia by causing serum to jellify
• Supporting findings — pancytopenia, a very high ESR, hypergammaglobulinaemia with a reversed albumin:globulin ratio
• PKDL is diagnosed by slit-skin smear or biopsy demonstrating amastigotes, together with the history of treated kala-azar

Differential Diagnosis of Fever with Splenomegaly and Pancytopenia

• A bone marrow aspirate answers several of these at once — amastigotes, granulomas, malignant cells and haemophagocytosis are all sought on the same specimen, which is why it is the pivotal investigation

Treatment, Control and Applied Aspects

• Liposomal amphotericin B is the treatment of choice for Indian visceral leishmaniasis, given as a single infusion in the national programme — effective, well tolerated and, crucially, requiring only one hospital visit
• Miltefosine is the first oral antileishmanial, but is teratogenic, requiring contraception for months, and resistance has emerged with its unsupervised use; paromomycin is also used
• Pentavalent antimonials (sodium stibogluconate) were the mainstay for decades but have been abandoned in Bihar because of widespread resistance — a clear instance of resistance rendering a drug class useless in a whole region
• Vector control — indoor residual spraying with insecticide, plastering of cracked mud walls, insecticide-treated nets with a fine mesh, and separating cattle sheds from dwellings
• India's kala-azar elimination programme targets Bihar, Jharkhand, West Bengal and Uttar Pradesh, aiming for fewer than 1 case per 10,000 population per block. It rests on active case detection with the rK39 test, single-dose liposomal amphotericin B, indoor residual spraying, and active search for PKDL. India has come close to elimination
• Elimination is feasible in India specifically because man is the only reservoir; in East Africa the disease is zoonotic and cannot be eliminated by treating people alone
• Distinguish PKDL from leprosy — both give hypopigmented patches, but PKDL lesions have normal sensation and there is a history of kala-azar; testing sensation settles it at the bedside
• The rK39 test cannot diagnose relapse, since antibody persists for years; recurrence must be confirmed parasitologically by marrow or splenic aspirate
• The sandfly passes through an ordinary mosquito net, so a fine-mesh or insecticide-treated net is required — a specific piece of advice that is often given wrongly
• Screen for HIV in visceral leishmaniasis; co-infection is increasing in Bihar, causes frequent relapse, and requires a different and longer regimen
• Kala-azar is a disease of poverty, concentrated in mud-walled housing with cattle nearby; plastering walls and separating cattle sheds reduce transmission without any drug
• Look for PKDL actively among treated patients; they feel well and will not present, yet they sustain transmission and their treatment is the key to elimination
• Miltefosine is teratogenic, so effective contraception must be assured in women of childbearing age for the duration of treatment and for months afterwards
• Antimonial resistance in Bihar shows how a drug class can be lost regionally through decades of unsupervised and incomplete treatment — a warning that applies equally to the drugs now replacing it
• The amastigote is distinguished from Histoplasma by its kinetoplast, which is the single feature to look for when both are possible in an HIV-positive patient with pancytopenia
• Splenic aspirate is the most sensitive test but carries real risk; marrow aspiration is preferred in routine practice and the rK39 test has made both unnecessary in most Indian cases
• Single-dose liposomal amphotericin B transformed the programme, because a treatment requiring one visit can be delivered in a district hospital to patients who cannot afford repeated travel
• The leishmanin test is negative in active disease and becomes positive after cure, so it is useless for diagnosis and useful for surveys and for assessing response
• Man is the only reservoir in India, which is precisely why elimination is achievable here and is not in East Africa — a point that explains the whole strategy
• Cutaneous leishmaniasis occurs in Rajasthan, and a chronic painless ulcer with a raised edge in a patient from there should prompt a slit-skin smear rather than repeated antibiotics