Write a short note on lymphatic filariasis.

Aetiology

Lymphatic filariasis is caused by tissue-dwelling nematodes transmitted by mosquitoes:

• *Wuchereria bancrofti* — responsible for about 99% of cases in India, transmitted by *Culex quinquefasciatus*, which breeds in polluted stagnant water
• *Brugia malayi* — focal areas of Kerala and the east coast, transmitted by *Mansonia*
• *Brugia timori* — Indonesia

India accounts for a large share of the global burden, with endemic districts across Uttar Pradesh, Bihar, Jharkhand, West Bengal, Odisha, Andhra Pradesh, Telangana, Tamil Nadu, Kerala and Gujarat. It is a leading cause of permanent disability and disfigurement, with profound social and economic consequences.

Life cycle: the mosquito injects infective larvae, which migrate to lymphatic vessels and lymph nodes, where they mature into adult worms over 6–12 months. Adults live 5–10 years, producing microfilariae that circulate in blood with nocturnal periodicity — peaking between 10 pm and 2 am, coinciding with the biting habits of the vector. This periodicity determines when blood samples must be taken.

Pathogenesis

Adult worms in the lymphatics cause lymphatic dilatation, dysfunction and obstruction, through:

• Direct mechanical effects and the host inflammatory response to living and, especially, dying worms
• Recurrent secondary bacterial and fungal infections of compromised skin — which are the main driver of progression to elephantiasis, and the reason hygiene is the mainstay of morbidity management
• *Wolbachia* endosymbiotic bacteria within the worms contribute to inflammation — and are the target of doxycycline therapy

Clinical Features

A key clinical point: filarial lymphangitis spreads RETROGRADE (peripherally, away from the node), in contrast to bacterial lymphangitis, which spreads towards the draining node. This distinction is asked in examinations.

Tropical pulmonary eosinophilia is a hypersensitivity response to microfilariae trapped in the lungs: nocturnal paroxysmal cough and wheeze, breathlessness, weight loss, with very high absolute eosinophil count (> 3000/µL), markedly raised IgE and filarial antibody, diffuse reticulonodular shadowing on chest X-ray, and absent microfilariae in blood. It responds dramatically to diethylcarbamazine (DEC), and is frequently misdiagnosed and treated as bronchial asthma or tuberculosis for months.

Investigations

• Night blood smear (thick film, taken between 10 pm and 2 am) — demonstrates sheathed microfilariae; Giemsa stained. The timing is essential and is the commonest reason for a false-negative result.
• DEC provocation test — a daytime dose of DEC provokes microfilariae into the peripheral blood, allowing daytime sampling.
• Circulating filarial antigen (ICT card test) — detects adult worm antigen, is not dependent on periodicity, can be done at any time of day, and is the test of choice for mapping and surveillance.
• Ultrasound with Doppler — visualises adult worms in dilated scrotal lymphatics as the "filarial dance sign"; useful for confirming live adult worms.
• Lymphoscintigraphy — demonstrates lymphatic dysfunction even in asymptomatic infection.
• Supporting: eosinophilia, raised IgE, and chylous urine analysis in chyluria.

Management

1. Antifilarial drugs

• Diethylcarbamazine (DEC) — the mainstay in India; kills microfilariae and has some effect on adult worms. Adverse effects are largely due to dying parasites — fever, headache, myalgia, and local nodular reactions — and are more marked with high microfilarial loads. DEC must be avoided where onchocerciasis or loiasis is co-endemic (not an issue in India, but examinable).
• Albendazole — used in combination; ivermectin in some regimens.
• Doxycycline — targets *Wolbachia* endosymbionts, producing macrofilaricidal (adult-worm-killing) effects and improving lymphoedema; given as a prolonged course.

2. Morbidity management — the most important intervention for those already disabled

Antifilarial drugs do not reverse established lymphoedema. The evidence-based measures are:

• Meticulous skin hygiene — daily washing of the affected limb with soap and water, careful drying, especially between toes and in skin folds
• Prompt treatment of entry lesions — fungal infection, fissures, wounds — with topical antifungals and antiseptics
• Antibiotics for acute attacks, and prophylactic antibiotics for frequent recurrences
• Limb elevation and exercise to promote lymph flow; compression bandaging in suitable cases
• Appropriate footwear; nail care
• Surgery — hydrocelectomy (which transforms quality of life and is provided under the national programme), and rarely lymphovenous anastomosis or debulking for advanced elephantiasis
• Psychosocial support — the stigma of elephantiasis is severe and is itself a major source of suffering

3. Elimination programme

India's National Filaria Elimination Programme rests on:

• Mass drug administration (MDA) — annual single-dose treatment of the entire at-risk population with DEC plus albendazole, or the more effective triple-drug therapy (ivermectin + DEC + albendazole, "IDA"), continued for several years to interrupt transmission
• Vector control — reducing *Culex* breeding by managing polluted water, drainage and sanitation
• Morbidity management and disability prevention for those already affected
• Surveillance with transmission assessment surveys to confirm interruption