Wuchereria bancrofti Morphology

• Nematodes in the superfamily Filarioidea are slender, thread-like worms.
• The worms are transmitted by the bite of blood-sucking insects.
• Female worms are viviparous, giving birth to larvae called microfilariae.
• Infected vertebrate hosts harbor both adult and larval stages.
• Microfilariae in some species retain their egg membranes as sheaths resulting in sheathed microfilariae.
• In other filarial nematodes, the egg membrane ruptures, leading to unsheathed microfilariae.

Wuchereria bancrofti Morphology

• Adults are whitish, translucent, thread-like worms with a smooth cuticle and tapering ends.
• The female worm is larger than the male.
• Males and females remain coiled together, typically in abdominal, inguinal lymphatics, and testicular tissues.
• The female worm is viviparous and directly releases sheathed microfilariae into the lymph.
• Adult worms can live for 10–15 years or more.
• Microfilariae have a colorless, translucent body with a blunt head and pointed tail.
• They are covered by a hyaline sheath, allowing them to move forwards and backwards.

Life Cycle

• W. bancrofti requires two hosts to complete its life cycle.
• The definitive host is humans.
• The intermediate host is the female mosquito of Culex spp.
• The actively motile third-stage filariform larva is the infective form.
• Humans get infected through the bite of a mosquito carrying the filariform larva.

Development in the Mosquito

• Microfilariae are ingested by a mosquito when it feeds on a carrier.
• Within 2–6 hours, the microfilariae cast off their sheaths (exsheathing) and penetrate the stomach wall.
• 4–17 hours later, they migrate to the thoracic muscles for further development.
• The worms become first-stage larvae within the next 2 days with a sausage shape and spiky tail.
• Within a week, it molts once or twice, increases in size and becomes the second-stage larvae.
• In another week, they develop internal structures and become the elongated third-stage filariform larva (actively motile).
• The infective larva enters the mosquito's proboscis.
• Larvae get deposited on the skin near the puncture site, usually in pairs when an infected mosquito feeds.

Development in Humans

• Larvae enter through the puncture wound or penetrate the skin themselves.
• Third-stage larvae enter the lymphatic vessels.
• Then travel to abdominal or inguinal lymph nodes.
• In the lymph nodes the larvae develop into adult forms.
• There is no multiplication; only one adult develops from one larva (male or female).
• Worms become sexually mature at about 6 months and mate.
• The gravid female releases up to 50,000 microfilariae per day.
• Microfilariae then pass through the thoracic duct and pulmonary capillaries to the peripheral circulation.

Pathogenicity

• Lymphatic filariasis is the disease caused by the worms.
• The typical manifestations arise from adult worms blocking lymph nodes and vessels.
• Blockage either occurs mechanically or more commonly due to allergic inflammatory reactions to worm antigens and secretions.
• Worms inside lymph nodes and vessels can stimulate granuloma formation and calcification.

Diagnosis

• Diagnosis of filariasis relies on clinical features.
• Laboratory tests that can be used:
  • Demonstration of microfilaria in peripheral blood.
  • Demonstration of the adult worm in biopsy specimens.
  • Skin tests with filarial antigens.
  • Serological tests.

Types of Female Nematodes

• Female nematodes are divided into the following:
  • Oviparous: laying eggs.
    • Unsegmented eggs: Ascaris, Trichuris.
    • Segmented eggs: Ancylostoma, Necator.
    • Eggs containing larvae: Enterobius.
  • Viviparous: producing larvae; Trichinella, Wuchereria.
  • Ovoviviparous: laying eggs containing fully formed larvae (hatch immediately); Strongyloides.