Wuchereria Bancrofti | Zoology Optional Notes for UPSC PDF Download

Systematic Position of Wuchereria bancrofti

• Phylum: Nematoda
• Class: Phasmidia
• Order: Filaroidea
• Family: Filariidae
• Genus: Wuchereria
• Species: bancrofti

Habits and Habitat of Wuchereria bancrofti

• Endoparasitic Nature: Wuchereria bancrofti, a filarial worm, is a parasitic organism that resides within the body of humans.

• Digenetic Parasite: This parasite undergoes a digenetic life cycle, involving two hosts. The final host is humans, where the adult worms live, while the intermediate host is blood-sucking insects. Female mosquitoes from the genera Culex, Aedes, or Anopheles serve as the intermediate hosts.

• Residence in Humans: Adult worms coil up within the lymph glands and lymph passages of humans, often causing obstruction to the flow of lymph.

• Presence in Peripheral Blood: Microfilariae, the larval stage of Wuchereria bancrofti, are found in the peripheral blood. Occasionally, they may also be detected in chylous urine or in hydrocele fluid.

  

Morphology of Wuchereria bancrofti

• Long, Thread-like Appearance: Adult worms have a long, hair-like, transparent, and translucent appearance, resembling thread-like structures with smooth cuticles and tapering ends.
• Filiform and Cylindrical: Their bodies are filiform and cylindrical, with both ends tapering.
• Sexual Dimorphism: Male and female worms are separate and exhibit sexual dimorphism. Females are larger (70–100 × 0.25 mm) compared to males (25–40 × 0.1 mm).
• Distinguishing Characteristics: The posterior end of the female worm is narrow and pointed, bearing an anus. In contrast, the male's posterior end is curved vertically and contains multiple genital papillae and two copulatory spicules of unequal length.
• Coiling Together: Males and females usually coil together in the abdominal and inguinal lymphatics and in the testicular tissues.
• Enlarged Head: The head end is slightly enlarged, and their mouth aperture is simple, lacking lips.
• Digestive System: The pharynx is divisible into an anterior muscular portion and a posterior glandular portion. There is no esophageal bulb, and the intestine is simple.
• Reproductive Features: The vulva of the female is ventrally located in the pharyngeal region and is equipped with a pyriform ejector mechanism. The female worm is viviparous and releases sheathed microfilariae directly into the lymph.
• Longevity: Adult worms have a long lifespan, estimated to be around 10–15 years or more.

2. Microfilaria

• First Stage Larva: Microfilariae represent the first stage larva of Wuchereria bancrofti.
• Colorless and Transparent: They are colorless, transparent, and have blunt anterior ends and pointed tails.
• Active Mobility: Microfilariae are very active and can move forward and backward within a sheath that is much longer than the embryo.
• Microscopic Size: They are microscopic, measuring about 250–300 μm in length and 6–10 μm in thickness.
• Structural Features: Microfilariae have a body covered with a hyaline sheath followed by a cuticle with distinct striations. Somatic cells or nuclei appear as granules in the central axis of the body, except for the terminal end. Their head-end has a clear space called the cephalic space.
• Structural Elements: Important structures include the future mouth or oral stylet, nerve ring (an oblique area devoid of granules), nephridiopore, renette cell, dark-colored inner mass, and four cells of the future anus.
• Development Requirement: Microfilariae do not undergo further development within the human body unless they are ingested by a suitable host, such as mosquitoes.
• Lifespan: Their lifespan within the human body is estimated to be around 70 days.

Periodicity of Microfilariae

1. Nocturnal Periodicity in India and Asian Countries

   • Nighttime Activity: In regions like India, China, and several Asian countries, microfilariae show a nocturnal periodicity in peripheral circulation. They are primarily visible in large numbers in the peripheral blood only at night, typically between 10 pm and 4 am.
   • Daytime Retreat: During the rest of the day, these microfilariae tend to retreat to deeper blood vessels. It is believed that this behavior is correlated with the night-biting habit of the mosquito vector, specifically, female mosquitoes from the Culex, Aedes, or Anopheles genera.

2. Nonperiodic or Diurnal Subperiodic in Pacific Islands and Malaysian Archipelago

   • Continuous Presence: In contrast, in the Pacific islands and certain parts of the Malaysian archipelago, microfilariae exhibit a nonperiodic or diurnal subperiodic pattern. This means they occur in peripheral circulation at all times, with a slight peak during the late afternoon or evening.
   • Day-Biting Mosquitoes: This continuous presence of microfilariae is related to the day-biting habits of the local vector mosquitoes in these regions.

Third Stage of the Larva (Infective Form)

• Elongated and Filariform: The third-stage larva is elongated, filariform, measuring approximately 1.5 mm in length and 18-23 µm in diameter.

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Life Cycle of Wuchereria bancrofti

• Definitive Host: The definitive host is humans. There is no known animal host or reservoir for W. bancrofti.
• Intermediate Host: Female mosquitoes of the Culex, Aedes, and Anopheles genera serve as the intermediate hosts.
• Infective Form: The third-stage filariform larva is the infective form and is only found in mosquitoes.

Life Cycle Details

• Copulation occurs when both male and female worms are present in the same lymph gland.

• Female worms are viviparous or ovoviviparous, releasing numerous larvae known as microfilariae into the bloodstream.
• Microfilariae are born in an immature stage but enter the lymph vessels and then blood vessels.
• They do not undergo further development until they are ingested by a mosquito, typically a night-biting mosquito, in the case of India and Asian countries.

• Microfilariae are ingested by mosquitoes.
• They lose their sheath in the mosquito's stomach, penetrate the stomach wall, and migrate to thoracic muscles, where they undergo metamorphosis.
• Over several days, they develop into first-stage larvae (L1), followed by second-stage larvae (L2), which grow rapidly.
• Finally, within 10-11 days, they reach the third-stage filariform larvae (L3), which are actively motile and infective.

• When an infected mosquito with L3 larvae pierces its proboscis into human skin, the larvae crawl out and penetrate the skin, settling into lymphatics.
• In the lymph vessels, they mature into fully adult and sexually mature worms over a period of 5 to 18 months.
• These mature worms start reproduction, continuing the life cycle.