WUCHERERIA BANCROFTI

 

2. WUCHERERIA BANCROFTI

1. Morphology

• Wuchereria bancrofti is a filarial nematode responsible for lymphatic filariasis (elephantiasis).
• Adult worms:
• Slender, thread-like, and white.
• Males: 2.5–4 cm in length; Females: 8–10 cm in length.
• They reside in the lymphatic vessels and lymph nodes.
• Microfilariae:
• Present in the peripheral blood.
• They are sheathed (a thin covering around the larva).
• The tail is pointed, with no nuclei at the tip (useful for microscopic identification).
• Length: ~245–300 µm.

2. Life Cycle

Phase 1: Infection of Humans (Definitive Host)

1. Mosquito Bite and Larval Entry (L3 Stage):
• During a blood meal, an infected mosquito (Culex, Aedes, or Anopheles) deposits L3 larvae on the skin of a human host.
• The larvae penetrate the bite wound and enter the human bloodstream or lymphatic system.
2. Migration and Maturation in Lymphatics:
• The L3 larvae migrate to lymphatic vessels and lymph nodes, especially in the lower extremities (e.g., legs, groin, scrotum, breasts).
• Inside the lymphatic system, the larvae molt and mature into L4 larvae, and eventually into adult worms.
• The process of larval development into mature adults takes approximately 6-12 months.
3. Adult Worms Reside in the Lymphatic System:
• Adult worms (both male and female) live within the lymphatic vessels.
• A female worm can live up to 5-7 years, continuously producing microfilariae.
• Adult worms cause blockage of lymphatic vessels, resulting in symptoms like lymphedema and elephantiasis over time.

 

 

4. Production of Microfilariae:
• Once fertilized by males, female worms release microfilariae into the bloodstream.
• These microfilariae are nocturnally periodic, meaning they circulate primarily during the night (10 PM to 2 AM). This aligns with the feeding behavior of mosquitoes that transmit the parasite.

 

Phase 2: Infection of Mosquito (Intermediate Vector)

1. Ingestion of Microfilariae by Mosquito:
• When a mosquito bites an infected person, it ingests microfilariae present in the peripheral blood.
• Inside the mosquito’s midgut, the microfilariae shed their sheath and penetrate the midgut wall.
2. Development of Microfilariae in the Mosquito:
• Once inside the thoracic muscles of the mosquito, the microfilariae develop through two molts:
• First, they transform into L1 larvae.
• Next, they molt into L2 larvae.
• Finally, they develop into infective L3 larvae (the stage capable of infecting humans).
• This entire development process takes approximately 10-14 days, depending on environmental conditions like temperature and humidity.
3. Migration of L3 Larvae to the Mosquito’s Proboscis:
• The L3 larvae migrate to the proboscis (the mosquito’s feeding apparatus), ready to be transmitted during the next blood meal.

 

Phase 3: Reinfection of Human Host

1. Mosquito Transmits L3 Larvae to Another Human:
• During the next blood meal, the infected mosquito deposits L3 larvae onto the skin of a new host.
• The cycle begins again as the larvae enter the bite wound and travel through the lymphatic system.

 

 

3. Clinical Manifestations

Infection with Wuchereria bancrofti can remain asymptomatic for years, but in chronic cases, it can cause significant disability.

1. Acute Phase:

• Lymphangitis and lymphadenitis (inflammation of lymphatic vessels and nodes).
• Fever, chills, and body aches.
• Swelling and tenderness of lymph nodes (commonly in the groin and armpit).
• Episodes of filarial fever occur intermittently.

2. Chronic Phase:

• Elephantiasis:
• Persistent lymphatic blockage causes massive swelling of limbs, scrotum, breasts, or genitals.
• Skin thickening and hardening due to long-term swelling.
• Hydrocele:
• Fluid accumulation in the scrotum, leading to scrotal swelling.
• Chyluria:
• Presence of lymph in the urine, giving it a milky appearance.

 

4. Laboratory Diagnosis

1. Microscopy:
• Thick blood smear (using Giemsa or Wright’s stain) taken at night (10 PM–2 AM) to detect microfilariae.
• A daytime blood smear may be used if the patient is from an area with non-periodic filariasis.
2. Serology:
• Detection of anti-filarial antibodies or antigens using ELISA or immunochromatographic tests (ICT).
3. Ultrasound:
• Useful to detect adult worms in lymphatic vessels ("filarial dance sign" – characteristic movement of worms in scrotal lymphatics).
4. Urinalysis:
• In cases of chyluria, microscopic examination of urine may reveal lymph or microfilariae.
5. PCR:
• Highly sensitive for detecting filarial DNA in blood.

5. Treatment

Drug Therapy:

1. Diethylcarbamazine (DEC):
• Standard treatment for Wuchereria bancrofti infections.
• Dosage: 6 mg/kg/day for 12 days.
• Effective against both adult worms and microfilariae.
2. Albendazole:
• Often given in combination with DEC to increase efficacy.
• Single dose of 400 mg.

 

3. Ivermectin:
• Used in mass drug administration (MDA) programs.
• Effective in reducing microfilariae in the blood.

Supportive and Surgical Treatment:

• Hydrocele: Can be treated surgically through hydrocelectomy.
• Lymphedema management:
• Elevation of limbs, wearing compression bandages, and skin care to prevent secondary infections.
• Antibiotics (like doxycycline) are sometimes used to target Wolbachia, a symbiotic bacterium essential for the worm’s survival.

 

6. Prevention and Control

1. Mosquito control:
• Use of insecticide-treated bed nets (ITNs) and indoor residual spraying.
• Reducing mosquito breeding sites (stagnant water).
2. Mass Drug Administration (MDA):
• WHO recommends MDA programs using DEC + Albendazole or Ivermectin + Albendazole in endemic areas.
3. Health education:
• Raising awareness about prevention and early diagnosis in high-risk communities.