Study Filarial Worms with clear, structured coverage of the key concepts in Parasitology. Designed for MBChB students preparing for medical examinations.
Introduction Filarial worms reside in the subcutaneous tissues, lymphatic system, or body cavities of humans. Female worms are longer than the males. The male worm has perianal papillae and unequal spicules but lacks a caudal bursa. Female worms are viviparous , giving birth to larvae known as microfilariae . Microfilariae are detected in peripheral blood or cutaneous tissues , depending on the species. Some species have sheathed microfilariae , where the larvae retain their egg membranes. Periodicity of Microfilariae Nocturnal periodicity : Peak circulation at night, e.g., Wuchereria bancrofti . Diurnal periodicity : Peak circulation during the day, e.g., Loa loa . Non-periodic : Microfilariae circulate consistently throughout the day and night, e.g., Onchocerca volvulus . Sub-periodic : Microfilariae are present all day but peak in the late afternoon or night. Life Cycle Definitive host : Humans. Intermediate host : Blood-sucking arthropods (mosquitoes, blackflies, etc.). Microfilariae develop in the arthropod , reaching the infective larval stage. During the arthropod’s next bite, larvae are transmitted to humans. Adult worms live for many years , while microfilariae survive for 3–36 months. Species of Filarial Worms and Their Diseases Wolbachia spp. , an endosymbiotic bacterium in filarial worms, contributes to pathogenesis by inducing inflammatory responses. Life Cycle of Wuchereria bancrofti Development in Mosquito Mosquito ingests microfilariae from an infected human. Microfilariae reach the mosquito’s stomach, shed their sheaths, and migrate to thoracic muscles . They develop into: First-stage larvae (sausage-shaped). Second-stage larvae (after 1 week). Third-stage (L3) filariform larvae (infective form) in another week. L3 larvae migrate to the mosquito’s proboscis , ready for transmission. Development in Humans Mosquito bites a human and deposits L3 larvae near the skin. L3 larvae enter through the puncture wound and migrate to lymphatic vessels . They mature into adults in the lymph nodes, mate, and produce microfilariae . Microfilariae circulate in the peripheral blood , awaiting uptake by another mosquito. The cycle repeats. Prepatent period : 8–12 months (time from infection to detectable microfilariae). Clinical incubation period : 8–16 months (time from infection to symptoms). Pathogenesis Classical Filariasis Blockage of lymphatic vessels by adult worms leads to: Lymphangitis (inflammation of lymphatic vessels). Lymphadenitis (inflamed, swollen lymph nodes). Lymphedema (fluid accumulation in limbs and other tissues). Elephantiasis (severe tissue swelling and fibrosis). Hydrocele (fluid accumulation in the scrotum due to lymphatic obstruction). Lymphoangiovarix (dilation of lymphatic vessels). Chyluria (rupture of lymphatics leading to chyle in urine). Secondary bacterial infections exacerbate symptoms. Occult Filariasis Hypersensitivity reaction to filarial antigens. No microfilariae in blood (destroyed in tissues). Clinical features : Massive eosinophilia (30–80%) . Pulmonary eosinophilia (dry cough, dyspnea, asthma-like symptoms). Hepatosplenomegaly . Immune-mediated complications (arthritis, glomerulonephritis, thrombophlebitis). Tropical Pulmonary Eosinophilia (TPE) :Chronic cough, breathlessness, fever. Chest X-ray: Mottled shadows (similar to miliary TB) . High IgE and filarial antibodies . Responds well to DEC treatment . Diagnosis Demonstration of Microfilariae Specimens : Peripheral blood, chylous urine, lymphatic fluid, hydrocele fluid. Techniques : Thick and thin blood smears (stained with Giemsa, Leishman, or Methylene blue). Knott’s concentration technique : Formalin-centrifugation method. Nucleopore filtration : Blood filtered through microporous membranes. DEC provocation test : Diethylcarbamazine induces microfilariae in blood. Other Diagnostic Methods Ultrasound : Detects filarial dance sign (movement of adult worms in lymphatics). X-ray : Shows calcified worms . Serology : ELISA, IFAT, IHA for filarial antigen detection. Molecular tests : PCR for filarial DNA. Treatment 1. Diethylcarbamazine (DEC) Drug of choice : Kills both microfilariae and adult worms . Dosage : 6 mg/kg/day for 12 days (total 72 mg/kg). Side effects : Allergic Mazzotti reaction due to dying microfilariae. 2. Ivermectin 200 µg/kg dose. Effective against microfilariae but not adult worms . 3. Tetracyclines (Doxycycline) Targets Wolbachia spp. , reducing worm fertility and inflammation. Other Management Surgery : For hydrocele and severe lymphedema. Supportive care : Compression therapy, physiotherapy for lymphedema. Prevention and Control 1. Vector Control Eradication of mosquito breeding sites . Larvicidal measures :Chemical control ( temephos, fenthion ). Mosquito larvicidal oils. Removal of aquatic plants ( Pistia ) to control Mansonia mosquitoes. Adult mosquito control :Use of DDT, pyrethroids, insecticides . Personal protection (mosquito nets, repellents). 2. Mass Drug Administration (MDA) DEC mass t