What is Filariasis?
Filariasis is a parasitic infection caused by thread-like nematode worms transmitted through insect bites, primarily affecting the lymphatic system and skin, with the potential to cause severe chronic disability including lymphedema (elephantiasis), blindness, and significant social stigma. 1, 2
Types of Filariasis
Filariasis is classified according to where the parasites reside in the human host:
Lymphatic Filariasis
- Caused by three species: Wuchereria bancrofti, Brugia malayi, and Brugia timori 3, 1
- W. bancrofti accounts for 90% of cases, while B. malayi causes the remaining 10% 4
- Transmitted by mosquito bites, including Aedes, Anopheles, and Culex species 3
- Currently affects an estimated 67.88 million people globally, including 36.45 million microfilaria carriers, 19.43 million hydrocele cases, and 16.68 million lymphedema cases 1
- Endemic in parts of Africa, West Pacific, Caribbean, with sporadic cases in South America, India, and Southeast Asia 3
Onchocerciasis (River Blindness)
- Caused by Onchocerca volvulus 3
- Transmitted through the bite of Simulium black flies 3
- Endemic near fast-flowing rivers in Africa south of the Sahara, Yemen, and parts of South America (Brazil, Venezuela, Bolivia) 3
- Ninety-nine percent of the global burden is in Africa 3
Other Filarial Infections
- Loa loa (African eye worm) - transmitted by deer flies 5
- Multiple filarial species can co-exist in the same geographic areas and even the same individuals 3
Clinical Manifestations
Lymphatic Filariasis
- Asymptomatic phase: Most infections are initially asymptomatic but cause progressive lymphatic damage nevertheless 3
- Acute phase: Fever with localized skin inflammation, lymphadenitis, and lymphangitis 3
- Chronic phase: Lymphedema (previously called elephantiasis) affecting arms, legs, and genitals; scrotal edema and hydrocele 3, 1
- Tropical pulmonary eosinophilia: Non-immune travelers may present with fever, dry cough, wheeze, and respiratory symptoms due to hypersensitivity reaction 3
- Incubation period is highly variable, ranging from 4 weeks to 16 months 3
- Prepatent period (time to microfilariae appearance): 7-8 months for W. bancrofti, 2 months for B. malayi 3
Onchocerciasis
- Skin manifestations: Diffuse dermatitis with severe pruritus and excoriation leading to hypo/hyperpigmentation, lichenification, and depigmentation 3
- Subcutaneous nodules (onchocercomas) on bony prominences, head, and trunk 3
- Ocular disease: Keratitis, uveitis, and choroidoretinitis leading to blindness ("river blindness") 3
- Travelers typically present with mild to intense pruritus and limb swelling only 3
- Incubation period: 8-20 months 3
Pathogenesis
- Adult filarial worms reside in lymphatics and lymph nodes, causing dilatation and thickening of lymphatic vessel walls 1
- Progressive lymphatic damage results from tissue alterations induced by both living and dead adult parasites 1
- Disease pathogenesis is linked to host inflammation invoked by parasite death 2
- Most filarial species co-exist with Wolbachia bacteria in mutualistic symbiosis, which are essential for parasite growth and contribute to inflammatory disease pathogenesis 2
Diagnosis
Lymphatic Filariasis
- Serology and nocturnal blood microscopy (10 pm to 2 am) using 4x citrated blood bottles (20 ml total volume, not refrigerated) 3
- Filaria IgG4 antibody tests are highly specific (99% specificity) 6
Onchocerciasis
- Microscopic visualization of microfilariae from skin snips incubated in normal saline (low sensitivity in recent infection) 3
- Slit lamp examination (rarely positive in travelers) 3
- Serology 3
Loa loa
- Daytime blood microscopy (10 am to 2 pm) using citrated blood samples 5
Treatment Principles
Lymphatic Filariasis
- Specialist consultation is advised 3
- Exclude onchocerciasis and loiasis if the patient has traveled to co-endemic areas before treatment 3
- Recommended regimen: Diethylcarbamazine (DEC) 6 mg/kg in 3 divided doses for 14 days PLUS doxycycline 200 mg daily for 6 weeks 3
- Doxycycline targets symbiotic Wolbachia bacteria, delivering macrofilaricidal activity and improving pathological outcomes 2
Onchocerciasis
- Specialist input and urgent ophthalmology assessment advised 3
- Exclude loiasis prior to treatment - seek expert advice in co-infection 3
- Recommended regimen: Doxycycline 200 mg once daily for 6 weeks to target Wolbachia, starting on day one with ivermectin 200 μg/kg monthly for 3 months 3
- Repeat ivermectin every 3-6 months until asymptomatic, then annually if necessary for several years 3
Available Medications
- Diethylcarbamazine (DEC): Effective against microfilariae and adult worms in lymphatic filariasis 2
- Ivermectin: Primarily microfilaricidal, used at 150-200 μg/kg 6, 7
- Albendazole: Used in combination therapy 2
- Doxycycline: Targets Wolbachia endosymbionts, providing macrofilaricidal activity 2
Important Clinical Pitfalls
- Co-infection screening is critical: The same mosquito vectors can transmit both malaria and lymphatic filariasis; multiple parasitic infections frequently cluster in the same communities and individuals 3
- DEC contraindications: Can cause severe reactions including encephalopathy, blindness, hypotension, and erythema in patients with onchocerciasis or high Loa loa microfilarial loads 5
- Prolonged exposure required: Filariasis is rarely diagnosed in short-term travelers, as prolonged stays in endemic areas are usually required to acquire infection 8
- Asymptomatic infections cause damage: Even without symptoms, infections cause progressive lymphatic damage 3
Public Health Context
- Filariasis is classified as a neglected tropical disease affecting millions and producing severe disability and social stigma 8
- Next to psychiatric illness, lymphatic filariasis is the leading cause of permanent and long-term disability globally 4
- Global programs for control and elimination use mass drug administration to interrupt transmission 2
- Individual treatment is effective, but community interventions have helped diminish incidence 8