Filarial and Related Infections¶
Chapter 240 | Part 5: Infectious Diseases
KEY CLINICAL POINTS¶
- Filarial infections are caused by nematodes (roundworms) with 8 human-pathogenic species, including Wuchereria bancrofti, Brugia malayi, Loa loa, and Onchocerca volvulus.
- Lymphatic filariasis (W. bancrofti/B. malayi) is a major global health issue affecting ~170 million people, transmitted by mosquitoes, with chronic complications like elephantiasis and hydrocele.
- Tropical pulmonary eosinophilia (TPE) is a syndrome caused by microfilariae of Wuchereria bancrofti or Brugia malayi, characterized by eosinophilia and pulmonary symptoms.
- Loiasis (Loa loa) is zoonotic, with microfilariae circulating in blood with diurnal periodicity, and can cause severe encephalopathy with high microfilaremia.
- Treatment options include DEC, albendazole, doxycycline, and ivermectin, with special considerations for pregnancy, drug interactions, and co-endemic regions.
1. DEFINITION & OVERVIEW¶
Filarial infections are caused by nematodes (roundworms) that dwell in subcutaneous tissues and lymphatics. Eight species infect humans, with four (W. bancrofti, B. malayi, L. loa, O. volvulus) causing most symptomatic disease. These infections are transmitted by arthropods (mosquitoes, blackflies, deerflies) and include lymphatic filariasis, onchocerciasis, loiasis, and tropical pulmonary eosinophilia (TPE).
Table 240-1 Characteristics of the Filariae¶
| ORGANISM | PERIODICIT Y | DISTRIBUTI ON | VECTOR | LOCATION OF ADULT | MICROFILAR IAL LOCATION | SHEATH |
|---|---|---|---|---|---|---|
| Wuchereria bancrofti | Nocturnal | Cosmopolitan areas worldwide, including South America, Africa, southern Asia, Papua New Guinea, China, Indonesia | Culex, Anopheles (mosquitoes) | Lymphatic tissue | Blood | + |
| Wuchereria bancrofti | Subperiodic | Eastern Pacific | Aedes (mosquitoes) | Lymphatic tissue | Blood | + |
| Brugia malayi | Nocturnal | Southeast Asia, Indonesia, India | Mansonia, Anophe,les (mosquitoes) | Lymphatic tissue | Blood | + |
| ORGANISM | PERIODICIT Y | DISTRIBUTI ON | VECTOR | LOCATION OF ADULT | MICROFILAR IAL LOCATION | SHEATH |
|---|---|---|---|---|---|---|
| Brugia malayi | Subperiodic | Indonesia, Southeast Asia | Coquillettidia, Mansonia (mosquitoes) | Lymphatic tissue | Blood | + |
| Brugia timori | Nocturnal | Indonesia | Anopheles (mosquitoes) | Lymphatic tissue | Blood | + |
| Loa loa | Diurnal | West and Central Africa | Chrysops (deerflies) | Subcutaneou s tissue | Blood | + |
| Onchocerca volvulus | None | South and Central America, Africa | Simulium (blackflies) | Subcutaneou s tissue | Skin, eye | - |
| Mansonella ozzardi | None | South and Central America | Culicoides (midges) | Undetermined site | Blood | - |
| Mansonella perstans | None | South and Central America, Africa | Culicoides (midges) | Body cavities, mesentery, perirenal tissue | Blood | - |
| Mansonella streptocerca | None | West and Central Africa | Culicoides (midges) | Subcutaneou s tissue | Skin | - |
1.1 Lymphatic Filariasis¶
Caused by W. bancrofti, B. malayi, or B. timori. Adult worms reside in lymphatics, causing chronic inflammation, lymphatic obstruction, and complications like elephantiasis, hydrocele, and scrotal lymphedema.
1.2 Onchocerciasis (River Blindness)¶
Caused by O. volvulus, leading to skin nodules (onchocercomata), ocular damage (blindness), and lymphadenopathy. Transmitted by blackflies.
1.3 Loiasis¶
Caused by L. loa, with microfilariae circulating in blood with diurnal periodicity. Characterized by Calabar swellings, pruritus, and potential severe encephalopathy with high microfilaremia.
1.4 Tropical Pulmonary Eosinophilia (TPE)¶
A syndrome caused by microfilariae of W. bancrofti or B. malayi, presenting with eosinophilia, pulmonary symptoms, and allergic reactions to cleared parasites.
2. EPIDEMIOLOGY¶
Lymphatic filariasis affects ~51 million people globally, with W. bancrofti in tropical/subtropical regions. Onchocerciasis affects ~37 million in 31 countries, primarily in Africa. Loiasis is endemic in West/Central Africa. TPE is reported in India, Pakistan, Brazil, and Southeast Asia. Microfilariae are transmitted via mosquito bites, with periodicity (nocturnal/subperiodic) influencing diagnostic timing.
2.1 Lymphatic Filariasis¶
W. bancrofti: Cosmopolitan (Asia, Africa, Americas). B. malayi: Southeast Asia, India, Indonesia. Subperiodic forms are more common in forested areas; nocturnal in rice fields.
2.2 Onchocerciasis¶
O. volvulus endemic in equatorial Africa (Atlantic to Red Sea) and Venezuela/Brazil. Transmission via blackflies breeding in rivers/streams.
2.3 Loiasis¶
L. loa endemic in West/Central Africa. Microfilariae circulate with diurnal periodicity, peaking 10 AM–2 PM.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Filarial infections are caused by nematodes with complex life cycles. Adult worms reside in lymphatics (lymphatic filariasis) or subcutaneous tissues (onchocerciasis). Microfilariae circulate in blood, with periodicity (nocturnal/subperiodic) influencing diagnostic timing. Wolbachia endosymbionts are present in many species and are targets for antifilarial therapy.
3.1 Lymphatic Filariasis¶
Adult worms cause lymphatic dilation and obstruction. Microfilariae trigger inflammatory granulomas and fibrosis. Wolbachia endosymbionts are critical for adult worm survival.
3.2 Onchocerciasis¶
Microfilariae cause skin inflammation and ocular damage. Adult worms reside in subcutaneous nodules. Wolbachia endosymbionts are targeted by doxycycline.
3.3 Loiasis¶
Microfilariae circulate with diurnal periodicity. Adult worms migrate subcutaneously, causing Calabar swellings. High microfilaremia can lead to severe encephalopathy.
4. CLINICAL FEATURES¶
Lymphatic filariasis presents with subclinical microfilaremia, hydrocele, acute adenolymphangitis (ADL), and chronic elephantiasis. Onchocerciasis causes skin pruritus, nodules, and ocular damage. Loiasis features Calabar swellings, pruritus, and potential encephalopathy. TPE presents with eosinophilia, cough, and pulmonary symptoms.
4.1 Lymphatic Filariasis¶
Subclinical microfilaremia, hydrocele, ADL (fever, lymphangitis), and chronic elephantiasis (brawny edema, scrotal lymphedema).
4.2 Onchocerciasis¶
Skin pruritus, papular rash, onchocercomata, ocular lesions (blindness), and lymphadenopathy.
4.3 Loiasis¶
Calabar swellings, pruritus, skin rashes, and potential encephalopathy with high microfilaremia (>30,000/mL).
4.4 Tropical Pulmonary Eosinophilia (TPE)¶
Eosinophilia, paroxysmal cough, wheezing, and pulmonary infiltrates. Caused by cleared microfilariae triggering allergic reactions.
5. DIFFERENTIAL DIAGNOSIS¶
ADL must be distinguished from thrombophlebitis, infection, and trauma. TPE differentiates from asthma, Löffler’s syndrome, and allergic bronchopulmonary aspergillosis. Onchocerciasis differentiates from other helminthic infections and dermatological conditions.
5.1 Lymphatic Filariasis¶
Distinguish from malignancy-related lymphedema, postoperative scarring, and congenital lymphatic abnormalities.
5.2 Onchocerciasis¶
Differentiate from other helminthic infections, dermatological conditions, and autoimmune diseases.
5.3 Loiasis¶
Differentiate from other filarial infections and allergic reactions.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnosis involves microscopic examination of blood, skin snips, or hydrocele fluid for microfilariae. Antigen detection (W. bancrofti), PCR, and serology (ELISA/lateral flow assays) are used. Imaging (ultrasound, lymphoscintigraphy) helps detect adult worms and assess lymphatic damage.
6.1 Microscopic Examination¶
Stool, blood, or skin snips examined for microfilariae. Knott’s concentration technique improves sensitivity.
6.2 Antigen Detection¶
ELISA and rapid lateral flow assays for W. bancrofti antigens (specificity >98%). No tests available for B. malayi.
6.3 Imaging¶
Ultrasound detects adult worms in lymphatics. Lymphoscintigraphy assesses lymphatic abnormalities.
7. MANAGEMENT & TREATMENT¶
Treatment includes DEC, albendazole, doxycycline, and ivermectin. DEC is first-line for lymphatic filariasis, while ivermectin is used for onchocerciasis and loiasis. Doxycycline targets Wolbachia endosymbionts. Management of complications includes supportive care, antihistamines, and surgical intervention for hydrocele.
7.1 Lymphatic Filariasis¶
DEC (6 mg/kg x12 days), albendazole (400 mg x21 days), or doxycycline (200 mg x6 weeks).
7.2 Onchocerciasis¶
Ivermectin (150 µ g/kg x12 months), with doxycycline for macrofilaricidal effect.
7.3 Loiasis¶
DEC (8–10 mg/kg x21 days), with apheresis and corticosteroids for severe cases.
7.4 Tropical Pulmonary Eosinophilia¶
DEC (4–6 mg/kg x14 days) or albendazole (400 mg x21 days).
8. PROGNOSIS & COMPLICATIONS¶
Lymphatic filariasis can lead to chronic elephantiasis, hydrocele, and secondary infections. Onchocerciasis causes blindness, skin lesions, and lymphadenopathy. Loiasis may result in severe encephalopathy with high microfilaremia. TPE is generally self-limiting but can cause pulmonary fibrosis if untreated.
8.1 Lymphatic Filariasis¶
Chronic complications include elephantiasis, hydrocele, and scrotal lymphedema. Mortality from secondary infections or cardiac failure.
8.2 Onchocerciasis¶
Blindness, skin nodules, and lymphadenopathy. Chronic morbidity and increased mortality in endemic areas.
8.3 Loiasis¶
Severe encephalopathy with high microfilaremia (>30,000/mL).
9. SPECIAL CONSIDERATIONS¶
Ivermectin is contraindicated in pregnancy, breastfeeding, and co-endemic areas with Loa loa. DEC is used for loiasis with caution. Doxycycline is safe in pregnancy for Wolbachia targeting. Zoonotic filariasis (e.g., Dirofilaria) requires vector control and anthelmintic therapy.
9.1 Pregnancy¶
Avoid ivermectin and DEC in pregnancy. Doxycycline is safe for Wolbachia targeting.
9.2 Co-endemic Regions¶
Ivermectin contraindicated in areas with Loa loa due to risk of severe encephalopathy.
10. KEY POINTS & CLINICAL PEARLS¶
- Use DEC, albendazole, or doxycycline for lymphatic filariasis. 2. Ivermectin is first-line for onchocerciasis but contraindicated in Loa loa-endemic areas. 3. TPE is diagnosed by eosinophilia and microfilariae in blood. 4. Loiasis requires careful monitoring for encephalopathy with high microfilaremia. 5. Zoonotic filariasis (e.g., Dirofilaria) is treated with anthelmintics and vector control.