Filarial worms, which infect >170 million people worldwide, are nematodes that dwell in the SC tissue and lymphatics. Usually, infection is established only with repeated and prolonged exposures to infective larvae; however, filarial disease is characteristically more intense and acute in newly exposed individuals than in natives of endemic areas.
- Filarial parasites have a complex life cycle, including infective larval stages carried by insects and adult worms that reside in humans.
- - The offspring of adults are microfilariae (200-250 µm long, 5-7 µm wide) that either circulate in the blood or migrate through the skin.
- - Microfilariae are ingested by the arthropod vector and develop over 1-2 weeks into new infective larvae.
- A bacterial endosymbiont, Wolbachia, is found in all stages of Brugia, Wuchereria, Mansonella, and Onchocerca spp. and has become a target for antifilarial chemotherapy.
Lymphatic Filariasis
Microbiology Lymphatic filariasis is caused by Wuchereria bancrofti (most commonly), Brugia malayi, or B. timori, which can reside in and cause inflammatory damage to lymphatic channels or lymph nodes.
Clinical Manifestations Subclinical microfilaremia, hydrocele, acute adenolymphangitis (ADL), and chronic lymphatic disease are the main clinical presentations.
- ADL is associated with high fever, lymphatic inflammation, and transient local edema. Both the upper and lower extremities can be involved in both bancroftian and brugian filariasis, but W. bancrofti almost exclusively affects genital lymphatics.
- ADL may progress to more chronic lymphatic obstruction and elephantiasis with brawny edema, thickening of the SC tissues, and hyperkeratosis. Superinfection is a problem.
Diagnosis Detection of the parasite is difficult, but microfilariae can be found in peripheral blood, hydrocele fluid, and occasionally other body fluids.
- Timing of blood collection is critical and should be based on the periodicity of the microfilariae in the endemic region involved (primarily nocturnal in many regions).
- Two assays are available to detect W. bancrofti circulating antigens, and a PCR has been developed to detect DNA of both W. bancrofti and B. malayi in the blood.
- High-frequency ultrasound (with Doppler techniques) of the scrotum or the female breast can identify motile adult worms.
- The presence of antifilarial antibody supports the diagnosis, but cross-reactivity with other helminthic infections makes interpretation of this finding difficult.
Treatment: Lymphatic Filariasis - Pts with active lymphatic filariasis (defined by microfilaremia, antigen positivity, or adult worms on ultrasound) should be treated with diethylcarbamazine (DEC, 6 mg/kg daily for 12 days), which has macro- and microfilaricidal properties. Albendazole (400 mg bid for 21 days), albendazole and DEC both given daily for 7 days, doxycycline (100 mg bid for 4-6 weeks), and the addition of DEC to a 3-week course of doxycycline are alternative regimens with macrofilaricidal efficacy.
- A single dose of albendazole (400 mg) combined with DEC (6 mg/kg) or ivermectin (200 µg/kg) has sustained microfilaricidal activity and is used in lymphatic filariasis eradication campaigns.
- For pts with chronic lymphatic filariasis, treatment regimens should focus on hygiene, prevention of secondary bacterial infections, and physiotherapy. Drug treatment should be reserved for individuals with evidence of active infection, although a 6-week course of doxycycline improves filarial lymphedema irrespective of disease activity.
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Onchocerciasis
Microbiology and Epidemiology Onchocerciasis (“river blindness”) is caused by Onchocerca volvulus, which infects 37 million people worldwide and is transmitted by the bite of an infected blackfly near free-flowing rivers and streams.
- Larvae deposited by the blackfly develop into adult worms (females and males are ~40-60 cm and ~3-6 cm in length, respectively) that are found in SC nodules (onchocercomata). About 7 months to 3 years after infection, the gravid female releases microfilariae that migrate out of the nodules and concentrate in the dermis.
- In contrast to lymphatic filariasis, onchocerciasis is characterized by microfilarial induction of inflammation.
Clinical Manifestations Onchocerciasis most commonly presents as dermatologic manifestations (an intensely pruritic papular rash or firm nontender onchocercomata), but visual impairment is the most serious complication in pts with moderate or heavy infections.
- Conjunctivitis with photophobia is an early ocular finding.
- Sclerosing keratitis (the leading cause of onchocercal blindness in Africa, affecting 1-5% of pts), anterior uveitis, iridocyclitis, and secondary glaucoma due to pupillary deformities are more serious ocular complications.
Diagnosis A definitive diagnosis is based on the finding of an adult worm in an excised nodule or of microfilariae in a skin snip.
- Specific antibody assays and PCR to detect onchocercal DNA are available in reference laboratories.
- Eosinophilia and elevated serum IgE levels are common but nonspecific.
Treatment: Onchocerciasis - Ivermectin (a single dose of 150 µg/kg), given yearly or semiannually, is microfilaricidal and is the mainstay of treatment.
- - In African regions where O. volvulus is coendemic with Loa loa, ivermectin is contraindicated because of the risk of severe posttreatment encephalopathy.
- - Doxycycline therapy for 6 weeks is macrofilaristatic, rendering adult female worms sterile for long periods, and also targets the Wolbachia endosymbiont.
- Nodules on the head should be excised to avoid ocular infection.
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