Onchocerciasis (Onchocerca Volvulus)

Infection with Onchocerca volvulus leads to onchocerciasis or river blindness. Onchocerciasis occurs primarily in West Africa but also Central and East Africa and is the world’s 2nd leading infectious cause of blindness. There are scattered foci in Central and South America. O. volvulus larvae are transmitted to humans by way of the bite of Simulium black flies that breed in fast-flowing streams. The larvae penetrate the skin and migrate through the connective tissue and eventually develop into adult worms that can be found tangled in fibrous tissue. Adult worms can live in the human body for up to 14 yr. Female worms produce large numbers of microfilariae that migrate through the skin, connective tissue, and eye. Most infected individuals are asymptomatic. In heavily infected subjects, clinical manifestations are due to localized host inflammatory reactions to dead or dying microfilariae. These reactions produce pruritic dermatitis, punctate keratitis, corneal pannus formation, and chorioretinitis. Adult worms in subcutaneous nodules are not painful and tend to occur over bony prominences of the hip. The diagnosis can be established by obtaining snips of skin covering the scapulae, iliac crests, buttocks, or calves. The snips are immersed in saline for several hours and examined microscopically for microfilariae that have emerged into the fluid. The diagnosis can also be established by demonstrating microfilariae in the cornea or anterior chamber on slit-lamp examination or finding adult worms on a nodule biopsy specimen. Ophthalmology consultation should be obtained before treatment of eye lesions. A single dose of ivermectin (150 µg/kg PO) is the drug of choice and clears microfilariae from the skin for several months but has no effect on the adult worm. Treatment with ivermectin should be repeated at 3-6 mo intervals if there are continuing symptoms or evidence of eye infection. Adverse effects of ivermectin therapy include fever, urticaria, and pruritus and are more frequent in individuals not born in endemic areas who acquired the infection following periods of intense exposure, such as Peace Corps volunteers. Patients with concurrent loiasis may develop encephalopathy with ivermectin therapy. Doxycycline, which kills endosymbiont bacteria (Wolbachia) of O. volvulus, may contribute to amicrofilaremia. Personal protection includes avoiding areas where biting flies are numerous, wearing protective clothing, and using insect repellent. Vector control and mass ivermectin distribution programs have been implemented in Africa in a successful effort to reduce the prevalence of onchocerciasis.

Loiasis (LOA LOA)

Loiasis is caused by infection with the tissue nematode Loa loa. The parasite is transmitted to humans via diurnally biting flies (Chrysops) that live in the rain forests of West and Central Africa. Migration of adult worms through skin, subcutaneous tissue, and subconjunctival area can lead to transient episodes of pruritus, erythema, localized edema known as Calabar swellings, which are nonerythematous areas of subcutaneous edema 10-20 cm in diameter typically found around joints such as the wrist or the knee (Fig. 289-1), or eye pain. They resolve over several days to weeks and may recur at the same or different sites. Although lifelong residents of endemic regions may have microfilaremia and eosinophilia, these individuals are often asymptomatic. In contrast, travelers to endemic regions may have a hyperreactive response to L. loa infection characterized by frequent recurrences of swelling, high-level eosinophilia, debilitation, and serious complications such as glomerulonephritis and encephalitis. Diagnosis is usually established on clinical grounds, often assisted by the infected individual reporting a worm being seen crossing the conjunctivae. Microfilariae may be detected in blood smears collected between 10 AM and 2 PM. Adult worms should be surgically excised when possible. Diethylcarbamazine is the agent of choice for eradication of microfilaremia, but the drug does not kill adult worms. Because treatment-associated complications such as pruritus, fever, generalized body pain, hypertension, and even death may occur, especially with high microfilarial levels, the dose of diethylcarbamazine should be increased gradually (children, 1 mg/kg PO on day 1, 1 mg/kg tid PO on day 2, 1-2 mg/kg tid PO on day 3, 6 mg/kg/day divided tid PO on days 4-21; adults, 50 mg PO on day 1, 50 mg tid PO on day 2, 100 mg tid PO on day 3, 6 mg/kg/day divided tid PO on days 4-21). Full doses can be instituted on day 1 in persons without microfilaremia. Individuals concurrently infected with O. volvulus are at increased risk for developing encephalopathy with ivermectin treatment. A single dose of ivermectin (150 µg/kg) decreases microfilarial densities in the blood in persons with high-density microfilaremia. A 3 wk course of albendazole can also be used to slowly reduce microfilarial levels as a result of embryotoxic effects on the adult worms. Antihistamines or corticosteroids may be used to limit allergic reactions secondary to killing of microfilariae. Personal protective measures include avoiding areas where biting flies are present, wearing protective clothing, and using insect repellents. Diethylcarbamazine (300 mg PO once weekly) prevents infection in travelers who spend prolonged periods of time in endemic areas. L. loa do not harbor Wolbachia endosymbionts, and therefore doxycycline has no effect on infection.

Figure 289-1 Calabar swelling of the right hand.

(From Guerrant RL, Walker DH, Weller PF, et al: Tropical infectious diseases, Philadelphia, 1999, Churchill Livingstone, p 863.)

Infection with Animal Filariae

The most commonly recognized zoonotic filarial infections are caused by members of the genus Dirofilaria. The worms are introduced into humans by the bites of mosquitoes containing 3rd stage larvae. The most common filarial zoonosis in the USA is Dirofilaria tenuis, a parasite of raccoons. In Europe, Africa, and Southeast Asia, infections are most commonly caused by the dog parasite Dirofilaria repens. The dog heartworm, Dirofilaria immitis, is the 2nd most commonly encountered filarial zoonosis worldwide. Other genera, including Dipetalonema-like worms, Onchocerca, and Brugia, are rare causes of zoonotic filarial infections.

Animal filariae do not undergo normal development in the human host. The clinical manifestations and pathologic findings correspond to the anatomic site of infection and can be categorized into 4 major groups: subcutaneous, lung, eye, and lymphatic. Pathologic examination of affected tissue reveals a localized foreign body reaction around a dead or dying parasite. The lesion consists of granulomas with eosinophils, neutrophils, and tissue necrosis. D. tenuis