Laboratory Diagnosis

Pruritus is a common and complex problem in the tropics, and may be the result of many infectious and noninfectious etiologies. Skin snips and scrapings are commonly obtained whenever infection is considered. Skin snips are best taken from the upper body in the South American tropics while iliac crest and lower limb skin snips are preferred sites in Africa. A razor blade and pin can be used to shave an elevated mount of skin or a corneascleral punch can be used to obtain a consistent sample volume (which is often bloodless). The biopsy is immersed in isotonic saline, incubated, and examined microscopically. In 1-2 hours, unsheathed microfilariae begin to emerge and can be seen wriggling in fluid under low-power magnification. Giemsa or hematoxylin stains can then be employed to differentiate microfilaria of O. volvulus from Mansonella streptocerca. The latter disease occurs in West Africa and has some similar clinical manifestations in the skin to onchocerciasis. Other biopsy specimens may reveal adult or larval stages in lymph nodes or nodules.

Microfilariae may be easily seen in the cornea or the anterior chamber of the eye, using a slit-lamp. Administration of a low dose of diethylcarbamazine has in the past been used as a provocative test for the diagnosis of onchocerciasis. Positive tests are referred to as a positive Mazzotti reaction. Infected persons develop cutaneous edema and pruritus, with or without a maculopapular rash. Systemic features, such as fever and arthralgia, may be provoked. Ocular disease may also be exacerbated to the point where vision is further impaired, especially in heavily infected people. The Mazzotti test is therefore no longer recommended for diagnosis.

Clinical Characteristics

Early or minor infection can be asymptomatic, but onchocerciasis can be a serious infection. The blackfly bites are painful and may bleed. Multiple bites in the nonimmune result in urticaria and itching. The dermatitis is due to the degenerating microfilaria, usually over the upper half of the body. These lesions can be secondarily infected. Skin pigmentation may result in chronic lesions with a spotty distribution over the legs (leopard spots), going on to skin thickening (peau d'orange). The skin becomes very thin and progresses to severe wrinkling; the end result is always fibrosis.

In hyperimmune patients the itching can be intense with swelling, hyperpigmentation, a diffuse papular eruption, and regional lymphadenopathy. Usually only one limb is affected, but the symptoms and signs can be more widespread. This is known as Sowda (the Arabic for black) in Saudi Arabia, Yemen, and parts of Africa.

A typical lesion is an onchocercoma, which is a subcutaneous nodule containing adult worms (Figs. 26.22, 26.23). These nodules can be small or several centimetres across, and are flattened, firm, mobile, and often tender. They may be present in groups, forming a mass. They can occur all over the body but are most frequent over bony prominences, e.g., the elbows, iliac crests, sacrum, knees, and skull. They may also lie deep in muscles, usually near joints. In Central America the nodules are most common over the head and the upper body, while in Africa they are most often found around the pelvis and lower body. The reaction and the size of the nodules depend on their age and the patient's immunity.

Lymph node obstruction from migrating microfilariae results in nontender lymphadenopathy, lymphedema, and protrusion of superficial nodes in pendulous skin. This may form a pouch, often quite large, which may be unilateral or bilateral and arise from inguinal or femoral lymph nodes or both. This is the characteristic adenolymphocele, called a "hanging groin," which is typically associated with onchocerciasis. Genital elephantiasis can be seen and may be confused with Wuchereria or Brugia spp. infections.

Eye lesions are due to penetration by microfilariae and involve both the anterior and posterior segments. Ocular migration into the anterior segment and the associated inflammation leads first to punctate keratitis. This is more common in younger people and is reversible. As the disease becomes chronic, over years there is progressive corneal inflammation leading to sclerosing keratitis and iridocyclitis which ultimately results in blindness. In the posterior segment, chorioretinitis and optic nerve atrophy may also cause blindness. Of particular note is that patients with a high microfilarial load treated with diethylcarbamazine will likely show an exacerbation of ocular pathology due to heightened inflammation around dying larvae throughout the globe. In Central and South America, nodulectomy programs have been successful in reducing the prevalence of ocular disease (Fig. 26.23 A).

Because microfilariae cause pathogenic lesions everywhere they migrate, the kidney may suffer from acute diffuse glomerulonephritis and there may be microabscesses in the lungs and inflamed sinusoids in the liver. The spleen, nerves, and other organs may also be damaged. Eventually many patients, including especially the immunocompromised, are severely weakened and wasted, becoming walking skeletons; a high percentage of these patients will be blind as well.