Onchocerca volvulus

Author: MBLOGSTU

Introduction

Onchocerciasis, also known as river blindness, infects 37 million people living near rivers and fast‐moving streams in Africa. A small number of cases have also been reported in Yemen and the Americas.

Onchocerciasis is the world’s fourth leading cause of preventable blindness. Approximately 500,000 infected individuals are severely visually impaired while another 270,000 have been rendered permanently blind.

Morphology

Adult worms are commonly located in nodules in the subcutaneous connective tissue; however, sometimes they are so deeply situated that they cannot be easily palpated. The living worms appear white, opalescent, and transparent. Although nodules may sometimes contain worms of only one sex, both sexes are usually present and may in fact be numerous.

• Male: 19–42 mm in length by 130–210 μm in diameter.
• Female: 33.5–50 cm in length and 270–400 μm in diameter.

Life Cycle

During a blood meal, an infected blackfly (genus Simulium) releases third-stage filarial larvae into the skin of the human host. These larvae then develop into adult filariae in the subcutaneous tissues. The adults become encased in connective tissue and can later be palpated as nodules. Key points of the life cycle include:

1. The adult worms may live up to 15 years.
2. Females produce microfilariae (their offspring) for about 9 years.
3. Microfilariae measure 220–360 μm in length and 5–9 μm in diameter, and can survive up to 2 years in the skin and lymphatics.
4. During a blood meal, blackflies ingest these microfilariae.
5. Inside the blackfly, microfilariae migrate from the midgut through the hemocoel, invade the thoracic muscles, and develop through a sausage stage to second- and then third-stage infective larvae.
6. The infective third-stage larva migrates to the blackfly’s proboscis awaiting the next blood meal, thereby transmitting the infection to a human host.

Pathogenesis and Clinical Manifestations

General Overview

Onchocerciasis is transmitted by the bite of blackflies of the genus Simulium. It is a cumulative and chronic infection, with clinical features depending on both the exposure period to fly bites and the density of microfilariae (mf) in the skin. This disease results in a broad spectrum of manifestations ranging from dermatological changes to blindness and carries serious socioeconomic consequences.

Mechanism of Disease

The primary mechanism involves the host’s allergic reaction to the adult worms and the microfilariae. Adult worms may exist singly, in pairs, or as coiled masses in the deep fascia or subcutaneous tissue. Often, the worms provoke a fibroblastic response, resulting in a fibrous capsule (nodule or onchocercoma) that is typically seen over bony prominences such as the scalp, scapulae, ribs, elbows, trochanters, iliac crests, sacrum, and knees.

Onchocercomata (Nodules)

Adult worms become encapsulated in nodules, which are most often found over the head, scapular girdle, ribs, pelvic girdle, trochanters, knees, and ankles. Although these nodules can be surgically removed (nodulectomy), studies suggest that many female worms may hide in deeper, non-visible nodules—potentially sustaining a high microfilarial load which continues to contribute to both transmission and disease severity.

Dermatological Lesions (Onchocercal Dermatitis)

Skin involvement is largely linked to the presence of microfilariae. Initially, patients may experience papular eruptions reflecting intra-epithelial abscesses. In regions such as Mexico and Guatemala, an acute inflammatory facial reaction (erisipelas de la costa) has been noted. Chronic cases give rise to skin changes that appear similar to premature ageing, featuring:

• Lichenoid changes
• Hyperkeratosis
• Exaggerated skin wrinkling
• Epidermal atrophy with loose, shiny, depigmented skin

A grading system for recording these cutaneous changes was devised by Murdoch et al. (1993). Moreover, in Africa, a rapid epidemiological assessment based on the number of individuals with pre-tibial depigmentation is sometimes used, though its utility in Latin America is limited by similar skin changes occurring in other conditions like treponemal infections.

Some patients develop an atypical, localized, and asymmetrical dermatitis known as “sowda” — this condition, characterized by itchy, swollen, darkened, scaling papules and regional lymphadenopathy, is seen primarily in Yemenites, and less frequently in patients from Sudan, West Africa, Guatemala, and Ecuador (Schwartz et al., 1983).

Ocular Lesions (Ocular Pathology)

The most serious consequence of onchocerciasis is visual damage. The death of microfilariae in the eyes triggers inflammatory responses leading to ocular pathology. Studies have shown that in regions like Africa, onchocerciasis has caused blindness in up to 270,000 people (with almost 99% of cases occurring there), and epidemiological studies have linked higher microfilarial loads to increased blindness rates. In early or light infections, corneal invasion by a few mf may cause reversible snowflake opacities; however, persistent or intense reactions can lead to chronic inflammation with fibrovascular pannus formation (sclerosing keratitis) and subsequent irreversible visual impairment. Additionally, atrophy of the optic nerve may lead to constricted visual fields, keyhole vision, or total loss of light perception.

Laboratory Diagnosis

A variety of laboratory techniques are employed for diagnosing onchocerciasis:

• Microscopy (Skin Snip): Skin snips taken from sites such as the buttocks, iliac crests, calves (Africa and South America), behind the shoulders and trunk (Mexico), and lower limbs (Yemen) are examined for the presence of microfilariae.
• Serodiagnosis: ELISA and other serological tests are used for cases where microfilariae are not detected in the skin, and these methods are less invasive than skin snips.
• Molecular Methods: PCR can detect parasitic genomes within skin-snip specimens.
• Imaging Techniques: Ultrasound is useful for detecting deep onchocercomas and vitreous changes in the eye.
• Histological Diagnosis: Identification of adult worms in biopsies from subcutaneous nodules confirms the diagnosis.
• Slit-Lamp Examination: Live microfilariae may be observed in the cornea and anterior chamber of the eye.
• DEC (Mazzotti) Patch Test: Topical application of diethylcarbamazine (DEC) on the skin produces a local reaction to dead microfilariae; although specific, this test is less sensitive than the skin-snip examination.

Prevention and Control

To reduce the public health impact of onchocerciasis, both vector control and drug treatment initiatives have been employed:

Vector Control: Ground larviciding has been used in small and accessible river basins to control blackfly populations. For example, although DDT was successfully used in a limited area in Guatemala and in Kenya (eliminating S. neavei in the Kodera valley), efforts in Mexico were abandoned due to labor intensity and modest fly population effects.

Drug Treatment: Mass drug administration has been implemented since 1987.

Treatment

The primary treatments for onchocerciasis include:

• Diethylcarbamazine (DEC)
• Ivermectin