eMedicine Specialties > Infectious Diseases > Parasitic Infections

Onchocerciasis

Author: Mary Nettleman, MD, MS, Chair, Department of Medicine, Michigan State University
Coauthor(s): Apoorv Kalra, MD, Assistant Professor of Medicine, Michigan State University
Contributor Information and Disclosures

Updated: Apr 16, 2009

Introduction

Background

Onchocerciasis is an infection caused by the nematode Onchocerca volvulus. Humans acquire onchocerciasis through the bite of Simulium blackflies.1,2,3 Because the fly develops and breeds in flowing water, onchocerciasis is commonly found along rivers and is sometimes referred to as river blindness.

<EM>Simulium</EM> fly (black fly).

Simulium fly (black fly).

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<EM>Simulium</EM> fly (black fly).

Simulium fly (black fly).


In the human host, the adult nematodes live in subcutaneous nodules and produce microfilariae, which are found throughout the body but preferentially reside in the skin and eye. Repeated exposures to infected flies increase the number of adult worms and microfilariae in the host. Chronic cutaneous onchocerciasis (onchodermatitis) causes pruritus, a papular rash, scarring, and lichenification. Over time, affected skin may begin to sag, leading to terms such as "hanging groin." Patchy depigmentation on the legs leads to a condition known as leopard skin. The term sowda is used to describe severe pruritus with darkening of the skin, often confined to one limb. Chronic ocular onchocerciasis may lead to sclerosing keratitis and iridocyclitis, and finally to blindness.

Onchocerciasis is endemic in Africa, Yemen, and in small foci in Central America and South America. The burden of the disease has been reduced by prevention efforts, including control of the fly vector and periodic ivermectin therapy in at-risk individuals. More recently, attention has been focused on Wolbachia organisms, which are endosymbiotic bacteria carried by adult worms and microfilariae. Treatment of Wolbachia infection has been shown to disrupt microfilariae production by the adult female nematode.

Pathophysiology


Simplified life cycle of <EM>Onchocerciasis volvu...

Simplified life cycle of Onchocerciasis volvulus.

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Simplified life cycle of <EM>Onchocerciasis volvu...

Simplified life cycle of Onchocerciasis volvulus.


Frequency

United States

Onchocerciasis is not acquired in the United States. Occasional cases are found in immigrants or travelers from endemic areas.4,5 However, symptomatic onchocerciasis usually requires heavy infestations and repeated exposure to the vector fly. Short-term travelers are at little or no risk of the disease. Pruritus, dermatitis, and eosinophilia may occur in travelers who stay longer than 3 months in endemic areas of Africa. Symptoms may occur months to years after leaving the endemic area.

International

Currently, onchocerciasis is endemic to 30 African countries, Yemen, and in localized foci of 6 Central and South America countries. Globally, approximately 18-36 million individuals have onchocerciasis, 99% of whom reside in Africa. The World Health Organization (WHO) estimates that 750,000 people are blind or have reduced vision as a result of the disease.

Since 1975, the WHO, international foundations, nongovernmental organizations, and governments have worked cooperatively to reduce the burden of onchocerciasis.6,7,8,9,10,11,12,13,14 Initial efforts focused on insecticide sprays and habitat control to reduce the numbers of black fly vectors. With the introduction of effective treatment, the program became focused on periodic treatment of at-risk persons.

Since 1988, ivermectin has been provided free of charge by Merck through the Mectizan Donation Program. By 2002, most affected countries had introduced population-based programs to supply ivermectin at least annually to at-risk individuals. The drug temporarily reduces the microfilarial burden, resulting in reduced morbidity and a reduced number of flies becoming infected when they bite humans. Reports suggest that this has been highly effective in the Americas, where transmission has been interrupted entirely in several areas and ocular disease has been eliminated in 9 of the 13 foci. In Africa, morbidity and transmission have been reduced but not eliminated. This may be due, at least in part, to migration of infected people into new areas, as well as the challenges inherent in educating and motivating large numbers of people.

Despite the challenges they face, control programs have had a significant impact. In Africa alone, an estimated 600,000 cases of blindness had been prevented by 2002 and 18 million children were living in risk-free areas. In 2007, 69 million doses of ivermectin were supplied through the Mectizan Donation Program to reduce the burden of onchocerciasis.

Mortality/Morbidity

  • Onchocerciasis is the second-leading infectious cause of blindness in the world.15
  • Skin disease and subcutaneous nodules can be intensely pruritic.16
  • Long-term onchodermatitis may cause scarring, depigmentation, loss of skin elasticity, and disfigurement.
  • Although not directly fatal, blindness and skin disease caused by onchocerciasis affect the hosts’ ability to assimilate into their societies, perform daily tasks, and care for themselves.17
  • Affected persons often have a low body mass.
  • Blindness alone has been estimated to reduce life expectancy by 4-10 years.
  • In the West African savanna, up to 10% of villagers may be blind from the disease.

Race

Onchocerciasis does not have a racial predilection. For an unclear reason, the symptoms caused by O volvulus infection appear to differ from region to region. For example, onchodermatitis is more common in forested areas, while blindness is more common in savanna areas. Some evidence has suggested that genetic variation in the host may explain part of this geographic specificity.18

Sex

Onchocerciasis does not have a sexual predilection.

Age

Onchocerciasis does not have an age predilection. Children born to mothers with onchocerciasis may be immunotolerant to O volvulus infection, potentially leading to a higher microfilarial burden. Transplacental transmission of microfilariae may occur.

Clinical

History

Symptoms of onchocerciasis reflect the developmental stage of the parasite and the degree of immune response by the host. Clinical manifestations are highly variable.

  • Symptoms of onchocerciasis do not appear until after the L3 larvae mature into adult worms. On average, symptoms appear between 9 months and 2 years after the initial infecting bite. The interval between acquisition of the parasite and onset of symptoms is sometimes referred to as the prepatent phase.
  • Once developed, adult worms cluster in subcutaneous nodules (onchocercomata).
  • Generalized pruritus may occur early in the infection and may be severe. A papular rash known as onchodermatitis may be present. Initially, the rash may be transient, but chronic infection over several years may lead to lichenification, loss of skin elasticity, atrophy, and/or depigmentation.
  • Itchy eyes, redness, or photophobia may be early symptoms of ocular onchocerciasis. Over years, the scarring progresses to cause visual loss and ultimately blindness. Acute optic neuritis is less common but may also cause blindness.
  • Weight loss and generalized myalgias may occur.

Physical

  • Skin examination in patients with onchocerciasis may reveal subcutaneous nodules, diffuse onchodermatitis, lymphedema, and/or atrophic changes.
    • Onchodermatitis consists of raised papules that are intensely pruritic. Vesicles and pustules may also be present. Scratching may cause secondary infection.
    • In its extreme form, skin atrophy may cause drooping of the inguinal skin, termed hanging groin.
    • In some cases, the skin is dry and resembles ichthyosis.
    • Sowda refers to severe pruritus and darkened skin, usually confined to one limb. It is most commonly described in Yemen but also occurs in Africa.
    • Leopard skin refers to bilateral, symmetric, patchy depigmentation of the shins.
    • Lymphadenopathy may occur.
    • Subcutaneous nodules are firm, nontender, and mobile and are several millimeters to centimeters in size. They develop most commonly over bony prominences on the trunk and hip (Africa) or head and shoulders (Americas).
  • In the eye, the inflammatory response to dying microfilariae and Wolbachia antigens causes punctuate keratitis (snowflake opacities). Advanced cases may result in corneal fibrosis or opacification. Slit-lamp examination may reveal microfilariae in the cornea and anterior chamber. Other ocular manifestations include iridocyclitis, glaucoma, chorioiditis, and optic atrophy.19

Causes

  • O volvulus is transmitted by the bite of infected Simulium flies. The fly bites during daylight hours. Simulium flies breed near fast-flowing rivers and streams.
  • Ocular symptoms are caused by the inflammatory response invoked by the release of Wolbachia antigens when microfilariae die.

More on Onchocerciasis

Overview: Onchocerciasis
Differential Diagnoses & Workup: Onchocerciasis
Treatment & Medication: Onchocerciasis
Follow-up: Onchocerciasis
Multimedia: Onchocerciasis
References
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References

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Further Reading

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Keywords

onchocerciasis, onchocercosis, river blindness, volvulosis, craw-craw, Robles disease, Onchocerca volvulus, O volvulus, Simulium fly, blinding disease, papular dermatitis, leopard skin, sowda, sowdah, Nakalaga syndrome, ocular onchocerciasis, ocular onchocercosis, onchodermatitis, onchocercomata

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Contributor Information and Disclosures

Author

Mary Nettleman, MD, MS, Chair, Department of Medicine, Michigan State University
Mary Nettleman, MD, MS is a member of the following medical societies: American College of Physicians, Association of Professors of Medicine, Central Society for Clinical Research, Infectious Diseases Society of America, and Society of General Internal Medicine
Disclosure: Nothing to disclose.

Coauthor(s)

Apoorv Kalra, MD, Assistant Professor of Medicine, Michigan State University
Disclosure: Nothing to disclose.

Medical Editor

Daniel R Lucey, MD, MPH, Chief, Fellowship Program Director, Department of Internal Medicine, Division of Infectious Diseases, Washington Hospital Center; Professor, Department of Internal Medicine, Uniformed Services University of the Health Sciences
Daniel R Lucey, MD, MPH is a member of the following medical societies: Alpha Omega Alpha and American College of Physicians
Disclosure: Nothing to disclose.

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: Nothing to disclose.

Managing Editor

John W King, MD, Professor of Medicine, Section of Infectious Diseases, Louisiana State University Health Sciences Center; Director, Viral Therapeutics Clinics for Hepatitis; Consulting Staff, Department of Infectious Diseases, Overton Brook Veterans Affairs Medical Center
John W King, MD is a member of the following medical societies: American Association for the Advancement of Science, American College of Physicians, American Federation for Medical Research, American Society for Microbiology, Association of Subspecialty Professors, Infectious Diseases Society of America, and Sigma Xi
Disclosure: emedicine $50.00 author of chapter

CME Editor

Eleftherios Mylonakis, MD, Clinical and Research Fellow, Department of Internal Medicine, Division of Infectious Diseases, Massachusetts General Hospital
Eleftherios Mylonakis, MD is a member of the following medical societies: American Association for the Advancement of Science, American College of Physicians, American Society for Microbiology, and Infectious Diseases Society of America
Disclosure: Nothing to disclose.

Chief Editor

Burke A Cunha, MD, Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital
Burke A Cunha, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, and Infectious Diseases Society of America
Disclosure: Nothing to disclose.

 
 
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