What is Onchocerciasis?
It is a common parasitic infection of the skin and eye. It forms an important constituent of Neglected Tropical Diseases – a group of disorders that persist exclusively in marginalized communities. It is globally the second leading cause of infectious blindness and has very less curative options.
The condition is pronounced as “On-ko-ser-ciasis”.
The disease is also known by other names like:
Picture 1 – Onchocerciasis
- River blindness
- Robles’ disease
The condition usually progresses through various phases with distinct manifestations. These include:
- Erisipela de la costa
- Mal morando
Onchocerciasis Incidence/ Epidemiology
Nearly 17.7 million individuals are infected with this condition worldwide. About 99% of the cases occur in Africa. It is currently endemic in 30 African countries, Yemen, and isolated parts of South America. Males are more likely to be affected by the disorder than females.
The infection normally begins in the skin and gradually invades other tissues, including the eyes. The symptoms of this condition vary with each stage, but are more alarming in the last phase. These signs include:
Erisipela de la costa
Intense swelling of the face and itching marks the first phase of the infection. The skin undergoes a series of modifications, which differ from region to region. The acute phase is commonly reported in Central and South America.
The dermal layer is prone to severe inflammatory pain, and hyperpigmentation in which the infected area of skin darkens due to increased production of melanin.
At this stage, the dermatological condition takes a chronic turn and becomes hyperactive. It is more often described as a localized type of River Blindness.
For better understanding, medical specialists have formulated a grading system that describes the systemic changes in the skin of individuals affected with the condition. The following stages are:
Acute papular onchodermatitis
Small, raised, solid lesions called papules are scattered all over the skin and produce pruritic sensations.
Chronic popular onchodermatitis
The papules grow large in size and cause hyperpigmentation of the skin.
The skin undergoes abnormal thickening and appear quite dark due to the hyperpigmented papules. The inflamed skin is covered with silvery scales called plaque. Secondary bacterial infections invade the skin, resulting in Lymphadenopathy, which is marked by swollen lymph nodes. The dry and leathery look is similar to the appearance of the skin of an elephant.
Skin atrophy or Lizard skin
The infectious disorder could be associated with dry, scaly, wrinkled and depigmented skin, which depicts loss of elasticity.
Depigmentation or Leopard skin
Small pigmented spots called macules with clearly-defined edge surrounded by normal-appearing skin characterizes the advance stages of the infection. This unusual appearance of the skin can be attributed to hyperpigmentation of the skin in a hypopigmented background.
After infecting the skin, the causative microorganisms attack several parts of the eye such as conjunctiva, cornea and the inner layer called uvea. The infection may even spread to the retina and optic nerve. Keratitis occurs when the cornea or the front part of the eye becomes swollen, red and itchy. Inflammation of the superficial layers does not leave a scar, unlike deep keratitis that involves the deeper areas of the cornea. In the case of chronic infection, the scarring slowly progresses towards visual impairment and subsequently causes blindness.
Several tropical diseases are caused by filarial worms and their larvae. These are usually thread-like parasitic roundworms that are known to give rise to a host of infections. Onchocerca volvulus is one such filarial worm, responsible for causing river blindness. The nematode shares an endosymbiotic relationship with Wolbachia pipientis, a parasitic bacterium that infects many arthropod species, including roundworms. These parasitic worms live inside the filarial nematode and receive benefits by deriving nutrients at the expense of the host. Wolbachia plays an essential role in the development of the disorder. Interestingly, the bacteria increase the pathogenecity of the worms by triggering an immune response in patients. The filarial worms encounter death when these microbes are removed from their body. The transmission of the disorder however, happens through the bite of an infected black fly of the genus Simulium. The black flies are predominantly found in fertile riverside areas and breeds largely near the fast-flowing streams or rivers.
Onchocerciasis Life Cycle
The microfilaria is an early stage in the life cycle of Onchocerca volvulus. Innumerable microfilariae are present in the bloodstream of a human infected with this parasitic disorder. A female black fly feeds on the blood of an infected human host and ingests microfilaria. The microfilariae enter the muscles of the gut and thoracic of the fly, and this marks the first larval stage denoted as “J1”. Over a short period of time, the larvae enter the second larval stage called J2. In the third larval stage (J3), the larvae migrate to the mouth parts of the fly and ultimately move into the saliva where they undergo maturation for 7 days. At this stage, if the infected black fly takes another blood meal then the mature larvae are passed into the human’s blood. The larvae now enter the subcutaneous tissue and exhibit two stages of molting. They form nodules and begin to mature into adult worms in a span of 6 to 12 months. The adult male worms then begin to mate with the female worms in the subcutaneous tissue to produce a large number of microfilariae per day. The microfilariae migrate to the skin, which is the most accessible site for the female black flies to have a blood meal. In this way, the black flies again ingest a new set of microfilaria to restart the cycle. In due course of time, these immature worms inhabit the eyes, causing visual loss. The symptoms occur when theses larvae die inside the skin or eyes.
Onchocerciasis in horses
Onchocerca cervicalis is another species of Onchocerca that causes the infection in horses. The condition occurs when the animals show an allergic response to the dying microfilariae. In addition to the skin, the larvae also affect the face, eyelids, chest, abdomen, groin and legs of animals.
Mild infections are often difficult to diagnose, owing to the lack of visible symptoms. Individuals who reside in areas where river blindness is endemic are normally the suspected candidates for this condition. Individuals who travel to such infectious zones could also be at a potential risk. The most commonly used diagnostic techniques for this disorder involve the following:
This is a part of a physical examination of the skin and eyes. A healthcare practitioner can feel the subcutaneous nodules with his hands in this simple technique. However, the method does not always provide a concrete proof of the condition due to which other techniques are required to confirm it.
Slit lamp exam
In this procedure, an instrument that provides a magnified, three-dimensional view of the various parts of the eye is used. The anterior region of the eye, consisting of the cornea, lens, iris and the vitreous gel are examined for the presence of free floating intraocular microfilaria. In case of severe infections, the deeper structures of the eye (such as the optic nerve, retina and drainage gel) can be viewed with special lenses to check for further damage.
A small portion of the infected skin is surgically removed and placed in a saline solution, or incubated for 4 hours in a cultured medium. The microfilariae emerge from the skin snip and float in the saline or grow rapidly on the medium. Microscopic examination of these larvae helps in the estimation of the infection.
An oral dose of diethylcarbamazine (DEC) is administered to patients for inhibiting the neuromuscular transmission in nematodes. Affected individuals begin to display acute skin rash and experience itching within few hours. The dermatologic presentations are attributed to the death of the microfilariae in the skin. The diagnostic method is, however, associated with several side effects like severe systemic reactions and ocular complications.
The antibodies, which are produced against the nematodes in the blood, can be detected using tests like ELISA, polymerase chain reaction and rapid-format antibody card. These techniques require only a small sample of a patient’s blood from a simple finger prick in order to carry out the pathological examination.
The goal of medical therapy is to eliminate the microfilarial stage of the condition to improve the symptoms and prevent further progression of the infection. A broad-spectrum anti-parasitic drug called ivermectin is normally given to the infected patients to paralyze and kill the larvae responsible for causing the symptoms. Generally, two doses of the medicine are advisable for effective results. The frequent bouts of intense itching and visual problems associated with the syndrome can be resolved to a great extent with the help of this drug. Doxycycline, a member of tetracycline antibiotic can kill the Wolbachia bacteria, living inside the adult worms. Surgical removal of the worms through the process of nodulectomy can aid in the elimination of nodules and reduction of microfilarial load in the body.
Presently, there are no vaccines or medications available that could help in preventing this condition. Individuals can however, avoid the condition by taking some personal protection measures such as:
Picture 2 – Onchocerciasis Image
- Applying insect repellant like N, N-Diethyl-meta-toluamide (DEET)
- Wearing long pants and sleeves during the day to avoid black fly bite
- Treating the clothes with permlethrin that kills black flies on contact
The condition was introduced in several regions of South America and the Arabian Peninsula from Africa through slave trade. The microfilariae were first observed by an Irish naval surgeon named John O’Neill in 1875 during an examination of a skin sample of an affected patient in Ghana. In 1890, a Scottish medical scientist named Patrick Manson discovered and identified the adult worms.
Personal efforts can prevent the transmission of Onchocerciasis. Many public healthcare initiatives have been taken by the governments of these tropical countries to control the spread of the disease. Aerial application of insecticides and prompt administration of ivermectin can certainly benefit the mass treatment approach in such regions.