What is River Blindness (Onchocerciasis)?
Onchocerciasis is a parasitic disease caused by an infection with Onchocerca volvulus that commonly thrives in rivers that the resulting blindness is also referred to as river blindness.Infection with Onchocerca volvulus is endemic in South Africa and was later on reported to be occurring in Yemen and some isolated areas of Central and South America.
Onchocerciasis is transmitted in human through bites from black fireflies commonly breeding in swiftly flowing rivers. The cycle of infection begins with human host infected with prelarval forms called microfilariae. The microfilariae will develop into larva and is then transmitted to another person through bite of black fly carrying the larva.
The transmitted larvae will migrate to the subcutaneous tissue and will undergo two more molts and take the form of a nodule as they mature into adult worms over the period of 6 to 12 months. Male and female worms can reproduce up to a thousand of microfilaria a day. This microfilaria will migrate to the skin during the day where it could be ingested by female fly that feeds at daytime.
Onchocerciasis is regarded as the second-leading infectious disease causing blindness. Prolonged exposure to fly bites and parasite introduction will put one potentially at high risk for the disease. Infection alone will not lead to death but the reduction in immunity of the host is at risk for resistance to other disease that can reduce life expectancy.
Symptoms
Single bite from black fly will not necessarily produce symptoms of onchocerciasis. Prolonged exposure to fly bites will lead to the disease that the prevalence is high to inhabitants near swiftly flowing rivers where the flies breed. Manifestation of infection occurs when microfilariae dies and releases Wolbachia symbionts triggering a host immune system response causing severe itching and can destroy the optical tissue of the eyes.Symptoms of Onchocerciasis are reflective of the developmental stage of the parasite and the degree of immune response of the host. Prepatent phase starts from initial infection until occurrence of symptoms that manifest between 9 months to 24 months from acquisition of infection.
Skin involvement characterized by general pruritus occur in early infection and onchodermatitis or popular rash may also occur which is initially transient in characteristic and becomes chronic over the years. Skin involvement is categorized according to degree and these are:
- Acute papular onchodermatitis described with scattered pruritic papules
- Skin atrophy characterized by loss of elasticity or resembling the lizard skin
- Chronic papular onchodermatitis described as larger papules causing hyperpigmentation
- Depigmentation usually occur in the anterior lower leg resembling leopard skin
- Lichenified onchodermatitis characterized with hyperpigmentation of papules and plaques associated with edema, pruritus and secondary bacterial infections
Lymph nodes become inflamed and swollen including those under the genital area. These nodules containing the adult worm can be seen and felt under the skin.
Skin changes due to onchocerciasis may be described as follows:
- Leopard skin – spotted skin depigmentation similar to that of the leopards
- Elephant skin – thickening of the human skin
- Lizard skin – thickened and wrinkled skin
Ocular onchocerciasis involves any part of the eye such as conjunctive, cornea, uvea, posterior segment, retina and the optic nerve. Early symptoms of ocular involvement are itchiness and redness of the eyes and may also include photophobia. Visual loss and blindness occurs over the years as the scarring progresses.
Inflammatory response of the dying microfilariae and Wolbachia agents results to punctuate keratitis which may result to corneal fibrosis and opacification in advance case.
Causes
Onchocerciasis is caused by Onchocera volvulus infection transmitted by infected Similium flies thriving in the areas surrounded with swiftly flowing river and stream. The spread to humans is carried out by female black flies carrying microfilariae through biting one person to another.A person can get infected through long exposure with black fly bite that people living near or within the areas of breeding ground of black flies are highly at risk of being infected.
Diagnosis
The condition of onchocerciasis may be determined through the following method:Skin-snip biopsy. The test is done by obtaining skin sample from patient to determine the existence of microfilariae.
ELISA test. This is an immunodiagnosis carried out through combination of 3 antigens tried in blood sample of the patient. This test can determine O. volvulus infection.
DEC patch test. This test is based on Mazzoti principle through application of DEC in cream base to elicit localized cutaneous reaction and result is gained within 2 hours.
Slit lamp. This is a method of determining microfilariae in the ocular infection of O. volvulus.
Treatment
The goal of treating onchocerciasis is to eliminate the microfilarial stage of the disease to improve symptoms, eliminate progression of eye lesions and most importantly, to prevent or interrupt transmission or spread of the disease.Ivermectin. This is a drug of choice used for treating onchocerciasis. It is given orally and is well tolerated. The dosage recommended for this treatment is once every six months until the symptoms appeased. Repeated dosage may be recommended for an interval of 3-12 months for the next 10 to 12 years.
Doxycycline. This is an antibiotic drug that can kill the Wolbachia bacteria thriving in adult worms. Significant decrease in the microfilarial loads in the host has been shown with the use of this therapy.
Prevention
Preventing onchocerciasis endemic in a population requires eradication of the fly vector and spread of the disease. Control programs utilized larvicide in eliminating the population of black fly thriving in the fast-moving rivers and streams.Mass treatment of Ivermectin is given regularly for those affected with the disease to treat and control the spread of the disease.
People in the areas with heavy infestation can prevent and avoid the disease through use of insect repellants. Wearing of long sleeve shirts and pants and head nets are encouraged for outdoor activity in areas with infestation.
