Oncocerciasis
Abstract
Oncocerciasis is a parasitic disease that primarily affects economically disadvantaged communities in Africa and Latin America. It results from infection with filarial nematode Oncocerca volvulus, transmitted to man through the bite of infected black fly of genius simulium. It is the second commonest infectious cause of blindness responsible for an estimated 340,000 cases of blindness and one million cases of visual impairment. The disease is endemic in 30 countries in Africa, six countries in Latin American and in Yemen. More than 85% of the world population is at risk of infection while 18 million are already suffering from the disease. Infection intensity and ocular morbidity including blindness is highest along fast flowing rivers where the vector breeds. Oncocercal blindness has serious social and economic impact. It strikes mainly at economically active adults at the prime of their life thereby leading to reduction in productivity, death and abandonment of fertile agricultural lands. The systemic and ocular manifestations of Oncocerciasis are discussed in this review in order to bring awareness of the importance of this disease in the epidemiology of blindness and visual impairment in South Sudan where it is endemic.
Introduction
Nematodes are unsegmented round worms. They have an elongated cylindrical bodies covered by tough cuticle and a cavity in which the organs lie. Phylum Nematoda includes a great number of species some of which are free leaving and others parasitic. There are seven super families, four of which are of clinical importance to man [fig 1].
The super family Filarioidea is composed of parasites of subcutaneous or connective tissue, lymphatic system or serous cavities. These worms do not lay eggs but give birth to larvae. In order to complete their development the larvae require a second host (vector) in which development has to occur to produce infective form. Man cannot therefore acquire the parasite directly from an infected person. Transmission has to be trough the bite of an infected black fly in which the larvae has to developed. The four species of medical importance, their main vector and distribution are shown in [fig 2]
Fig 2: Species of Filarial Nematodes of Medical Importance
|
Species |
Adult worm |
Microfilariae |
Vector |
Distribution |
|
Wuchereria bancrofti |
Lymphatic |
Blood |
Culex species |
Tropics |
|
Bruglia malayi |
Lymphatic |
Blood |
Mansonia |
South East Asia, India, Sri Lanka |
|
Loa loa |
Subcutaneous |
Blood |
Chrysops species |
West/Central Africa |
|
Oncocerca Volvulus |
Subcutaneous |
Skin / Eyes |
Simulium |
Africa/South America |
Oncocerca volvulus
Oncocerca volvulus is almost exclusively a parasite of man. The adult worm lives encysted in fibrous subcutaneous nodules (fig 3) although some nodules may be so deeply situated as to be impalpable 1. Each nodule contains 1-2 males and 2-3 female worms lying in a twisted tangled mass. The male worms are shorter 3.5cm) while the females are much longer (50-70cm). Female adult worms can live for up to 9-10 years and can produce close to 1600 microfilariae/day, resulting in total microfilaria load of 150 million or more2.
Fig 3: Oncocerca nodule containing mass of worms
Tangled worms in a nodule whose wall has been digested. Note the twisted mass of male and female worms
Parasite life cycle
Microfilaria released by the female worm migrates to skin and subcutaneous tissue where they are picked up by the vector during a blood meal. In the vector they develop after 3 moulds into third stage infective larvae ready to be transmitted during the next blood meal and thereby completing their life cycle. After entering the new host, the infective form moulds twice to become immature adults. These become encysted in fibrous nodules were the male fertilizes the female which then begin producing millions of microfilaria.
The Vector
The main African vector of Oncocerca. Volvulus is Simulium Damnosum. It is made of several cytospecieces. These dominate different environments and many do not feed on man and therefore do not transmit disease. The other important vector is S neavei which occurs mainly in East Africa. The larvae are always attached to bodies of crabs in contrast to s damnosum whose larvae live on rocks and vegetations. One other African specieces, S Albivirgulatum is found in the Republic of Congo Simulium bites by day and can make long wind assisted flights covering several kilometers. Exposure to the bite of simulium depends on several factors including distance of the dwellings from the breeding site of the fly, the age and sex of the victim, occupation and habits of the individual. Boys are exposed to transmission 2 times more than girls while there is no sex difference in the exposure of men and women [fig 4]. Occupation is another factor that may expose a person to intense transmission and high risk of blindness. Fishing or farming activities associated with the river as well as Ferry men and those who have to dig sand along river banks have high chance of transmission. Levels of transmission among expatriate workers living in Tent hotels along the Nile is not currently known
Epidemiology of Oncocerciasis in South Sudan
Nearly all parts of south Sudan have oncocerciasis of varying intensity. High prevalence can be found along the Nile basin and its distributaries. The Bhar El Gazel States, Western and Eastern Equatoria States and some areas of Jongole State have high prevalence. These areas are currently targeted by the oncocerciasis control program (fig 4)
Fig 4: Map of S. Sudan showing CDTI priority areas [red] where there is high prevalence of oncocerciasis.
Clinical Features of Oncocerciasis
There is a wide spectrum of clinical manifestations and marked geographic variation in clinical picture which may be related to pathogen city of Oncocerca volvulus, vector biting habits and host immune response. Signs and symptoms of oncocerciasis can be classified into Dermal, lymphatic, ocular and systemic.
Dermal oncocerciasis
Pruritis is the most common early manifestation of oncocerciasis. It may affect up to 30% of the population in hyperendemic areas. Itching can be so intense as to render the patient sleepless, fatigued and depressed. Many patients suffer from low self esteem and may be socially ostracized as a result of social stigma. Patients may scratch the skin with objects such as sticks and stones leading excoriation and secondary infection. The first visible signs in the skin other than evidence of scratching are an alteration in pigmentation with areas of hyper or hypo pigmentation. After years of chronic infection atrophy of skin develops leading to an appearance called Lizard skin. This is characterized by thin epidermis with shiny fragile surface. The normal dermal structure is replaced by thin un-elastic scar tissue. Another characteristic aspect of oncocercal skin disease is leopard skin, a spotty depigmentation occurring in the anterior aspects of lower extremities. In Africa oncodermatitis is typically generalized, diffuse and maximal on the lower trunk, pelvic girdle, and thighs [fig 5]
Fig 5: Dermal changes in Oncocerciasis
Left to Right: Leopard Skin and Lizard Skin
Adult worms of oncocerca volvulus are found encapsulated in skin nodules which are usually subcutaneous but may also be found in deep layers, near to joint capsules, bones and fasciae and therefore often impalpable. A nodule contains on average 1-2 males and 2-3 females. A typical onco-sarcomata is easy to recognize and differentiate from Lipoma, lymph node, Dermoid cyst, Ganglia or histoplasmosis. They appear as firm round elongated non tender subcutaneous tumor. The size may vary from half a centimeter to 10 cm. Nodules are usually freely mobile but may be fixed to fascia or skin. Distribution of nodules may vary according to regions which may relate to frequency of bites to different parts of the body in different regions and availability of clothing to cover the body. In Africa for instance, people rarely cover their trunks or wear huts hence high frequency of bites occur on the trunk, buttocks, pelvis and legs where most nodules are found [fig 6]. In Central America nodules are found mainly on neck and head and in lower parts of the body in Yemen. Presence of nodules on or near the head has importance significance for ocular involvement and blindness rates. Microscopically a nodule has an outer scar tissue enclosing the adult
worms. Soft tissue composed of granuloma, fibrin, macrophages and ploymorphonuclear neutrophils may surround the worms in some nodules where portions of the worm may lie free.
Fig 6: Skin nodules on trunk, pelvis and abdomen
Ocular manifestations
Ocular manifestations due to oncocerciasis may involve any part of the eye from conjunctiva and cornea to uvea and posterior segment including retina and optic nerve. The first sign of ocular involvement is the inversion of the anterior segment where micorfilariae can be seen swimming freely in the anterior chamber. Conjunctiva reaction with inflammation and chemosis can be seen. The cornea is involved in punctuate Keratitis which appear as snowflake opacities of 0.5mm in diameter representing dead microfilaria surrounded by inflammatory infiltrate. In later stages Sclerosing Keratitis may develop usually as a result of large number of micorfilariae in the cornea. This is a fibrovascular tissue growing initially in the inter palperbral fissure and inferiorly and progressing to reach central cornea, causing
blindness. Anterior Uveitis can lead to posterior synechie especially occurring at 6 o’clock position causing the characteristic pear shaped pupil also called Onco pupil. Uveitis may cause cataract and glaucoma which can contribute to visual loss in oncocerciasis [fig 7]
A wide spectrum of retinal changes may exist. In the early stages of retinal involvement cotton wool sports (soft exudates) may be observed. In advanced cases chorioretinal atrophy may occur associated with retinal pigment clumping. Optic atrophy is usually associated with peripheral visual field loss, key hole vision and total loss of sight. Blindness results from sclerosing Keratitis, iridocyclitis, chorioretinitis or optic atrophy.
