Onchocerciasis

Onchocerciasis

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ICD-10	B73
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ICD-9	125.3
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? Onchocerca volvulus
240px O. vovulus, the causative agent of river blindness.
Scientific classification
Kingdom: Animalia Phylum: Nematoda Class: Secernentea Order: Spirurida Family: Filariidae Genus: Onchocerca Bickel, 1982 Species: O. volvulus

Onchocerciasis , also known as river blindness , Roble’s disease or sowda , is the world's second leading cause of blindness due to infection,with trachoma being number one. It is a chronic non fatal disease caused by a parasitic worm , Onchocerca volvulus , that enters the body through the bite of a black fly. It is estimated that 37 million people are infected at present and that among these more than 250,00 are blind (WHO 2005).

Distribution of Vectors

       insert map of disease distribution here

The distribution and spread of onchocerciasis is a result of blackflies that carry the infectious nematode,onchocerca volvulus. The primary agent for transmission (the vector) of this parasite is the blackfly of the genus Simulium. In Africa , where 99% of the cases occur, the most widespread vector is Simulium damnosum (sensu stricto). There are some regional species that are also important such as S.ethiopiense in Ethiopia , S. woodi in Malawi and Tanzania and S.neavei in east Africa.

In Latin America ,S.ochraceum,S.metallicum and S.callidum are important vectors in Mexico and Guatemala. In Colombia , S. exiguum is the primary vector with S. amazonicum being predominant in Brazil.

The most likely vector in Yemen is S.damnosum , but the most important transmitter in Saudi Arabia is not certain (Awadzi 1989).

Blackfly Life Cycle

Blackflies ,of the Simulium damnosum complex, lay their eggs in swiftly moving water where they attach to submerged objects such as rocks and plants. When the eggs hatch, within 36 to 48 hours,they become black larvae that remain attached to stationary objects in the fast flowing water. The larvae feed and molt over a period of 8 to 10 days and pupate for 2-5 days without losing their anchorage. After metamorphosis , the adult fly emerges within four weeks.

The emergence of blackflies from swiftly moving rivers and streams makes any human beings living or working nearby a prime target for feeding. Those who spend more time near the river are more likely to become infected by blackflies. The infection often develops into blindness and thus came to be known as ‘river blindness’, before anyone realized what was the causative agent.

a parasitic worm that can live for up to fourteen years in the human body.

O.volvulus life cycle

When a female blackfly is carrying eggs that have been fertilized by a male, she will seek a blood meal from a warm-blooded mammal. Although there are a few reported cases of onchocerciasis developing in gorillas and spider monkeys, human beings are the most important host for O. volvulus. (Awadzi 1989).

The female fly bites the subject and causes a pool of blood to form on the skin surface. When she drinks this blood from an infected person, she will also receive the first stage microfilariae of the parasite, which will pass through the mouth (proboscis) and into the stomach. Most of the parasites will be digested by the stomach of the fly but the few that survive will migrate into the cells of the flight muscles. They will molt twice,within seven days, to form stage three infective microfilariae, which will then migrate back to the proboscis of the mouth.

When the fly has another blood meal , the third stage microfilariae will be carried into the human host. They will enter the subcutaneous tissues where, within 3 months, they will mature into male and female adults. They will then mate and produce offspring that are stage one microfilaria. The formation of these microfilariae requires usually more than one year after the initial infection. These stage one infective larvae will then migrate up to a subepidermal capillary where they will wait , from 6 to 30 months, for the invading proboscis of the blackfly vector.

From here the larvae migrate to the subcutaneous tissue where they form nodules and then mature into adult worms over a period of one to three months. After the worms have matured they mate, the female worm producing between 1000 and 1900 eggs per day. The eggs mature internally to form stage one microfilariae, which are released from the female's body one at a time.

The microfilariae migrate from the location of the nodule to the skin where they wait to be taken up by a black fly. Once in the black fly they moult twice within seven days and then move to its mouthparts to be retransmitted.

Causes of Morbidity

When the microfilariae migrate to the skin they are a target for the immune system. White blood cells release various cytokines that have the effect of damaging the surrounding tissue and causing inflammation. This kills the microfilariae but is the cause of the morbidity associated with this disease.

In the skin this can cause intense itching that leads to the skin becoming swollen and chronically thickened, a condition often called lizard skin. The skin may also become lax as a result of the loss of elastic fibres. Over time the skin may lose some of its pigment; on dark skin this gives rise to a condition known as leopard skin.

The symptom that gives the disease its common name river blindness is also caused by the immune system's reaction to the microfilariae. The surface of the cornea is another area to which the microfilariae migrate, where they are also attacked by the immune system. In the area that is damaged, punctate keratitis occurs, which clears up as the inflammation subsides. However, if the infection is chronic, sclerosing keratitis can occur, making the affected area become opaque. Over time the entire cornea may become opaque, thus leading to blindness.

Treatment and control

The treatment for onchocerciasis is ivermectin (mectizan); infected people can be treated once every twelve months. The drug paralyses the microfillariae and prevents them from causing itching. In addition, while the drug does not kill the adult worm, it does prevent them from producing additional offspring. The drug therefore prevents both morbidity and transmission.

Since 1988, ivermectin has been provided free of charge by Merck & Co. through the Mectizan Donation Program (MDP). The MDP works together with ministries of health and non-governmental development organsations such as the World Health Organisation to provide free mectizan to those who need it in endemic areas.

There are various control programs that aim to stop onchocerciasis from being a public health problem. The first was the Onchocerciasis Control Program (OCP), which was launched in 1974 and at its peak covered 30 million people in eleven countries. Through the use of larvicide spraying of fast flowing rivers to control black fly populations and, from 1988 onwards, the use of ivermectin to treat infected people, the OCP eliminated onchocerciasis as a public health problem. The OCP, a joint effort of the World Health Organisation, the World Bank, the United Nations Development Programme and the UN Food and Agriculture Organization, was considered to be a success and came to an end in 2002. Continued monitoring ensures that onchocerciasis cannot reinvade the area of the OCP.

In 1992 the Onchocerciasis Elimination Programme for the Americas (OEPA) was launched. The OEPA also relies on ivermectin.

In 1995 the African Programme for Onchocerciasis Control (APOC) began covering another nineteen countries and mainly relying upon the use of ivermectin. Its goal is to set up a community-directed supply of ivermectin for those who are infected. In these ways, transmission has declined.

Recent research suggests that the Wolbachia bacteria carried by O. volvulus may actually provoke the damaging inflammatory response rather than the worm itself, opening the possibility for antibiotic treatment.

See also

• Carter Center River Blindness Program

External link

• Pathology of Onchocerciasis(River Blindness)

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