Onchocerciasis

Onchocerciasis or river blindness is the world's second leading infectious cause of blindness. It is caused by Onchocerca volvulus, a parasitic worm that can live for up to fourteen years in the human body.

Life Cycle

The life cycle of Onchocerca Volvulus begins when a parasitised female black fly of the genus Simulium takes a blood meal. Saliva containing L3 (stage 3) Onchocerca larvae passes into the blood. From here the larvae migrate to the subcutaneous tissue where they form nodules, they then mature into adult female and male worms. This takes 1-3 months. After the worms have matured they mate, the female worm produces between 1000 and 1900 eggs per day, these mature internally to form stage 1 microfilariae. These 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 7 days of the black fly taking its meal, they then move to the mouth pieces of the black fly and wait for their oportunity to be transmitted.

Causes of morbidity

When the microfilariae migrate to the skin they are a target for the immune system. Cells such as Natural Killer cells and phagocytes release various inflammatory 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 in the skin structure. 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, here they are also attacked by the immune system and the tissue surrounding the microfilariae becomes damaged. In the area that is damaged Punctate Keratitis occurs, this clears up as the inflammation subsides. However if infection is chronic then Scelerosing Keratitis can occur, this makes the area of the cornea affected become opaque and over time the whole of the cornea can become opaque, this of course leads to blindness.

Tretment and Control

The treatment for onchocerciasis is Ivermectin (Mectizan) infected people can be treated once every 12 months. The drug paralyses the microfillariae and prevents them from causing itching. In addition whilst 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.

Today there are various control programs that aim to stop onchocerciasis from being a public health problem. The first was the Onchocerciasis control program (OCP), this was laucned in 1974 and at its peak covered 30 million people in 11 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 was considered to be a success and came to an end in 2002, monitoring is continues to ensure that onchocerciasis cannot reinvade the area of the OCP.

In 1995 the African Programme for Onchocerciasis control (APOC) bega. This took in another 19 countries and mainly relies upon the use of Ivermectin. The aim is to set up community directed supply of Ivermectin to those who are infected, in this way transmission has declined.

In the Americas the Onchocerciasis Elimination Programme for the Americas (OEPA) was launched in 1992, this is also relying on Ivermectin for its success.