By the BBC's Daniel Dickinson in Dar es Salaam

Celestina Hiza, a 60-year-old grandmother who works on her family's small-holding, was the recipient of the symbolic 250 millionth free dose of a drug to prevent river blindness.

A ceremony was held in the village of Bombani in Tanzania, attended by the country's Vice President, Dr Ali Mohammed Shein, and 2,000 villagers to mark the event.

The Tanzania initiative is part of a worldwide bid by health experts to eradicate the disease, which is endemic in some Latin American countries and Yemen.

But it is most prevalent in sub-Saharan Africa, where 28 countries are affected.

Eradicating the threat of the disease has given a new lease of life to villages

Dr William Mwengee, Regional Medical Officer, Tanga, Tanzania

The World Health Organization estimates that onchocerciasis, known as river blindness, afflicts 18m people worldwide with a further 120m at risk of contracting it.

It is the leading cause of blindness in the developing world.

Most of those cases are found in Africa in countries stretching from Senegal in West Africa to Ethiopia in the east and from Sudan to the southern extremities of Mozambique and Angola.

Skin rashes

Celestina has suffered from the condition for some years, although it has not reached its most mature stage at which irreversible blindness can occur.

But she has suffered uncontrollable itching and developed "lizard skin", a chronic swelling and thickening of the skin.

She told BBC News Online: "I have been taking the drug for three years now and the itching has stopped.

"This has been a relief to me. I am now able to go back to farming my land and looking after my family."

Merck CEO Raymond Gilmartin says the company will continue to donate the drug free to those who need it

River blindness is a parasitic disease transmitted to humans through the bite of the common blackfly found along riverbanks.

Unlike many other waterborne diseases, river blindness only occurs around fast flowing rivers and not in pools of stagnant water.

The parasite once in the bloodstream, multiplies and spreads throughout the body.

The adult parasite, which can survive for up to 15 years, produces offspring called microfilarie.

It is the microfilarie which cause the acute skin rashes, itching, disfigurement and in many cases blindness.

Villages abandoned

In addition to the disease's effect on health, river blindness can also have long-term social implications.

In some cases, entire villages have been abandoned with the inhabitants fleeing fertile land along river banks in the search for safer, although less fertile soil.

Young people especially have left their communities to avoid the disease.

Dr William Mwengee, the Regional Medical Officer for Tanga region, where Bombani is located said: "Eradicating the threat of the disease has given a new lease of life to villages like Bombani.

"It has meant that the inhabitants are healthier and are more productive from an economic point of view.

"Most importantly, village life has not been destroyed by people moving elsewhere.

Free medication

The drug that Celestina Hiza has been taking is called Mectizan. One dose a year is enough to prevent the onset of river blindness.

Doctors believe that if taken over 15 years, the life of the adult parasite, it is almost 100% effective.

Celestina, like 30m other people in Africa and Latin America, have received Mectizan free of charge.

It has been donated by the US-based pharmaceutical company Merck, which has been running its programme in conjunction with WHO, non-governmental organisations and national and regional governments since 1987.

Merck said it donated 100-120 million tablets at a cost of $1.50 each in 2001.

Raymond Gilmartin, chief executive of Merck, was in Bombani to administer the 250 millionth dose.

He told BBC News Online: "The image of a young boy leading a blind man through an African village is now largely a thing of the past due to the Mectizan programme.

"We are committed to continuing the supply of Mectizan free of charge to whoever needs it wherever they are in the world."