Most people have heard of ‘river blindness’ and seen the devastating, memorable images of people suffering from the disease. Also known as onchocerciasis, the disease is transmitted by blackflies (Simulium spp.) that breed in fast-flowing rivers. When an infective blackfly bites a person, a nematode worm (Onchocerca volvulus) enters the person’s body, and nine months later may become an adult worm. Mated adult female worms release offspring known as microfilariae at a rate of 1,600 per day. These microscopic worms invade the person’s skin, eyes and other organs, causing intense irritation, sight loss and even blindness.
What is NRI's solution?
NRI has been at the forefront of efforts to eradicate onchocerciasis since the early 1970s, when the disease was most prevalent in west Africa. This 45-year research story has culminated in the protection of many thousands of people from the disease and freed many millions from the risk of blindness. Importantly, the true research impact of the project is just reaching the point where it can be demonstrated, as the nematode worms can survive for 12 years.
Working with the World Health Organization’s (WHO) Onchocerciasis Control Programme in West Africa (OCP) and the African Programme for Onchocerciasis Control (APOC), the institute’s innovative research involved helping the OCP to address a variety of scientific problems that threatened the success of the programme, such as reinvasion of controlled zones by migrant flies and resistance to the insecticides of choice. OCP’s two-pronged approach involved providing medication to attack microscopic worms developing inside the bodies of infected people whilst attacking the fly with insecticide treatments.
In 1974, the institute contributed to calculating that 17.8 million people were at risk of infection, of whom 7.5 million were infected, in the countries of Benin, Burkina Faso, Côte d’Ivoire, Ghana, Mali, Niger and Togo. In that year, the WHO mounted a vector control operation in these seven countries, the OCP, targeting the larval stages of the vector in fast-flowing sections of rivers with aerial applications of insecticide, and later in four more countries.
Before the OCP operation began, the institute conducted insecticide trials which led to the development of a rapid release system for depositing insecticide into rivers from aircraft. Shortly after OCP started, it became apparent that flies were re-invading the controlled zone from as far away as 500 km and so the institute undertook biogeographical studies on the migrations.
In 1975 it had been established that the vectors in west Africa were members of a species complex, identifiable from patterns in the chromosomes of the larvae, but adults of the different forms remained inseparable. Therefore, in order to investigate the migrations, it was necessary to develop means of identifying adults. Given advances in identification methods, it then became possible to study which forms were the best and worst vectors, with implications for control decisions since they bred in different habitats.
By 1980, one of the vectors became resistant to the main insecticide being used, so alternatives were needed quickly. In 1981, the team conducted helicopter-based trials of various candidate products for WHO and showed that the biocide Bacillus thuringiensis H-14, a naturally occurring soil-borne bacteria, was effective and it later became used operationally for ecologically friendly insect control. From 1987 onwards, the OCP supplemented vector control with mass distributions of the microfilaricidal drug ivermectin. When the OCP ended in 2002, it had succeeded in eliminating onchocerciasis as a public health problem in 10 out of the 11 countries in which it operated. In the 11th country, Sierra Leone, years of armed conflict had prevented completion of the task.
During its 28 years of operations, OCP:
- freed 18 million children from the risk of blindness
- prevented 600,000 people from becoming blind
- re-claimed 250,000 km2 of abandoned land and made it safe for cultivation and resettlement.
Innovations and outcomes
Since 1974, the institute’s work on onchocerciasis has centred on understanding the biology of the vector blackflies, in order to prevent and control spread of the disease. Innovations include:
- development of a method for aerial application of insecticides
- demonstrations of the extent of migrations by the vectors and the distances moved
- understanding of the effects of deforestation on different forms of the vectors
- models of different vectors’ population dynamics and their transmission abilities, including predictions of likely effects of climate change on onchocerciasis transmission
- understanding of the influences of environmental factors on the distribution of different vectors in West Africa
- DNA-based investigations of the taxonomy of vectors and parasites.
In 1996, Professor Robert Cheke, NRI's expert entomologist on blackfly, visited Bioko Island, an island off Equatorial Guinea, to assist with research surveys of its blackfly populations. These concluded that the only vector on the island was a unique endemic form of Simulium yahense, its ability to transmit disease was assessed, and a plan was drawn up to eliminate it. The institute participated in insecticide trials, mapping of target breeding sites and training of local staff, after which, the African Programme for Onchocerciasis Control (APOC) mounted a ground-based control campaign supported by helicopter-based treatments of the extensive inaccessible sites. The vector’s eventual elimination in 2005 protected 90,000 people.
As the adult worms can only live up to 12 years, the lack of any flies able to transmit the disease on the island until mid-2018 provides optimism that the disease will soon be declared as having been eliminated in Bioko.