Modelling the destructive path of a biting fly
Ever heard of Robles disease, craw-craw, or onchocerciasis? No? They are all aliases for ‘river disease’, which is probably still little known to most Europeans. But to travellers and people who live in one of the many regions listed as endemic to the parasitic disease, it means painful skin lesions leading to blindness for many. European scientists have come up with mathematical models which, they predict, will help guide health campaigns aimed at stamping out this chronic illness.
Human onchocerciasis is caused by a biting black fly found across large parts of Africa and the Americas. The disease is transmitted by certain species of female Simulium flies which bite by day and are found near fast-flowing rivers and streams. River disease is a major public health problem in many parts of the world. It is thought to be endemic in 36 countries. According to the World Health Organisation (WHO), it is the world’s second leading infectious cause of blindness. The vast majority of cases are recorded in Africa.
|Tropical conditions like malaria and river disease are a major burden on the developing world’s health systems.|
© TUFTS – New England Medical Center
The international community’s response to treating river blindness focuses on three programmes: a general onchocerciasis scheme and specific ones for Africa and the Americas. The total number of people treated – with ivermectin, a drug that attacks the microfilariae (pre-larva) that cause the disease – has increased from around 11 million worldwide in 1996 to some 45 million in 2004. But the disease is still a major concern for health communities.
A team of European and South American researchers is calling for better public health campaigns to reduce the numbers affected by river blindness. They have developed a mathematical model, published on-line in Proceedings of the National Academy of Sciences, which measures exposure to the disease by looking at how often people were bitten by the flies carrying the parasite. Unlike previous models, which only looked at one geographical area, the new model can be applied globally.
“This new model could be an important tool in developing effective health campaigns to reduce the numbers affected by river blindness,” Dr João Filipe of Imperial College London (ICL), and first author of the paper says. “Currently there are at least 18 million people worldwide affected by this parasite, and more action is urgently needed. This model will help in the fight against the disease by providing a better understanding of the role of exposure to the biting flies that transmit river blindness.”
Prevention is key
The flies carry a parasite called Onchocerca volvulus, which lays microscopic worms in the human host. The body’s immune response towards these worms can lead to eye opacities, eventually causing blindness, and in many cases, skin disease. Symptoms are usually visible on the skin and can occur months even years after exposure, which makes treatment difficult. At present, no vaccine is available.
Prevention is the most effective measure to cut back the impact of river blindness on badly affected countries and their overburdened healthcare systems. The best forms of prevention, according to leading travel health websites, is to avoid black fly habitats or otherwise wear protective gear during the day in the fly’s habitat. Travellers may be able to take measures to avoid contact with the flies, but local populations are more exposed and ill-informed about the dangers.
The new model uses data from three regions – Cameroon, central Guatemala, and southern Venezuela – and looks at human age and sex. The team estimated entomological factors, such as the number of times people were bitten. This could be affected by anthropological factors, such as the level of protection against bites afforded by clothing.
Co-author of the paper, Dr María-Gloria Basáñez of ICL, says this disease is often overlooked because it only affects the poorest tropical areas. “Greater investment needs to be made in public awareness campaigns to reduce exposure to it in the affected countries,” she asserts. Joining ICL in the research were the Institut de Recherche pour le Développement (France), Institut für Tierphysiologie (Germany), Universidad Central de Venezuela, and the Centro Amazónico de Enfermedades Tropicales (Venezuela). The work was supported by the UK’s Medical Research Council.
The EU also supports research into major poverty-related diseases and the EDCTP (European and Developing Countries Clinical Trials Partnership) scheme aims to develop new medical products, microbiocides and vaccines for HIV/Aids, tuberculosis and malaria. At present, river disease research is not directly covered by EU funding.
Participation of research institutions from developing countries is also highly encouraged in the EU’s Sixth Research Framework Programme (FP6). “Such participation is particularly justified in the context of research activities to confront the major communicable diseases linked to poverty, which are endemic in these countries and heavily affecting their populations,” notes the Research DG’s poverty-related diseases website.
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