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[NAIROBI] The first ever mapping of the geographic distribution and prevalence of loa loa filariasis (loiasis) disease in Africa could help in designing better treatment strategies for river blindness and lymphatic filariasis.

A study published in PLoS Neglected Tropical Diseases (28 June), estimated that more than 14 million people live at high-risk areas in ten African countries. The disease affects the skin and eyes, and is caused by the Loa loa worm, also known as the African eye worm, transmitted by deer fly bites (Chrysops spp.).*

Honora Gustave Marie Zouré of WHO's African Programme for Onchocerciasis Control (APOC) which undertook the mapping, said the studyprovides a first estimate of geographic distribution and prevalence, based on large-scale field survey data.

Loiasis, one of the neglected tropical diseases, is considered important in Africa because those infected can suffer additionally from efforts to treat other diseases.

During the 1990s, for example, several patients with a high intensity loa loa infection developed severe adverse neurological reactions after treatment with ivermectin, which is also used to treat river blindness (onchocerciasis) and lymphatic filariasis (elephantitis), another worm disease.

The risk of severe side effects in people co-infected with Loa loa was a major obstacle to elimination of lymphatic filariasis and control of river blindness in central Africa, the paper says. Now that the areas of low risk have been mapped, the campaign for the elimination of lymphatic filariasis can continue.

Zouré told SciDev.Net that the main strategy for eliminating river blindness is community-directed treatment with ivermectin. The community selects volunteers, who collect ivermectin at the nearest health facility, and decide how and when to distribute the drug.

"This map will help inform decision makers on the treatment strategy that they can employ on whether ivermectin distribution under mass drug administration can be undertaken safely with a minimum of precautions, or if there is need to take measures before treating," said Zouré.

David Taylor, a professor of immunology at Edinburgh University, United Kingdom said: "Community-directed treatment programmes have been a great help in reaching those who require treatment. Given the work required to implement such schemes, it is very important to know precisely where co-infections exist."

"The problem is you cannot use mass ivermectin treatment in areas with loa loa over a certain level of prevalence, [as] a significant number of people have both loa loa and onchocerciasis," said Taylor.

"Mapping will help in designing intervention because of the concern over adverse side reactions which do seem to be related to prevalence and intensity."

The countries involved in the study were Angola, Cameroon, Central African Republic, Chad, Congo, DR Congo, Equatorial Guinea, Ethiopia, Gabon, Nigeria and Sudan.

Link to full paper in PLoS Neglected Tropical Disease

 *Updated, to clarify, on 9 August 2011


PLoS Neglected Tropical Diseases doi:10.1371/journal.pntd.0001210