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Population dynamics of Onchocerca volvulus after 7 to 8 years of vector control in West Africa
M Karam1, H Schulz-Key, J Remme
1Onchocerciasis Control Programme, Ouagadougou, Burkina Faso.
Abstract:
In an attempt to describe the changing population dynamics of Onchocerca volvulus during a period of vector control, nodulectomies were undertaken in 256 patients from ten villages in the Onchocerciasis Control Programme (OCP) and in 74 patients from two villages in an area with ongoing transmission. A total of 1198 nodules were excised and 4350 adult worms were isolated and examined for viability and productivity. In the OCP villages, the worm population is ageing and dying without replacement by new generations of parasites and various findings signal a breakdown of the worm population after about 12 years interruption of transmission. The sexual activity of the worms was significantly reduced. A Productivity Index was developed to measure the microfilariae production at the nodule level. The reduction in this index for the OCP villages correlates closely with the decline over the control period in the community microfilarial loads in the skin. The results show that it is not only the longevity of the parasite which will determine the duration of vector control, but that the reduced productivity of the ageing parasite population is of equal importance.
Insights
Vector control for Onchocerca volvulus (river blindness) leads to an ageing parasite population with reduced productivity. This breakdown, observed after 12 years, is key to controlling the disease.
Area of Science:
- Parasitology
- Tropical Medicine
- Public Health
Background:
- Onchocerciasis, or river blindness, is a parasitic disease transmitted by blackflies.
- Effective vector control programs are crucial for disease eradication.
- Understanding parasite population dynamics under control is essential for long-term success.
Purpose of the Study:
- To investigate the population dynamics of Onchocerca volvulus in response to vector control.
- To assess worm viability, productivity, and population structure after prolonged control efforts.
- To evaluate the impact of reduced parasite productivity on disease transmission.
Main Methods:
- Nodulectomies performed on 256 patients in Onchocerciasis Control Programme (OCP) villages and 74 patients in ongoing transmission areas.
- Isolation and examination of 4350 adult worms from 1198 excised nodules for viability and productivity.
- Development and application of a "Productivity Index" to measure microfilariae production.
Main Results:
- In OCP villages, the Onchocerca volvulus population is ageing and declining without replacement after 12 years of transmission interruption.
- Significant reduction in worm sexual activity and microfilariae production was observed.
- The "Productivity Index" reduction closely correlated with decreased community microfilarial loads.
Conclusions:
- Parasite longevity and reduced productivity of ageing worm populations are critical factors in determining the duration of effective vector control.
- The breakdown of the parasite population, evidenced by reduced productivity, is a significant indicator of successful disease control.
- Findings emphasize the importance of monitoring parasite health beyond just transmission interruption for onchocerciasis elimination.