Factors affecting onchocerciasis transmission: lessons for infection control

ABSTRACT

Introduction: Onchocerca volvulus infects in excess of 15 million people. The vectors are Simulium blackflies, varieties of which differ in their ecologies, behavior and vectorial abilities. Control of the vectors and mass administrations of ivermectin have succeeded in reducing prevalences with elimination achieved in some foci, particularly in Central and southern America. In Africa, progress towards elimination has been less successful.

Areas covered: Even with community directed treatment with ivermectin (CDTI), control has been difficult in African areas with initial prevalences in excess of 55%, especially if only annual treatments are dispensed. This is partly attributable to insufficient coverage, but the appearance of incipiently resistant non-responding parasites and lack of attention to vector biology in modeling and planning outcomes of intervention programmes have also played their parts, with recrudescence now appearing in some treated areas.

Expert commentary: The biology of onchocerciasis is complex involving different vectors with differing abilities to transmit parasites, diverse pathologies related to geographical and parasite variations and endosymbionts in both parasite and vector. Modeling to predict epidemiological and control outcomes is addressing this complexity but more attention needs to be given to the vectors’ roles to further understanding of where and when control measures will succeed.

KEYWORDS:

• Onchocerca volvulus
• Simulium damnosum complex
• vector biology
• ivermectin
• doxycycline
• moxidectin

Acknowledgments

I dedicate this paper to Professor Rolf Garms of the Bernhard Nocht Institute for Tropical Medicine, Hamburg, on the occasion of his 85th birthday on 21 October 2015.

I am very grateful to Professor Rolf Garms for his comments on an earlier draft that led to substantial improvements, as did the suggestions of three anonymous referees.

Declaration of interest

The author has no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Additional information

Funding

This paper was not funded.