River Blindness

By Jeremy Aguinaldo
April 4, 2014

Onchocerciasis, also known as River Blindness, is considered a neglected tropical disease (NTD) and caused by Onchocerca volvolus. It is predominantly found in Africa, South America, and some parts of Yemen. This parasitic worm is transmitted through multiple repeated bites by blackflies of the genus Simulium that inhabit near running bodies of water. Clinical symptoms include visual impairments and skin disease. Mass drug administration of ivermectin is currently employed, as well as the utilization of larvicide in Africa and South America, with the latter currently on the path to the elimination of the disease.

Epidemiology:

African and American forms of Onchocerciasis show certain differences such as vector biting habits. It is presumed that Onchocerciasis was introduced to the Americas by the slave trade. It was estimated in 2006 that about 37 million people were infected. Majority of the cases occurred in 27 sub-Saharan African countries. Infections have also been found in Yemen. Currently, transmission has been eliminated or interrupted in most of the Americas [1].

By the end of 2012, transmission was interrupted or eliminated in 11 of the 13 foci in the Americas. Colombia was the first country to achieve country-wide interruption of transmission, and ivermectin distribution was discontinued in 2008. Ecuador was the second country to suspend treatment, in 2010. Surveillance is required three years following cessation of treatment to achieve certification of elimination. The Onchocerciasis Elimination Program in the Americas (OEPA) is on track to eliminate onchocerciasis in the region by 2015 [2].

In Africa, the African Programme for Onchocerciasis Control (APOC) had shown a remarkable impact from 1995 to 2010. The program utilizes efficient and coordinated annual mass treatment with ivermectin in sixteen countries. Since then, about 8.2 million disability-adjusted life years (DALYs) had been averted. By 2015, it is speculated that about 9.2 million DALYs will also be prevented [3].

Pathogenesis:

The blackfly genus Simulium is responsible for transmitting the parasite. The adult filarial worms live in subcutatenous fibrous nodules. It is here where they become coiled and the microfilariae are born. The microfilariae migrates throughout the infected nodules, subcutaneous tissues, skin, and into the eye; and is rarely seen in the blood [4].

Wolbachia spp in the parasite seem to have evolved as symbiotic relationship that is essential for normal development of larvae and embryos and may support the long-term survival of adult worms [5]. Corneal inflammation appears to occur in response to both Wolbachia and Onchocerca antigens. O. volvulus adults and microfilariae harbor endosymbiotic Wolbachia bacteria, which are essential for the parasite’s fertility and survival. The release of Wolbachia-derived antigens from dying parasites can activate innate immune responses and are considered to play an important role in the development of onchocercal eye disease. Wolbachia bacteria mediate corneal pathology by activating Toll-like receptors on mammalian cells, which in turn stimulate recruitment and activation of neutrophils and macrophages [6].

Eye disease continues to progress despite the elimination of the parasite from the eye, which suggest an autoimmune response. Cross-reactivity has been found between O. volvulus antigen Ov39 and the antigen hr44 found in the retinal pigment epithelium. Hr44, however, is also found in the optic nerve and other ocular tissues.

Dermatologic manifestations appear to occur in response to Onchocerca antigens. Secretory proteins from the surface of O. volvulus may induce down-regulation of host immunity. The secretory omega-class glutathione transferase 3 from O. volvulus (OvGST3) has a protective role against intracellular and environmental reactive oxygen species.

 

CITATIONS
[1]   Progress Toward Elimination of Onchocerciasis inthe Americas- 1993-2013. (2013, May 24). Morbidity and Mortality Weekly Report, 62(20), pp. 405-408.

[2]   World Health Organization. (2010) Report from the 2009 InterAmerican Conference on Onchocerciasis: Progress towards eliminating river blindness in the Region of the Americas. Weekly Epidemiological Record 85:321-327. Available from http://www.who.int/wer/2010/wer8533.pdf

[3]   Coffeng, L. E., Stolk, W. A., Zoure, H. G., Veerman, L., Agblewonu, K. B., Murdoch, M. E., . . . Amazigo, U. V. (2013). African Programme for Onchocerciais Control 1995-2015: Model-Estimated Health Impact and Cost. PLOS: Neglected Tropical DIseases, 7(1), e2032.

[4]   Zeibig, E. A. (1997). Clinical Parasitology: A Practical Approach. Philadelphia, Pennsylvania: W.B. Saunders Company.

[5]   Tamarozzi, F., Halliday, A., Gentil, K., Hoerauf, A., Pearlman, E., & Taylor, M. J. (2011). Onchocerciasis: the Role of Wolbachia Bacterial Endosymbionts in Parasite Biology, Disease Pathogenesis, and Treatment. Clinical Microbiology, 24(3), 459-468.

[6]   Roberts, L. S., & Schmidt, G. D. (1996). Foundations of Parasitology. Dubuque, IA: Wm. C. Brown.

Jeremy Aguinaldo is a Medical student at St. George’s University with an MPH from the University of South Florida. Learn more about river blindness here.

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