Blindness by Onchocera Volvulus parasites

Onchocera volvulus is nematode parasite that is known to cause a disease called onchocerciasis or “river blindness.” The river blindness is common in many countries in Africa, South America and some Arabian countries. Currently, there are about 37 million people identified to be affected by O.volvulus which can eventually lead to blindness and about 89 million are vulnerable to get the disease (2). Many people who contract onchocerciasis disease are known to live near rivers, where the reproductions of O. volvulus larva occur. O. volvulus larva transmission to humans is caused by an insect known as Simulium fly (black flies). The O. volvulus parasite has two life cycles: one in the flack flies and another after it transmitted to humans. The parasites are known to cause blindness and skin irritation (itching) after it develops into an adult stage known as microfilariae in humans. There is no cure or effective treatments for the onchocerciasis, however, drugs known as Ivermectin have been used for treatment in the last 15 years. Ivermectin is known to slow down the O. volvulus reproduction process, which can finally reduce transmissions (2). Unfortunately, Onchocra volvulus becomes resistant to Ivermectin by making it less effective. Another approach that is in progress is the use of Ov-CPI-2, also known as onchocystatin that can be used as vaccine against Onchocera volvulus in progress (5).

Black flies play a substantial role in spreading the onchocerciasis among many people mainly in West Africa. Flies transmit the microfilariae of O. volvulus from human to human. During the night the  parasites stay down in the tissue but daytime come up at the surface of the skin. The black flies get attached humans by the carbon dioxide in the breath. As the flies come to take a blood meal, they take the parasites with them from the surface of the skin (figure 1). Then, the parasites go through the flies’ gut, muscles and finally to flies head and mouthpart (salivary gland) where they can easily be ejected out of the flies into the skin and continue its second life cycle in humans. Subsequently, upon inter into humans, the parasites get into the subcutaneous tissues. At this stage the larva become adults and forms nodule in the subcutaneous tissues (4).

Finally, the parasites are able to get into the lymphoid tissues, and blood; at this step of their growth they are known as microfilariae. As a result of heavy infection by the microfilariae, tissues get damaged. Then, the infection spread into many parts of the body including eyes. When alive the microfilariae appear not to cause infection. However, the dead ones cause impairment eyes tissues by initiating inflammation (4). Eventually, the accumulations of dead microfilariae prevent the movement of nutrients through the vessels to the optic nerves result in blindness.

Moreover, there is no cure or effective treatment for this deadly disease at the present time. The main obstacle for the lack of onchocerciasis treatment is the long-term parasites existence in tissues. For instance, microfilariae can live in human tissue (nodules) for more than 15 years (1). It is hard waiting for this long period of time to eliminate the existence of the parasite in human tissues. Nevertheless, for nearly 18 years, Ivermectin was the only drug that had been used for the treatment of onchocerciasis (3). Recently, the drug became less effective because of the resistance. The genetic alterations in β-tubulin protein is in nematodes genes found to be associated with ivermectin resistance (3). The β–tubulin accociated with O. volvulus genotypes and fertility. Now, it seems that ivermectin is becoming less effective to reduce the process of O. volvulus reproduction.

The only strategy people are using now to reduce or prevent the incidence of onchocerciasis is to control the vectors of black flies. Since, transmission of the onchocerciasis is only caused by black flies during binding of human skins, the insecticide can stop the spread of onchocera volvulus from person to person when black flies killed by insecticide. However, the insecticide spread can kill many other insects that are beneficial to the environment.

Currently, a number of experiments have been done to indicate the possibility of vaccination against onchocerciasis. One of the experiments that is in progress is Ov-CPI-2 which is also called onchocystatin. Ov-CPI-2 is a proteinase inhibitor and plays important role during the development of larva in the uterus of female worm (7). It is known to increase recognition with aging and a process that has been previously associated with the growth of concomitant immunity (5). Ov-CPI-2 was able to get expressed in almost all the stages of the O. volvulus parasites and able to disrupt of O. volvulus life cycles. Therefore, Ov-CPI-2 is a hopeful target of defensive immunity against river blindness that is killing and disabling millions of people in the world.

Submitted by Ibsa Idris

Reference:

1. Hise, Amy G., Ferguson, and Pearlman. "Immunopathogenesis of Onchocerca volvulus keratitis (river blindness): a novel role for TLR4 and endosymbiotic Wolbachia bacteria." Endotoxin Research 96 Nov. (2003).

2. Osei-Atweneboana, Mike Y., Awadz, Attah, Boakye, and Gyapong. "Phenotypic Evidence of Emerging Ivermectin Resistance in Onchocerca volvulus." PLos Mar. (2011).

3. Bourguinat, Catherine, Pion, Kamgno, Gardon, and Duke. "Genetic Selection of Low Fertile Onchocerca volvulus by Ivermectin Treatment." PLos 30 Aug. (2007).

4. Cho-Ngwa, Fidelis, Liu, and Lustigman. "The Onchocerca volvulus Cysteine Proteinase Inhibitor, Ov-CPI-2, Is a Target of Protective Antibody Response That Increases with Age." PLos 24 Aug. (2010).

5. Scho¨nemeyer, Annett, Lucius, Sonnenburg, Brattig, and Sabat. "Modulation of Human T Cell Responses and Macrophage Functions by Onchocystatin, a Secreted Protein of the Filarial Nematode Onchocerca volvulus1." Immunology 6 Nov. (2011).

6. "Life Cycle of Onchocerca volvulus." Parasites and Health. N.p., n.d. Web. 2 Dec. 2011. <http://dpd.cdc.gov/dpdx/html/Frames/A-F/Filariasis/body_Filariasis_o_volvulus.htm>.

7. Lustigma, Sara, Brotman, Huima, Prince, and McKerrow. "Molecular Cloning and Characterization of Onchocystatin, a Cysteine Proteinase Inhibitor of Onchocerca uoZuuZus." THE JOURNAL OF BIOLOGICAL CHEMISTRY 9 Apr. (1992).