Determination Of Onchocerca Volvulus Strains Prevalent In The Nkwanta North District Of Ghana

ABSTRACT

Background

Onchocerca volvulus is a filarial parasite that causes onchocerciasis or ‘river blindness’. Two strains of the parasite exist in West Africa namely, savannah and forest strains. They differ significantly in epidemiology, disease severity and are specific to different vectors. The savannah strain found in West Africa is associated with blindness while the forest strain, on the other hand, causes less severe ocular diseases even in individuals with high parasite load. Information obtained from some workers of the Onchocerciasis Chemotherapy Research Centre who carried out some investigations in the Nkwanta North district suggested that the MF of the parasite appear morphologically longer, a character that is associated with the savannah strain. However, the preponderance of the ocular manifestations in patients that are usually associated with the savannah strain was absent in the patients. The lack of empirical data to address this issue calls for further investigation and research in this area. Therefore, this study was aimed at characterizing the strain types of O. volvulus present in these communities and evaluating clients for clinical lesions of onchocerciasis.

Methodology

Subjects who consented to participate in the study were physically examined for clinical signs of onchocerciasis, particularly; skin rashes, depigmentation (leopard skin), visible and palpable nodules as well as visual acuity assessment using the Snellen chart. Skin snips were collected and examined microscopically for O. volvulus MF. The residual skin snips were analyzed for O. volvulus DNA using conventional PCR. A nested-PCR was performed on positive samples with a forest strain specific primer to further characterize the strain type.

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Results

A total of 218 participants were enrolled. The most predominant clinical manifestations among the participants was rashes/itches 15.1% (33/218) followed by visual impairment (low vision, severe low vision and profound low vision) 8.3% (18/218). Palpable nodules were found in only 0.5% (1/218) of the study participants while lizard and leopard skin presentations were absent. About 9.2% (20/218) participants were positive for O. volvulus DNA PCR as compared with 3.7% (8/218) by microscopy (p< 0.05). All the 20 O. volvulus samples were classified as savannah strains by the nested PCR analysis.

Conclusion

The results from this study suggest that the Nkwanta North district is endemic for savannah strains of O. volvulus. The prevalence of the savannah strains in these communities may indicate a changing trend in the vector population as a consequence of deforestation and climate change.

The prevalent clinical manifestations found among the study subjects were predominantly skin rashes/itches and ocular lesions with blindness in just 0.5% of the participants. The generally low prevalence of clinical manifestations and MF in skin snip microscopy is an indication of success of several years of control activities in these communities in spite of evidence of disease transmission in the area.