Taking the strain out of onchocerciasis? A reanalysis of blindness and transmission data does not support the existence of a savannah blinding strain of onchocerciasis in West Africa.

Onchocerciasis (also known as 'river blindness'), is a neglected tropical disease (NTD) caused by the (Simulium-transmitted) filarial nematode Onchocerca volvulus. The occurrence of 'blinding' (savannah) and non-blinding (forest) parasite strains and the existence of corresponding, locally adapted Onchocerca-Simulium complexes were postulated to explain greater blindness prevalence in savannah than in forest foci. As a result, the World Health Organization (WHO) Onchocerciasis Control Programme in West Africa (OCP) focused anti-vectorial and anti-parasitic interventions in savannah endemic areas. In this paper, village-level data on blindness prevalence, microfilarial prevalence, and transmission intensity (measured by the annual transmission potential, the number of infective, L3, larvae per person per year) were extracted from 16 West-Central Africa-based publications, and analysed according to habitat (forest, forest-savannah mosaic, savannah) to test the dichotomous strain hypothesis in relation to blindness. When adjusting for sample size, there were no statistically significant differences in blindness prevalence between the habitats (one-way ANOVA, P=0.68, mean prevalence for forest=1.76±0.37 (SE); mosaic=1.49±0.38; savannah=1.89±0.26). The well-known relationship between blindness prevalence and annual transmission potential for savannah habitats was confirmed and shown to hold for (but not to be statistically different from) forest foci (excluding data from southern Côte d'Ivoire, in which blindness prevalence was significantly lower than in other West African forest communities, but which had been the focus of studies leading to the strain-blindness hypothesis that was accepted by OCP planners). We conclude that the evidence for a savannah blinding onchocerciasis strain in simple contrast with a non-blinding forest strain is equivocal. A re-appraisal of the strain hypothesis to explain patterns of ocular disease is needed to improve understanding of onchocerciasis epidemiology and disease burden estimates in the light of the WHO 2030 goals for onchocerciasis.

Genome-wide analysis of ivermectin response by Onchocerca volvulus reveals that genetic drift and soft selective sweeps contribute to loss of drug sensitivity.

BACKGROUND: Treatment of onchocerciasis using mass ivermectin administration has reduced morbidity and transmission throughout Africa and Central/South America. Mass drug administration is likely to exert selection pressure on parasites, and phenotypic and genetic changes in several Onchocerca volvulus populations from Cameroon and Ghana-exposed to more than a decade of regular ivermectin treatment-have raised concern that sub-optimal responses to ivermectin's anti-fecundity effect are becoming more frequent and may spread. METHODOLOGY/PRINCIPAL FINDINGS: Pooled next generation sequencing (Pool-seq) was used to characterise genetic diversity within and between 108 adult female worms differing in ivermectin treatment history and response. Genome-wide analyses revealed genetic variation that significantly differentiated good responder (GR) and sub-optimal responder (SOR) parasites. These variants were not randomly distributed but clustered in ~31 quantitative trait loci (QTLs), with little overlap in putative QTL position and gene content between the two countries. Published candidate ivermectin SOR genes were largely absent in these regions; QTLs differentiating GR and SOR worms were enriched for genes in molecular pathways associated with neurotransmission, development, and stress responses. Finally, single worm genotyping demonstrated that geographic isolation and genetic change over time (in the presence of drug exposure) had a significantly greater role in shaping genetic diversity than the evolution of SOR. CONCLUSIONS/SIGNIFICANCE: This study is one of the first genome-wide association analyses in a parasitic nematode, and provides insight into the genomics of ivermectin response and population structure of O. volvulus. We argue that ivermectin response is a polygenically-determined quantitative trait (QT) whereby identical or related molecular pathways but not necessarily individual genes are likely to determine the extent of ivermectin response in different parasite populations. Furthermore, we propose that genetic drift rather than genetic selection of SOR is the underlying driver of population differentiation, which has significant implications for the emergence and potential spread of SOR within and between these parasite populations.

Genomic diversity in Onchocerca volvulus and its Wolbachia endosymbiont.

Ongoing elimination efforts have altered the global distribution of Onchocerca volvulus, the agent of river blindness, and further population restructuring is expected as efforts continue. Therefore, a better understanding of population genetic processes and their effect on biogeography is needed to support elimination goals. We describe O. volvulus genome variation in 27 isolates from the early 1990s (before widespread mass treatment) from four distinct locales: Ecuador, Uganda, the West African forest and the West African savanna. We observed genetic substructuring between Ecuador and West Africa and between the West African forest and savanna bioclimes, with evidence of unidirectional gene flow from savanna to forest strains. We identified forest:savanna-discriminatory genomic regions and report a set of ancestry informative loci that can be used to differentiate between forest, savanna and admixed isolates, which has not previously been possible. We observed mito-nuclear discordance possibly stemming from incomplete lineage sorting. The catalogue of the nuclear, mitochondrial and endosymbiont DNA variants generated in this study will support future basic and translational onchocerciasis research, with particular relevance for ongoing control programmes, and boost efforts to characterize drug, vaccine and diagnostic targets.

Emergence of Onchocerca volvulus from skin mimicking Dracunculiasis medinensis.

We describe 11 cases of suspected Dracunculus medinensis infection in which the worm recovered was identified as Onchocerca volvulus. Identification was based on morphology of the examined specimen.

An atypical microfilaria in blood samples from inhabitants of Brazilian Amazon.

An unidentified microfilaria sharing characteristics with Mansonella ozzardi and Onchocerca volvulus was detected in blood samples from seven human volunteers, inhabitants of a community in the border of Amazonas and Acre State. They were detected during epidemiological studies carried out in some communities along Antimary, Acre, and Purus Rivers in the Brazilian Amazon. The most striking difference was presented in the shape of the cephalic space from this microfilaria which was different from those of M. ozzardi and with similarities to O. volvulus in this region, but no remarkable differences were observed at the caudal region. More accurate studies are being carried out in order to provide additional data and supporting evidences before establishment of a new species can be done.

DNA-based detection of onchocerca volvulus.

During the 1980s, the idea of using deoxyribonucleic acid probes for specific identification and diagnosis of infectious agents became very fashionable. There was therefore an explosion in the development of these tools and one particular group of organisms which received much attention was the parasitic nematodes. This review traces the development and use of such probes with the filarial nematode Onchocerca volvulus, with emphasis on their application to resolving certain 'problems' associated with this parasite, e.g. whether or not strains exist and difficulties in distinguishing the infective larval stage morphologically from related species.

Characterisation of nuclear ribosomal DNA sequences from Onchocerca volvulus and Mansonella ozzardi (Nematoda: Filarioidea) and development of a PCR-based method for their detection in skin biopsies.

The internal transcribed spacer region (ITS1, 5.8S gene and ITS2) of the two filarial nematodes Onchocerca volvulus and Mansonella ozzardi was sequenced, and two species-specific primers designed in the ITS2 to develop a PCR-based method for their specific detection and differentiation. When used with a universal reverse primer, the two species-specific primers gave amplification products of different size, which were readily separated in an agarose gel. The PCR was tested on skin biopsies from 51 people from three localities in Brazil where M. ozzardi is present, and results have been compared with those of parasitological examination of blood. The species-specific PCR gave a higher percentage of detection of infection by M. ozzardi than the parasitological examination of blood. No infection with O. volvulus was detected by PCR. This PCR-based assay may assist in determining the nature of infection in areas where both filarial species exist in sympatry.

Distribution of the blinding and nonblinding strains of Onchocerca volvulus in Nigeria.

Onchocerciasis remains an important public health problem throughout much of sub-Saharan Africa. Nigeria is the country whose population is most afflicted by onchocerciasis; however, little is known concerning the epidemiology of onchocerciasis in this country. Previous studies demonstrated that onchocerciasis in West Africa exists in two forms, which differ in their clinical and epidemiologic characteristics. This is believed to be due to the existence of 2 strains of Onchocerca volvulus, the causative agent of onchocerciasis. The O-150 polymerase chain reaction has been developed to differentiate these 2 strains, and this method has been used to map the distribution of the blinding and nonblinding strains of O. volvulus in Nigeria. The strain distribution is consistent with what is known concerning the ecology and epidemiology of onchocerciasis in this country. The results also suggest that migration may be affecting the historic distribution of the 2 strains of the parasite in Nigeria.

DNA probe-based classification of Simulium damnosum s. l.-borne and human-derived filarial parasites in the onchocerciasis control program area.

The development of polymerase chain reaction-based methods using strain- and species-specific DNA probes for Onchocerca volvulus has permitted classification of individual parasites from every stage of the parasite's life cycle. This technology has been applied on a large scale basis by Onchocerciasis Control Program (OCP) in West Africa. The primary objective of the OCP in using the DNA probes was to obtain accurate estimates of the annual transmission potential of the blinding strain of O. volvulus. The DNA probe classification of larvae collected throughout the OCP area demonstrated that larvae of less pathogenic strains of O. volvulus and other filarial parasites carried by Simulium damnosum s.l. have resulted in a significant overestimation of the annual transmission potential for blinding onchocerciasis. This effect is particularly pronounced along the southern border of the OCP, where the blinding and less pathogenic strains of O. volvulus coexist, and in the north of the control area, where animal parasites, particularly O. ochengi, may even predominate. A second objective of the OCP in applying the DNA probe technology was to determine the distribution of blinding and less pathogenic O. volvulus in infected individuals along the southern border of the control area. Results obtained from these studies have generally confirmed the distribution pattern established by previous epidemiologic studies. In addition, DNA probe classifications have demonstrated that in areas where the blinding and less pathogenic strains of O. volvulus coexist, a single individual may simultaneously be infected with both strains of the parasite.

Recent evolutionary history of American Onchocerca volvulus, based on analysis of a tandemly repeated DNA sequence family.

Polymerase chain reaction (PCR) products were characterized for a repeated sequence family (designated "O-150") of the human filarial parasite Onchocerca volvulus. In phylogenetic inferences, the O-150 sequences clustered into closely related groups, suggesting that concerted evolution maintains sequence homology in this family. Using a novel mathematical model based on a nested application of an analysis of variance, we demonstrated that African rainforest and savannah strain parasite populations are significantly different. In contrast, parasites collected in the New World are indistinguishable from African savannah strains of O. volvulus. This finding supports the hypothesis that onchocerciasis was recently introduced into the New World, possibly as a result of the slave trade.