Onchocerca-Simuliumcomplexes

Onchocerciasis (river blindness) - more than a century of research and control

This review summarises more than a century of research on onchocerciasis, also known as river blindness, and its control. River blindness is an infection caused by the tissue filaria Onchocerca volvulus affecting the skin, subcutaneous tissue and eyes and leading to blindness in a minority of infected persons. The parasite is transmitted by its intermediate hosts Simulium spp. which breed in rivers. Featured are history and milestones in onchocerciasis research and control, state-of-the-art data on the parasite, its endobacteria Wolbachia, on the vectors, previous and current prevalence of the infection, its diagnostics, the interaction between the parasite and its host, immune responses and the pathology of onchocerciasis. Detailed information is documented on the time course of control programmes in the afflicted countries in Africa and the Americas, a long road from previous programmes to current successes in control of the transmission of this infectious disease. By development, adjustment and optimization of the control measures, transmission by the vector has been interrupted in foci of countries in the Americas, in Uganda, in Sudan and elsewhere, followed by onchocerciasis eliminations. The current state and future perspectives for control, elimination and eradication within the next 20-30 years are described and discussed. This review contributes to a deeper comprehension of this disease by a tissue-dwelling filaria and it will be helpful in efforts to control and eliminate other filarial infections.

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.

Indices of onchocerciasis transmission by different members of the Simulium damnosum complex conflict with the paradigm of forest and savanna parasite strains

Onchocerciasis in savanna zones is generally more severe than in the forest and pathologies also differ geographically, differences often ascribed to the existence of two or more strains and incompatibilities between vectors and strains. However, flies in the forest transmit more infective larvae than their savanna counterparts, even in sympatry, contradicting expectations based on the forest and savanna strains paradigm. We analysed data on the numbers of Onchocerca volvulus larvae of different stages found in 10 different taxonomic categories of the Simulium damnosum complex derived from more than 48,800 dissections of flies from Sierra Leone in the west of Africa to Uganda in the east. The samples were collected before widespread ivermectin distribution and thus provide a baseline for evaluating control measures. Savanna species contained fewer larvae per infected or per infective fly than the forest species, even when biting and parous rates were accounted for. The highest transmission indices were found in the forest-dwelling Pra form of Simulium sanctipauli (616 L3/1000 parous flies) and the lowest in the savanna-inhabiting species S. damnosum/S. sirbanum (135) and S. kilibanum (65). Frequency distributions of numbers of L1-2 and L3 larvae found in parous S. damnosum/S. sirbanum, S. kilibanum, S. squamosum, S. yahense, S. sanctipauli, S. leonense and S. soubrense all conformed to the negative binomial distribution, with the mainly savanna-dwelling species (S. damnosum/S. sirbanum) having less overdispersed distributions than the mainly forest-dwelling species. These infection patterns were maintained even when forest and savanna forms were sympatric and biting the same human population. Furthermore, for the first time, levels of blindness were positively correlated with infection intensities of the forest vector S. yahense, consistent with relations previously reported for savanna zones. Another novel result was that conversion rates of L1-2 larvae to L3s were equivalent for both forest and savanna vectors. We suggest that either a multiplicity of factors are contributing to the observed disease patterns or that many parasite strains exist within a continuum.

Guide to blackflies of the Simulium damnosum complex in eastern and southern Africa

At least 26 members of the Simulium damnosum Theobald complex (Diptera: Simuliidae) are known from eastern Africa, most of which are assumed to be non-anthropophilic and some are restricted to small areas of endemicity. Their discovery was based on polytene chromosome inversion polymorphisms and since then they have been cited as cytological entities, but, in many cases, with inadequate descriptions (e.g. without illustrations of the chromosomes). The present distribution, taxonomy and vector identity have been re-evaluated during the last decade in conjunction with ongoing onchocerciasis control programmes. Here, a summary of the relevant data is presented, integrating results of cytotaxonomic, morphotaxonomic and molecular investigations into a guide that provides a comprehensive system of identification for the cytoforms 'Kagera', 'Kasyabone', 'Kibwezi', 'Kisiwani', 'Kisiwani E', 'Kulfo', 'Linthipe', 'Mombo', 'Mutonga', 'Njombe', 'Nkusi', 'Nkusi SA', 'Nkusi SW', 'Nyika', 'Pienaars', 'Sanje', 'Sebwe', 'Turiani', Simulium damnosum sensu stricto, Simulium kaffaense Hadis et al., Simulium kilibanum Gouteux, Simulium kipengere Krueger, Simulium latipollex (Enderlein), Simulium pandanophilum Krüger, Nurmi & Garms, Simulium plumbeum Krueger, Simulium thyolense Vajime et al. and for five newly recognized molecular forms of 'Nkusi' and S. kilibanum. All taxa except S. pandanophilum are assigned to one of five subcomplexes (damnosum, Ketaketa, Kibwezi, Sanje, squamosum) within the S. damnosum complex. The guide encompasses anthropophilic and non-anthropophilic segregates for the whole of eastern and southern Africa, and is not restricted to larval chromosome criteria; in many cases it also allows the typing of adult flies.

Cytotaxonomy, morphology and molecular systematics of the Bioko form of Simulium yahense (Diptera: Simuliidae)

Cytotaxonomic analysis of the polytene chromosomes from larvae of the Simulium damnosum Theobald complex from the island of Bioko in Equatorial Guinea is reported, and a new endemic cytoform is described. Chromosomally this cytoform is close to both S. squamosum (Enderlein) and S. yahense Vajime & Dunbar, but is not identical to either. However, it is morphologically and enzymatically identical to S. yahense. The Bioko form was also found to differ from other cytoforms of the S. damnosum complex in West Africa in the copy number or RFLP pattern of several different repetitive DNA sequences. It is clear that the Bioko form is genetically distinct from other populations of the S. damnosum complex, and whilst it is closest to S. yahense, it shows features that suggest a high degree of geographical and genetic isolation. Such isolation is an important consideration in the assessment of the potential for onchocerciasis vector eradication on Bioko.

Multivariate morphotaxonomy in the identification of adult females of the Simulium damnosum Theobald complex (Diptera: Simuliidae) in the Onchocerciasis Control Programme area of West Africa

Adult female Simulium damnosum s.l. flies obtained in nine countries of the Onchocerciasis Control Programme were examined for 14 morphological characters with the aim of selecting specific characters to identify the main species that form the complex. Two approaches were used, the first involved discriminant function analysis of the species groups using all the characters which were both qualitative (the colour of the fore coxa, the antennae, the wing tuft, the arculus, the ninth abdominal tergite setae and the scutellar setae) and quantitative (the lengths of thorax, antenna, and femur, tibia and first basitarsus of both the fore and hind legs) in nature. The second sought colour characters unique to the S. damnosum subcomplex and to S. yahense, to separate them from the rest, and progressively applied discriminant analyses in the dichotomous stepwise manner to within the subcomplex level. The combination of pale arculus and pale antennae with pale or dark fore coxae were unique to the S. damnosum subcomplex, whilst those of black arculus together with wholly dark or mixed ninth abdominal tergite setae and scutellar setae were unique to S. yahense. Further discrimination was achieved between subcomplexes and species by entering the qualitative and quantitative characters into discriminant function analysis. The two schemes were both successful at separating the main species subcomplexes of the S. damnosum complex, with 90-100% correct classification. Within the subcomplex level, correct identification of sibling species was 85-100% successful.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on onchocerciasis in forest-savannah mosaic areas of Nigeria. I. Investigations in Gbaragu, Oji River

Reports on onchocerciasis in Nigeria have been scanty, despite the size of the country and of the problem. A survey of the prevalence and transmission of the disease in the south-eastern communities of Nigeria was commenced, with a year-long study of the vector and the clinical manifestations of the disease in a forest-savannah mosaic area on the Oji river. Microfilaraemia was found in 76% of the population, and nodules in 62%; there were no significant differences between male and female prevalence rates. Skin and ocular lesions of onchocerciasis, including 'Sowda', were common. Flies of the Simulium damnosum complex were found breeding along the entire length of the Oji river and tributaries. Biting occurred all the year round, but peaked in the months of March, April and May.

Characterization of an Onchocerca-specific DNA clone from Onchocerca volvulus

A genomic library of a savanna isolate of Onchocerca volvulus was screened to detect recombinant plasmids containing highly repeated DNA sequences of this parasite. Four recombinant plasmids were identified which hybridized specifically to Onchocerca DNA, but not to DNA from humans, black flies, Brugia malayi, B. pahangi, or Wuchereria bancrofti. The recombinant plasmids had a low level of homology to Dirofilaria immitis. All recombinant plasmids contain related DNA sequences based on Southern hybridization analysis. Sequences related to these recombinant plasmids are present in different geographic isolates of O. volvulus and O. ochengi, an animal parasite. Two of the recombinant plasmids contain sequences also found in O. lienalis. One recombinant plasmid, puOvs3, has been characterized in detail, including DNA sequence determination. Radiolabeled puOvs3 is able to detect 100 pg of genomic DNA isolated from O. volvulus worms from both savanna and forest regions. It can differentiate O. volvulus from O. ochengi by Southern blot analysis.

The status of human filariasis in relation to clinical signs in endemic areas of the Niger Delta

A study was carried out in selected parts of the Niger Delta Basin on human filariasis and its relationship to clinical signs. One hundred and seventy-six (13.0%) of 1351 individuals examined were infected with microfilaria. Forty-six percent of microfilaria positive cases had M. perstans; 25.6% had W. bancrofti, 19.3% had L. loa, and 9.0% had O. volvulus. The prevalence of filariasis increased with age. Microfilarial density was generally low, 28.4% of infected individuals had counts of 50-100 microfilaria per 20 mm3 blood, 12.5% had counts less than five microfilaria per 20 mm3 blood. Onchocerca microfilariae did not exceed five microfilaria per snip. Microfilarial density was high among individuals aged between 20 and 49 years, but declined with increasing age above 50 years. Acute clinical signs of febrile attack, sink eruptions, and chronic clinical signs of chyluria, hydrocoele, elephantiasis and ocular lesions were observed in 61.9% of infected individuals, 73.4% of clinical cases were febrile attack and skin eruptions, 5.5% were chyluria, 12.8% were elephantiasis of the leg and scrotum, 7.3% were hydrocoele and 0.9% were ocular lesions. Chronic clinical signs were more prevalent in individuals over 40 years of age.

The distribution of the Simulium damnosum complex in Sierra Leone and its relation to onchocerciasis

Because of the paucity of literature concerning onchocerciasis and its vectors in Sierra Leone, new information of the geographical distribution of the Simulium damnosum complex is presented in the context of a comprehensive review of what is known generally about S. damnosum s.l. in Sierra Leone, and how the data relate to the transmission of onchocerciasis. Neither biting adults nor breeding sites have been found on the coastal plain and Freetown peninsula, and these area seem to be free from infestation. Throughout the rest of the country S. damnosum s.l. breeds extensively in all the major river systems. The close proximity of the major rivers to one another facilitates inter-river migration, and ensures that most villages are well within the fly's dispersal range. In accordance with this, onchocerciasis is widespread throughout Sierra Leone and vector biting has been recorded away from riverine breeding sites. Seven cytospecies have been recorded in Sierra Leone but of these S. sirbanum and S. damnosum s.str. are rare outside the far north, in the narrow savanna belt. Simulium squamosum and S. yahense are widely distributed, although S. squamosum is somewhat less common than S. yahense which unexpectedly occurs in large rivers of the forest and savanna as well as small forest streams. Simulium sanctipauli has been found in the south-east of the country, where the forest is most dense, whereas S. soubrense occurs farther north. A new species referred to as S. soubrense 'B' is restricted to the south and west of the country. Over most of Sierra Leone onchocerciasis shows a 'forest' epidemiology and is transmitted by 'forest' vector cytospecies. However, data from the narrow savanna belt in the extreme north are sparse.