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[
Parasitology,
1993]
The use of a selective schedule of tests to identify a viable population of isolated adult Onchocerca volvulus (Nematoda: Filarioidea) has been investigated in a large worm population. The study was initiated to develop methodology appropriate to test new candidate macrofilaricides for their in vitro activity against O. volvulus. After removal from the host the viability of isolated intact parasites was estimated by assessing the motility indices of male worms, and the colorimetric quantification of the reduction of the bioreducible tetrazolium reagent XTT and lactate output by female worms. Additionally the motility of whole females and the movement of inner organs of female worms were scored quantitatively. These response parameters were used to sort the adult worms into viability groups at the start of the in vitro culture. The adult worms were then observed for 6 days and viability was assessed regularly during the culture period. At the end of the culture period, the reduction of the water-insoluble tetrazolium reagent MTT was used to determine the formazan formed by the entire male and female worms. The response parameters used at the start of the culture proved to be highly predictive for detecting viable and non-viable adult worms. In the group of worms selected as 'viable' around 70% kept their motility and metabolic activity at a high level until the end of the culture compared to the initial level. In contrast, none of the female worms and only 13% of the male worms categorized as 'poorly viable' demonstrated a motility index or metabolic level at the end of the culture period that was comparable to that of the worms in the 'viable' groups. For female worms the lactate output correlated significantly with weight whereas no correlation was seen between MTT reduction and weight.
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[
Trop Med Parasitol,
1985]
Calcified worm fragments of adult Onchocerca volvulus from patients in West Africa were collected for a chemical analysis. The material contained predominantly calcium carbonate. Some results of investigations on the occurrence of calcified worms in untreated populations are reported on. Relations between the numbers of old parasites and those of calcified worms are discussed. The significance of possible resorption of calcified worm fragments for the interpretation of the effects of drug trials or other control measures are mentioned.
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[
Tropenmed Parasitol,
1980]
The paper describes observations made on 32 chimpanzees experimentally infected with Onchocerca volvulus. The mean pre-patent intervals for the Cameroon forest and the Guatemalan strains of O. volvulus were 13-16 months and 12-15 months respectively. That for the Cameroon Sudan-savanna strain was much longer, i.e. 22-23 months. The numbers of microfilariae found in the skins of animals infected with the Cameroon Sudan-savanna strain were also much lower than in animals infected with the other two strains. Long-term observations on infected animals showed that microfilarial infections had virtually died out 6.5-9 years after the last inoculation with infective larvae. Those animals which were inoculated with infective larvae in the head or above the waist tended to show a higher proportion of microfilariae in the upper parts of the body, than did those inoculated with infective larvae below the waist. In animals which showed adult worm-bundles on only one side of the body, the concentration of microfilariae was usually greater on that side of the body. Worm-bundles in the chimpanzee varied in size from 8 x 5 x 2 mm to 4 x 3 x 2 cm. Out of 47 worm-bundles found, only two were subcutaneous. The remainder lay deep in the tissues, most commonly adjacent to the posterior surface of the capsule of the hip joint. No onchocerciasis eye lesions were seen in any of the infected animals.
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[
Int J Parasitol,
2000]
Onchocerca volvulus, the filarial parasite that causes onchocerciasis or river blindness, contains three distinct genomes. These include the nuclear genome, the mitochondrial genome and the genome of an intracellular endosymbiont of the genus Wolbachia. The nuclear genome is roughly 1.5x10(8) bp in size, and is arranged on four chromosome pairs. Analysis of expressed sequence tags from different life-cycle stages has resulted in the identification of transcripts from roughly 4000 O. volvulus genes. Several of these transcripts are highly abundant, including those encoding collagen and cuticular proteins. Analysis of several gene sequences from O. volvulus suggests that the nuclear genes of O. volvulus are relatively compact and are interrupted relatively frequently by small introns. The intron-exon boundaries of these genes generally follow the GU-AG rule characteristic of the splice donor and acceptors of other vertebrate organisms. The nuclear genome also contains at least one repeated sequence family of a 150 bp repeat which is arranged in tandem arrays and appears subject to concerted evolution. The mitochondrial genome of O. volvulus is remarkably compact, only 13747 bp in size. Consistent with the small size of the genome, four gene pairs overlap, eight contain no intergenic regions and the remaining gene pairs are separated by small intergenic domains ranging from 1 to 46 bp. The protein-coding genes of the O. volvulus mitochondrial genome exhibit a striking codon bias, with 15/20 amino acids having a single codon preference greater than 70%. Intraspecific variation in both the nuclear and mitochondrial genomes appears to be quite limited, consistent with the hypothesis that O. volvulus has suffered a genetic bottleneck in the recent past.
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[
Trop Med Parasitol,
1988]
Instructions are provided for selection of nodules, isolation of female Onchocerca volvulus and shipping to obtain suitable, intact and live worms for drug-screening and metabolic studies in vitro as well as material for isolation of enzymes, receptors, antigens etc.
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[
Harefuah,
1998]
The parasite Onchocerca volvulus is well-known in its endemic areas in South and Central America and West Africa. It is transmitted to man by simulium flies and causes systemic infection with skin, lymphatic and ophthalmic manifestations and can cause blindness (river blindness). Treatment with Ivermectin is effective but sometimes there is need for surgical intervention to prevent or treat complications. We describe an 11-year-old girl, a new immigrant from Ethiopia, who had a firm mass in her left thigh, caused by Onchocerca volvulus. It was completely excised. This is a very rare condition in Israel, which must be considered in patients coming from endemic areas.
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[
Science,
1986]
Ivermectin, given to onchocerciasis patients as a single oral dose of 200 micrograms per kilogram of body weight, substantially reduced the uptake of Onchocerca volvulus microfilariae by Simulium yahense, an efficient black fly vector of the parasite in the tropical rain forests of West Africa. Three months after treatment, patients given ivermectin infected flies at a significantly lower rate than those who had received diethylcarbamazine or placebo, thereby reducing the number of developing larvae in the vector population. This diminished rate of infectiousness was also evident 6 months after treatment. These results strongly suggest that ivermectin could be effective in interrupting transmission of Onchocerca volvulus for epidemiologically important periods of time.
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[
Acta Leiden,
1990]
This study compiles observations on the reproductive capacity of O. volvulus. Adult parasites enzymatically isolated from excised onchocercomata of untreated and chemotherapeutically treated patients, and from inhabitants living in areas with long vector control, were investigated to assess their fecundity. Changes of microfilaria development in utero and microfilaria release were assessed or estimated after treatment of patients with micro-filaricidal drugs that interfered with the development of intra-uterine stages. Intra-uterine production of microfilariae: After treatment of patients with ivermectin a daily development of 2500 to 4000 uterine microfilariae per female worm was observed. Actual output of microfilariae: Microfilariae left actively the female worms. The daily microfilaria release in vivo was 700 to 900 microfilariae, assessed after treatment of patients with mebendazole. In vitro most worms isolated from untreated patients shed between 500 to 1500 microfilariae per day.-The microfilarial load of 56 adult patients calculated from microfilarial skin counts was 12 million on the average. Taken for granted a mean life span of a microfilaria of 1.0 to 1.5 years, 22 to 47 female worms per patient would suffice to maintain this microfilarial load on a constant level. Excision of all palpable nodules showed a geometric mean of 15.9 female worms in these patients. It is suggested that factors intrinsic in the host and the adult worms partially operate together to regulate and maintain a stable microfilarial density.
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[
Parasitology,
2001]
A model is presented which describes the aggregation of female Onchocerca volvulus in nodules and their distribution in the human population. The basic model is based on a single parameter, the formation probability q, which represents the probability with which incoming larvae form a new nodule. This parameter describes parasite behaviour which cannot easily be recognized in available data without modelling. The estimate for the average formation probability of muq = 0.39 suggests an attraction of the invading infective larvae to already existing nodules or resident worms with probability 0.61. No significant difference in muq was found between the forest and savanna parasite strains. The model can be used inversely to estimate the worm burden of persons from palpation data. The observed variance in the number of nodules per person requires the assumption of a variance-increasing mechanism which was implemented by heterogeneity within the host population (extended model with 2 parameters). Possible reasons for this heterogeneity are presented and its implications concerning the reproductive biology of the parasite are discussed.
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[
Trop Med Parasitol,
1993]
The quantitative aspects of onchocerciasis that focus on the relationship between the number of adult female Onchocerca volvulus in the human host, the total numbers of microfilariae (mfs) in the body at any one time, and the numbers of infective larvae (L3) to which a person is exposed over time, have received little attention. This paper attempts to investigate the problem from three starting points:--(a) using the numbers of palpable nodules to estimate the numbers of productive female worms in the body; (b) using the concentrations of mfs in skin snips to estimate the total load of mfs in the body; and (c) using the Annual Transmission Potential to estimate the numbers of L3 developing to productive female maturity. By analysing published and unpublished data relating to the forest and savanna zones of West Africa, these three approaches indicate that a relatively large proportion of adult female worms lie deep in the body and are impalpable from the surface. They also provide numerical results, of the same order to magnitude, which can be linked to provide estimates of the total numbers of adult female worms and mfs in the bodies of persons with infections of different intensities.