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[
PLoS Negl Trop Dis,
2011]
BACKGROUND: Ivermectin (IVM) has been used in Ghana for over two decades for onchocerciasis control. In recent years there have been reports of persistent microfilaridermias despite multiple treatments. This has necessitated a reexamination of its microfilaricidal and suppressive effects on reproduction in the adult female Onchocerca volvulus. In an initial study, we demonstrated the continued potent microfilaricidal effect of IVM. However, we also found communities in which the skin microfilarial repopulation rates at days 90 and 180 were much higher than expected. In this follow up study we have investigated the reproductive response of female worms to multiple treatments with IVM. METHODS AND FINDINGS: The parasitological responses to IVM in two hundred and sixty-eight microfilaridermic subjects from nine communities that had received 10 to 19 annual doses of IVM treatment and one pre-study IVM-naive community were followed. Skin snips were taken 364 days after the initial IVM treatment during the study to determine the microfilaria (mf) recovery rate. Nodules were excised and skin snips taken 90 days following a second study IVM treatment. Nodule and worm density and the reproductive status of female worms were determined. On the basis of skin mf repopulation and skin mf recovery rates we defined three categories of response--good, intermediate and poor--and also determined that approximately 25% of subjects in the study carried adult female worms that responded suboptimally to IVM. Stratification of the female worms by morphological age and microfilarial content showed that almost 90% of the worms were older or middle aged and that most of the mf were produced by the middle aged and older worms previously exposed to multiple treatments with little contribution from young worms derived from ongoing transmission. CONCLUSIONS: The results confirm that in some communities adult female worms were non-responsive or resistant to the anti-fecundity effects of multiple treatments with IVM. A scheme of the varied responses of the adult female worm to multiple treatments is proposed.
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[
Lancet,
2007]
BACKGROUND: Ivermectin has been used for onchocerciasis control since 1987. Because of the long-term use of this drug and the development of resistance in other nematodes, we have assessed Onchocerca volvulus burdens, effectiveness of ivermectin as a microfilaricide, and its effect on adult female worm reproduction. METHODS: For the first phase of the study, 2501 individuals in Ghana, from 19 endemic communities who had received six to 18 annual rounds of ivermectin and one ivermectin naive community, were assessed for microfilarial loads 7 days before the 2004 yearly ivermectin treatment, by means of skin snips, and 30 days after treatment to assess the ivermectin microfilaricidal action. For the second phase, skin snips were taken from 342 individuals from ten communities, who were microfilaria positive at pretreatment assessment, on days 90 and 180 after treatment, to identify the effects of ivermectin on female worm fertility, assessed by microfilaria repopulation. FINDINGS: 487 (19%) of the 2501 participants were microfilaria positive. The microfilaria prevalence and community microfilarial load in treated communities ranged from 2.2% to 51.8%, and 0.06 microfilariae per snip to 2.85 microfilariae per snip, respectively. Despite treatment, the prevalence rate doubled between 2000 and 2005 in two communities. Microfilaria assessment 30 days after ivermectin treatment showed 100% clearance of microfilaria in more than 99% of people. At day 90 after treatment, four of ten communities had significant microfilaria repopulation, from 7.1% to 21.1% of pretreatment counts, rising to 53.9% by day 180. INTERPRETATION: Ivermectin remains a potent microfilaricide. However, our results suggest that resistant adult parasite populations, which are not responding as expected to ivermectin, are emerging. A high rate of repopulation of skin with microfilariae will allow parasite transmission, possibly with ivermectin-resistant O volvulus, which could eventually lead to recrudescence of the disease.
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Int J Parasitol Drugs Drug Resist,
2012]
Ivermectin (IVM) has been in operational use for the control of onchocerciasis for two decades and remains the only drug of choice. To investigate the parasitological responses and genetic profile of Onchocerca volvulus, we carried out a 21month epidemiological study to determine the response of the parasite to IVM in 10 Ghanaian endemic communities. Onchocerca nodules were surgically removed from patients in three IVM response categories (good, intermediate and poor) and one IVM naive community. DNA from adult worms was analyzed to determine any association between genotype and IVM response phenotypic. Embryogramme analysis showed significantly higher reproductive activity in worms from poor response communities, which had up to 41% of females with live stretched microfilaria (mf) in utero, despite IVM treatment, compared with good response communities, which had no intra-uterine stretched mf. -tubulin isotype 1 gene has been shown to be linked to IVM selection in O. volvulus and also known to be associated with IVM resistance in veterinary nematodes. We have genotyped the full length genomic DNA sequence of the -tubulin gene from 127 adult worms obtained from the four community categories. We found SNPs at 24 sites over the entire 3696bp. Eight of the SNPs occurred at significantly higher (p<0.05) frequencies in the poor response communities compared with the good response communities and the IVM naive community. Phenotypic and genotypic analyses show that IVM resistance has been selected and the genotype (1183GG/1188CC/1308TT/1545GG) was strongly associated with the resistance phenotype. Since the region in the -tubulin gene where these four SNPs occur is within 362bp, it is feasible to develop a genetic marker for the early detection of IVM resistance.
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[
J Neurogenet
]
I did not set out to study <i>C. elegans</i>. My undergraduate and graduate training was in Psychology. My postdoctoral work involved studying learning and memory in 1mm diameter juvenile <i>Aplysia californica</i>. As a starting Assistant Professor when I attempted to continue my studies on Aplysia I encountered barriers to carrying out that work; at about the same time I was introduced to <i>Caenorhabditis elegans</i> and decided to investigate whether they could learn and remember. My laboratory was the first to demonstrate conclusively that <i>C. elegans</i> could learn and in the years since then my lab and many others have demonstrated that <i>C. elegans</i> is capable of a variety of forms of learning and memory.
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[
PLoS One,
2014]
Onchocerciasis is a debilitating neglected tropical disease caused by infection with the filarial parasite Onchocerca volvulus. Adult worms live in subcutaneous tissues and produce large numbers of microfilariae that migrate to the skin and eyes. The disease is spread by black flies of the genus Simulium following ingestion of microfilariae that develop into infective stage larvae in the insect. Currently, transmission is monitored by capture and dissection of black flies and microscopic examination of parasites, or using the polymerase chain reaction to determine the presence of parasite DNA in pools of black flies. In this study we identified a new DNA biomarker, encoding O. volvulus glutathione S-transferase 1a (OvGST1a), to detect O. volvulus infection in vector black flies. We developed an OvGST1a-based loop-mediated isothermal amplification (LAMP) assay where amplification of specific target DNA is detectable using turbidity or by a hydroxy naphthol blue color change. The results indicated that the assay is sensitive and rapid, capable of detecting DNA equivalent to less than one microfilaria within 60 minutes. The test is highly specific for the human parasite, as no cross-reaction was detected using DNA from the closely related and sympatric cattle parasite Onchocerca ochengi. The test has the potential to be developed further as a field tool for use in the surveillance of transmission before and after implementation of mass drug administration programs for onchocerciasis.
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Parasit Vectors,
2016]
BACKGROUND: Genetic surveillance of the human filarial parasite, Onchocerca volvulus, from onchocerciasis endemic regions will ideally focus on genotyping individual infective larval stages collected from their intermediate host, Simuliid blackflies. However, blackflies also transmit other Onchocerca species, including the cattle parasite O. ochengi, which are difficult to distinguish from the human parasite based on morphological characteristics alone. This study describes a versatile approach to discriminate between O. volvulus and O. ochengi that is demonstrated using parasite infective larvae dissected from blackflies. RESULTS: A speciation assay was designed based on genetic differentiation between O. volvulus and O. ochengi mitochondrial genome sequences that can be performed in high-throughput high-resolution melt (HRM)- or lower throughput conventional restriction fragment length polymorphism (RFLP) analyses. This assay was validated on 185 Onchocerca larvae dissected from blackflies captured from 14 communities in Ghana throughout 2011-2013. The frequency of O. ochengi was approximately 67% of all larvae analysed, which is significantly higher than previously reported in this region. Furthermore, the species distribution was not uniform throughout the study region, with 25%, 47% and 93% of O. volvulus being found in the western-most (Black Volta, Tain and Tombe), the central (Pru) and eastern-most (Daka) river basins, respectively. CONCLUSIONS: This tool provides a simple and cost-effective approach to determine the identity and distribution of two Onchocerca species, and will be valuable for future genetic studies that focus on parasites collected from blackflies. The results presented highlight the need to discriminate Onchocerca species in transmission studies, as the frequency of each species varied significantly between the communities studied.
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[
Annu Rev Genet,
2018]
My trajectory to becoming a plant biologist was shaped by a complex mix of scientific, political, sociological, and personal factors. I was trained as a microbiologist and molecular biologist in the late 1960s and early 1970s, a time of political upheaval surrounding the Vietnam War. My political activism taught me to be wary of the potential misuses of scientific knowledge and to promote the positive applications of science for the benefit of society. I chose agricultural science for my postdoctoral work. Because I was not trained as a plant biologist, I devised a postdoctoral project that took advantage of my microbiological training, and I explored using genetic technologies to transfer the ability to fix nitrogen from prokaryotic nitrogen-fixing species to the model plant Arabidopsis thaliana with the ultimate goal of engineering crop plants. The invention of recombinant DNA technology greatly facilitated the cloning and manipulation of bacterial nitrogen-fixation (nif) genes, but it also forced me to consider how much genetic engineering of organisms, including human beings, is acceptable. My laboratory has additionally studied host-pathogen interactions using Arabidopsis and the nematode Caenorhabditis elegans as model hosts. Expected final online publication date for the Annual Review of Genetics Volume 52 is November 23, 2018. Please see
http://www.annualreviews.org/page/journal/pubdates for revised estimates.
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[
Philosophy of Science,
1998]
In this rejoinder to the three preceding comments, I provide some additional philosophical warrant for the biomedical sciences' focus on model organisms. I then relate the inquiries on model systems to the concept of 'deep homology', and indicate that the issues that appear to divide my commentators and myself are in part empirical ones. I cite recent work on model organisms, and especially C. elegans that supports my views. Finally, I briefly readdress some of the issues raised by Developmental Systems Theory.
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[
Mol Phylogenet Evol,
1994]
We have begun to reconstruct the ancient history of the nematode phylum based on cytochrome c and globin amino acid sequences. The data suggest that the nematode ancestor diverged from a line leading to mammals about 1 billion years ago and that the most recent common ancestor of the extant species Caenorhabditis elegans, Trichostrongylus colubriformis, Nippostrongylus brasiliensis, Ascaris suum, and Pseudoterranova decipiens lived about 550 MY ago. The rhabditids and strongylids emerged as one offshoot of this ancestor, the ascarids as another. Rhabditids and strongylids diverged some 400 MY ago, whereas the genera Trichostrongylus and Nippostrongylus diverged slightly over 200 MY ago. A gene duplication event in the strongylid branch is predicted to have occurred around 250-335 MY ago. There are two globin genes in Nippostrongylus, expressed in anatomically distinct compartments (body and cuticle), and the single sequence from Trichostrongylus is most like the Nippostrongylus body globin gene. A strikingly different duplication event occurred within the same period in the line leading to the extant ascarid genera, creating a single polypeptide containing two globin domains. The genera Ascaris and Pseudoterranova diverged some 150-250 MY ago. Interestingly, the second globin repeat evolved at a faster rate in both species examined. This is possibly related to the acquisition of an unusual carboxyterminal extension, composed of alternating positively and negatively charged residues, that is necessary for the assembly of several monomers into the native polymeric molecules.
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Proc Natl Acad Sci U S A,
2009]
Identification of drug resistance before it becomes a public health concern requires a clear distinction between what constitutes a normal and a suboptimal treatment response. A novel method of analyzing drug efficacy studies in human helminthiases is proposed and used to investigate recent claims of atypical responses to ivermectin in the treatment of River Blindness. The variability in the rate at which Onchocerca volvulus microfilariae repopulate host's skin following ivermectin treatment is quantified using an individual-based onchocerciasis mathematical model. The model estimates a single skin repopulation rate for every host sampled, allowing reports of suboptimal responses to be statistically compared with responses from populations with no prior exposure to ivermectin. Statistically faster rates of skin repopulation were observed in 3 Ghanaian villages (treated 12-17 times), despite the wide variability in repopulation rates observed in ivermectin-naive populations. Another village previously thought to have high rates of skin repopulation was shown to be indistinguishable from the normal treatment response. The model is used to generate testable hypotheses to identify whether atypical rates of skin repopulation by microfilariae could result from low treatment coverage alone or provide evidence of decreased ivermectin efficacy. Further work linking phenotypic poor responses to treatment with parasite molecular genetics markers will be required to confirm drug resistance. Limitations of the skin-snipping method for estimating parasite load indicates that changes in the distribution of microfilarial repopulation rates, rather than their absolute values, maybe a more sensitive indicator of emerging ivermectin resistance.