Use this URL to cite or link to this record in EThOS:
Title: Circulating biomarkers of Onchocerca volvulus for diagnosis of infection and antifilarial treatment efficacy
Author: Macfarlane, C. L.
ISNI:       0000 0004 7428 6662
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
Availability of Full Text:
Access from EThOS:
Access from Institution:
Onchocerciasis, or "river blindness", is a parasitic disease caused by the filarial worm Onchocerca volvulus. Significant progress in onchocerciasis control in Latin America and Africa, where 99% of cases occur, has made elimination of the disease feasible in some circumstances. However, progression to an elimination programme poses new challenges for diagnosis, as no test can detect current infection with the O. volvulus adult worm, and the detection of microfilariae (mf) in the skin lacks sufficient sensitivity after long-term exposure to treatment with ivermectin. To achieve the new global health goals for onchocerciasis, O. volvulus biomarkers with high sensitivity are needed to map areas with low levels of ongoing transmission and monitor infection recrudescence, while high specificity is required to enable discrimination between closely related filarial worms in areas with overlapping geographic distributions. Although the adult worms live in subcutaneous nodules, these nodules are highly vascularised, allowing potential biomarkers of O. volvulus to present in the host circulation. In this study, a longitudinal plasma sample set collected in Cameroon from individuals with onchocerciasis pretreatment, and at four, 12 and 21 months after following one of three antifilarial treatment regimens, was used to screen for circulating protein, DNA and miRNA markers of infection and macrofilaricidal treatment efficacy in the host. Following the development of a discovery proteomic method for plasma, five individuals infected at baseline and amicrofilaridermic or with low mf burden (four patients and one patient, respectively) at 21 months were analysed. Sixteen circulating O. volvulus proteins were identified, of which 15 were detected 21 months post-doxycycline treatment. Eight proteins were detected in almost every individual at each time point over 21 months, while three parasite proteins changed in detection frequency among individuals by the final follow-up. An uncharacterised O. volvulus membrane protein enriched in female worms, A0A044VCM8, was detected in all individuals and may be a circulating marker for female worm infection. However, the protein was detected consistently over the 21 month follow-up, suggesting it is unlikely to be useful as a marker for treatment efficacy. An analysis of 18 participants before and after treatment with either doxycycline, doxycycline + ivermectin, or ivermectin, detected both parasite-derived miRNAs and O. volvulus-specific DNA in the circulation of the host using RT-qPCR and qPCR, respectively. However, the two parasite-derived miRNAs associated with O. volvulus, miR-71 and lin-4, were negative in almost all plasma samples, and did not have the specificity or sensitivity to be circulating markers for onchocerciasis. The O. volvulus-specific O-150 DNA marker was detected in plasma in almost half of the same 18 individuals pre-treatment, with a decline in the proportion of positive patients detected in all treatment groups over the follow-up timeframe. Of the 58 plasma samples negative for O-150 by qPCR, 36 (62.1%) had microfilaridermia detected by parasitological evaluation. Detection of O. volvulus DNA in the host plasma was therefore was not sufficiently sensitive. No suitable circulating protein, DNA or miRNA markers of infection clearance and treatment efficacy were identified by 21 months post-treatment among the individuals tested. Several factors may have confounded our longitudinal biomarker analyses, such as large gaps in time between sampling in an area of ongoing transmission, where reinfections can occur and influence the prevalence or abundance of circulating biomarkers. Individuals may also have occult L. loa and/or M. perstans infection, and therefore potential biomarkers identified may not be specific for onchocerciasis. Participants in the study may have had incomplete responses to macrofilaricidal treatment, and the variable persistence of adult worms among individuals influenced the circulating biomarker profile. Ideally, biomarkers of active O. volvulus infection and infection clearance following macrofilaricidal treatment would be validated in clinical sample sets from areas of low endemicity or in a confirmed elimination setting, in order to reduce the possibility of reinfections over follow-up. These areas should also be free of coinfective parasites, such as L. loa, M. perstans or W. bancrofti, to ensure that biomarker(s) are detected due to infection with O. volvulus only. Additionally, patients would consistently respond to treatment and show a total macrofilaricidal response. In the absence of a perfect human sample set, animal models, such as new immunodeficient mouse models for onchocerciasis, would also be useful for conducting preclinical studies, where samples can be readily obtained and conditions optimally controlled. Future work should determine in an optimised sample set whether the O. volvulus proteins consistently detected here in plasma are indeed novel circulating markers of infection, in order to progress specific and sensitive targets for future diagnostic development.
Supervisor: Taylor, Mark ; Wagstaff, Simon Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral