A comparative study on the impact of two artemisinin derivatives, artemether and artesunate, on the female reproductive system of Fasciola hepatica
Introduction
Triclabendazole (TCBZ) remains the most effective flukicide for the treatment of fasciolosis in ruminants because of its high activity against the most clinically damaging immature, migratory stage of infection. However, the over-dependence on TCBZ has led to the almost inevitable development of drug-resistant fluke populations. Since the first report of TCBZ resistance (in Australia, by Overend and Bowen, 1995), further well-documented reports have identified resistance in Australia (Walker et al., 2004, Brockwell et al., 2014), in South America (Olaechea et al., 2011, Ortiz et al., 2013) and in a number of European countries, including The Netherlands, UK and Ireland (e.g. Coles et al., 2000, Gaasenbeek et al., 2001, Flanagan, 2010, Mooney et al., 2009, Gordon et al., 2012, Hanna et al., 2015). Taken on its own, the existence of TCBZ resistance in diverse areas of the World is a matter of considerable concern and represents a challenge to management strategies in the future. Other drugs are available commercially, but their activity is confined to the adult stage and so they do not share the same range or spectrum of activity as TCBZ. Moreover, the position of TCBZ as market leader is being compromised by farmers responding to anecdotal reports of resistance by switching to alternative (and less effective or potentially inappropriate) drugs, even when this may not be warranted. However, there is another area of concern, namely, the recent and likely continued future increase in the incidence and spread of the disease (discussed by Fairweather, 2011b). These two issues (of resistance and altered epidemiology) should serve as drivers in the search for new drugs. One group of potential new compounds is the artemisinins.
The artemisinins are important antimalarial agents and anti-schistosomal compounds (e.g. Woodrow et al., 2005, Utzinger et al., 2007, Cui and Su, 2009, Ding et al., 2011, Keiser and Utzinger, 2012). More recently, they have received a considerable amount of attention as potential fasciolicides. A number of semi-synthetic and fully-synthetic artemisinin derivatives have been tested against Fasciola hepatica, and activity demonstrated under in vitro conditions and in rodent (rat) infections, even against a TCBZ-resistant isolate of F. hepatica (Keiser et al., 2006a, Keiser et al., 2006b, Keiser et al., 2007, Duthaler et al., 2010, Duthaler et al., 2012, Zhao et al., 2010, Kirchhofer et al., 2011, Kirchhofer et al., 2012, Wang et al., 2011). This activity has not been fully or consistently translated to infections in larger ruminants such as sheep (Keiser et al., 2008, Keiser et al., 2010a, Keiser et al., 2010b, Meister et al., 2013). Nevertheless, it is important for the future design and testing of new artemisinin derivatives that there is a better understanding of their action against fluke. There have been few morphological studies and they have concentrated on external surface changes to flukes following treatment (Keiser and Morson, 2008a, Keiser and Morson, 2008b, Halferty et al., 2009, O’Neill et al., 2009, O’Neill et al., 2015, Kirchhofer et al., 2011, Duthaler et al., 2012).
The aim of the present investigation was to assess the impact of two semi-synthetic derivatives, artemether and artesunate, on the reproductive capacity of the fluke, by observing changes in those components of the female reproductive system that are involved in egg production, namely, the ovary, vitellaria, Mehlis’ gland and uterus. The main focus of the paper will be on the vitelline cells, which provide shell protein and energy reserves for egg formation. They have a very high level of secretory activity and rapid turnover, are very sensitive to drug action and previously have been good models for studies on drug action against F. hepatica (Fairweather et al., 1988, Skuce and Fairweather, 1990, Stitt and Fairweather, 1996, Colhoun et al., 1998, Halferty et al., 2009, Toner et al., 2011, McConville et al., 2012). Additional histological observations on changes induced by artemether in other female tissues will provide a more complete picture of drug action and the downstream impact on egg production. Furthermore, the results may help to explain the faecal egg count reductions recorded post-treatment with these two artemisinins (Keiser et al., 2008, Keiser et al., 2010a). Two well-defined TCBZ-resistant fluke isolates were used in the study.
Section snippets
Fluke material
Two laboratory-maintained TCBZ-resistant isolates of F. hepatica were used for these experiments: the Sligo and Oberon isolates. The reader is referred to the review on fluke provenance by Fairweather (2011a) for details of in vivo efficacy trials and in vitro experiments carried out with the two isolates, the results of which demonstrate their resistance status. (See also more recent papers by Devine et al., 2011, Devine et al., 2012, Fairweather et al., 2012, Meaney et al., 2013, Savage et
Artemether
Details of visual observations on flukes recovered following treatment with artemether are given in O’Neill et al. (2009). Briefly, flukes were highly active at 24 and 48 h pt, resembling control flukes in terms of their motility but, by 72 h pt, the flukes had become very sluggish and erratic in their movement, although none were totally inactive.
Discussion
It is clear that treatment of TCBZ-resistant isolates of F. hepatica with two artemisinin derivatives, artemether and artesunate, caused progressively severe degenerative changes to the vitelline cells and follicles. The changes had a knock-on effect on egg production, which declined over a 3-day period following treatment with artemether. The results with the two drugs will be compared and will be discussed in relation to the reproductive capacity of the flukes following treatment.
Taking
Acknowledgements
This work was supported by a postgraduate studentship awarded to James O’Neill by the Department of Agriculture and Rural Development, Northern Ireland (DARDNI).
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