Medicinal plants for helminth parasite control: facts and fiction

Abstract

The use of medicinal plants for the prevention and treatment of gastro-intestinal parasitism has its origin in ethnoveterinary medicine. Although until recently the majority of the evidence on the antiparasitic activity of medicinal plants was anecdotal and lacked scientific validity, there is currently an increasing number of controlled experimental studies that aim to verify and quantify such plant activity. There are indeed a large number of plants whose anthelmintic activity has been demonstrated under controlled experimentation, either through feeding the whole plant or administering plant extracts to parasitised hosts. However, contrary to traditional expectation, there are also a great number of plants with purported antiparasitic properties, which have not been reproduced under experimental conditions. In this paper, we discuss the source of such inconsistencies between ethnoveterinary wisdom and scientific experimentation. We focus on the strengths and weaknesses of the existing methodologies used in the controlled studies to determine the activity of antiparasitic plants. We discuss issues like the seasonal and environmental variability of the plant composition, and how this can affect their antiparasitic properties and highlight the importance of identifying the mechanisms of action of such plants and the target parasite species. In addition to their antiparasitic properties, medicinal plants may also have anti-nutritional properties, which can affect animal performance and behaviour. For this reason, we emphasise the need for considering additional dimensions when evaluating medicinal plants. We also question whether using similar criteria as those used for the evaluation of anthelmintics is the way forward. We propose that a holistic approach is required to evaluate the potential of medicinal plants in parasite control and maximise their benefits on parasitised hosts. Keywords: bioactive plants, ethnoveterinary medicine, medicinal plants, parasites, ruminants IntroductionFor centuries, medicinal plants have been used to combat parasitism, and in many parts of the world are still used for this purpose. In ethnoveterinary medicine, which draws inspiration from traditional practice, there seems to be a range of plant/s or plant extract suitable for treating almost every parasitic disease of livestock (International Institute of Rural Reconstruction (IIRR), 1994). For example, seeds of garlic, onion and mint have been used to treat animals that suffer from gastro-intestinal parasitism, whereas extracts of the tobacco plant have been used to treat the skin of livestock afflicted with external parasites (Guarrera, 1999). Leaves, dried flowers and oil from Chenopodium ambrosioides, a shrub that originated from Central America and has been distributed around the world, have all been used as anthelmintics since the early 1900s (Guarrera, 1999).Reports from around the world include exhaustive lists of plants that have been reported to have medicinal properties (Hammond et al., 1997;Akhtar et al., 2000;Waller ...

Exploring the anthelmintic properties of Australian native shrubs with respect to their potential role in livestock grazing systems

“…The imperative to identify alternatives to drugs for control of parasites is apparent in the areas targeted by the Enrich project which show widespread resistance to existing anthelmintic drugs (Besier and Love, 2003). There is extensive literature on the use of plants to provide an anthelmintic component to the diet of grazing livestock (reviewed by Athanasiadou and Kyriazakis, 2004 ;Hoste et al 2006 ;Athanasiadou et al 2007) ; however, there has been no study in the past of the potential role of Australian native shrub species in this regard. The present study was therefore initiated to use in vitro bioassays to identify shrub species that possess significant anthelmintic activity.…”

Assessing the usefulness of mineral licks containing herbal extracts with anti-parasitic properties for the control of gastrointestinal helminths in grazing sheep – a field trial

“…Furthermore, in the trial conducted by Luginbuhl et al (2006), another commercially available herbal de-wormer was ineffective in reducing EPG counts in goats, and in the study by Bouilhol et al (2001) there were no apparent anti-parasitic effects of fi ve different herbal preparations on gastrointestinal helminths in grazing sheep. Contrary to the aforementioned fi eld studies, a number of in vivo and in vitro experiments have been supportive of the effi cacy of using plant extracts for the control of gastrointestinal parasites, especially the nematodes, in small ruminants (Githiori et al, 2006;Athanasiadou et al, 2007) and others reported inconsistent outcomes (Torres-Acosta & Hoste, 2008). A high dose of A. sativum extracts -one of the plants used in the preparation of mineral licks tested in the present trial -reduced the nematodes' EPGs in sheep (Masamba et al, 2010).…”

“…It is, therefore, feasible that the doses of extracts used by the manufacturer of the Star Bloc Phyto Vers mineral supplements are not high enough to produce therapeutic or anti-parasitic effects. On the other hand, continual oral administration of the larger amounts of some herbal extracts contained in this product could be harmful to animals because of their potential toxicity and hence relatively narrow ranges of therapeutic doses (Waller et al, 2001;Athanasiadou et al, 2007). In sheep with the mixed nematode infections and presence of blood-suckling species such as H. contortus or T. circumcincta the estimated threshold levels for helminthiases are 50 -800 EPGs for light, 800 -1200 EPGs for moderate, and >1200 EPGs for heavy infections (Hansen & Perry, 1994).…”

“…There are a few other methods of parasite control currently used in organic production systems, such as rational pasture management (Cabaret et al, 2002;Torres-Acosta & Hoste, 2008), exploitation of intrinsic/genetic resistance of sheep (Nowosad et al, 2003), or nutritional modulation of the host's immune responses and gut environment (Waller & Thamsborg, 2004). The latter refers to controlled grazing on bioactive forages or feeding condensed tannins as nutraceuticals (Min & Hart, 2003;Tzamaloukas et al, 2005;Torres-Acosta & Hoste, 2008;Houdijk et al, 2012), although controversy regarding the use of tannins is constantly rising as their excessive consumption has been associated with the reduction in food intake and digestibility as well as impaired rumen metabolism (Athanasiadou et al, 2007). Based on several earlier studies, certain plants are more effective against specifi c parasites than the others (Tzamaloukas et al, 2005;Athanasiadou et al, 2007).…”

“…However, the incidents of drug-resistant populations of worms along with the emerging trend to limit the application of chemical agents, especially in organic farming, has prompted the development of alternative approaches to anti-parasitic therapies (Cabaret et al, 2002;Waller & Thamsborg, 2004;Torres-Acosta & Hoste, 2008). A broad range of ethnoveterinary therapeutic products, most of them utilizing various medicinal plants with anti-parasitic effects, are currently being evaluated (Akhtar et al, 2000;Waller et al, 2001;Githiori et al, 2006;Athanasiadou et al, 2007). However, the effectiveness of herbal formulations marketed for parasite control in small ruminants under fi eld conditions has been scarcely documented (Bouilhol et al, 2001;Luginbuhl et al, 2006;Burke et al, 2009a).…”