Reports from the world of equine science
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Australian search for anthelmintic plants
Anthelmintic resistance is a widespread and growing problem. Many horses now carry parasitic worms that are resistant to one or more of the major groups of anthelmintics. What's more there are no new synthetic products in the pipeline. So it is unlikely that any alternative products will become available in the near future. We will have to rely increasingly on other methods of control.
Certain plants have been considered by tradition and folklore to have action against internal parasites. If it turns out that they do indeed inhibit internal parasites, they may eventually yield new anthelmintic compounds.
In the meantime, if they are effective, it might be possible to include them in pasture for horses or to add them to the feed.
Scientists in Western Australia have been investigating the anthelmintic effect of various plant extracts. In a laboratory investigation, they screened extracts of 37 plants to see if they inhibited the development of cyathostome eggs. The results have been published in the journal Veterinary Parasitology.
Extracts from seven species completely inhibited larval development. A further ten species resulted in 90% inhibition compared with controls.
The research team then took the seven most effective plants extracts and tested how their inhibitory effects were affected by diluting them. The most effective plants, Alectryon oleifolius and Duboisia hopwoodii, had IC50 (concentration that resulted in a 50% inhibition of development) of 47.2μg/ml and 30.9 μg/ml respectively. In comparison, ivermectin had an IC50 of 0.0000817μg/ml -
Further tests showed that the effective constituents of many of the plant extracts were likely to be tannins. Tannins may limit palatability, so plants whose anthelmintic properties rely on them may not be ideal candidates.
However, the researchers identified two plants, Acacia melanoxylon and Duboisia hopwoodii, with anthelmintic properties that were not reliant on tannins.
A further consideration was that the plants may contain other constituents with possible adverse effects. For example, D hopwoodii, the most effective plant tested in this study, contains alkaloids such as nicotine and nornicotine, which are toxic for animals.
The search for plants with anthelmintic properties for use in horses is still in the early stages. This study looked at the effect on larval stages, while any anthelmintic would have to be effective against adult worms. Further work also needs doing to identify the active constituents of the plant that are responsible for the anthelmintic effect.
Eventually, it may be possible to isolate and concentrate the active compound. The researchers calculated that a daily intake of 120g of the anthelmintic plants would expose parasitic worms in the horse's colon to concentrations of 1400 μg/ml – the concentration used in the initial screening test. So it would be feasible exert control over the helminth population by grazing pasture containing the plant or including it in the diet as a feed additive.
They conclude: this study “suggests that Australian plants may be useful in forming part of an integrated parasite management program for horses, but more studies are needed before developing appropriate applications.”
For more details see:
Australian plants show anthelmintic activity toward equine cyathostomins in vitro.
SE Payne, AC Koetze, Z Durmic, PE Vercoe
Veterinary Parasitology(2013) 196, 153-
Written by Mark Andrews. Published on line 29.08.13