Scientists in Western Australia have investigated the parasite-killing potential of 37 plants in horses.
In a laboratory investigation, the scientists 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.
Their investigation comes amid growing concern over resistance among parasites to the drugs used for deworming, which are known as anthelmintics.
Many horses now carry parasitic worms that are resistant to one or more of the major groups of dewormers.
No new products are in the pipeline, so it is unlikely that any alternative commercial preparations will become available in the near future.
The lastest issue of Equine Science Update reports that of the 37 plants assessed under laboratory conditions, extracts from seven species completely inhibited larval development. A further 10 species resulted in 90 percent inhibition compared with controls.
The research team then took the seven most effective plant extracts and tested how their inhibitory effects were affected by diluting them.
The most effective plants, Alectryon oleifolius and Duboisia hopwoodii, had an IC50 – that’s the concentration that resulted in a 50 percent inhibition of development – of 47.2μg/ml and 30.9 μg/ml respectively. By comparison, ivermectin had an IC50 of 0.0000817μg/ml – considerably more effective.
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 factor for 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, the researchers stressed.
This study looked at the effect on larval stages, while any anthelmintic would have to be effective against adult worms.
Further work is also needed 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 120 grams 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.
Therefore, it would be feasible to exert control over the worm population by grazing pasture containing the plant or including it in the diet as a feed additive.
The researchers concluded that some Australian plants may be useful in forming part of an integrated parasite management program for horses, but more studies were needed before developing appropriate applications.
The study was conducted by University of Western Australia researchers Stephanie Payne, Zoey Durmic and Philip Vercoe; and Andrew Kotze from the Australian Animal Health Laboratory in Geelong, Victoria.
Australian plants show anthelmintic activity toward equine cyathostomins in vitro.
SE Payne, AC Koetze, Z Durmic, PE Vercoe
Veterinary Parasitology(2013) 196, 153-160