Abstract
Cyathostomins are common intestinal parasites of horses worldwide. Control of these
parasites relies largely on the use of synthetic anthelmintic pastes or drenches, however,
anthelmintic resistance is increasing in prevalence. Hence, there is a need to investigate
new methods of control. The overall objective of this research was to identify
Australian plants with potential to control cyathostomin parasites. The main hypothesis
tested in this thesis was that there are Australian plants with anthelmintic activity
against cyathostomins, and the activity is due to plant secondary compounds. This
hypothesis was addressed in three experiments.
In the first experiment (Chapter 4), a number of Australian plants were screened in vitro
to identify species with anthelmintic activity against cyathostomin larvae. Twenty-nine
out of the 37 crude plant extracts tested had significant inhibitory effects on larval
development. Seven plants, including Alectryon oleifolius and Acacia melanoxylon,
completely inhibited larval development and had IC50 values ranging from 30.9 - 196
μg of extractable solids/mL. This first experiment therefore confirmed that anthelmintic
activity exists amongst Australian plants and that there was potential for them to be
used as novel control methods for cyathostomins in horses.
In the second experiment (Chapter 5), the plant A. oleifolius was investigated further.
Procyanidin A2, a condensed tannin, was isolated from the plant through a process of
bioassay guided fractionation and demonstrated a significant anthelmintic activity in
larval development assays, completely inhibiting development at concentrations as low
as 50 μg/mL and having an IC50 value of 12.57 μg/mL. Procyanidin A2 also
significantly inhibited larval migration at concentrations of 25 μg/mL.
The anthelmintic activity demonstrated by procyanidin A2 in the in vitro assays suggests that it could
play a useful role in controlling cyathostomins in the future.
In the third experiment (Chapter 6), the compound(s) responsible for the anthelmintic
properties of A. melanoxylon was investigated further. A specific structure of an active
compound was unable to be obtained via bioassay-guided fractionation, however,
spectroscopic evidence suggested the compounds were similar to hydroxycinnamic acid
esters known to be present in A. melanoxylon. Analogous compounds were synthesised
or purchased from a commercial source and tested in vitro. Four hydroxycinnamic ethyl
esters had significant anthelmintic activity in the larval development assays. Ethyl
caffeate and ethyl ferulate completely inhibited development at 200 μg/mL, while ethyl
p-coumarate and ethyl isoferulate completely inhibited development at 100 μg/ml.
Additionally, ethyl p-coumarate significantly inhibited larval migration at 800 μg/mL,
and other compounds also showed some activity at higher doses.
The work presented in this thesis demonstrates that Australian plants have anthelmintic
activity against cyathostomin parasites of horses. This is also the first time that
compounds such as procyanidin A2 and hydroxycinnamic ethyl esters have been
demonstrated to have anthelmintic properties. Further investigation, particularly in
vivo, is needed in order to develop appropriate applications. However, the results
indicate that either through using whole plant preparations or purified compounds,
Australian plants could be a useful part of cyathostomin control programs in the future.
parasites relies largely on the use of synthetic anthelmintic pastes or drenches, however,
anthelmintic resistance is increasing in prevalence. Hence, there is a need to investigate
new methods of control. The overall objective of this research was to identify
Australian plants with potential to control cyathostomin parasites. The main hypothesis
tested in this thesis was that there are Australian plants with anthelmintic activity
against cyathostomins, and the activity is due to plant secondary compounds. This
hypothesis was addressed in three experiments.
In the first experiment (Chapter 4), a number of Australian plants were screened in vitro
to identify species with anthelmintic activity against cyathostomin larvae. Twenty-nine
out of the 37 crude plant extracts tested had significant inhibitory effects on larval
development. Seven plants, including Alectryon oleifolius and Acacia melanoxylon,
completely inhibited larval development and had IC50 values ranging from 30.9 - 196
μg of extractable solids/mL. This first experiment therefore confirmed that anthelmintic
activity exists amongst Australian plants and that there was potential for them to be
used as novel control methods for cyathostomins in horses.
In the second experiment (Chapter 5), the plant A. oleifolius was investigated further.
Procyanidin A2, a condensed tannin, was isolated from the plant through a process of
bioassay guided fractionation and demonstrated a significant anthelmintic activity in
larval development assays, completely inhibiting development at concentrations as low
as 50 μg/mL and having an IC50 value of 12.57 μg/mL. Procyanidin A2 also
significantly inhibited larval migration at concentrations of 25 μg/mL.
The anthelmintic activity demonstrated by procyanidin A2 in the in vitro assays suggests that it could
play a useful role in controlling cyathostomins in the future.
In the third experiment (Chapter 6), the compound(s) responsible for the anthelmintic
properties of A. melanoxylon was investigated further. A specific structure of an active
compound was unable to be obtained via bioassay-guided fractionation, however,
spectroscopic evidence suggested the compounds were similar to hydroxycinnamic acid
esters known to be present in A. melanoxylon. Analogous compounds were synthesised
or purchased from a commercial source and tested in vitro. Four hydroxycinnamic ethyl
esters had significant anthelmintic activity in the larval development assays. Ethyl
caffeate and ethyl ferulate completely inhibited development at 200 μg/mL, while ethyl
p-coumarate and ethyl isoferulate completely inhibited development at 100 μg/ml.
Additionally, ethyl p-coumarate significantly inhibited larval migration at 800 μg/mL,
and other compounds also showed some activity at higher doses.
The work presented in this thesis demonstrates that Australian plants have anthelmintic
activity against cyathostomin parasites of horses. This is also the first time that
compounds such as procyanidin A2 and hydroxycinnamic ethyl esters have been
demonstrated to have anthelmintic properties. Further investigation, particularly in
vivo, is needed in order to develop appropriate applications. However, the results
indicate that either through using whole plant preparations or purified compounds,
Australian plants could be a useful part of cyathostomin control programs in the future.
| Original language | English |
|---|---|
| Qualification | Doctor of Philosophy |
| Awarding Institution |
|
| Award date | 13 Jul 2016 |
| Publication status | Unpublished - 2015 |