Prevalence, characterisation and management of anthelmintic resistance in gastro-intestinal nematodes of Scottish sheep
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Date
2008Author
Bartley, David Jon
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Abstract
The studies within this thesis have made a valuable contribution to our understanding
of anthelmintic resistance in Scotland and in particular to the prevalence of
benzimidazole (BZ) and ivermectin (IVM) resistance, the expression of multiple
resistance and its management. Parasitic gastroenteritis (PGE) is a major welfare
issue not only for Scottish, UK and European farmers but also for livestock
producers throughout the world. Parasites such as Haemonchus, Trichostrongylus
and Teladorsagia are estimated to cost the sheep industry hundreds of millions of
dollars annually. To date control has largely been achieved using anthelmintics, but
over reliance on anthelmintics has led to the development of multi class anthelmintic
resistance (AR) and the realization that intensive chemoprophylaxis is not a
sustainable approach for the control of nematodoses.
The first two papers contributing to this thesis assessed the prevalence of
benzimidazole (BZ) and ivermectin (IVM) resistance within ovine gastrointestinal
nematode populations in Scotland. The prevalence of BZ resistance in selected
Scottish lowland sheep farms was around 24% in 1991 but this had risen to over 80%
by 2001. The first cases of ivermectin resistance in sheep were only detected in 2001
but a small scale survey in 2004 showed that 35% of the farms (6 from 17) surveyed
had IVM resistance, with Teladorsagia and Trichostrongylus being identified as the
resistant genera. The isolation of a triple class resistant T. circumcincta (MTci5)
population has enabled research to focus on the important issue of the therapeutic
and prophylactic management of this emerging problem. The third and fourth
papers detail a series of controlled efficacy tests conducted on MTci5 that confirmed,
in the short term at least, it should be possible to use a milbemycin (moxidectin;
MOX) or combination treatments, with IVM and one other class of anthelmintic to
control nematodoses (>90% efficacy) caused by adult and/or immature worms.
However the study examining larval susceptibility highlighted the important role that
immature stages can play in the selection and transmission of resistance. Currently
there are no tests that can detect the presence of these resistant larval stages.
The fifth paper outlines parasitological findings from the farm where MTci5
was isolated following the confirmation of multiple class resistance. Substantial efforts were made to find solutions to maintain sustainability and profitability of the
enterprise though ultimately the use of MOX selected for a, predominately
Teladorsagia, population against which the persistent activity of the compound was
only negligible with the reappearance of eggs in faeces occurring between 21 and 28
days post treatment.
Effective sustainable control of AR populations not only requires an
understanding of the phenotypic and genotypic mechanisms that underpin resistance
but also improved means of ensuring that our farmers are made aware of and utilize
identified best practice approaches. The written and verbal responses of the farmers
to questions relating to best practice advice (papers six and seven) would suggest that
many of the recommendations for delaying the selection and transmission of AR
(ACME, Moredun Foundation and sustainable control of parasites of sheep
(SCOPS), DEFRA) are not being followed, recommendations such as the effective
quarantine treatment of newly purchased animals and dosing animals at the
manufacturers’ recommended dose rate were followed by only 20% and 56% of
farmers respectively.