Parasitology
Characterisation studies on anthelmintic resistance in
Teladorsagia
Jackson, F
Roundworm infections have a significant impact globally on the performance
and welfare of livestock. Teladorsagia circumcincta is one
of the commonest parasitic worms found in small ruminants of the UK
and is associated with many cases of parasitic gastroenteritis in
temperate regions. Drug resistance in this parasite species is extensive
and is increasing. Surveys conducted on predominantly Scottish sheep
farms in 2000 and 2004, found the prevalence of benzimidazole resistance
and ivermectin resistance to be around 65% and 35% respectively and
within both surveys Teladorsagia was the pre-dominant resistant nematode.
More worryingly from the UK farmers perspective is that multiple drug
resistance [resistance to all three of the broad-spectrum drug classes
i.e. the Benzimidazoles (white drenches), imidazothiazoles / tetrahydropyrimidines
(yellow/pink drenches) and avermectins (clear drenches)] has been
detected on several farms.
The current bioassays used to detect resistance in the field are relatively
insensitive and cannot be used to detect the earliest stages of developing
resistance against the avermectins. Unless we can detect resistance
at an early stage when the genes for resistance or resistance mechanisms
are at a low frequency in the population then we cannot effectively
manage developing resistance. Work being conducted within this project
is characterising the phenotypic and genotypic expression of resistance
in a MDR Teladorsagia isolate. The studies are seeking molecular,
biochemical and/or genetic markers that can be used to examine individual
worm susceptibility to anthelmintics which would provide tools for
population genetic studies examining rates of change in resistance
gene frequencies.