Skip to main content
Doctoral thesis, 2022

The genetic basis and monitoring of anthelmintic resistance in the pathogenic endoparasites of small ruminants

Baltrusis, Paulius


By parasitizing livestock, gastrointestinal nematodes inflict a significant monetary
burden on the producers and compromise animal health as well as the sustainability
of animal farming systems. To date, the most reliable way to control parasitic
infections in grazing livestock is through the use of anthelmintic drugs. However,
decades of use have led to the parasites developing resistance. In small ruminants,
one of the most pathogenic and prone to developing resistance is the parasite
Haemonchus contortus. This thesis aims to describe our most recent efforts to update
the current knowledge on anthelmintic resistance in sheep nematodes by studying H.
contortus in order to i) develop molecular tools to screen for mutations associated
with resistance, ii) use these tools to carry out country-wide screenings for resistance
mutations on Swedish sheep farms as well as iii) investigate potential, resistanceassociated
candidate loci and genes through both the single-candidate and the genome-
wide comparison approaches. We developed a sensitive droplet digital PCRbased
tool to screen for benzimidazole resistance-associated mutations and subsequently
found extremely high frequencies of the F200Y mutation but not F167Y in
Swedish, field isolates of H. contortus collected over 6 years from 67 sheep farms.
We also investigated the 63bp deletion in the hco-acr-8 as well as its role as a possible
marker for levamisole resistance and found it to correlate with (but be an unreliable
predictor of) the resistant phenotype. Finally, we carried out a genome-wide
comparison study between the ivermectin treated and untreated population and
found a region in chromosome V to show signs of recent selection, indicating the
importance of that locus in the development of ivermectin resistance in sheep nematodes.


benzimidazole; levamisole; ivermectin; anthelmintic resistance; F200Y; hco-acr-8; droplet digital PCR; sequencing; GWAS; genome

Published in

Acta Universitatis Agriculturae Sueciae
2022, number: 2022:38
ISBN: 978-91-7760-951-3, eISBN: 978-91-7760-952-0
Publisher: Swedish University of Agricultural Sciences

Authors' information

Baltrusis, Paulius
Swedish University of Agricultural Sciences, Department of Biomedical Science and Veterinary Public Health

UKÄ Subject classification


URI (permanent link to this page)