title: Worms, sheep, and environment: integrating ecological perspectives into anthelmintic resistance management
Creators: name: Brown, Samuel
Publisher: name: University of Glasgow
Year: 2023

description: Haemonchus contortus is a gastrointestinal nematode (GIN) that infects small ruminants around the globe. Control of H. contortus infections relies on anthelmintics, with the macrocyclic lactone ivermectin (IVM) being frequently used since its release. The mode of action of IVM is unknown in parasitic helminths but is predominantly attributed to the irreversible opening of glutamate-gated chloride channels. However, less than a decade after its commercialisation, resistance to IVM had developed and its prevalence has only increased since. Methods to control GIN infections have been developed to impede the rate of selection for resistance, with Targeted Selective Treatment (TST) being increasingly used. However, various aspects of parasite ecology will have an impact on the effectiveness of TST, such as the capacity of resistant and sensitive worms to interbreed, the mode of resistance inheritance and whether there is a cost to resistance in terms of life history traits or survivorship. Despite a large body of research on IVM resistance in GIN of livestock, the interactions between resistance and parasite ecology are poorly understood. To investigate these interactions, this project utilised two experimental infections to explore how IVM resistance and treatment may impact the ecology of well-characterised laboratory isolates of H. contortus. Firstly, using various genetic markers to genotype broods from a mixed co-infection comprised of sensitive and resistant isolates in equal proportions, evidence of interbreeding between the isolates was shown. However, there was insufficient evidence to detect a barrier to mating, as had been previously identified in a different resistant H. contortus isolate. During the development of the genetic markers, sequences in intronic regions were found to be highly polymorphic relative to the reference genome and to mapped Illumina sequencing data of both the resistant and sensitive populations, suggesting that exonic regions with lower within-strain variability could be more useful for the development of genetic markers. Utilising morphometric analysis of adult worms, host-parasite interactions were found to significantly impact the worm size, with differences in host age between experimental infections resulting in variable responses to IVM treatment. In older donors, where an immune response can occur, IVM treatment significantly reduced the size of resistant adult male and female worms, but in an immune naïve host treatment did not affect the size or resulted in larger female worms from both isolates. Additionally, treatment of resistant females with IVM did not significantly affect the number of eggs in utero, and resistant females were found to have approximately double the egg content compared to sensitive females. To further explore these changes in morphology, the impact of IVM treatment on the metabolism of the resistant isolate was investigated using metabolomic analysis. Although highly resistant to IVM, detrimental metabolic changes in response to IVM treatment were detected, such as increased lipid mobilisation, oxidative stress and increased neurotransmitter turnover. Interestingly, compounds with neuroprotective and antioxidant properties were upregulated in resistant worms, offering further insight into how resistant worms survive IVM treatment. This could suggest a cost to resistance that could be explored in future studies. Lastly, treatment of the first generation of a genetic cross between the resistant and sensitive isolate with IVM indicated that resistance is a partially dominant trait, providing further evidence that IVM resistance is neither recessive nor completely dominant. The impact of IVM treatment on the offspring of the resistant and sensitive isolate was assessed using a larval development test, which found no significant effect of treatment on the viability of offspring from either isolate. However, it was observed that offspring from worms reaching maturity early had reduced viability at the temperature limits for development, which may impact how resistance spreads, given that the sensitive isolate reaches maturity earlier than the resistant isolate. This project has highlighted the importance of parasite ecology in combatting the spread and development of resistance, providing the foundations for future research to further explore the complex interactions between resistance, treatment, and parasite ecology.