Investigations into the population structure of the ovine parasitic nematode Haemonchus contortus: A comparison of isolates from differing climatic and geographical regions of New South Wales

Alice Bunyan

Research output: ThesisDoctoral Thesis

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Abstract

Haemonchus contortus (barber’s pole worm) is an economically important and highly pathogenic parasite of small ruminants (sheep and goats) principally residing in regions with summer dominant rainfall. In recent years, anecdotal reports of a change in the distribution and epidemiology of H. contortus have emerged, with an apparent increase of outbreaks occurring in the cooler, winter rainfall regions of southern New South Wales (NSW). The high level of genetic diversity within H. contortus populations has long been recognised, and is considered to be an attributing factor to the adaptive capabilities of the parasite to a range of climates, including those previously considered less favourable for the development of the free-living stages.

The extent of genetic diversity within and among a set of 26 isolates of H. contortus originating from both haemonchosis endemic and sporadic regions of northern and southern NSW respectively, were examined to gain insight into the population structure of H. contortus on a regional scale. The aim was to identify differences between the isolates characterised by their contrasting climatic and geographical origins, using both phenotypic and molecular genetic markers.
Three morphological characters were studied, namely the female vulva region and male bursa, including spicule lengths and distance of the left and right hooks from the spicule tips. Overall, a high level of morphological variation was apparent with some significant differences observed between isolates based on male morphologies. However, no distinct clustering of either male or female morphological traits could be associated with isolate origin or climate.
Some phenotypic divergence was observed in the second study in which a cool climate isolate, Tooma 1 2016, appeared to be more tolerant of cold temperatures in comparison to isolates from warmer areas. Eggs of isolates Bundarra 2014, Book Book 2016 and Tooma 1 2016 were examined for their ability to hatch at different temperatures with Tooma 1 2016 the more successful isolate in which some hatching occurred at 8°C (1.8%) and 11°C (1.7%).
To further test the hypothesis of discontinuity between isolates from northern and southern NSW, two population genetic studies utilising nuclear DNA makers were performed. In the first study, amplification of the guanosine triphosphate (GTP)-binding protein gene (Hc cgta) was undertaken to identify sequence variations and population genetic diversities amongst 17 isolates of H. contortus representing northern NSW/Queensland and southern NSW/Victoria. Genetic variation was high and predominately within isolates (95.33%) with only minor differentiation (3.09%) between the two study groups. In the proceeding genetic study, a panel of single nucleotide polymorphisms (SNPs) was used to assess and compare the genetic diversity and population structure of H. contortus in NSW on a more detailed scale. Differences in allele frequencies were made by allelotyping pooled DNA samples from 16 different H. contortus isolates. No geographical differentiation between northern and southern NSW isolates was evident following a principal component analysis (PCA). Furthermore, pairwise FST values between each of the 16 isolates was low (<0.05).
The high level of diversity observed across the successive studies, is in agreement with previous studies of H. contortus in Australia and many other regions of the world, and can be attributed to their large population size. The low levels of population structure observed in this research, are suggestive of high gene flow between northern and southern regions of NSW. Although mechanisms of gene flow were not evaluated within this study, migration of parasites through anthropogenic livestock movement is highly likely. This conclusion has important implications regarding the emergence and spread of drug resistance in nematode populations and serves as a timely reminder of the importance of appropriate worm control measures, including the use of quarantine treatments to prevent further anthelmintic resistance in not only H. contortus but other important gastrointestinal nematodes of sheep.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Charles Sturt University
Supervisors/Advisors
  • Woodgate, Rob, Principal Supervisor
  • Hunt, Peter, Co-Supervisor, External person
  • Shamsi, Shokoofeh, Co-Supervisor
  • Peters, Andrew, Co-Supervisor
Place of PublicationAustralia
Publisher
Publication statusPublished - 2021

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