Doctor of Philosophy (PhD)


Veterinary Medical Sciences - Pathobiological Sciences

Document Type



The first event in the infection process of Ostertagia ostertagi in cattle is the process of exsheathment. Before trichostrongylid nematodes can transition from a free-living infective stage larva (L3) on pasture to a parasitic existence within the ruminant host, it must first undergo exsheathment. Exsheathment is the process whereby the L2 cuticle retained from the previous molt is cast from the L3. Exsheathment enables the developmental transition from a free-living stage on pasture to a parasitic existence in the bovine host. For those species with a predilection site in the abomasum, such as O. ostertagi, exsheathment is initiated as the larvae pass through the rumen. Although the stimulus for exsheathment is not known, previously reported biochemical studies on exsheathment suggest the role of a carbonic anhydrase (CA). Partial support for this hypothesis comes from the reported failure of the Haemonchus contortus L3 to exsheath following pretreatment with ethoxzolamide, a known inhibitor of CA's. Although convincing, a CA has not been previously reported from a trichostrongylid nematode. Therefore, the objective of this work was to isolate a CA gene from O. ostertagi L3 and subsequently quantitatively measure its expression during in vivo exsheathment of O. ostertagi L3. This work resulted in the successful isolation, cloning and sequencing of a gene that showed 90.5 % sequence identity with the CA eukaryotic consensus sequence and was 78% and 55% similar to the Caenorhabiditis elegans cah-6 and human CAIII isozyme, respectively. This is the first CA isolated from a gastrointestinal nematode parasite. The enzyme was consequently named OoCAIII. The expression pattern of OoCAIII in O. ostertagi L3 suggests this particular CA is not responsible for initiating exsheathment, but perhaps has a role in immediate early developmental events following initiation of exsheathment. Analysis of the first 1,758 bases of the proximal and distal promoter regions of the gene suggested OoCAIII is regulated in part by transcription factors associated with hypoxic signaling and development.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

James C. Williams