Doctor of Philosophy (PhD)


Veterinary Medical Sciences - Pathobiological Sciences

Document Type



To study the protective responses of cyathostome-infected ponies, two challenges were performed employing animals with different histories of exposure to these parasites. The hypothesis developed and to be tested in these experiments was that ponies that had longer exposure to cyathostome contaminated pastures would express acquired resistance to infection. The assumption behind this hypothesis was that helminth-naïve ponies infected with cyathostomes would eliminate the infection using only innate immune responses. Whereas previously exposed ponies would eliminate the infection with acquired immune responses, and these would be more effective in ponies with longer exposure to cyathostomes. Thus, helminth-naïve animals would acquire the largest number of worms, followed in decreasing order by young and then adult ponies. Two types of challenges were used: an experimental infection with 150,000 cyathostome infective third stage (L3) given over a period of 5 days, and the natural acquisition of infection by grazing a cyathostome contaminated pasture for 7 weeks. The natural challenge was performed to confirm the data obtained with the experimental challenge, therefore to validate its use. The parasitological data recovered showed that ponies with acquired resistance to cyathostome infections had reduced total number of worms, of developing larvae, luminal fourth stage larvae, adult parasites and of cyathostome species. The acquisition of resistance was also observed as elevations in the hypobiotic larvae numbers and of intestinal mast cells, intestinal and peripheral eosinophils, and antibody responses. These responses were consistent with increases in Th2 type cytokines, principally IL-4. The data obtained suggest that the immune mechanisms of resistance developed in ponies with acquired protection to cyathostome are slow to develop and are targeted against each parasite stage present in the host. These results warrant further research in the area, especially in the difference between immune mechanisms of helminth naïve ponies and animals with short exposures to cyathostome contaminated pastures.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Thomas R. Klei