Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Veterinary Medical Sciences - Pathobiological Sciences

First Advisor

Thomas R. Klei


Protective resistance to Strongylus vulgaris infection was examined by immunization of ponies with radiation-attenuated larvae or soluble parasite homogenates. Twelve yearling ponies raised and maintained under parasite-free conditions were divided into four groups which received either radiation-attenuated L$\sb3$; aqueous-soluble adult somatic antigens plus adjuvant; aqueous-soluble larval somatic antigens with excretory/secretory products and adjuvant; or media with adjuvant. Ponies were immunized twice; radiation-attenuated L$\sb3$ were administered orally and soluble antigens or control injections were given intramuscularly. Six weeks following the second immunization, ponies were challenged per os with virulent S. vulgaris L$\sb3.$ Ponies were monitored twice daily for rectal temperatures and signs of discomfort. Hematologic exams were performed weekly. Six weeks following challenge ponies were euthanized and necropsy performed. Challenge larvae were recovered from dissections of the cranial mesenteric artery (CMA) and major branches. Ponies immunized with radiation-attenuated L$\sb3$ had fewest post-challenge febrile episodes, developed anamnestic eosinophilias and were greater than 91% protected from challenge determined by larval recoveries from arterial dissections. Ponies immunized with either Adult or Larval antigens or Controls had similar larval burdens recovered from arterial dissections, thus were equally susceptible to homologous challenge. Ponies which received Adult and Larval immunizations, however, had more severe clinical signs and necropsy lesions, suggesting that prior sensitization to these antigens exacerbated their post-challenge inflammatory response. Histological differences included the number and staining characteristics of eosinophils. Tissues from radiation-attenuated recipients demonstrated severe eosinophilic infiltrates with dark-staining eosinophils which lacked distinct cytoplasmic granules. Sections of the CMA showed that these recipients maintained arterial architectural integrity while ponies which received either Adult or Larval immunizations and Controls all suffered severe arterial destruction secondary to larval migration. Analysis by enzyme-linked immunosorbent assay (ELISA), Western blot and indirect fluorescent antibody (IFA) assay demonstrated differences which further characterized these groups. Radiation-attenuated larvae recipients developed higher antibody recognition of larval surface antigens determined by IFA than their counterparts, but recognized fewer somatic antigens on Western blots. Protection was associated with recognition of antigens on larval surfaces.