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Immunology of lungworm (Protostrongylus) infections of the Rocky Mountain bighorn sheep Hudson, Robert John


Protostrongylus stilesi, the parenchymal lungworm, has been attributed an important role in the widespread respiratory diseases of the Rocky Mountain bighorn sheep (Ovis canadensis canadensis). This study was conducted to delineate some of the immunologic and non-specific interactions between parasite and host. A procedure was developed for- the immunochemical quantitation of the ovine immunoglobulins IgG(1), IgG(2) and IgM and for the semi-quantitative analysis of IgA. This technique obviated, the preparation of highly specific antisera required for single radial immunodiffusion. The lungworm did not appear to be a significant part of the total antigenic environment of infected animals since no relationship between immunoglobulin levels and parasite activity was detected. Genetic influences were suggested in the levels of IgG(2) which remained at a relatively constant level characteristic of individual animals. The seromucoids were evaluated as correlates of the inflammatory reaction to parasitism. These proteins were useful in detecting changes in parasite activity and bacterial infection. Gastrointestinal disorders associated with severe scouring were accompanied by the disappearance of circulating orosomucoid (α-1 acid glycoprotein). The loss of this low molecular weight protein appeared to be related to vascular leakage. Lungworm infections induced the appearance of homocytotropic antibodies which could be detected in vitro by their ability to sensitize peripheral polymorphonuclear leucocytes for adherence to the larval cuticle. The elution of this antibody from sensitized cells and the inhibition of the adherence reaction with specific antiserum suggested that the reaction was mediated at least partly by IgG(1). The adherence reaction was used to assay homocytotropic activity of serum from infected animals (washed-cell test). This test was correlated with the ability of sera to sensitize skin for anaphylaxis. The effective hypersensitive response, accounting for both sensitizing and blocking activity, was determined by exposing normal cells to larvae in a medium containing serum (decomplemented-serum test). The results of this test paralleled inflammatory changes in parasitized animals, monitored by the levels of serum orosomucoid. Using this method, levels of homocytotropic and blocking antibodies were measured throughout the annual parasite cycle. Preliminary observations indicated that immunogenic inflammation, associated with the "spring rise" and "self cure," resulted from a shift in a dynamic balance between competing antibodies rather than a proportionate increase in homocytotropic activity. The response of peripheral lymphocytes to the presence of larvae or larval extracts was cursorily examined. Although ovine lymphocytes did not respond well enough in culture to draw definitive conclusions, the presence of larval extracts appeared to have a detrimental effect on cell survival and transformation. Even in the presence of EDTA, minute amounts of antibody sensitized lymphocytes for adherence to the larval cuticle. Sera from a wide variety of sources had an inhibitory effect on this reaction. Treatment of larvae with neuraminidase and high concentrations of NaCl engendered improved adherence. This suggested that Protostrongylus had adopted mechanisms similar to the trophoblast or the neoplastic cell which allegedly evade immunologic recognition with a strongly anionic coat. These aspects are discussed in terms of the possible role of Protostrongylus in respiratory diseases of the bighorn sheep, and the exploitation of molecular mechanisms in the control of parasitic diseases.

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