| Abstract |
- Hemorrhagic bowel syndrome (HBS) is a devastating disorder affecting high producing dairy cattle. As an important emerging disease (Animal and Plant Health Inspection Service, 2003), HBS is now estimated to account for > 2% of the deaths of dairy cattle (Kirkpatrick et al, 2001). However, little is known about the causative agents or factors contributing to HBS. In the first part of this study, I tested the hypothesis that HBS is associated with an invasive mold: A. fumigatus. To test this hypothesis, I completed a series of epidemiological studies. They included: an analysis of control and HBS samples collected from local herds; a broader analysis of control and HBS samples collected from across the US; and also a collaborative study with the Wisconsin Veterinary Diagnosis Laboratory. For these studies, I developed a real-time quantitative polymerase chain reaction (PCR) assay to determine the amount of A. fumigatus DNA present in blood and tissues of HBS and healthy cows. We also adopted a multiplex PCR assay to detect C. perfringens toxin genes in the same samples. From the first study, it was determined that a positive association exists between HBS and A. fumigatus, but not with C. Perfringens. In the second analysis using blood samples collected from across the US, I found a high association of A. fumigatus DNA with HBS (X² = 27.6836, P < 0.00 1) but not with sudden death, downer cows, bloody diarrhea and other diseases (X² = 0.0442 - 2.42, P < 0.1198 - 0.8335). In the collaborative experiment with the Wisconsin Veterinary Diagnostic Laboratory (WVDL), we evaluated the associations between various gastrointestinal diseases (including HBS) with GI pathogens (A. fumigatus, C. perfringens, Salmonella and BVDV). A unique aspect of this study was the use of cows dying from other gastrointestinal diseases as controls. Results indicated there is an association between A. fumigatus and HBS (X² = 12.15, P [less than or equal to] 0.001) but not between C. perfringens Type A and HBS (X² =0.532, P [less than or equal to] 0.466). A. fumigatus has potential to infect immunocompromised individuals. Hence, I proposed that HBS arises from a combination of A. fumigatus exposure and immunocompromise. To test whether a combination of A. fumigatus exposure and immunosuppression could induce HBS, we conducted one sheep study. I also tested the ability of a commercial feed product to eliminate "HBS" and to regulate innate immune function. In this study, We failed to develop an HBS model in sheep. However, I discovered mechanisms-of-action of a commercially-available feed additive. I determined that dexamethasone injection provides a model for immunosuppression in growing sheep. Specifically, daily injection of dexamthasone reduced concentration of both L-selectin and IL-1[beta]. The feed additive restored the L-selectin and interleukin- 1[beta] (IL-1[beta]) expression levels in immune-suppressed sheep. In order to further understand the mechanisms by which the nutritional product stimulates innate immune function, I used cDNA microarray analysis to examine gene expression in neutrophils of post-parturient cows. More than 20 genes, including IL-4 receptor, IL-1 [beta] converting enzyme, angiopoietin-2, epithelin-1 and -2, nucleoporin, and wilms tumor 1, were influenced (P<0.05) by addition of the feed product to the diets of dairy cattle. Independent confirmation of altered expression of these genes used quantitative real-time RT-PCR (Q-RT-PCR). I found that the feed product up-regulated IL-4 receptor, IL-1[beta] converting enzyme, upstream stimulatory factor, nucleoporin and down-regulated angiopoietin, hepatitis B virus-associated factor receptor and forkhead transcription factor. Many of these gene products are involved in life-expectancy of neutrophils through control of apoptosis. I propose that the feed product enhances neutrophil function, in part, via increasing L-selectin and IL-1 [beta] expression and via inhibition of apoptotic signaling. This should enhance circulation of neutrophils to effectively access infection sites and kill pathogens. In summary, I determined that HBS is significantly associated with A. fumigatus. I also evaluated a commercial feed additive which is currently marketed to prevent HBS. The additive was able to increase markers of innate immune function in both sheep and dairy cattle and I propose that these mechanisms account for its efficacy.
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