Liver abscess in feedlot cattle: association with gastrointestinal histopathology, finishing diet composition, and other risk factors

Abstract

Liver abscess (LA) is an ongoing issue in feedyard cattle and has served as a crucial topic of research in recent years for several reasons. Dairy-influenced animals have become a greater proportion of slaughtered fed cattle and have an increased prevalence of LA. Additional movements to increase sound antimicrobial stewardship practices when managing LA has increased the need for alternative control methods. As the significance of the LA issue has increased, the need has increased for better understanding of LA as a disease process to develop improved mitigation strategies. The objective of this thesis is to evaluate risk factors and potential pathophysiological associations of liver abscess in feedyard cattle. The first chapter is dedicated to a meta-regression of the current literature to evaluate potential associations of LA with finishing cattle performance, diet ingredient composition, and the digestion rate of dietary starch and neutral detergent fiber (NDF). The second study is an evaluation of feedyard mortalities across the feeding period to determine potential associations of liver abscess with comorbidities, cattle demographics, and gastrointestinal histopathology. For the first study, a literature search was performed to identify published cattle finishing studies that provided LA prevalence, finishing diet composition, and other dietary and demographic information. After a literature search, 676 treatment groups (TG) were included from 160 different experiments across 121 different publications. Both LA prevalence and severe LA (SLA) prevalence were evaluated across three meta-regression model structures: total LA prevalence relative to the total cattle in TG, SLA prevalence relative to the total cattle with LA in TG, and SLA prevalence relative to the total cattle in TG. Diet-related moderators significantly associated with LA prevalence were grain (% dry matter), starch digestibility (%²/hour), and fibrous byproduct (FB) NDF digestibility (%²/hour). The only diet-related moderator associated with SLA prevalence relative to the total cattle in TG was grain (% dry matter). The only diet-related moderator significantly associated with SLA prevalence relative to the total cattle with LA in TG was FB NDF digestibility (%²/hour). Overall, grain and fibrous byproduct inclusion rate and digestibility, but no moderators related to roughage ingredients or nutrient digestibility, were associated with LA prevalence and severity. The second study evaluated findings from gross necropsy and histopathology evaluations of feedyard mortalities to identify factors associated with the presence of LA including cattle demographic factors, and gross and histologic gastrointestinal (GI) morphologic abnormalities. A cross-sectional, observational study was performed via gross necropsies of 935 feedyard cattle mortalities across 6 High Plains feedyards during the months of June and July of two consecutive years. The LA status of each mortality was recorded and samples for GI histopathological evaluation were collected from ventral sac of the rumen, duodenojejunal junction, and spiral colon. Grossly, GI pathology was recorded as either abnormal or normal for the mucosa, serosa, and contents of the rumen, small intestine (SI), and spiral colon (SC). Tissues collected were analyzed to collect multiple histologic measurements of various structures. Overall, LA prevalence was lower than expected at 5.3%. Liver abscesses were observed in mortalities from across the feeding period, however there was a higher probability of LA if the animal was 100+ days on feed (DOF) compared to less than 50 DOF at death. Dairy-influenced mortalities, steer mortalities, and mortalities with peritoneal adhesions present at necropsy had a higher probability of LA at time of necropsy, though the observational nature of the study prevents any temporal conclusions of causation. Cases with LA had thinner rumen papillae width, reduced rumen papillae keratin thickness, reduced small intestine (SI) crypt depth, or reduced total SI proprial thickness. Although LA occurred at a lower rate than expected in feedyard mortalities, LA occurred throughout the feeding period and was associated with several demographic factors and with histologic GI morphologic lesions but not with gross abnormalities in the GI tract. In conclusion, both studies focused on liver abscess as the primary health outcome and sought to better quantify potential associations with concurrent pathologies, comorbidities, demographics, diet, and other risk factors. This thesis has contributed to the body of literature evaluating risk factors and comorbidities of liver abscess in feedyard cattle. By investigating LA disease process from multiple perspectives, at different time points, and both on the macroscopic and microscopic level this work will provide information to fill in knowledge gaps surrounding liver abscess development. Further work is needed to utilize risk factor and disease progression knowledge to develop novel mitigation strategies for LA in feedyard cattle.