Ruminal Epithelial Integrity and Liver Abscesses in Feedlot Cattle

Ruminal epithelium and vascular interface allows absorption of microbial fermentation products from the rumen and serves as a protective barrier to prevent translocation and systemic dissemination of bacteria and bacterial toxins. Ruminal epithelium, in contrast to intestinal epithelium, is composed of stratified squamous epithelial cells and lacks specialized cells, such as goblet cells M cells, Paneth cells, and immune cells. Ruminal bacteria, equipped with an array of degradative enzymes, are evolutionarily adapted to extract nutrients from the diet. Unlike known gut pathogens, ruminal bacteria do not possess virulence factors to facilitate invasion and exploitation of host tissue for nutritional benefit and subvert the host immune system. Ruminal dysbiois in feedlot cattle, which occurs because of rapid rate of fermentation of grain and lack of adequate roughage, promotes the proliferation of opportunistic bacteria and their products and compromise ruminal epithelial integrity leading to pathogenic outcomes and inflammatory responses. Liver abscesses are focal infections resulting from entry, via portal blood that drains the gut, a source of bacteria, and establishment of pyogenic bacteria. The accepted pathogenesis of liver abscesses includes chronic ruminal acidosis that leads to rumenitis, which then allows bacteria to cross the epithelial barrier to enter the portal circulation. The bacterial flora in liver abscesses is dominated by Gram negative bacterial species. Fusobacterium necrophorum, a ruminal bacterium, is the primary causative agent. In recent studies, occurrence of Salmonella enterica has been a common finding, however, the role of Salmonella in liver abscesses is not known. The economic impact of liver abscesses includes liver condemnation, and reduced performance, particularly in cattle with severe abscesses with adhesions. The control of liver abscesses has depended on the use of antibiotics in the feed combined with sound nutritional management to minimize occurrence of ruminal acidosis and subsequent rumenitis. Tylosin, a macrolide, is the most commonly used antimicrobial feed additive. Although there is no evidence of resistance development in F. necrophorum, the use of tylosin in the feed has uncertain future because of the veterinary feed directive. Therefore, there is interest in evaluating antibiotic alternatives, such as essential oils, probiotics, etc. to control liver abscesses. Because liver abscess is an infection and the pathogenicity and virulence factors of F. necrophorum have been characterized, there have been interest and efforts to develop an effective vaccine. Leukotoxin, an exotoxin, and an outer membrane protein of F. necrophorum have been the antigens targeted for the development of vaccines.