By Russell Daly
While we usually think of a cow’s gut simply as the organ system responsible for digestion and absorption of nutrients, it also plays other critical roles. Essentially a long tube from the mouth to rectum, open to the outside world, the digestive tract is the most substantial interface between the cow’s external environment and her body. The gut is home to the great majority of bacteria found in or on the cow’s body, and, significantly, functions as the largest immune organ of the body.
When the gut functions properly then, the cow benefits from much more than optimal nutrient digestion and absorption. The gut also provides an appropriate barrier against harmful insults (toxins and pathogenic germs) from the external environment. In a healthy gut, the bacterial population is stable, favoring beneficial commensal bacteria at the expense of pathogens. In terms of the immune system, the gut must mount a response that’s neither under-responsive nor over-responsive to potential pathogens.
A recent dairy nutrition meeting held on campus at SDSU focused on the immune function of the bovine gut. Dr. Chris Chase, SDSU Veterinary and Biomedical Sciences Department, along with other participants, spelled out the factors that enhance or undermine this important function.
The gut protects the body from the harmful effects of toxins or pathogens through three lines of defense:
- The physical barrier the gut provides between the outside environment and the cow’s body;
- Biochemical features of innate immunity that neutralize pathogens that breach the barrier; and
- The adaptive immune system, protecting the body against specific threats.
Components of the gut’s physical barrier include a layer of gut surface cells (epithelium) that feature a rapid regenerative ability when damaged or aged, as well as firm physical connections (tight junctions) between cells. A mucus layer, as well as the persistent action of peristalsis, also help prevent pathogens and toxins from binding to epithelial cells. When inflammation (breakdown of tight junctions), dehydration (decreasing mucus secretion), or factors that decrease gut motility are present, the physical aspects of this barrier can be compromised.
Chemical and biological substances of the innate immune system make up the second line of gut defense against pathogens. These include antibodies secreted into the gut lumen that bind and tag pathogens for removal by immune cells. Interestingly, 70% of all antibodies in the cow’s system can be found in the gut. Antimicrobial peptides, molecules secreted by the cells of the epithelium, have direct lethal effects on certain harmful bacteria. Cytokines and other immune messengers regulate the ability of the body’s immune cells to respond to pathogens.
Finally, the active, adaptive immune system is very active at the gut level as well. Immune cells called dendritic cells sample the microbes in the gut and communicate with other immune cells in the lymph nodes. It’s there that B-lymphocytes (responsible for antibody production) and T-lymphocytes (cell-based immunity) are activated to respond to specific antigens.
The meeting focused on ways that nutrition experts, veterinarians, and dairy producers can manage and encourage healthy gut function, such as:
- Preventing dehydration. Besides diminishing the mucus layer, dehydration hampers the activity and flow of immune cells through the bloodstream and lymphatic systems.
- Probiotics. These are beneficial gut bacteria fed to the cow, that help promote a healthy commensal bacterial population at the expense of potential pathogens in the gut.
- Prebiotics. Certain feed additives promote the health and function of gut epithelial cells. Common examples of prebiotics include yeast and various components of yeast cells. These compounds, particularly refined carbohydrates such as mannan oligosaccharide, beta-glucans, and amine-carbohydrates, appear to have direct effects on certain pathogens as well as helping gut epithelial cells carry out their immune capabiliites. These include clumping of certain enteric pathogens so they cannot efficiently bind to the surface of the gut cells, providing nutrients to beneficial commensal bacteria, dampening toxic effects of mycotoxins on epithelial cells, and decreasing the ability of some protozoa to bind to epithelial cell surfaces.
Much work remains to define the particular role(s) these particular feed additives may have in dairy cow and calf productivity and disease prevention. If they can modulate the effects of subclinical diseases and reduce mortality and morbidity, herd profitability and animal well-being could greatly benefit.