ABSTRACT
Most pathogens of poultry and other vertebrates enter the body via a mucosal portal of entry, and for the vast majority, this means either the respiratory or gastrointestinal tract (GIT). It is instructive to remember that the intestinal tract of vertebrate animals is essentially a ‘tube within a tube’, and that which is within the lumen of the intestinal tract is completely external to the host. Given the vast numbers of bacteria, including some opportunistic pathogens, viruses and protozoa that are ubiquitous in the digesta of commercial poultry, it is obviously advantageous that this is so. On the other hand, it is remarkable that a fragile, single cell layer of enteric epithelium is all that separates this enormously complex microbiota within the intestinal tract from the submucosa and systemic entry into the body. While the intestinal mucosa is the target for a plethora of viral and protozoal pathogens, it is also remarkable how few obligatory bacterial pathogens directly target the enteric mucosa, with most bacterial causes of enteropathies being opportunistic in nature.
