mechanistic overview of the key steps involved in pathogenesis of microbial infections

Diseases caused by microbes (i.e., bacteria, viruses, fungi, protozoa, and prions) can have a catastrophic impact on livestock health and production and affect countries economies. typical sequences of steps that lead to microbial diseases are 1) acquiring access to a portal entry 2) encountering targets in mucosae, mucocutaneous junctions, or skin 3) colonizing targets to sustain or amplify the encounter or cross the barrier system to gain access to targets such as leukocytes located in the lamina propria, submucosae or dermis. entry into neurons especially for neurotropic microbes 4) spread locally in the extracellular matrix 5) enter regional blood or lymphatic vessels and leukocyte trafficking or as cell-free microbes within these vessels 6) spread to regional lymph nodes or systemically within lymphatic vessels and or to the blood vascular system 7) colonize and invade new populations of target cells that are unique to a specific organ system. these steps are controlled by virulence factors expressed by their genes that allow microbes to complete the above steps more rapidly, evade or reduce the effects of animal defense mechanisms, and develop resistance to specific antimicrobials. microbes gain access to the portals of entry via ingestion, inhalation, ascending entry, penetration, and direct contact. microbes commonly colonize and injure target cells and or biological substances they synthesize. target cells spread microbes locally, regionally, or systemically via leukocyte trafficking. microbes can use nerve endings and fibers to spread within the nervous system or systemically. pathways of spread used by microbial infections are mucus layer, cilia, microvilli, mucosal epithelial cells, mucosa-associated lymphoid tissue via macrophages, lymphocytes, dendritic cells, transcytosis or junctional complexes, m cells, nerve endings, and retrograde axonal transport. microbes use virulence factors to attach to cells, kill phagocytic cells, block phagocytosis, evade fusion with lysosomes, block killing within phagocytes, weaken or inhibit the host immune response, and enhance replication within phagocytes. adhesion, colonization, invasiveness, and toxigenesis occur during bacterial pathogenesis. enzymatic lysis, pore formation, inhibition of protein synthesis, dysfunction of ion pumps, and selective protein inactivation are examples of the effects of toxins causing diseases in animals.