FIGURE 2.

Bacterial load exposed to AMDs during and after treatment, and the duration of exposure. One important category of bovine respiratory disease is pasteurellosis. Roof (2011) estimated the pathogen load for the entire lung to be 2 × 10⁷–2 × 10⁸ colony forming units (CFU) for Pasteurella multocida and 9 × 10⁶–9 × 10⁸ CFU for Mannheimia haemolytica. Therefore, the estimated pathogen biomass in the lung of an infected cow does not exceed a few mg in toto, whereas the bacterial mass in the animal’s commensal GIT microbiota amounts to several kg. The duration of exposure of the target pathogen can be nil (prophylactic use) or very short (with rapid pathogen eradication during metaphylaxis). In contrast, the duration of intestinal microbiota exposure will never be less than the duration of treatment plus the delay in fully clearing the AMD, together with any newly formed resistant bacteria from the intestinal microbiota, i.e., several weeks (Hansen et al., 2002). The GIT microbiota are continually eliminated at a high rate into the environment, often into an aqueous matrix, thereby allowing further dissemination of the excreted and potentially resistant bacteria. Furthermore, this process will favor horizontal exchanges of resistance factors between organisms within this vast ecosystem. In consequence, this pathway of bacterial elimination (together with their genes) via the excreta is by far the largest connection between animal and human resistomes.