Simple SummaryZinc oxide remains a widely used compound for the control and prevention of post-weaningEscherichia coli(E. coli) diarrhoea in piglets. It is sometimes administered concomitantly with oralE. colivaccines. In this research, we assessed the influence of the administration of zinc oxide in piglet feed in combination with a bivalent vaccine againstE. coli. We studied the immune activation, intestinal integrity, production of secretory IgA, and excretion ofE. colivia faecal samples at different times post-vaccination. Although the main difference observed was the excretion of theE. colivaccine strain, the immune response determined in both vaccine groups was similar, irrespective of the presence of ZnO in the feed.AbstractBackground: Although zinc oxide has been banned at therapeutic doses in the EU, its use is still legal in most countries with industrial pig farming. This compound has been shown to be very effective in preventingE. coli-related diseases. However, another strategy used to control this pathogen is vaccination, administered parenterally or orally. Oral vaccines contain live strains, with F4 and F18 binding factors. Since zinc oxide preventsE. coliadhesion, it is hypothesised that its presence at therapeutic doses (2500 ppm) may alter the immune response and the protection of intestinal integrity derived from the vaccination of animals. Methods: A group of piglets were orally vaccinated at weaning and divided into two subgroups; one group was fed a feed containing 2500 ppm zinc oxide (V + ZnO) for the first 15 days post-vaccination (dpv) and the other was not (V). Faeces were sampled from the animals at 6, 8, 11, 13, and 15 dpv. Unvaccinated animals without ZnO in their feed (Neg) were sampled simultaneously and, on day 15 post-vaccination, were also compared with a group of unvaccinated animals with ZnO in their feed (ZnO). Results: Differences were found inE. coliexcretion, with less quantification in the V + ZnO group, and a significant increase in secretory IgA in the V group at 8 dpv, which later equalised with that of the V + ZnO group. There was also some difference inIFNα,IFNγ,IL1α,ILβ, andTNFαgene expression when comparing both vaccinated groups (p< 0.05). However, there was no difference in gene expression for the tight junction (TJ) proteins responsible for intestinal integrity. Conclusions: Although some differences in the excretion of the vaccine strain were found when comparing both vaccinated groups, there are no remarkable differences in immune stimulation or soluble IgA production when comparing animals orally vaccinated againstE. coliin combination with the presence or absence of ZnO in their feed. We can conclude that the immune response produced is very similar in both groups.