In-Feed vs. In-Water Chlortetracycline Administration on the Fecal Prevalence of Virulence Genes and Pathotypes of Escherichia coli Involved in Enteric Colibacillosis in Piglets

Colibacillosis in nursery pigs, caused by <i>Escherichia coli</i> (ETEC, EPEC, and STEC pathotypes), remains a major economic concern in the swine industry. This study evaluated the effects of in-feed or in-water chlortetracycline (CTC) administration on the fecal prevalence of virulence genes and pathotypes associated with colibacillosis. A total of 1296 weaned piglets (21 days old) were allocated to 48 pens (16 pens/treatment; 27 piglets/pen) and assigned randomly to no CTC, in-feed CTC, or in-water CTC groups. CTC was administered from days 0 to 14. Fecal samples from five piglets per pen on days 0, 14, and 28 were enriched, screened by 11-plex PCR, cultured for pathotypes, and tested for CTC susceptibility and tetracycline resistance genes. None of the 360 fecal samples or 3267 <i>E. coli</i> isolates were positive for <i>bfpA</i> or <i>aggA</i>. Prevalence of <i>estB</i> (96.9%) and <i>astA</i> (92.8%) was highest. ETEC was the dominant pathotype (41.2%), with <i>astA</i> (29%) and <i>estB</i> (21.9%) as predominant enterotoxin genes. CTC administration had no significant effect on fecal prevalence of virulence genes or pathotypes (<i>p</i> > 0.05). <i>stx2</i> and STEC were detected only at day 28, all harboring <i>stx2e</i>. All pathotypes were CTC-resistant, with <i>tetA</i> as the predominant resistance gene.