INTRODUCTION: Previous research projects have demonstrated the need for better diagnostic tools to support decisions on medication strategies for infections caused by Escherichia coli F4 (F4) and F18 (F18), Lawsonia intracellularis (LI) and Brachyspira pilosicoli (PILO). This study was carried out as a randomised clinical trial in three Danish pig herds and included 1047 nursery pigs, distributed over 10 batches and 78 pens. The objectives of this study were: (1) to assess the effect of four 5-day treatment strategies (initiated at clinical outbreak of diarrhoea or at fixed time points 14, 21, or 28days after weaning) on average daily weight gain (ADG); (2) to compare the effect of treatment with doxycycline or tylosine on diarrhoea prevalence, pathogenic bacterial load, and ADG; (3) to evaluate PCR testing of faecal pen floor samples as a diagnostic tool for determining the optimal time of treatment.

RESULTS: (1) The four treatment strategies had a significant overall effect on ADG (p=0.01). Pigs starting treatment 14days after weaning had a significantly higher ADG (42 g) compared to pigs treated on day 28 (p=0.01). (2) When measured 2days after treatment, doxycycline treatment resulted in fewer LI-positive pens (p=0.004), lower excretion levels of LI (p=0.013), and fewer pens with a high level of LI (p=0.031) compared to pens treated with tylosine. There was no significant difference in F4, F18 and PILO levels after treatment with the two antibiotic compounds. There was a significant difference (p=0.04) of mean diarrhoea prevalence on day 21 of the study between pens treated with tylosine (0.254, 95% CI: 0.184-0.324), and doxycycline (0.167, 95% CI: 0.124-0.210). The type of antibiotic compound was not found to have a significant effect on ADG (p=0.209). (3) Pigs starting treatment on day 14 in pens where F4, F18, LI or PILO were detected by qPCR on the pen floor had a statistically significant increase in ADG (66g) compared to pigs treated on day 14 in pens where no enteric pathogens were detected (p=0.04).

CONCLUSIONS: The results of this study showed that the highest ADG was achieved when treatment was initiated 14days after weaning in pens where intestinal pathogens were detected. Doxycycline was more effective in reducing diarrhoea and LI excretion levels than treatment with tylosine.