Abstract: The purpose of the study was to evaluate therapeutic effects of dry suspension of andrographolide against diarrhea in broiler challenged with pathogenic Escherichia coli, and thus to provide experimental evidence supporting its clinical application. Three hundred healthy broilers at 12 days old were randomly divided into 5 groups with 4 replicates in each group and 15 broilers per replicate. Including negative control (no challenge), positive control (E. coli ETECO101 challenge only) and 3 treatment groups (ETECO101 challenge plus 100 or 200 mg·L^-1 of dry suspension of andrographolide, or ETECO101 challenge plus 60 mg·L^-1 of colistin). Starting from day 12 of age, broilers in the 3 treatment groups received colistin or dry suspension of andrographolide for 6 days in drinking water consecutively. Amount of 700 mL drinking water containing colistin or andrographolide were supplied twice a day for each replication. On day 15 of age, broilers in the 3 treatment groups and the positive control group were challenged with 0.2 mL saline containing 5×10⁸ cfu·mL^-1 pathogenic E. coli ETECO101 via intraperitoneal injection, while the negative control was intraperitoneally injected with 0.2 mL of normal saline. Among 4 replicates, one of the replicates in each group (15 broilers) was used for blood and organs sampling at 24 h and on day 5 post challenge (6 broilers for each time point). Liver histopathology, small intestine microstructure, immune organ index, and serum biochemical parameters were determined. The other 3 replicates in each group (45 broilers) were used to evaluate the efficacy of the drugs, including survival rate, fecal score, average daily gain(ADG), and average daily feed intake(ADFI) until day 7 post challenge. The results showed that, compared with the positive control, treating with 100 or 200 mg·L^-1 of dry suspension of andrographolide resulted in an increase in broiler survival rate by 13.3 and 17.8 percentage point, respectively (P<0.05), alleviated diarrhea caused by E. coli ETECO101 challenge, and significantly improved broiler ADG (P<0.01) and ADFI (P<0.01). In comparison with the positive control, treating with 200 mg·L^-1 of dry suspension of andrographolide significantly increased the spleen and bursa indexes (P<0.01) of broilers at 24 h post challenge, decreased levels of serum GLB and ALB on day 5 post challenge (P<0.05), effectively suppressed the increase of MDA content in serum caused by E. coli challenge, and enhanced activities of serum SOD (P<0.01) and GSH-Px (P<0.01). Meanwhile, the height of jejunum villi (P<0.05) and the ratio of villi height to crypt depth in the group of 200 mg·L^-1 of dry suspension of andrographolide were significantly higher than those in the positive control. In summary, supplementation of dry suspension of andrographolide in drinking water could reduce broiler death rate caused by E. coli ETECO101 infection, improve broiler growth performance, alleviate serum oxidative stress, improve intestinal morphology, increase bursa index, and thereby show remarkable therapeutic effects against broiler diarrhea caused by E. coli ETECO101 infection.

Key words: dry suspension of andrographolide, broiler, Escherichia coli, diarrhea, therapeutic effect evaluation