Effect of nitrofurazone or novobiocin on Salmonella enteritidis cecal colonization and organ invasion in Leghorn hens.

Nitrofurazone (433 mg/liter drinking water) administration to leghorn laying hens for 72 hours with a 48-hour withdrawal period before nitrofurazone-sensitive Salmonella enteritidis challenge resulted in a S. enteritidis culture-positive rate significantly (P less than 0.05) higher than that of unmedicated controls when hens were cultured 6 days following challenge. In a similar experiment, simultaneous nitrofurazone administration and S. enteritidis challenge resulted in no significant differences in S. enteritidis isolation frequency. However, unchallenged nitrofurazone-medicated contact control hens showed a significantly higher S. enteritidis culture-positive rate than the unchallenged unmedicated controls. This increase in apparent S. enteritidis susceptibility was associated with significantly lower cecal propionic acid and fewer anaerobic colony-forming units (CFU). In a third experiment, nitrofurazone treatment had no effect when the antibiotic was withdrawn 72 hours before S. enteritidis challenge. In a subsequent experiment, novobiocin (385 mg/kg) or nitrofurazone (300 mg/kg) was administered to adult hens for 7 days before challenge with nitrofurazone and novobiocin-resistant S. enteritidis. Both nitrofurazone and novobiocin administration resulted in a significantly higher S. enteritidis culture-positive rate than unmedicated controls at end of the experiment 7 days post-challenge. Results indicate that administration of some antibiotics may enhance susceptibility to S. enteritidis in leghorn hens.