Tetracycline consumption and occurrence of tetracycline resistance in Salmonella typhimurium phage types from Danish pigs.

The aims of the present study were to investigate at the farm-owner level the effect of prescribed tetracycline consumption in pigs and different Salmonella Typhimurium phage types on the probability that the S. Typhimurium was resistant to tetracycline. In this study, 1,307 isolates were included, originating from 877 farm owners, and data were analyzed using logistic regression. The analysis showed that both the S. Typhimurium phage type (p < 0.0001) and an increase in tetracycline consumption (p = 0.0007) were significantly associated with tetracycline resistance. In particular, the phage type was strongly associated with tetracycline resistance. A further analysis of data from the Danish Integrated Antimicrobial Resistance Monitoring and Research Programme (DANMAP) indicates that the tetracycline-susceptible phage types only slowly become tetracycline resistant, although tetracycline consumption more than doubled at the national level from 12,000-13,000 kg of active compound in 1996-1998 to 29,000 kg of active compound in 2004. Instead, tetracycline-resistant S. Typhimurium phage types became more prevalent. This suggests that the spread of already established or new resistant clones, rather than conversion of "old" well-established susceptible clones to resistant clones by uptake of resistance genes, explains most of the increased levels of tetracycline resistance in S. Typhimurium in Danish swine production in response to increased tetracycline consumption.