An increasing body of research has indicated that the relatively intensive therapeutic use of ceftiofur in dairy cattle has led to selection and dissemination of multi drug-resistant (MDR) Salmonella containing the blaCMY-2 gene. This resistance determinant confers reduced susceptibility or resistance to extended-spectrum β–lactams (ESBL, such as ceftiofur and ceftriaxone).
Ceftiofur usage in feedlots has been somewhat limited prior to the introduction of a long-acting formulation (EXCEDE; this formulation has an approximate 7-day duration of action). This product is labeled for both treatment and control of bovine respiratory disease in cattle. Since its launch, ceftiofur crystalline free acid has been widely used metaphylactically in high-risk cattle at arrival. Consequently, the intensity of ceftiofur’s use in feedlots has increased appreciably.
Evaluating the effect of ceftiofur on generic E. coli may be a good model for its effect on Salmonella because 1) E. coli are universally present in the GIT of cattle; 2) E. coli and Salmonella are very closely related; and 3) resistance determinants are shared between E. coli in a similar manner to the mechanisms by which they are shared among Salmonella. We did not evaluate Salmonella primarily because its prevalence is much lower than E. coli and to maintain study power, therefore, many more animals and samples would be required making the study cost prohibitive.
The stated objectives for this work were: