Increasing incidence of resistance to gentamicin and related aminoglycosides in Salmonella typhimurium phage type 204c in England, Wales and Scotland.

Phage type 204c of Salmonella typhimurium (DT 204c) appeared in bovine animals in 1979. It is now the predominant type in cattle in England, Wales and Scotland and ranks in the 10 most common phage types in humans. All strains of DT 204c have been resistant to at least four antimicrobial drugs. In 1979 and 1980 the most common resistance pattern was that of chloramphenicol, streptomycin, sulphonamides, tetracyclines and trimethoprim (CSSuTTm) but since 1981 strains with additional resistance to ampicillin and neomycin-kanamycin (AK) have predominated. Strains resistant to furazolidone (Fu) have caused sporadic outbreaks. Gentamicin resistance (G) appeared in DT 204c in 1983 and gentamicin-resistant strains are increasing in incidence. With the exception of resistance to furazolidone, drug resistance in DT 204c has been plasmid-mediated. Characterisation of gentamicin resistance plasmids in DT 204c of R-type ACGKSSuTTm has demonstrated the existence of three distinct lines, two of which have been found exclusively in cattle and one in cattle and humans. The misguided and often inappropriate use of antimicrobial drugs in calves has contributed to the appearance of multiresistant strains of DT 204c and positive measures to limit range and levels of antimicrobials available to feed manufacturers may be necessary.