An eleven-year study of drug resistance in Salmonella in the Netherlands

From 1959 to 1969 a total of 123 070 strains of salmonellae, representing nearly all the strains that have been isolated from animals and man, were collected in the Netherlands and tested for antibiotic resistance.In the course of the study, only a few strains of S. typhi and S. paratyphi B were found to be resistant to chloramphenicol, ampicillin, or tetracycline.From 1959 to 1966 there was a sharp increase in the prevalence of tetracycline resistance in both the human and the animal strains of S. typhimurium and S. panama; subsequently, however, the prevalence declined. During 1966-70 approximately one-third of the tetracycline-resistant human and animal strains of S. typhimurium were also resistant to ampicillin. This type of multiple resistance was only occasionally encountered in the other serotypes. In contrast to resistance to tetracycline and ampicillin, the incidence of S. typhimurium resistant to chloramphenicol remained low during the whole period of observation. In all other serotypes drug resistance remained at a low level.It is shown in this study that the greatly increased use of broad-spectrum antibiotics for medical and veterinary purposes and for animal feeding over the last decade in the Netherlands has not led to the development of serious drug resistance from the medical point of view. An important factor involved in the low incidence of resistance to chloramphenicol may be that R-factors in salmonellae usually lose this resistance determinant rapidly.In the Netherlands the annual mortality figures for all forms of salmonellosis decreased from 0.84 (per 100 000 inhabitants) in 1961 to 0.25 in 1969.