Antibiotic resistance in human Salmonella isolates are related to animal strains.

In my opinion, the conclusions given in the recent paper published on antimicrobial-resistant Salmonella in your Journal [1] are erroneous and opposite to what emerges from any objective analysis of the data. In contrast to their conclusions, there seems an obvious and clear association between antibiotic-resistant Salmonella isolates in animal and human ones. Their major antibiotic resistance profiles were types 3, 4, 17, 20 and 24. These involved more than 4000 of their isolates and were clearly commonly found in both animals and humans. All had the same resistance phenotype in both humans and animals. Overall, more than 95 per cent of the human isolates were also found in animal isolates. There are many methodological limitations in this paper. The most important is that they have selectively given similar weights to resistance phenotypes with just single isolates as to phenotypes containing hundreds of isolates or more. Importantly, other limitations are that it is based on data from only sick animals in Scotland. This means they were likely causing disease in animals (probably calves) and are unlikely to reflect what is being carried in other food animals asymptomatically. The authors early on appropriately acknowledge and reference papers that show Salmonella DT104 is predominantly spread to humans from food animals via foods. However, they then seem very ambivalent on this issue. The deleterious effects of the spread of non-typhoidal Salmonella isolates to people from food animals were established decades ago. In Britain, in the 1960s, hundreds of people were infected with antibiotic-resistant Salmonella from cattle with six deaths [2]. In Denmark, the vast majority of Salmonella infections are clearly linked to food animals mainly poultry and pigs [3]. By using only isolates from Scottish animals, the authors have assumed that nearly all Salmonella that infect people in Scotland have come from foods that are locally produced and hence captured by their sampling. This is clearly very unlikely. Another bias is that their animal isolates predominantly came from cattle (80%). Only 20 per cent of meat consumed in Scotland is beef [4] and only 62 per cent of raw beef purchased in Scotland is of Scottish origin (and thus about 12% of meats consumed there). Thus, the vast majority of food animals that are the source of foods for people in Scotland (foods imported from England or elsewhere) were not sampled.