Abstract
Analysis of long-term anti-microbial resistance (AMR) data is useful to understand source and transmission dynamics of AMR. We analysed 5124 human clinical isolates from Washington State Department of Health, 391 cattle clinical isolates from the Washington Animal Disease Diagnostic Laboratory and 1864 non-clinical isolates from foodborne disease research on dairies in the Pacific Northwest. Isolates were assigned profiles based on phenotypic resistance to 11 anti-microbials belonging to eight classes. Salmonella Typhimurium (ST), Salmonella Newport (SN) and Salmonella Montevideo (SM) were the most common serovars in both humans and cattle. Multinomial logistic regression showed ST and SN from cattle had greater probability of resistance to multiple classes of anti-microbials than ST and SN from humans (P < 0.0001). While these findings could be consistent with the belief that cattle are a source of resistant ST and SN for people, occurrence of profiles unique to cattle and not observed in temporally related human isolates indicates these profiles are circulating in cattle only. We used various measures to assess AMR diversity, conditional on the weighting of rare versus abundant profiles. AMR profile richness was greater in the common serovars from humans, although both source data sets were dominated by relatively few profiles. The greater profile richness in human Salmonella may be due to greater diversity of sources entering the human population compared to cattle or due to continuous evolution in the human environment. Also, AMR diversity was greater in clinical compared to non-clinical cattle Salmonella, and this could be due to anti-microbial selection pressure in diseased cattle that received treatment. The use of bootstrapping techniques showed that although there were shared profiles between humans and cattle, the expected and observed number of profiles was different, suggesting Salmonella and associated resistance from humans and cattle may not be wholly derived from a common population.
Highlights
Non-typhoidal Salmonella (NTS) are related bacterial pathogens transmitted mainly via food (Majowicz et al, 2010), and via non-foodborne mechanisms (Hoelzer et al, 2011)
Some anti-microbial resistance (AMR) profiles in the three common serovars (ST, Salmonella Newport (SN) and Salmonella Montevideo (SM)) were similar, and unique profiles were observed in each host population
AMR profile richness was greater in the common serovars from humans, both source data sets were dominated by relatively few profiles
Summary
Non-typhoidal Salmonella (NTS) are related bacterial pathogens transmitted mainly via food (Majowicz et al, 2010), and via non-foodborne mechanisms (Hoelzer et al, 2011). Worldwide, they are associated with significant morbidity and mortality in humans and animals. The serovars that cause typhoid fever are restricted to humans while NTS infect multiple hosts. Food animals are considered the main reservoirs of NTS for human infections (Angulo et al, 2004; Hoelzer et al, 2010). Salmonella mostly causes self-limiting gastroenteritis, severe systemic infections do occur in infants, the elderly and immune compromised individuals (Gordon, 2008; Crump et al, 2011). One of the challenges for treating these infections is anti-microbial resistance (AMR) (CDC, 2013)
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