**5. Conclusion**

246 Salmonella – A Dangerous Foodborne Pathogen

humans, suggesting a difference in timing of outbreak and possibly, the direction of infection from domestic animals to humans. Recent evidence of clustering of *S. Typhimurium*  infection in domestic animals and correspondingly high case reports of the same serovars in humans in the same counties of ND (Oloya et al. 2007), concurs with an earlier observation that region and infection of domestic animals influence S*almonella* occurrence in humans

AMR profiles showed that most domestic animal strains were multidrug resistant (Oloya et al, 2007). Cattle isolates were resistant (>76.5%) to Amoxicillin/clavulanic acid, ampicillin, chloramphenicol, streptomycin and tetracycline, while human isolates were of comparatively lower resistance to the similar individual drugs (1.6-8.1%) or drug combinations. Only 1 human isolate with similar PFGE profile as the main group of cattle isolates, had similar range of multidrug resistance, providing a single evidence of a possible AMR transmission from cattle to humans. Whereas parallel development of resistance in humans as result of using antibiotics that are identical to those used in animals (Phillips et al. 2004; Tumuhairwe et al. 2007) could not be ruled out, this scenario is less likely. Various epidemiological studies (Besser et al. 2000; Padungtod and Kaneene 2006; Zhao et al. 2003) have provided insights into the roles of domestic animals or their products in the transmission of *Salmonella* and associated antimicrobial drug resistance to humans. Occurrence of serovars with similar PFGE profile may suggest that some cases of human salmonellosis are the results of the circulation of certain strains between animal and human hosts (Phillips et al. 2004). However, the occurrence of different AMR profiles within the similar PFGE patterns suggests fairly established strains in which the domestic animal isolates are more subjected to antimicrobial pressure in the production systems (Zhao et al. 2003), hence the higher resistance compared to the human isolates. If the widespread use of antimicrobial agents in animal husbandry is selecting for antimicrobial-resistant serotypes and there is transmission to humans, then these ought to be reflected in the resistance

The presence of resistance to chloramphenicol or drug patterns; amoxicillin-ampicillin and chloramphenicol-kanamycin-tetracycline combinations in humans but not in domestic animals could have equally resulted from use of these antibiotic drugs in humans (Phillips et al. 2004). The fact that most isolates with multi-drug resistance were from cattle and only a single human case had the similar resistance profile suggests that *Salmonella* in cattle or predominantly food animals may not play a significant role in transmitting AMR to S*almonella* in humans. This observation may also support the argument that adequate cooking destroys bacteria in the food (Phillips et al. 2004) and could be that one important barrier to both human infection and AMR transfer. Evidence linking antimicrobial use in food animals to human health risk points to but does not prove a human health threat (Barza and Travers 2002). Attempts could also be made to explain this difference in light of the time lag between time of outbreaks in cattle and humans. Reduction in the antibiotic selection pressure from cattle to humans could result in loss of expression of specific resistance genes (Dowd et al. 2008) as well as loss of the mobile genetic elements responsible

The diverse *Salmonella* serotypes observed infecting man, suggests other possible sources of infection in human environment. Differences could also arise from the fact that not all infections arise directly from farm animals in contact with the farmers, but also from other sources such as pets and contaminated produce (Johnston et al. 2006) or water sources (Phillips et al. 2004) that may not have been captured in this study. In conclusion, this study

profiles of salmonella isolates from humans in the same period.

for resistance (Kang et al. 2006), but this is beyond the scope of this study.

(Torpdahl et al. 2006).

The study on *Salmonella* occurrence from naturally infected feedlot cattle housed at the North Dakota State University cattle feedlot research facility highlighted the genotypic variation in *Salmonella* isolated in healthy feedlot steers and also supported previous reports that not all MDR *salmonella* Typhimurium do carry a wide variety of resistance genes, and also that isolates with the same resistance phenotype often have different resistance genotypes. Also the widespread AMR observed in the majority of *Salmonella* isolates was not matched with presence of integrons, an indication that besides integrons, AMR in *Salmonella* may be explained by other mechanisms that warrant further research. Prevalence

Antimicrobial Drug Resistance and Molecular Characterization

Microbiol. 2009 Apr;58(4):354-9. Epub 2009 Feb 14.

*Veterinary Medical Association*. 210:528–530.

at slaughter, *J. Food Prot.* 65 (2002), pp. 1172–1176

"Attributable Fraction". *Clin Infect Dis* 34 Suppl 3:S126-130.

antimicrobial drug susceptibility. J Food Prot 68:696-702, 2005.

carcasses. *J Food Prot* 65 (2):284-290.

States. *Epidemiol Infect* 124 (2):193-200.

*Dis*. 2:111–114.

hospital in eastern Saudi Arabia. *J Chemother* 19, 62–65.

fluoroquinolones in food animals. *Microb. Drug Resist*. 6:77–83.

(Food Safety Risk Assessment) grants.

*Pathog Dis* 4 (1):16-25.

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of *Salmonella* in grass fed cattle in ND was 7.1%, relatively higher than some studies have reported. *Salmonella* Typhimurium was the most common cause of salmonellosis in animals in North Dakota. *Salmonella* Typhimurium (Copenhagen) serotype was identified as the major serotype that was being shed by ranch beef cattle. The data show that multi-drug resistance was widespread among the *Salmonella* recovered from apparently healthy grass fed cattle. The emergence of multi-drug resistant *Salmonella* reduces the therapeutic options in cases of invasive infections and has been shown to be associated with an increased burden of illness.

The study of salmonella occurrence in dairy cattle demonstrated that a substantial percentage of cattle in this dairy was shedding *Salmonella* in the feces, and antimicrobial resistance among the five *Salmonella* isolates was widespread*.* It is possible that some management practices of dairies related to antimicrobial use may contribute to developing *Salmonella* serotypes that are resistant to antimicrobials. The study on *Salmonella* occurrence in a bison herd indicated that Salmonellae were shed in feces of bison at a comparable prevalence to that of cattle herds in the US, and that the isolates were multidrug resistant. The data contribute to risk assessment of *Salmonella* in bison and highlight the possible existence of antimicrobial resistance in bison. The multi-drug resistance reported among the *Salmonella* isolates warrants further study considering that the serotype *S.* Typhimurium is widely distributed and has the potential of greatly impacting human and animal health. The study on retail meats indicate that turkey meat products from retail stores may occasionally be contaminated with *Salmonella* possessing a varied spectrum of antimicrobial resistance. The contamination was dependent on the type of meat and the time of sampling. These data confirm that both raw and ready to eat retail turkey meat products may be vehicles for transmitting salmonellosis, some of which is resistant to antimicrobials justifying the need for sustained surveillance of foodborne pathogens in retail meats.

The study that compared *Salmonella* isolates from clinical cases of humans and animals reported that human isolates were more diverse than cattle or other domestic animal species. PFGE results confirmed occurrence of similar *Salmonella* genotypes in both domestic animals and humans, with the isolation in cattle preceding those in humans. This suggests a spread of infection from domestic animals to humans. AMR profiles showed that domestic animal strains were multidrug resistant. Only 1 human isolate had similar PFGE profile as cattle isolates with a similar range of multidrug resistance, providing a single evidence of a possible AMR transmission from cattle to humans. This study demonstrated that although there were similar *Salmonella* genotypes from domestic animals and humans, the AMR levels observed in domestic animal isolates was higher than in humans, implying that cattle or food animals may not play a significant role in transmitting AMR to S*almonella* in humans and that the occurrence of resistance in animal isolates may not translate directly into resistance in human isolates in this area.
