**4.3** *Salmonella* **from clinical cases of animals and humans**

In the study by Oloya et al (2007), more *Salmonella* isolates were recovered from feces of apparently healthy feedlot cattle (25.8%) than range or beef cattle (3.9%) or dairy (1.2%) cattle. A similar *Salmonella* prevalence in feedlot cattle had been reported before and been attributed to low hygiene in feedlots (Vanselow et al. 2007; Khaitsa et al. 2007a). Also, previous reports of *Salmonella* prevalence in range cattle (Ranta et al. 2005) and dairy cattle (Sorensen et al. 2003; Huston et al. 2002) have been comparable to what is reported by this study, and have been consistently lower than in feedlot cattle. However, the isolation of *Salmonella* in sick or dead cattle (13.6%) and sick humans (41.2%) was indicative of its increasing role in causing disease in both groups of hosts (Besser et al. 2000; Padungtod and Kaneene 2006). Previous studies have reported lower prevalence of salmonellosis in both humans and cattle in ND (Tumuhairwe et al. 2008) and the US (Tumuhairwe et al. 2007).

Human isolates were more diverse (32 different serotypes) than cattle (9 serotypes) or other domestic animal species with the following predominant serotypes; *S. Typhimurium* (cattle and man), S. *Newport* (cattle, man and turkey) and *S. Heidelberg* (man and turkey) (Oloya et al, 2007). The occurrence of *Salmonella* serovars; Agona, Anatum, Heidelberg, Newport, St. Paul and Typhimurium in turkey and man, Infantis, Mbandaka, Newport and Typhimurium in cattle and man and many other less frequently recovered serotypes in both domestic animals and man, highlights the scope and magnitude of risk of *Salmonella* infection from individual species of domestic animals to man (Besser et al. 2000; Gorman and Adley 2004; Oloya et al. 2007; Padungtod and Kaneene 2006). Previous studies had reported clonal relationships of *Salmonella* serovars from humans and non-animal and animal sources and products (Gorman and Adley 2004; Padungtod and Kaneene 2006; Zhao et al. 2003).

The PFGE results showed occurrence of similar genotypes of *Salmonella* isolates in both domestic animals and humans (Oloya et al, 2007). However, it was not possible to ascertain whether the transmission was from domestic animals to humans or either way. Previous studies (Besser et al. 2000; Gorman and Adley 2004) have provided incriminating evidence against food animals or their products as being responsible for transmission of *Salmonella* to humans. The most common PFGE fingerprint profiles I, II, III and IV had strong cattle and human involvement (Figure 2). Since *Salmonella* serovar Typhimurium was a major infection in both domestic animals and humans the isolation of *Salmonella* serotypes with similar PFGE fingerprints profiles in both groups confirms existence of common clones or genotypes between human and animal sources and suggests occurrence of an epidemic strain circulating between the two groups (Tsen et al. 2002). Interestingly, the isolation of serovars with the exact similar PFGE fingerprint patterns in cattle preceded those in

Antimicrobial Drug Resistance and Molecular Characterization

shown among isolates from a foodborne outbreak (Nielsen, 2009).

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

resistance in humans.

**5. Conclusion** 

of *Salmonella* Isolated from Domestic Animals, Humans and Meat Products 247

demonstrated that although there were similarities in *Salmonella* genotypes responsible for infection in both domestic animals and humans in the 2000-2005 period, both the AMR and multidrug resistance levels in animals were higher than in humans suggesting that resistance acquired in domestic animals did not translate directly into the burden of

Greene et al (2008) conducted a nationwide study in the US to test for regional differences in risk factors for human infection with salmonellosis. The study analyzed distributions of the two most prevalent MDR Salmonella phenotypes in the United States, 2003-2005: (i) MDR-ACSSuT (resistant to at least ampicillin, chloramphenicol, streptomycin, sulfonamides, and tetracycline) Typhimurium; (ii) MDR-AmpC (resistant to at least ampicillin, chloramphenicol, streptomycin, sulfonamides, tetracycline, amoxicillin/clavulanic acid, and ceftiofur, and with decreased susceptibility to ceftriaxone) Newport. Participating public health laboratories in all states forwarded every 20th Salmonella isolate from humans to the National Antimicrobial Resistance Monitoring System for Enteric Bacteria for antimicrobial susceptibility testing. Among the serotypes Typhimurium and Newport isolates submitted 2003-2005, pansusceptible, MDR-ACSSuT Typhimurium, and MDR-AmpC Newport were identified. Patterns of resistance, demographic factors, and cattle density were compared across regions. Of 1195 serotype Typhimurium isolates, 289 (24%) were MDR-ACSSuT. There were no significant differences in region, age, or sex distribution for pansusceptible versus MDR-ACSSuT Typhimurium. Of 612 serotype Newport isolates, 97 (16%) were MDR-AmpC, but the percentage of MDR-AmpC isolates varied significantly across regions: South 3%, Midwest 28%, West 32%, and Northeast 38% (p < 0.0001). The South had the lowest percentage of MDR-AmpC Newport isolates and also the lowest density of milk cows. More Newport isolates were MDR-AmpC in the 10 states with the highest milk cow density compared with the remaining states. Overall, 22% of pansusceptible Newport isolates but only 7% of MDR-AmpC Newport isolates were from patients <2 years of age. For both serotypes, MDR phenotypes had less seasonal variation than pansusceptible phenotypes. This was the first analysis of the distribution of clinically important MDR Salmonella isolates in the United States. MDR-ACSSuT Typhimurium was evenly distributed across regions. However, MDR-AmpC Newport was less common in the South and in children <2 years of age. Information on individuals' exposures was needed to fully explain the observed patterns. Moreover, another study (Nielsen, 2009) reported variation in antimicrobial resistance in sporadic and outbreak-related Salmonella enterica serovar Typhimurium from patients in Denmark. Variation in antimicrobial resistance and corresponding changes of SGI1 were

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 (Torpdahl et al. 2006).

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 profiles of salmonella isolates from humans in the same period.

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

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 demonstrated that although there were similarities in *Salmonella* genotypes responsible for infection in both domestic animals and humans in the 2000-2005 period, both the AMR and multidrug resistance levels in animals were higher than in humans suggesting that resistance acquired in domestic animals did not translate directly into the burden of resistance in humans.

Greene et al (2008) conducted a nationwide study in the US to test for regional differences in risk factors for human infection with salmonellosis. The study analyzed distributions of the two most prevalent MDR Salmonella phenotypes in the United States, 2003-2005: (i) MDR-ACSSuT (resistant to at least ampicillin, chloramphenicol, streptomycin, sulfonamides, and tetracycline) Typhimurium; (ii) MDR-AmpC (resistant to at least ampicillin, chloramphenicol, streptomycin, sulfonamides, tetracycline, amoxicillin/clavulanic acid, and ceftiofur, and with decreased susceptibility to ceftriaxone) Newport. Participating public health laboratories in all states forwarded every 20th Salmonella isolate from humans to the National Antimicrobial Resistance Monitoring System for Enteric Bacteria for antimicrobial susceptibility testing. Among the serotypes Typhimurium and Newport isolates submitted 2003-2005, pansusceptible, MDR-ACSSuT Typhimurium, and MDR-AmpC Newport were identified. Patterns of resistance, demographic factors, and cattle density were compared across regions. Of 1195 serotype Typhimurium isolates, 289 (24%) were MDR-ACSSuT. There were no significant differences in region, age, or sex distribution for pansusceptible versus MDR-ACSSuT Typhimurium. Of 612 serotype Newport isolates, 97 (16%) were MDR-AmpC, but the percentage of MDR-AmpC isolates varied significantly across regions: South 3%, Midwest 28%, West 32%, and Northeast 38% (p < 0.0001). The South had the lowest percentage of MDR-AmpC Newport isolates and also the lowest density of milk cows. More Newport isolates were MDR-AmpC in the 10 states with the highest milk cow density compared with the remaining states. Overall, 22% of pansusceptible Newport isolates but only 7% of MDR-AmpC Newport isolates were from patients <2 years of age. For both serotypes, MDR phenotypes had less seasonal variation than pansusceptible phenotypes. This was the first analysis of the distribution of clinically important MDR Salmonella isolates in the United States. MDR-ACSSuT Typhimurium was evenly distributed across regions. However, MDR-AmpC Newport was less common in the South and in children <2 years of age. Information on individuals' exposures was needed to fully explain the observed patterns. Moreover, another study (Nielsen, 2009) reported variation in antimicrobial resistance in sporadic and outbreak-related Salmonella enterica serovar Typhimurium from patients in Denmark. Variation in antimicrobial resistance and corresponding changes of SGI1 were shown among isolates from a foodborne outbreak (Nielsen, 2009).
