**4.1.2** *Salmonella* **from ranch cattle**

The study by Theis et al (2005, 2007) reported a prevalence of *Salmonella* in ranch cattle of 7.1%. Other researchers (Dargatz et al., 2000) have reported a lower prevalence (1.4 to 4.5%) than that observed by Theis et al (2005, 2007) while others (Fegan et al, 2004) have reported *Salmonella* prevalence as high as 16%. It is possible that the lower prevalence reported by Theis et al (2005, 2007) could have been attributed to the smaller sample (N =212) of cattle compared to that of other researchers. It is also possible that the time of sampling may have influenced the prevalence of *Salmonella* reported. Seasonal changes have been reported to affect *Salmonella* prevalence. Samples collected during the period of April to June and July to September were more likely to be positive than those collected during October to December and January to March (Fegan et al, 2004). The study by Theis et al (2005, 2007) was conducted from September to November, 2004.

The *Salmonella* serotypes identified in beef cattle (Theis et al, 2005, 2007) were *Salmonella* Typhimurium (Copenhagen) (87%) and *Salmonella* Worthington (13%). The presence of *S.* Typhimurium in cattle and the consequent cross contamination of beef carcass tissue are of particular concern as this serotype is one of the most common causes of *Salmonella* infection in developed countries (Gomez et al, 1997). Of the twenty most common *Salmonella* serotypes identified by the Centers for Disease Control and Prevention (CDC) eight (*Salmonella* Typhimurium, Heidelberg, Agona, Montevideo, Braenderup, Enteritidis, Saint Paul, and Thompson) are found in both human and non-clinical nonhuman isolates (Chen et al, 2004). All 15 *Salmonella* isolates recovered by Theis et al (2005, 2007) were resistant to more than 10 antimicrobials which is an indication that multiple antimicrobial resistance was widespread. This should be of concern because of the potential for therapeutic failures. Other studies have found various levels of antimicrobial resistance. For example one study of *Salmonella* isolates in food animals found that of the 209 *Salmonella* isolates tested 112 (53.6%) were resistant to more than one antimicrobial (Johnson et al., 2005). AMR has been a topic of interest in many studies and the results of those studies vary widely. For instance one study of AMR patterns of *Salmonella* isolated from beef cattle (Dargatz et al., 2000) showed that all of the 1314 *Salmonella* isolates tested were susceptible to amikacin, cefotaxime, and ciprofloxacin with only 14% susceptible to all antimicrobials tested. The remaining 86% showed resistance to at least one antimicrobial agent. The most common resistance observed was to tetracycline with ampicillin, and co-amoxiclav was the second most common class that the *Salmonella* serotypes were resistant.

#### **4.1.3** *Salmonella* **from dairy cattle**

In the study by Khaitsa et al (2004) five out of 30 (17%) of the cattle sampled tested positive for *Salmonella*. This result was similar to what had been reported in other dairies (NAHMS, 1996; USDA, 2001) with prevalence values ranging from 5.4% to 75%. This result demonstrated that dairies are a potential source of *Salmonella* for susceptible animals/humans.

dynamics of foodborne pathogens in food animals preharvest and demonstrates their variability in terms of shedding and environmental contamination (Edrington et al., 2004). In order to reduce the prevalence of foodborne pathogens in food animals at slaughter (which could produce significant reductions in the food supply; Hynes et al., 2000), a thorough understanding of the population dynamics of *Salmonella* at the farm level is crucial before implementation of pathogen reduction strategies can be expected to be successful

The study by Theis et al (2005, 2007) reported a prevalence of *Salmonella* in ranch cattle of 7.1%. Other researchers (Dargatz et al., 2000) have reported a lower prevalence (1.4 to 4.5%) than that observed by Theis et al (2005, 2007) while others (Fegan et al, 2004) have reported *Salmonella* prevalence as high as 16%. It is possible that the lower prevalence reported by Theis et al (2005, 2007) could have been attributed to the smaller sample (N =212) of cattle compared to that of other researchers. It is also possible that the time of sampling may have influenced the prevalence of *Salmonella* reported. Seasonal changes have been reported to affect *Salmonella* prevalence. Samples collected during the period of April to June and July to September were more likely to be positive than those collected during October to December and January to March (Fegan et al, 2004). The study by Theis et al (2005, 2007) was

The *Salmonella* serotypes identified in beef cattle (Theis et al, 2005, 2007) were *Salmonella* Typhimurium (Copenhagen) (87%) and *Salmonella* Worthington (13%). The presence of *S.* Typhimurium in cattle and the consequent cross contamination of beef carcass tissue are of particular concern as this serotype is one of the most common causes of *Salmonella* infection in developed countries (Gomez et al, 1997). Of the twenty most common *Salmonella* serotypes identified by the Centers for Disease Control and Prevention (CDC) eight (*Salmonella* Typhimurium, Heidelberg, Agona, Montevideo, Braenderup, Enteritidis, Saint Paul, and Thompson) are found in both human and non-clinical nonhuman isolates (Chen et al, 2004). All 15 *Salmonella* isolates recovered by Theis et al (2005, 2007) were resistant to more than 10 antimicrobials which is an indication that multiple antimicrobial resistance was widespread. This should be of concern because of the potential for therapeutic failures. Other studies have found various levels of antimicrobial resistance. For example one study of *Salmonella* isolates in food animals found that of the 209 *Salmonella* isolates tested 112 (53.6%) were resistant to more than one antimicrobial (Johnson et al., 2005). AMR has been a topic of interest in many studies and the results of those studies vary widely. For instance one study of AMR patterns of *Salmonella* isolated from beef cattle (Dargatz et al., 2000) showed that all of the 1314 *Salmonella* isolates tested were susceptible to amikacin, cefotaxime, and ciprofloxacin with only 14% susceptible to all antimicrobials tested. The remaining 86% showed resistance to at least one antimicrobial agent. The most common resistance observed was to tetracycline with ampicillin, and co-amoxiclav was the second

In the study by Khaitsa et al (2004) five out of 30 (17%) of the cattle sampled tested positive for *Salmonella*. This result was similar to what had been reported in other dairies (NAHMS, 1996; USDA, 2001) with prevalence values ranging from 5.4% to 75%. This result demonstrated that dairies are a potential source of *Salmonella* for susceptible animals/humans.

(Edrington et al., 2004).

**4.1.2** *Salmonella* **from ranch cattle** 

conducted from September to November, 2004.

most common class that the *Salmonella* serotypes were resistant.

**4.1.3** *Salmonella* **from dairy cattle** 

The United States National Animal Health Monitoring System's Dairy '96 study reported 54% of milk cows shed *Salmonella* and 275% of dairy operations had at least one cow shedding *Salmonella* [Wells et al, 1998; NAHMS, 1996]. *Salmonella* has been isolated from all ages of dairy cattle and throughout the production process. Mature dairy cattle typically appear asymptomatic while shedding this pathogen in their faeces (Richardson, 1975; McDonough, 1986; Edrington, 2004; Edrington et al, 2004) and while young calves are more susceptible to salmonellosis, cases in adult cattle have been reported (Gay and Hunsaker, 1993; Anderson, 1997; Sato, 2001). Previous research demonstrated significant variation in the prevalence of faecal *Salmonella* in healthy, lactating dairy cattle, not only among farms across the United States (Edrington et al, 2008) but also in farms within a small geographic area and in individual farms from season to season (Edrington et al, 2004 ) . Additional research examined production parameters (heifers *vs*. mature cows, lactation status, stage of lactation and heat stress) on *Salmonella* prevalence (Edrington, 2004; Fitzgerald et al, 2003). While minor differences were noted in *Salmonella* shedding, results were generally inconsistent with no significant trends noted.

As part of a national study of US dairy operations, another study (Blau et al 2005) conducted between March and September 2002, in 97 dairy herds in 21 states reported an overall prevalence of 7.3% of fecal samples that were culture positive for *Salmonella.* In another study of dairy cattle (Warnick et al. 2003) , *Salmonella* was isolated from 9.3% of 4049 fecal samples collected from a 2 months study of 12 dairy herds originating from Michigan, Minnesota, New York and Wisconsin(Warnick et al, 2003). Also, Fossler et al (2004) sampled dairy cattle to describe the occurrence of fecal shedding, persistence of shedding over time, and serogroup classification of *Salmonella* spp on a large number of dairy farms of various sizes. The design was that of a longitudinal study and the sample population comprised 22,417 fecal samples from cattle and 4,570 samples from the farm environment on 110 organic and conventional dairy farms in Minnesota, Wisconsin, Michigan, and NewYork. Five visits were made to each farm at 2-month intervals from August 2000 to October 2001. Fecal samples from healthy cows, calves, and other targeted cattle groups and samples from bulk tank milk, milk line filters, water, feed sources, and pen floors were collected at each visit. *Salmonella* spp were isolated from 4.8% of fecal samples and 5.9% of environmental samples; 92.7% of farms had at least 1 *Salmonella*-positive sample.

Results from the various studies conducted indicated some variability in the prevalence of fecal shedding of *Salmonella* among the different cattle and production systems sampled possibly due to several factors such as state of origin, treatment with antimicrobials, herd size and season that have previously been reported (Fossler et al, 2005). The study by Fossler et al (2005) that investigated environmental sample-level factors associated with the presence of *Salmonella* in a multi-state study of conventional and organic dairy farms reported that State of origin was associated with the presence of *Salmonella* in samples from cattle and the farm environment; Midwestern states were more likely to have *Salmonella*positive samples compared to New York. Cattle treated with antimicrobials within 14 days of sampling were more likely to be *Salmonella*-negative compared with nontreated cattle (OR=2.0, 95% CI: 1.1, 3.4). Farms with at least 100 cows were more likely to have *Salmonella*positive cattle compared with smaller farms (OR=2.6, 95% CI: 1.4, 4.6). Season was associated with *Salmonella* shedding in cattle, and compared to the winter period, summer had the highest odds for shedding (OR=2.4, 95% CI: 1.5, 3.7), followed by fall (OR=1.9, 95% CI: 1.2, 3.1) and spring (OR=1.8, 95% CI: 1.2, 2.6). Environmental samples significantly more likely to be *Salmonella*-positive (compared to bulk tank milk) included, in descending order,

Antimicrobial Drug Resistance and Molecular Characterization

from samples taken in September.

contamination of bison carcasses post-harvest.

(Beach et al 2002).

of *Salmonella* Isolated from Domestic Animals, Humans and Meat Products 241

a lower prevalence of (3/81 (3.70%), and 1/80 (1.25%) in goats and cattle, respectively, all

The *Salmonella* isolated from bison feces (Khaitsa et al, 2008) belonged to the serotypes *Salmonella* Typhimurium (Copenhagen) and *Salmonella* Worthington. This was not a total surprise since bovine are a common source of *Salmonella* Typhimurium (Cray et al, 2006). It is interesting to note that the same serotypes, *Salmonella* Typhimurium (Copenhagen) and *Salmonella* Worthington, were recovered from cattle on cow-calf operations in North Dakota during the same year35 (Theis, 2006). However, a larger study of beef cattle (Beach et al 2002), reported that the five serotypes most commonly associated with feedlot cattle and their environment were Salmonella Anatum (18.3% of the isolates), Salmonella Kentucky (17.5%), Salmonella Montevideo (9.2%), Salmonella Senftenberg (8.3%), and Salmonella Mbandaka (7.5%). The five serotypes most commonly associated with nonfeedlot cattle and their environment were Salmonella Kentucky (35.4%), Salmonella Montevideo (21.7%). Salmonella Cerro (7.5%), Salmonella Anatum (6.8%), and Salmonella Mbandaka (5.0%)

Other studies9, (Edrington et al 2004) have reported different *Salmonella* serotypes recovered from cattle originating from other states, possibly due to regional differences. In one study (Edrington et al 2004)9 mature dairy cattle were sampled over a 2-year period (2001-2002) on six farms in New Mexico and Texas. Fecal samples (n = 1560) were collected via rectal palpation and cultured for *Salmonella*, and one isolate from each positive sample was serotyped. Twenty-two different serotypes were identified from a total of 393 Salmonella isolates. Montevideo was the predominant serotype (27%) followed by Mbandaka (15%), Senftenberg (11.4%), Newport (6.4%), Anatum (4.8%), and Give (4.8%). Salmonella Typhimurium and Dublin, two frequently reported serotypes, accounted for only 1% of the observed serotypes in this study. A national *Salmonella* study of 97 dairy herds in 21 states in the US reported *Salmonella* Meleagridis (24.1%), Salmonella Montevideo (11.9%), and *Salmonella* Typhimurium (9.9%) as the three most frequently recovered serotypes (Blau et al 2005). It is noteworthy that *Salmonella enterica* serovar Hadar was the major *Salmonella* serotype isolated from processed bison carcasses originating in the same region as our sampled animals25 (Li et al, 2006). In the absence of studies that correlate recovery of *Salmonella* from the same bison pre and post-harvest, it is difficult to ascertain the sources of

In the study Khaitsa et al (2008) both *Salmonella* isolates were susceptible to at least 6 antimicrobials on the panel including the cephalosporin - ceftiofur and the quinolone/fluoroquinolone - enrofloxacin that are clinically important. However, both isolates (100%) demonstrated widespread multi-drug resistance (resitance to ≥ 13 antimicrobials) in a panel of 20.antimicrobials with resistance most frequently to tetracycline, streptomycin, and/or ampicillin). In a larger study (Dargatz et al 2003) of 73 feedlots in 12 states the antimicrobial resistance patterns of *Salmonella* spp recovered were determined. The susceptibilities of all isolates were determined using a panel of 17 antimicrobials. The majority of isolates (62.8%, 441/702) were sensitive to all of the antimicrobials tested. Resistance was most frequently observed to tetracycline (35.9%, 252/702) followed by streptomycin (11.1%, 78/702), ampicillin (10.4%, 73/702) and chloramphenicol (10.4%, 73/702). Multiple resistance (resistance to > or =2 antimicrobials) was observed for 11.7% (82/702) of the isolates. However, overall, most of the *Salmonella* isolates were sensitive to all the antimicrobials tested. Interestingly, antimicrobial testing of *Salmonella enterica* serovar

were; samples from sick pens (OR=7.4, 95% CI: 3.4, 15.8), manure storage areas (OR=6.4, 95% CI: 3.5, 11.7), maternity pens (OR=4.2, 95% CI: 2.2, 8.1), hair coats of cows due to be culled (OR=3.9, 95% CI: 2.2, 7.7), milk filters (OR=3.3, 95% CI: 1.8, 6.0), cow waterers (OR=2.8, 95% CI: 1.4, 5.7), calf pens (OR=2.7, 95% CI: 1.3, 5.3), and bird droppings from cow housing (OR=2.4, 95% CI: 1.3, 4.4). Parity, stage of lactation, and calf age were not associated with *Salmonella* shedding. Another study (Fitzgerald et al, 2003)

that examined factors affecting fecal shedding of *Salmonella* in dairy cattle reported that multiparous lactating cows tended to shed more (P = 0.06) *Salmonella* than primiparous lactating cows (39% vs 27%, respectively), and that parity did not influence (P > 0.10) *Salmonella* shedding in non lactating cows. Unfortunately, information on parity of the cows in Khaitsa et al (2004) was not obtained so comparisons of *Salmonella* prevalence by parity could not be made.

The fact that *Salmonella* isolates recovered by Khaitsa et al (2004) were resistant to more than 10 out of the 20 antimicrobials tested was a concern. Dairy cattle serve as an important reservoir for *Salmonella* and have been implicated in cases of human salmonellosis [CDC, 2003]. In the study by Edrington et al (2008), seven and nine different *Salmonella* serotypes were identified in the healthy and sick dairy cattle, respectively. The serotypes Senftenberg and Kentucky were not detected in any of the healthy cattle and accounted for 34% of the sick isolates. No differences in antimicrobial susceptibility patterns were observed in any the *Salmonella* isolates from sick and healthy cattle. Isolates were susceptible to all antimicrobials examined with the exception of spectinomycin, with three and five isolates resistant in the healthy and diarrhoeic groups, respectively. PFGE was used to compare the genetic relatedness of isolates cultured from the faecal samples of healthy and sick cattle. Seventeen serotypes representing 84 isolates were examined. No genotypic differences were noted when comparing sick *vs*. healthy isolates However, multiple genotypes within serotype were observed for a number of the isolates examined.

#### **4.1.4** *Salmonella* **from bison**

*Salmonella* prevalence of 15% reported in the bison herd was comparable to that reported in cattle herds (Beach et al, 2002; Huston et al, 2002; Warnick et al, 2003) and other livestock (Branham et al, 2005) from the US. This is an indication that *Salmonella* prevalence in bison may be more widespread than is currently known. Unfortunately, not many studies of Salmonella occurrence in bison have been reported; it is possible, Khaitsa et al (2008) was the first of such studies reported. A cross-sectional study of 212 cattle from 7 cow-calf operations in North Dakota reported *Salmonella* spp. shedding point prevalence of 7% (15 of 212) of cattle sampled (Theis, 2006). This prevalence was similar to that reported for bison given the limitation of number of animals sampled in both studies. It is also possible that the time of sampling may have influenced the prevalence of *Salmonella* reported. Seasonal changes have been reported to affect prevalence of *Salmonella* fecal shedding in cattle (Dargatz et al, 2003). Samples collected during the period of April to June and July to September were more likely to be positive than those collected during October to December and January to March (Dargatz et al, 2003). In this study we sampled bison in June 2005 while Theis (2006) sampled cattle from September to November, 2004. Another longitudinal study (Branham et al, 2005) that assessed *Salmonella* spp. presence in white-tailed deer (*Odocoileus virginianus*) and livestock simultaneously grazing the same rangeland, reported *Salmonella* prevalence of 2/26 (7.69%) and 6/82 (7.32%) in deer and sheep, respectively, and

were; samples from sick pens (OR=7.4, 95% CI: 3.4, 15.8), manure storage areas (OR=6.4, 95% CI: 3.5, 11.7), maternity pens (OR=4.2, 95% CI: 2.2, 8.1), hair coats of cows due to be culled (OR=3.9, 95% CI: 2.2, 7.7), milk filters (OR=3.3, 95% CI: 1.8, 6.0), cow waterers (OR=2.8, 95% CI: 1.4, 5.7), calf pens (OR=2.7, 95% CI: 1.3, 5.3), and bird droppings from cow housing (OR=2.4, 95% CI: 1.3, 4.4). Parity, stage of lactation, and calf age were not associated

that examined factors affecting fecal shedding of *Salmonella* in dairy cattle reported that multiparous lactating cows tended to shed more (P = 0.06) *Salmonella* than primiparous lactating cows (39% vs 27%, respectively), and that parity did not influence (P > 0.10) *Salmonella* shedding in non lactating cows. Unfortunately, information on parity of the cows in Khaitsa et al (2004) was not obtained so comparisons of *Salmonella* prevalence by parity

The fact that *Salmonella* isolates recovered by Khaitsa et al (2004) were resistant to more than 10 out of the 20 antimicrobials tested was a concern. Dairy cattle serve as an important reservoir for *Salmonella* and have been implicated in cases of human salmonellosis [CDC, 2003]. In the study by Edrington et al (2008), seven and nine different *Salmonella* serotypes were identified in the healthy and sick dairy cattle, respectively. The serotypes Senftenberg and Kentucky were not detected in any of the healthy cattle and accounted for 34% of the sick isolates. No differences in antimicrobial susceptibility patterns were observed in any the *Salmonella* isolates from sick and healthy cattle. Isolates were susceptible to all antimicrobials examined with the exception of spectinomycin, with three and five isolates resistant in the healthy and diarrhoeic groups, respectively. PFGE was used to compare the genetic relatedness of isolates cultured from the faecal samples of healthy and sick cattle. Seventeen serotypes representing 84 isolates were examined. No genotypic differences were noted when comparing sick *vs*. healthy isolates However, multiple genotypes within serotype

*Salmonella* prevalence of 15% reported in the bison herd was comparable to that reported in cattle herds (Beach et al, 2002; Huston et al, 2002; Warnick et al, 2003) and other livestock (Branham et al, 2005) from the US. This is an indication that *Salmonella* prevalence in bison may be more widespread than is currently known. Unfortunately, not many studies of Salmonella occurrence in bison have been reported; it is possible, Khaitsa et al (2008) was the first of such studies reported. A cross-sectional study of 212 cattle from 7 cow-calf operations in North Dakota reported *Salmonella* spp. shedding point prevalence of 7% (15 of 212) of cattle sampled (Theis, 2006). This prevalence was similar to that reported for bison given the limitation of number of animals sampled in both studies. It is also possible that the time of sampling may have influenced the prevalence of *Salmonella* reported. Seasonal changes have been reported to affect prevalence of *Salmonella* fecal shedding in cattle (Dargatz et al, 2003). Samples collected during the period of April to June and July to September were more likely to be positive than those collected during October to December and January to March (Dargatz et al, 2003). In this study we sampled bison in June 2005 while Theis (2006) sampled cattle from September to November, 2004. Another longitudinal study (Branham et al, 2005) that assessed *Salmonella* spp. presence in white-tailed deer (*Odocoileus virginianus*) and livestock simultaneously grazing the same rangeland, reported *Salmonella* prevalence of 2/26 (7.69%) and 6/82 (7.32%) in deer and sheep, respectively, and

with *Salmonella* shedding. Another study (Fitzgerald et al, 2003)

were observed for a number of the isolates examined.

**4.1.4** *Salmonella* **from bison** 

could not be made.

a lower prevalence of (3/81 (3.70%), and 1/80 (1.25%) in goats and cattle, respectively, all from samples taken in September.

The *Salmonella* isolated from bison feces (Khaitsa et al, 2008) belonged to the serotypes *Salmonella* Typhimurium (Copenhagen) and *Salmonella* Worthington. This was not a total surprise since bovine are a common source of *Salmonella* Typhimurium (Cray et al, 2006). It is interesting to note that the same serotypes, *Salmonella* Typhimurium (Copenhagen) and *Salmonella* Worthington, were recovered from cattle on cow-calf operations in North Dakota during the same year35 (Theis, 2006). However, a larger study of beef cattle (Beach et al 2002), reported that the five serotypes most commonly associated with feedlot cattle and their environment were Salmonella Anatum (18.3% of the isolates), Salmonella Kentucky (17.5%), Salmonella Montevideo (9.2%), Salmonella Senftenberg (8.3%), and Salmonella Mbandaka (7.5%). The five serotypes most commonly associated with nonfeedlot cattle and their environment were Salmonella Kentucky (35.4%), Salmonella Montevideo (21.7%). Salmonella Cerro (7.5%), Salmonella Anatum (6.8%), and Salmonella Mbandaka (5.0%) (Beach et al 2002).

Other studies9, (Edrington et al 2004) have reported different *Salmonella* serotypes recovered from cattle originating from other states, possibly due to regional differences. In one study (Edrington et al 2004)9 mature dairy cattle were sampled over a 2-year period (2001-2002) on six farms in New Mexico and Texas. Fecal samples (n = 1560) were collected via rectal palpation and cultured for *Salmonella*, and one isolate from each positive sample was serotyped. Twenty-two different serotypes were identified from a total of 393 Salmonella isolates. Montevideo was the predominant serotype (27%) followed by Mbandaka (15%), Senftenberg (11.4%), Newport (6.4%), Anatum (4.8%), and Give (4.8%). Salmonella Typhimurium and Dublin, two frequently reported serotypes, accounted for only 1% of the observed serotypes in this study. A national *Salmonella* study of 97 dairy herds in 21 states in the US reported *Salmonella* Meleagridis (24.1%), Salmonella Montevideo (11.9%), and *Salmonella* Typhimurium (9.9%) as the three most frequently recovered serotypes (Blau et al 2005). It is noteworthy that *Salmonella enterica* serovar Hadar was the major *Salmonella* serotype isolated from processed bison carcasses originating in the same region as our sampled animals25 (Li et al, 2006). In the absence of studies that correlate recovery of *Salmonella* from the same bison pre and post-harvest, it is difficult to ascertain the sources of contamination of bison carcasses post-harvest.

In the study Khaitsa et al (2008) both *Salmonella* isolates were susceptible to at least 6 antimicrobials on the panel including the cephalosporin - ceftiofur and the quinolone/fluoroquinolone - enrofloxacin that are clinically important. However, both isolates (100%) demonstrated widespread multi-drug resistance (resitance to ≥ 13 antimicrobials) in a panel of 20.antimicrobials with resistance most frequently to tetracycline, streptomycin, and/or ampicillin). In a larger study (Dargatz et al 2003) of 73 feedlots in 12 states the antimicrobial resistance patterns of *Salmonella* spp recovered were determined. The susceptibilities of all isolates were determined using a panel of 17 antimicrobials. The majority of isolates (62.8%, 441/702) were sensitive to all of the antimicrobials tested. Resistance was most frequently observed to tetracycline (35.9%, 252/702) followed by streptomycin (11.1%, 78/702), ampicillin (10.4%, 73/702) and chloramphenicol (10.4%, 73/702). Multiple resistance (resistance to > or =2 antimicrobials) was observed for 11.7% (82/702) of the isolates. However, overall, most of the *Salmonella* isolates were sensitive to all the antimicrobials tested. Interestingly, antimicrobial testing of *Salmonella enterica* serovar

Antimicrobial Drug Resistance and Molecular Characterization

best estimates of seasonal occurrence of *Salmonella.* 

may have explained the difference in results.

of *Salmonella* Isolated from Domestic Animals, Humans and Meat Products 243

to eat meat products was a concern as it indicated that control strategies for this pathogen post-processing in these ready to eat turkey products was not completely successful. This

Other researchers have reported similar low recovery of *Salmonella* in retail meats (Ono, 1999; , Mayrhofer et al, 2004, Whyte et al, 2004, Zhao et al, 2001*)*. It was also reported that among raw turkey meat products, ground turkey had higher *Salmonella* contamination rates than whole turkey or other turkey parts (drumsticks, thighs, breast, breast cutlets, wings, breakfast link, bratwurst, sausage and bacon). This was not a total surprise as ground turkey samples have traditionally had higher food borne pathogens compared to whole turkey or turkey parts *(*Cloak et al, 2001*)*. This is possibly due to the fact that ground turkey is an amalgamation of large numbers of meat parts from different sources that are eventually ground together. *Salmonella* contamination of poultry meat has been reported to be seasonal with higher prevalence in summer than other seasons (Wallace et al, 1997). Although *Salmonella* recovery was reported to be higher in spring than winter, the study was limited in that it spanned over a period of only 6 months so could not possibly provide us with the

While some previous researchers (Zhao et al, 2001) reported similar *Salmonella* prevalence (4.2%) to ours, others *(*Soultos et al, 2003*)* reported lower levels. Low *Salmonella* incidence rates in chicken of 1.5% were reported by Soultos et al *(2003)*. Another study (Zhao et al, 2006) of *Salmonella* from retail foods of animal origin reported a higher prevalence (6%) than what we observed. However, the *Salmonella* distribution within the meat products was similar to ours, with ground turkey and chicken having the highest *Salmonella* contamination rates; overall, six percent of 6,046 retail meat samples (n = 365) were contaminated with *Salmonella*, the bulk recovered from either ground turkey (52%) or chicken breast (39%). There are other studies that have reported higher *Salmonella* prevalence (16.4% to 35.8%) than reported here (Domínguez et al, 2002; Duffy et al, 199; Mayrhofer et al, 2004, White et al, 2001). In one study (White et al, 2001), 200 meat samples were processed and 41 (20 percent) contained *Salmonella*, with a total of 13 serotypes. The majority of *Salmonella* isolates (61.5%) in the Khaitsa et al (2007b) study were recovered from ground turkey. In the study by Kegode et al (2008), *Salmonella* prevalence was 3% (13/ 456) of all retail meat samples. The *Salmonella* contamination rate for chicken was 4.1% (5/123), which is strikingly similar to what Zhao et al (2001) reported for grocery stores in the Washington, DC metropolitan area. In that study, *Salmonella* was isolated from 3.0% of the 825 meat samples, and chicken had a *Salmonella* contamination rate of 4.2%. Furthermore, the percentage of *Salmonella* recovered in the assorted turkey and chicken parts was similar to findings of the larger FoodNet study conducted in 2002 to 2003 (Zhao et al, 2006). Kegode

et al (2008) did not report any *Salmonella* from beef and pork products tested.

Recovery of *Salmonella* from the retail meat products was not influenced by the store type (Khaitsa et al, 2007b). The possible explanations for this finding include; similar product batches within stores, the location of stores within one city, low number of stores sampled, short sampling time and the relatively smaller number of samples tested. It is possible that the relatively low prevalence of *Salmonella* recovered from our study hindered our ability to detect a significant difference among the stores. Also, the relatively smaller number of stores in our study (5 compared to 58 in that study (Zhao et al, 2001)

Khaitsa et al (2007b) reported the predominant *Salmonella* serotype in retail meats as *S. heidelberg* (30.8%) followed by *S. kentucky* (15.4%). Studies have reported different serotypes

may be attributed to the way the meats are handled after processing (CDC, 1998).

Hadar recovered from bison carcasses originating from the same region as our sample bison also demonstrated resistance to tetracycline, gentamicin, sulfamethoxazole, and streptomycin25, results that were quite similar to what we reported for isolates from apparently healthy bison. Additionally, both isolates recovered in our study were susceptible to apramycin. In comparison with human isolates, of the 2613 isolates tested in 1999-2000 at the 17 public health laboratories participating in NARMS, 26% (679) were resistant to >1 agent; 21% (546) were multidrug resistant (resistant to >2 agents)1 (Angulo et al, 2001). Three multidrug resistant strains accounted for 10% (263/2613) of all Salmonella isolates, 38% (263/679) of the resistant isolates and 48% (263/546) of the multidrug resistant isolates. In particular, 30% (162/546) of multidrug resistant *Salmonella* were *S*. Typhimurium R-type ACSSuT, 12% (63/546) were S. Typhimurium R-type AKSSuT, and 7% (38/546) were S. Newport R-type ACSSuT; no other multidrug resistant patterns accounted for more than 5% of multidrug resistant Salmonellae.

It was interesting to note that in spite of the reports that antibiotics were not routinely used in the study herd, and that no other animals were raised on the farm together with the bison, antimicrobial resistance was detected in the *Salmonella* isolates recovered. It is possible that since the animals were not housed, and the pasture was not completely fenced, wild life, birds and other domestic livestock had access to the animals. It is possible therefore that even when antibiotics were not used in the bison, *Salmonella* isolated from the bison could have acquired resistance through horizontal transfer from other multidrug resistant organisms originating from wild life, birds or other domestic livestock that had access to the bison. Hoyle et al., 2005 discuss the problem of possible transfer of resistance, which may occur horizontally or vertically from enteric organisms such as *Salmonella* to other organisms. Many pathogenic and commensal organisms are multidrug resistant due to exposure to various antibiotics. Often, this antimicrobial resistance is encoded by integrons that occur on plasmids or that are integrated into the bacterial chromosome. Integrons are commonly associated with bacterial genera in the family *Enterobacteriaceae* (Goldstein et al 2001). Most of the resistance integrons found to date in clinical isolates of *Enterobacteriaceae* are class 1 integrons, which are highly associated with resistance to antimicrobial agents (Norrby 2005). Multi-drug resistant phenotypes have been associated with large, transferable plasmids such as integrons (Schoeder et al 2003). These plasmids are stable, transfer readily to other microorganisms in the same environment, and often contain cassettes encoding resistance to one or more classes of antimicrobials (Schoeder et al 2003) thus, resistance to an antimicrobial not routinely used in clinical medicine can mean resistance to one that is (Schoeder et al 2003). This finding has implications for animal and public health due to the potential for failure to treat some infections in animals and humans with the drugs that are currently on the market.

#### **4.2** *Salmonella* **from meats**

In the study by Khaitsa et al (2007b) that investigated the occurrence of *Salmonella* in raw and ready to eat turkey meat products, in 959 turkey meat products (raw, n =614; and ready to eat (RTE), n = 345) purchased from four retail outlets in the Midwestern United States, overall, *Salmonella* was detected in 2.4% (23 of 959) of the retail meat samples with most 5% (16/329), recovered from raw meats and only 1% (7/607) from ready to eat meat samples. This finding was significant as it demonstrated that control strategies for this pathogen postproduction are meeting with some success. However, recovery of *Salmonella* from the ready

Hadar recovered from bison carcasses originating from the same region as our sample bison also demonstrated resistance to tetracycline, gentamicin, sulfamethoxazole, and streptomycin25, results that were quite similar to what we reported for isolates from apparently healthy bison. Additionally, both isolates recovered in our study were susceptible to apramycin. In comparison with human isolates, of the 2613 isolates tested in 1999-2000 at the 17 public health laboratories participating in NARMS, 26% (679) were resistant to >1 agent; 21% (546) were multidrug resistant (resistant to >2 agents)1 (Angulo et al, 2001). Three multidrug resistant strains accounted for 10% (263/2613) of all Salmonella isolates, 38% (263/679) of the resistant isolates and 48% (263/546) of the multidrug resistant isolates. In particular, 30% (162/546) of multidrug resistant *Salmonella* were *S*. Typhimurium R-type ACSSuT, 12% (63/546) were S. Typhimurium R-type AKSSuT, and 7% (38/546) were S. Newport R-type ACSSuT; no other multidrug resistant patterns accounted for more than 5%

It was interesting to note that in spite of the reports that antibiotics were not routinely used in the study herd, and that no other animals were raised on the farm together with the bison, antimicrobial resistance was detected in the *Salmonella* isolates recovered. It is possible that since the animals were not housed, and the pasture was not completely fenced, wild life, birds and other domestic livestock had access to the animals. It is possible therefore that even when antibiotics were not used in the bison, *Salmonella* isolated from the bison could have acquired resistance through horizontal transfer from other multidrug resistant organisms originating from wild life, birds or other domestic livestock that had access to the bison. Hoyle et al., 2005 discuss the problem of possible transfer of resistance, which may occur horizontally or vertically from enteric organisms such as *Salmonella* to other organisms. Many pathogenic and commensal organisms are multidrug resistant due to exposure to various antibiotics. Often, this antimicrobial resistance is encoded by integrons that occur on plasmids or that are integrated into the bacterial chromosome. Integrons are commonly associated with bacterial genera in the family *Enterobacteriaceae* (Goldstein et al 2001). Most of the resistance integrons found to date in clinical isolates of *Enterobacteriaceae* are class 1 integrons, which are highly associated with resistance to antimicrobial agents (Norrby 2005). Multi-drug resistant phenotypes have been associated with large, transferable plasmids such as integrons (Schoeder et al 2003). These plasmids are stable, transfer readily to other microorganisms in the same environment, and often contain cassettes encoding resistance to one or more classes of antimicrobials (Schoeder et al 2003) thus, resistance to an antimicrobial not routinely used in clinical medicine can mean resistance to one that is (Schoeder et al 2003). This finding has implications for animal and public health due to the potential for failure to treat some infections in animals and humans

In the study by Khaitsa et al (2007b) that investigated the occurrence of *Salmonella* in raw and ready to eat turkey meat products, in 959 turkey meat products (raw, n =614; and ready to eat (RTE), n = 345) purchased from four retail outlets in the Midwestern United States, overall, *Salmonella* was detected in 2.4% (23 of 959) of the retail meat samples with most 5% (16/329), recovered from raw meats and only 1% (7/607) from ready to eat meat samples. This finding was significant as it demonstrated that control strategies for this pathogen postproduction are meeting with some success. However, recovery of *Salmonella* from the ready

of multidrug resistant Salmonellae.

with the drugs that are currently on the market.

**4.2** *Salmonella* **from meats** 

to eat meat products was a concern as it indicated that control strategies for this pathogen post-processing in these ready to eat turkey products was not completely successful. This may be attributed to the way the meats are handled after processing (CDC, 1998).

Other researchers have reported similar low recovery of *Salmonella* in retail meats (Ono, 1999; , Mayrhofer et al, 2004, Whyte et al, 2004, Zhao et al, 2001*)*. It was also reported that among raw turkey meat products, ground turkey had higher *Salmonella* contamination rates than whole turkey or other turkey parts (drumsticks, thighs, breast, breast cutlets, wings, breakfast link, bratwurst, sausage and bacon). This was not a total surprise as ground turkey samples have traditionally had higher food borne pathogens compared to whole turkey or turkey parts *(*Cloak et al, 2001*)*. This is possibly due to the fact that ground turkey is an amalgamation of large numbers of meat parts from different sources that are eventually ground together. *Salmonella* contamination of poultry meat has been reported to be seasonal with higher prevalence in summer than other seasons (Wallace et al, 1997). Although *Salmonella* recovery was reported to be higher in spring than winter, the study was limited in that it spanned over a period of only 6 months so could not possibly provide us with the best estimates of seasonal occurrence of *Salmonella.* 

While some previous researchers (Zhao et al, 2001) reported similar *Salmonella* prevalence (4.2%) to ours, others *(*Soultos et al, 2003*)* reported lower levels. Low *Salmonella* incidence rates in chicken of 1.5% were reported by Soultos et al *(2003)*. Another study (Zhao et al, 2006) of *Salmonella* from retail foods of animal origin reported a higher prevalence (6%) than what we observed. However, the *Salmonella* distribution within the meat products was similar to ours, with ground turkey and chicken having the highest *Salmonella* contamination rates; overall, six percent of 6,046 retail meat samples (n = 365) were contaminated with *Salmonella*, the bulk recovered from either ground turkey (52%) or chicken breast (39%). There are other studies that have reported higher *Salmonella* prevalence (16.4% to 35.8%) than reported here (Domínguez et al, 2002; Duffy et al, 199; Mayrhofer et al, 2004, White et al, 2001). In one study (White et al, 2001), 200 meat samples were processed and 41 (20 percent) contained *Salmonella*, with a total of 13 serotypes. The majority of *Salmonella* isolates (61.5%) in the Khaitsa et al (2007b) study were recovered from ground turkey. In the study by Kegode et al (2008), *Salmonella* prevalence was 3% (13/ 456) of all retail meat samples. The *Salmonella* contamination rate for chicken was 4.1% (5/123), which is strikingly similar to what Zhao et al (2001) reported for grocery stores in the Washington, DC metropolitan area. In that study, *Salmonella* was isolated from 3.0% of the 825 meat samples, and chicken had a *Salmonella* contamination rate of 4.2%. Furthermore, the percentage of *Salmonella* recovered in the assorted turkey and chicken parts was similar to findings of the larger FoodNet study conducted in 2002 to 2003 (Zhao et al, 2006). Kegode et al (2008) did not report any *Salmonella* from beef and pork products tested.

Recovery of *Salmonella* from the retail meat products was not influenced by the store type (Khaitsa et al, 2007b). The possible explanations for this finding include; similar product batches within stores, the location of stores within one city, low number of stores sampled, short sampling time and the relatively smaller number of samples tested. It is possible that the relatively low prevalence of *Salmonella* recovered from our study hindered our ability to detect a significant difference among the stores. Also, the relatively smaller number of stores in our study (5 compared to 58 in that study (Zhao et al, 2001) may have explained the difference in results.

Khaitsa et al (2007b) reported the predominant *Salmonella* serotype in retail meats as *S. heidelberg* (30.8%) followed by *S. kentucky* (15.4%). Studies have reported different serotypes

Antimicrobial Drug Resistance and Molecular Characterization

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

and Adley 2004; Padungtod and Kaneene 2006; Zhao et al. 2003).

(Zhao et al, 2006).

of *Salmonella* Isolated from Domestic Animals, Humans and Meat Products 245

ceftriaxone. All isolates were susceptible to amikacin and ciprofloxacin; however, 3% of isolates were resistant to nalidixic acid and were almost exclusive to ground turkey samples (n = 11/12). All Salmonella isolates were analyzed for genetic relatedness using pulsed-field gel electrophoresis (PFGE) patterns generated by digestion with Xba1 or Xba1 plus Bln1. PFGE fingerprinting profiles showed that Salmonella, in general, were genetically diverse with a total of 175 Xba1 PFGE profiles generated from the 365 isolates. PFGE profiles showed good correlation with serotypes and in some instances, antimicrobial resistance profiles. Results demonstrated a varied spectrum of antimicrobial resistance and PFGE patterns, including several multidrug resistant clonal groups among Salmonella isolates, and signify the importance of sustained surveillance of foodborne pathogens in retail meats.

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

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

and proportions recovered from meat products. One study found that *S. heidelberg* was predominant in chicken, *S. Montevideo* in beef, *S. hadar* in turkey and *S. derby* in pork (Schlosser et al, 2000). The three major *Salmonella* serotypes (Heidelberg, Typhimurium and Kentucky) reported by Kegode et al (2008) were similar to major serotypes reported by the larger studies conducted by FoodNet and others (Zhao et al, 2001; CDC, 2005; CDC, 2006). For example, in 2005, the *Salmonella* serotypes accounting for 56% of human infections included Typhimurium (20%), Enteritidis (15%), Newport (10%), Javiana (7%), and Heidelberg (5%) (CDC, 2006). Another study found the predominant serotype to be *S. typhimurium* var Copenhagen (Sorensen et al, 2002). Other studies have reported the predominant serotype to be *S. enteritidis* (Domínguezet al, 2002; Mayrhofer et, 2004), *S. bredeney* (Duffy et al, 1999) and *S. anatum* (Mrema et al, 2006). The different results may reflect the different meat types examined (meat cuts vs ground meat) or different geographic locations of sampling. Regional variation in predominant serotypes of bacterial foodborne pathogens has previously been reported (CDC, 1998).

In the study by Tumuhairwe et al, 2007) that investigated the temporal and spatial distribution of 1465 salmonellosis outbreaks involving 49/50 states in the US , overall, when the incidence rates were computed, the states with higher rates were not necessarily those with higher outbreak occurrences, an indication that these states probably had better reporting systems. Membership in FoodNet (US federal agency that actively monitors seven foodborne disease trends including *Salmonella*) may have explained the comparatively large number of reports originating from California, Maryland, and New York. The four major *Salmonella* serotypes commonly isolated in humans in the US are: S. Enteritidis, S. Typhimurium, S. Heidelberg and S. Newport; Three of these serotypes (S. Enteritidis, S. Heidelberg and S. Newport) were the most implicated in both TMAOs and SOOVs compared to the other serotypes. Additionally, S. Reading was frequently isolated in TMAOs in this study. This observation was in agreement with other studies (CDC, 2005; CDC, 2006) that have cited S. Reading as a common serotype in turkey meats. Also, it is interesting to note that S. Reading and S. Heidelberg were among the serotypes recovered from turkey farms and their environment, where S. Heidelberg was relatively more common in both humans and turkeys than S. Reading.

The Centers for Disease Control Foodborne Diseases Active Surveillance Network (FoodNet) data indicate that outbreaks and clusters of food-borne infections peak during the warmest months of the year (CDC, 2006). Additionally, some studies have shown that the rate of microbial contamination of food products follows the same trend (CDC, 2003; CDC, 2006). Since our study was conducted during the warmest months of the year, the prevalence estimates of the food-borne pathogens obtained should be fairly representative of their true estimate. One limitation of the study was that we could not evaluate the seasonality of microbial contamination of retail meats due to the short sampling period; the study was conducted only during one season (summer). It has been suggested that future food safety studies focusing on seasonality components of microbial contamination of retail meats may require larger sample sizes and longer analysis periods (Zhao et al, 2006. Also, the location of sampling, the relatively smaller number of samples tested and low number of stores sampled may have influenced the results of this study. S. Heidelberg was the predominant serotype identified (23%), followed by S. Saintpaul (12%), S. Typhimurium (11%), and S. Kentucky (10%). Overall, resistance was most often observed to tetracycline (40%), streptomycin (37%), ampicillin (26%), and sulfamethoxazole (25%). Twelve percent of isolates were resistant to cefoxitin and ceftiofur, though only one isolate was resistant to

and proportions recovered from meat products. One study found that *S. heidelberg* was predominant in chicken, *S. Montevideo* in beef, *S. hadar* in turkey and *S. derby* in pork (Schlosser et al, 2000). The three major *Salmonella* serotypes (Heidelberg, Typhimurium and Kentucky) reported by Kegode et al (2008) were similar to major serotypes reported by the larger studies conducted by FoodNet and others (Zhao et al, 2001; CDC, 2005; CDC, 2006). For example, in 2005, the *Salmonella* serotypes accounting for 56% of human infections included Typhimurium (20%), Enteritidis (15%), Newport (10%), Javiana (7%), and Heidelberg (5%) (CDC, 2006). Another study found the predominant serotype to be *S. typhimurium* var Copenhagen (Sorensen et al, 2002). Other studies have reported the predominant serotype to be *S. enteritidis* (Domínguezet al, 2002; Mayrhofer et, 2004), *S. bredeney* (Duffy et al, 1999) and *S. anatum* (Mrema et al, 2006). The different results may reflect the different meat types examined (meat cuts vs ground meat) or different geographic locations of sampling. Regional variation in predominant serotypes of bacterial

In the study by Tumuhairwe et al, 2007) that investigated the temporal and spatial distribution of 1465 salmonellosis outbreaks involving 49/50 states in the US , overall, when the incidence rates were computed, the states with higher rates were not necessarily those with higher outbreak occurrences, an indication that these states probably had better reporting systems. Membership in FoodNet (US federal agency that actively monitors seven foodborne disease trends including *Salmonella*) may have explained the comparatively large number of reports originating from California, Maryland, and New York. The four major *Salmonella* serotypes commonly isolated in humans in the US are: S. Enteritidis, S. Typhimurium, S. Heidelberg and S. Newport; Three of these serotypes (S. Enteritidis, S. Heidelberg and S. Newport) were the most implicated in both TMAOs and SOOVs compared to the other serotypes. Additionally, S. Reading was frequently isolated in TMAOs in this study. This observation was in agreement with other studies (CDC, 2005; CDC, 2006) that have cited S. Reading as a common serotype in turkey meats. Also, it is interesting to note that S. Reading and S. Heidelberg were among the serotypes recovered from turkey farms and their environment, where S. Heidelberg was relatively more

The Centers for Disease Control Foodborne Diseases Active Surveillance Network (FoodNet) data indicate that outbreaks and clusters of food-borne infections peak during the warmest months of the year (CDC, 2006). Additionally, some studies have shown that the rate of microbial contamination of food products follows the same trend (CDC, 2003; CDC, 2006). Since our study was conducted during the warmest months of the year, the prevalence estimates of the food-borne pathogens obtained should be fairly representative of their true estimate. One limitation of the study was that we could not evaluate the seasonality of microbial contamination of retail meats due to the short sampling period; the study was conducted only during one season (summer). It has been suggested that future food safety studies focusing on seasonality components of microbial contamination of retail meats may require larger sample sizes and longer analysis periods (Zhao et al, 2006. Also, the location of sampling, the relatively smaller number of samples tested and low number of stores sampled may have influenced the results of this study. S. Heidelberg was the predominant serotype identified (23%), followed by S. Saintpaul (12%), S. Typhimurium (11%), and S. Kentucky (10%). Overall, resistance was most often observed to tetracycline (40%), streptomycin (37%), ampicillin (26%), and sulfamethoxazole (25%). Twelve percent of isolates were resistant to cefoxitin and ceftiofur, though only one isolate was resistant to

foodborne pathogens has previously been reported (CDC, 1998).

common in both humans and turkeys than S. Reading.

ceftriaxone. All isolates were susceptible to amikacin and ciprofloxacin; however, 3% of isolates were resistant to nalidixic acid and were almost exclusive to ground turkey samples (n = 11/12). All Salmonella isolates were analyzed for genetic relatedness using pulsed-field gel electrophoresis (PFGE) patterns generated by digestion with Xba1 or Xba1 plus Bln1. PFGE fingerprinting profiles showed that Salmonella, in general, were genetically diverse with a total of 175 Xba1 PFGE profiles generated from the 365 isolates. PFGE profiles showed good correlation with serotypes and in some instances, antimicrobial resistance profiles. Results demonstrated a varied spectrum of antimicrobial resistance and PFGE patterns, including several multidrug resistant clonal groups among Salmonella isolates, and signify the importance of sustained surveillance of foodborne pathogens in retail meats. (Zhao et al, 2006).
