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    Prevalence of Salmonella entericain fecal samples collected from preweaned dairy-source calves on 33 farms in California. Overall, 3,685 fecal samples were collected from 1,288 calves during their first 12 weeks of life. Values are mean ± SD.

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Prevalence and antimicrobial resistance patterns of Salmonella enterica in preweaned calves from dairies and calf ranches

Anna Catharina B. Berge DVM, MPVM, PhD1, Dale A. Moore DVM, MPVM, PhD2, and William M. Sischo DVM, MPVM, PhD3
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  • 1 Department of Population Health and Reproduction, Veterinary Medicine Teaching and Research Center, University of California, Tulare, CA 93274.
  • | 2 Department of Population Health and Reproduction, Veterinary Medicine Teaching and Research Center, University of California, Tulare, CA 93274.
  • | 3 Department of Population Health and Reproduction, Veterinary Medicine Teaching and Research Center, University of California, Tulare, CA 93274.

Abstract

Objective—To evaluate serovar and antimicrobial resistance patterns of Salmonella enterica isolated from preweaned calves and identify management risk factors associated with fecal shedding of S enterica.

Sample Population—Cohorts of 10 to 15 preweaned calves (1 to 84 days of age) on 26 dairies and 7 calf ranches and cross-sectional samples of preweaned calves on smaller farms.

Procedures—Calves were evaluated every 2 weeks during a 6-week period. Salmonella isolates obtained from rectal fecal swabs underwent antimicrobial susceptibility testing against 12 antimicrobials. Cluster analysis enabled description of antimicrobial susceptibility patterns. Calf, cohort, and farm risk factors associated with both the prevalence of S enterica and multiple-antimicrobial–resistant S enterica in preweaned calves were identified with repeated-measure logistic regression models.

ResultsSalmonella enterica was detected on > 50% of farms and in 7.5% of 3,686 fecal samples. Many isolates (33%) were resistant to multiple antimicrobials. Shedding of Salmonella spp was negatively associated with increasing calf age, herds being closed to incoming cattle, and antimicrobial supplementation of milk replacer; prophylactic antimicrobial treatment in day-old calves increased shedding. No association between farm management and presence of multiple-antimicrobial–resistant S enterica or between calving management and presence of S enterica in calves ≤ 1 week old was detected.

Conclusions and Clinical Relevance—In preweaned calves, the most important factors associated with decreased likelihood of fecal shedding of S enterica were the use of antimicrobial-supplemented milk replacer and maintenance of a closed herd. Infection with multiple-antimicrobial–resistant S enterica was not associated with antimicrobial administration.

Abstract

Objective—To evaluate serovar and antimicrobial resistance patterns of Salmonella enterica isolated from preweaned calves and identify management risk factors associated with fecal shedding of S enterica.

Sample Population—Cohorts of 10 to 15 preweaned calves (1 to 84 days of age) on 26 dairies and 7 calf ranches and cross-sectional samples of preweaned calves on smaller farms.

Procedures—Calves were evaluated every 2 weeks during a 6-week period. Salmonella isolates obtained from rectal fecal swabs underwent antimicrobial susceptibility testing against 12 antimicrobials. Cluster analysis enabled description of antimicrobial susceptibility patterns. Calf, cohort, and farm risk factors associated with both the prevalence of S enterica and multiple-antimicrobial–resistant S enterica in preweaned calves were identified with repeated-measure logistic regression models.

ResultsSalmonella enterica was detected on > 50% of farms and in 7.5% of 3,686 fecal samples. Many isolates (33%) were resistant to multiple antimicrobials. Shedding of Salmonella spp was negatively associated with increasing calf age, herds being closed to incoming cattle, and antimicrobial supplementation of milk replacer; prophylactic antimicrobial treatment in day-old calves increased shedding. No association between farm management and presence of multiple-antimicrobial–resistant S enterica or between calving management and presence of S enterica in calves ≤ 1 week old was detected.

Conclusions and Clinical Relevance—In preweaned calves, the most important factors associated with decreased likelihood of fecal shedding of S enterica were the use of antimicrobial-supplemented milk replacer and maintenance of a closed herd. Infection with multiple-antimicrobial–resistant S enterica was not associated with antimicrobial administration.

Salmonella enterica is one of many enteric disease–causing agents in humans and other animals. Salmonella spp are zoonotic and often foodborne bacteria; major reservoirs for human infection are believed to be livestock and poultry.1Salmonella spp are frequently isolated from the environment and livestock of dairy farms and calf ranches. Although Salmonella isolates can be obtained from nonclinically affected animals, these organisms are also associated with considerable illness and death among bovids.2–4Typically, Salmonella isolates from nonclinically affected animals are susceptible to antimicrobials, whereas the isolates from clinically affected animals have higher overall levels of antimicrobial resistance.5,6

There are conflicting data regarding the influence that calf and dairy farm management practices, specifically the use of antimicrobials, have on fecal shedding of Salmonella spp and the development of antimicrobial resistance in those organisms. Results of 1 study7 of herd-level risk factors for the recovery of Salmonella spp from the environments of free-stall housing occupied by adult dairy cows indicated that there was no relationship between production or management factors and the prevalence of Salmonella spp. Other studies8,9 in adult dairy cattle have revealed an association of increasing herd size with increased fecal shedding and prevalence of Salmonella spp. In a national US study9 that was focused on heifers, 1,063 farms were surveyed; Salmonella spp were detected in 145 of 6,861 (2.1%) fecal samples obtained from preweaned heifers and 79 of 1,063 (7.4%) dairies from which 1 or more Salmonella-positive fecal samples had been collected. Feeding hay or antimicrobial–supplemented milk replacer to calves during the preweaning period was associated with lower risk for fecal shedding of Salmonella spp, compared with calves that were fed nonsupplemented milk or no hay. In another study,10 fecal samples were collected from calves every 2 months for 1 year on 129 conventional and organic dairy farms in 4 states; 176 of 4,673 (3.8%) fecal samples from calves and 40 (31%) farms yielded positive results for Salmonella spp. Herd-level factors associated with isolation of Salmonella spp from calves included feeding milk replacer without antimicrobial supplementation, use of the maternity pen as a hospital area for sick cows, and high prevalence of Salmonella spp in the cows.

Antimicrobial use in calves and the risk for fecal shedding of Salmonella spp have been investigated fur ther. Hinton et al11 found that calves fed milk supplemented with furazolidone had peak fecal shedding of Salmonella spp from 2 to 3 weeks of age and that shedding rates decreased after 5 weeks of age. During the second week of life, diseased calves treated with antimicrobials were more likely to shed Salmonella spp than untreated calves. However, the level of Salmonella shedding in those calves was lower than that determined by those investigators in a previous study12 in which in-feed administration of antimicrobials to calves was used. The authors concluded that exposure to antimicrobials in feed decreased the probability of recovering Salmonella spp from fecal samples, but calves with clinical disease that were treated with antimicrobials were more likely to shed Salmonella spp than healthy, untreated calves. This latter effect may not be related to exposure to antimicrobials, but rather to the disease state. Most of these studies were crosssectional in design, and the findings are difficult to interpret when evaluating management practices as causal factors for fecal shedding of Salmonella spp. In addition, none of those studies assessed the impact of management practices and the use of antimicrobials on the recovery of multiple-antimicrobial–resistant Salmonella serovars.

The objective of the study reported here was to evaluate serovar and antimicrobial resistance patterns of S enterica isolated from preweaned calves and identify management risk factors associated with fecal shedding of Salmonella spp. In addition, calf-level risk factors associated with the presence of multipleantimicrobial–resistant Salmonella spp in fecal samples obtained longitudinally from preweaned calves were investigated.

Materials and Methods

Farm enrollment and sampling strategy—Thirty-three farm operations that reared calves for veal, fed beef, or heifer replacements were selected from a convenience sample identified by University of California Cooperative Extension farm advisors and private veterinarians. The farms were located throughout California and included animals from the 4 major milk production areas in the state (North Coast, North Valley, South Valley, and Southern California). The farms represented dairy and calf ranch operations. Farm management personnel responded to a questionnaire (30-minute completion time) that was administered by an investigator (ACB) and gave permission for sample collection from calves. The questionnaire addressed farm type and size, biosecurity management (animal introduction onto the farm), antimicrobial treatments of cows, calving and colostrum management, type of calf housing and cleaning of housing, calf feeding management, and the use of prophylactic and therapeutic calf treatments. Farm operators maintained records of all antimicrobial treatments administered to calves in the trial cohorts. On farms where treatment records were not in compliance with study requirements, a temporary recording system was established at the first farm visit.

For each farm, 1 to 3 cohorts were enrolled at approximately 6-month intervals (Table 1). At 5 farms, only 1 cohort was enrolled because of sampling logistics or changes in farm management. Ten to 15 newborn calves, or those newly arrived at a farm, were enrolled as a cohort. Each calf was individually identified. For smaller dairies, as many as 15 calves that were < 4 weeks of age were enrolled on the first visit; on subsequent visits to these smaller operations, calves born between visits were enrolled. Overall, 1,288 calves were included in the study. Fecal samples were collected from each cohort on 4 occasions at 2-week intervals.

Table 1—

Characteristics of study farms and enrolled calf cohorts included in a study of prevalence and risk factors for fecal shedding of Salmonella entericain preweaned calves in California.

Milk shedFarm typeFarm subtypeCohort 1Cohort 2Cohort 3
No. of samples collectedNo. of Salmonella-positive samplesNo. of samples collectedNo. of Salmonella-positive samplesNo. of samples collectedNo. of Salmonella-positive samples
NCDairyOrganic230510ND
 DairyConventional260610ND
 DairyConventional420470ND
 DairyConventional560190ND
 DairyConventional530570ND
 DairyConventional410530ND
 DairyConventional440580ND
 DairyOrganic220NDND
 DairyOrganic210NDND
NVDairyConventional370320ND
 Calf ranchHeifer340NDND
 DairyConventional7319510ND
 DairyConventional410370ND
 DairyConventional472390ND
 DairyConventional410402ND
SVDairyConventional5815500ND
 DairyConventional380410ND
 DairyConventional410570ND
 DairyConventional5625513ND
 DairyOrganic650412ND
 Calf ranchBeef580532ND
 Calf ranchVeal, heifer1296521ND
 Calf ranchBeef105012191414
 Calf ranchHeifer481NDND
 DairyConventional11529NDND
 DairyConventional674NDND
 DairyConventional1112011724ND
 DairyConventional1172211318ND
 Calf ranchVeal, heifer1066591570
SODairyConventional550660ND
 Calf ranchHeifer5531692ND
 DairyConventional5226557ND
 DairyConventional5006310ND
Total1,9271831,557911984

NC = North Coast. NV = North Valley. SV = South Valley. SO = Southern California. ND = Not done. — = Not applicable.

Bacteriologic culture and antimicrobial resistance typing—Rectal fecal samples were obtained with sterile cottontipped transportation swabs.a Results of a prior pilot study by one of the authors indicated that Salmonella isolates on the swabs remained viable and bacterial counts were unchanged for as long as 2 weeks at 20°C. Study investigators collected the samples and transported them directly to the laboratory in cool, insulated containers within 24 hours of sampling.

Fecal swabs were initially incubated for 18 to 24 hours at 37°C in 10 mL of tetrathionate broth.b Approximately 0.1 mL of the tetrathionate broth was then transferred to xylose lysine desoxycholate agarc and brilliant green agar platesc that were streaked for isolation and incubated for 18 to 24 hours at 37°C. Plates were evaluated for growth of Salmonella organisms, and 1 putative Salmonella colony from each Salmonella-positive plate was streaked onto blood agar and incubated 18 to 24 hours at 37°C. For biochemical confirmation, well-isolated colonies were inoculated to triple sugar ironc slants and urea agar slants and incubated at 37°C for 24 hours. Urease-negative, dextrose-fermenting, H2S-producing colonies were presumed to be Salmonella spp and transferred to blood agar plates for serotyping. Salmonella isolates were serogrouped by use of a commercial polyvalent A-I and Vi anti-sera as well as individual serogroups B, C1, C2, C3, D, and E.d One isolate/serogroup identified in each fecal swab was sent to the California Animal Health and Food Safety Laboratory for serovar determination. Salmonella isolates were kept in tryptic soy brothb with 10% glycerol (vol/vol) at −80°C.

Antibiograms—Antimicrobial disk susceptibility tests were performed as recommended by the Clinical and Laboratory Standards Institute (previously named the National Committee for Clinical Laboratory Standards [NCCLS]) and as previously described.13–15 Twelve antimicrobials from the panel used by the National Antimicrobial Resistance Monitoring System were selected for the tests.5 This selection represented the major antimicrobial classes in historical and present use, and the drugs were chosen because of their importance in animal and human health. The 12 antimicrobials and their concentrations were the following: amikacin, 30 μg; amoxicillin-clavulanic acid, 20 μg/10 μg; ampicillin, 10 μg; cephalothin, 30 μg; ceftiofur, 30 μg; chloramphenicol, 30 μg; gentamicin, 10 μg; nalidixic acid, 30 μg; streptomycin, 10 μg; sulfisoxazole, 250 μg; tetracycline, 30 μg; and sulfamethoxazole-trimethoprim 23.75 μg/1.25 μg.b For each batch of isolates tested, 2 quality-control strains were included in the assay set: Escherichia coli ATCC 25922e from the American Type Culture Collection and S enterica serovar Typhimurium (bovine clinical isolate from the Veterinary Medicine Teaching and Research Center, University of California).

Antimicrobial susceptibility patterns—Data from the antibiograms for each isolate consisted of a series of inhibition zones for each of the 12 antimicrobials. Zone sizes ranged from 6 mm in diameter (corresponding to the size of an antimicrobial disk) to 31 mm. For each antimicrobial, the distribution of zone sizes for the isolate set was determined.f Isolates were grouped into ARCs on the basis of patterns of similarity in the zone sizes to the 12 antimicrobials.g Clustering was identified by use of the squared Euclidean distance as a dissimilarity measure and Ward's minimum variance clustering algorithm.16,17 For each cluster, the mean antimicrobial zone size and SD were calculated for each of the 12 antimicrobials.18 Antimicrobial resistance clusters were ordered on the increasing sum of the mean zone sizes, which roughly corresponded to increasing levels of multiple resistance. The clustering algorithm performs most efficiently when the distribution of zone sizes is bimodal; therefore, the distribution of the zone sizes was tested prior to clustering. All antimicrobial inhibition zone sizes had a bimodal distribution and were included in the cluster analysis procedure.

For illustrative and descriptive purposes, the mean zone sizes for each antimicrobial within an ARC were described as susceptible or resistant according to the Clinical and Laboratory Standards Institute guidelines for human and animal E coli.15 Isolates in ARCs with resistance to 2 or more antimicrobials were described as multiple-antimicrobial–resistant isolates.

Statistical modeling—To analyze the relationship between prevalence of Salmonella spp and risk factors, GEE logistic regression was used to account for the sampling of multiple cohorts on a farm. The nonindependence of isolates was modeled as a repeated measure of samples in a cohort nested by farm. Three separate models were used to describe the risk factors for fecal shedding of Salmonella spp in preweaned calves, risk factors for fecal shedding of multiple-antimicrobial–resistant Salmonella spp in preweaned calves, and calving-associated risk factors on dairy farms for fecal shedding of Salmonella spp in calves ≤ 7 days of age.

The modeling approach has been previously described.18 Briefly, the distribution of the predictive variables, conditional on Salmonella shedding status, was initially assessed by use of bivariate analyses. Prior to developing the GEE model, the set of variables identified in the bivariate analyses (χ2 test, P value ≤ 0.1) was evaluated in a population-averaged maximum-likelihood logistic regression model.19,h A stepwise variable selection procedure was used to build models. The procedure was based on a score χ2 test; a value of P ≤ 0.30 was required for entry into the model, and a value of P ≤ 0.10 was required for retention. Two-way interaction effects between variables included in this initial selection process were also tested. The variables and interaction terms retained in the population-averaged model were introduced to a GEE logistic regression model with a repeated measure of samples in a cohort nested within farm. An independent correlation structure was used for the repeated-measures component.20,21,i A value of P ≤ 0.10 was required for a risk factor to be retained in the final model.

Results

Salmonella isolates were recovered from 278 of 3,686 (7.5%) fecal samples collected from preweaned calves, 18 of 33 (55%) farms, and 27 of 62 (44%) cohorts (Table 1). The fecal prevalence of Salmonella isolates was 8.3% (215/2,597) and 5.8% (63/1,089) from dairy calves and calf ranch calves, respectively. Salmonella Montevideo was the most common serovar recovered and accounted for 106 of 278 (38%) isolates. Other Salmonella serovars detected on several farms included the following: Give, Infantis, Meleagridis, Muenster, Newport, and Typhimurium. Salmonella serovar Minnesota was isolated only from calves on 3 farms in Southern California. The highest recovery of Salmonella organisms was in samples from 1-day-old calves, with declining prevalence as the calves aged (Figure 1). Multiple-antimicrobial–resistant Salmonella isolates were recovered from 92 of the 3,686 (2.5%) fecal samples. There were no detectable shifts in prevalence of multiple-antimicrobial–resistant Salmonella spp in calves with increasing age.

Figure 1—
Figure 1—

Prevalence of Salmonella entericain fecal samples collected from preweaned dairy-source calves on 33 farms in California. Overall, 3,685 fecal samples were collected from 1,288 calves during their first 12 weeks of life. Values are mean ± SD.

Citation: American Journal of Veterinary Research 67, 9; 10.2460/ajvr.67.9.1580

Fourteen ARCs described the patterns of resistance observed in the Salmonella isolates. Clusters were ordered from pan-susceptible (cluster A) to highly resistant (cluster N) on the basis of the sum of the mean zone sizes associated with the 12 antimicrobials (Table 2). Of the isolates, 169 of 278 (61%) were placed in ARC A and 92 of 278 (33%) were classified as multiple-antimicrobial–resistant organisms (ARCs D through N). The most common Salmonella serovar, Montevideo, was detected only in the most susceptible ARCs (A through C). Serovars with high levels of multiple-antimicrobial resistance (ie, resistant to 6 or more antimicrobials) included Dublin, Infantis, Minnesota, Muenster, Newport, Reading, and Typhimurium (Table 3).

Table 2—

Characteristics of ARCs in a study of prevalence and risk factors for fecal shedding of Salmonella enterica in preweaned calves in California.

ARCsNo. of antimicrobials to which the isolates were resistantAntimicrobialFrequency of Salmonella isolates belonging to susceptibility clusters
AMPAMCCEFCFFSTRGENAMISULFSXTTETCHLNAL 
A0232523251422222227212421169
B1242524268*2222202721252112
C1232523251222216*232025215
D2232624256*18226*231921229
E36*7*6*2712212319241920232
F46*1919256*22236*229*24216
G56*1719246*19246*198*8*214
H66*2016246*6*236*6*6*16171 
I86*7*6*14*6*21226*206*6*215
J96*8*6*13*6*6*236*226*6*222
K96*6*6*13*6*21236*6*6*6*2321
L106*7*6*15*6*8*226*7*6*6*223
M116*8*6*11*6*6*6*6*6*6*6*2215
N126*8*6*12*6*6*6*6*7*6*6*7*24
Total No. of isolates278

Mean zone sizes within a cluster were considered resistant to that antimicrobial; resistance classification was performed according to the Clinical and Laboratory Standards Institute guidelines for Escherichia coliisolated from humans.

Clusters classified as multiple-antimicrobial–resistant clusters.

AMP = Ampicillin. AMC = Amoxicillin-clavulanic acid. CEF = Cephalothin. CFF = Ceftiofur. STR = Streptomycin. GEN = Gentamicin. AMI = Amikacin. SULF = Sulfisoxazole. SXT = Sulfamethoxazole-trimethoprim. TET = Tetracycline. CHL = Chloramphenicol. NAL = Nalidixic acid.

Table 3—

Frequency of Salmonella entericaserovars and their ARC distribution in a study of the prevalence and risk factors for fecal shedding of Salmonella spp in preweaned calves in California.

Salmonella serovarsNo. of isolatesARCs
A*BCDEFGHIJKLMN
Agona211
Anatum77
Cerro55
Dublin814111
Give1515
Infantis1313
Kentucky7151
Lille11
Livingstone633
Mbandaka752
Meleagridis19181
Minnesota25124
Montevideo10610312
Muenster972
Newington11
Newport212172
Reading422
Schwarzengrund11
Typhimurium219624
Total No. of isolates27816912592641522131524

Pan-susceptible to 12 antimicrobials.

Pan-resistant to 12 antimicrobials.

See Table 1 for remainder of key.

Most dairy farms and all calf ranches purchased animals from other sources (Table 4). Eight dairy farms were closed herds with no reported introduction of bovids in the preceding year. Preweaned calves were generally raised in individual, wooden hutches. Most farms removed manure from the hutches by either scraping or flushing, but only after a calf was weaned. On dairy farms, calves were typically fed fresh unpasteurized milk, whereas on calf ranches, calves were typically fed milk replacer (Table 5). Nonsaleable milk (ie, milk not allowed for human consumption) was fed to calves on dairy farms and 2 calf ranches. Pasteurization of fresh and nonsaleable milk was practiced on several dairy farms and all calf ranches that fed nonsaleable milk. All calf ranches used antimicrobials in the feed, whereas only 5 of 26 dairies used antimicrobials in the feed. Approximately half of the dairies started feeding calves with bottles and then switched to buckets, whereas calf ranches fed calves with bottles throughout the preweaning period.

Table 4—

Categorization of 26 dairies and 7 calf ranches in which preweaned calves were reared in California by management and housing practices and number of fecal samples that were positive for S enterica.

Table 4—
Table 5—

Categorization of 26 dairies and 7 calf ranches in which preweaned calves were reared in California by calf feeding and antimicrobial use practices and number of fecal samples that were positive for S enterica.

Table 5—

The final GEE logistic regression model for whether preweaned calves would shed Salmonella ssp in their feces contained factors for calf age, administration of antimicrobials in feed, biosecurity, and administration of antimicrobials within 24 hours after birth (Table 6). The odds that preweaned calves would shed Salmonella spp in their feces decreased significantly as age increased; calves fed antimicrobials in milk replacer were less likely to shed Salmonella spp in their feces than were calves that did not receive antimicrobials in milk replacer. Calves receiving prophylactic antimicrobials on the first day of life (day 1 treatments) were more likely to shed Salmonella spp in their feces at some time in the sampling period, compared with untreated calves. Antimicrobials used for these day 1 treatments included ceftiofur, penicillin, and gentamicin. Furthermore, calves from open herds were significantly more likely to shed Salmonella spp in feces than were calves from closed herds; this effect was consistent whether herds received purchased calves, heifers, or adult replacements.

Table 6—

Odds ratios derived from a GEE multilevel logistic regression model for the risk of fecal shedding of S entericain preweaned calves on 26 dairies and 7 calf ranches in California.

VariableEstimateOdds ratio90% confidence intervalP value
Calf age (d)
   Continuous−0.040.970.95–0.98<0.01
Antimicrobials in feed
   Grain−0.100.900.35–5.600.87
   Milk−1.470.230.09–0.590.01
   NoReference
Biosecurity
   Open herd3.5735.527.11–178.50<0.01
   Closed herdReference
Antimicrobials administered during first 24 hours after birth
   Yes1.213.351.09–10.330.08
   NoReference

A value of P ≤ 0.10 was considered significant.

See Table 1 for key.

No management or feeding risk factors were associated with fecal shedding of multiple-antimicrobial–resistant Salmonella spp from preweaned calves. Calving management factors and colostrum regimen were not associated with Salmonella isolation in calves ≤ 1 week of age on dairy farms.

Discussion

In the present study, the prevalence of S enterica in fecal samples collected from preweaned calves was 7.5%. In addition, 55% of the dairies and calf ranches yielded Salmonella spp via bacteriologic culture at 1 or more of the sample collections. Many Salmonella serovars (Agona, Anatum, Dublin, Kentucky, Montevideo, Muenster, Newport, and Typhimurium) detected in our study have been identified in a national investigation5 of the prevalence of Salmonella spp in bovids. Most of the Salmonella isolates were pan-susceptible to the antimicrobials tested. Consistent with other reported data,5Salmonella Montevideo was the most common pan-susceptible serovar detected in the present study. Ten of the 19 serovars included multiple-antimicrobial–resistant isolates. Multiple-antimicrobial–resistant Salmonella Minnesota has not been previously described and was detected in only 1 geographic region; one of the authors is aware that it has since been isolated from cattle in other regions of California.

The focus of this research was to identify risk factors for fecal shedding of Salmonella spp in preweaned calves. The sampling and microbiologic methods (based on use of a fecal swab and a 1-step enrichment process) could identify animals that were shedding > 20 Salmonella CFUs/g of feces. Although it is likely that the sensitivity of the bacteriologic culture of Salmonella spp from feces can be improved with increased size of the fecal sample, we believe that many animals that are shedding low numbers of Salmonella organisms are not actively infected but that this low-level shedding reflects environmental contamination.

Certain risk factors, including maintenance of a closed herd, exposure of calves to antimicrobial-supplemented milk replacer, antimicrobial prophylactic treatment of day-old calves, and calf age, were associated with fecal shedding of Salmonella spp by calves. The strongest risk factor was associated with biosecurity; farms that received animals from other sources were 35 times as likely to have calves that were shedding Salmonella spp in feces than herds that did not receive such animals. This effect was consistent whether herds purchased calves, heifers, or adult replacements. This may be attributable to the introduction and dissemination of Salmonella spp into farms via animals. Closed herds may also have other biosecurity measures in place that prevent the introduction and dissemination of Salmonella spp, such as restrictions on incoming visitors, vehicles, or feed.

Calves that received antimicrobial-supplemented milk replacer throughout the preweaning period were less likely to shed Salmonella spp in feces, compared with calves that received no antimicrobials in milk or milk replacer or only antimicrobials in the starter grain. The lower recovery of Salmonella isolates from feces of calves exposed to antimicrobial-supplemented milk replacer during the preweaning period was likely attributable to suppression of pan-susceptible Salmonella serovars. In other studies,9,10 a similar association between the use of antimicrobial-supplemented milk replacer and decreased odds of fecal shedding of Salmonella spp by preweaned calves has been determined.

In contrast to the protection that prolonged use of prophylactic antimicrobials in the milk or milk replacer appeared to have on fecal shedding of Salmonella spp, a single prophylactic antimicrobial treatment of calves on their first day of life was associated with an increased recovery of Salmonella spp. Antimicrobial therapy can disrupt the normal, commensal enteric flora.22 This disruption may allow either colonization or overgrowth of nondominant organisms (such as Salmonella spp) and subsequent shedding. The class of antimicrobial and the singledose treatment may have different selective effects on the gut flora and result in a markedly different response on Salmonella shedding for the day 1 treatment versus in-feed antimicrobials. It is also possible that the day 1 prophylactic use may be a response to a farm history of Salmonella infection and not directly a cause of the shedding event.

In the present study of preweaned calves, no associations between the use of antimicrobials or farmor calf-specific management factors and fecal shedding of multiple-antimicrobial–resistant Salmonella isolates were identified. Because the power of the study was sufficient to detect management factors related to all Salmonella spp, it may be that the entry and presence of multiple-antimicrobial–resistant Salmonella spp are not linked to management practices. Furthermore, most multiple-antimicrobial–resistant Salmonella spp are resistant to antimicrobials commonly used in milk replacer and likely have no influence on the presence of theseSalmonella spp in calf feces.

Fossler et al10 determined that increased prevalence of Salmonella spp in cows and housing of sick or treated cows in maternity pens at least once per month increased the risk of fecal shedding ofSalmonella spp in calves. In the present study, prevalence of Salmonella spp in adult bovids was not assessed and our data could not confirm those findings. Housing of sick animals in maternity pens was uncommon in the present study; only 1 organic dairy housed sick cows with immediate prepartum cows.

The finding that the prevalence of fecal shedding ofSalmonella spp decreased with calf age may be attributable to age-related changes in commensal enteric flora, a result of which is microflora that is more effective at excluding Salmonella spp than the neonatal microflora. Diet changes during this period could be a reason for the enteric flora shift. Most liquid diets used on dairy farms and calf ranches are energy restricted and alone do not support calf growth during the preweaning period.23 Caloric and protein needs for growth are met by feeding calf starter feeds (primarily grain mixes). As the calf ages and its energy requirements increase, proportionately more of the diet is comprised of starter grain. This may result in changes in the commensal enteric flora. Results of 1 study9 corroborate this finding; an association exists between the early inclusion of hay in calf feed and a lower prevalence of fecal shedding of Salmonella spp.9 Other studies24,25 have revealed that as the calf's immune system develops, there is reduced fecal shedding of Salmonella spp.

Overall, our data suggest that the most important management factor associated with decreased fecal shedding of Salmonella spp by calves was maintenance of a herd that was closed to the introduction of new animals. The inclusion of antimicrobials in the milk replacer was associated with reduced prevalence of fecal shedding of Salmonella spp. Salmonella shedding prevalence was lower as calves aged. Calving or colostrum management did not appear to influence fecal shedding of Salmonella spp in neonatal calves. Antimicrobial use and management and feeding risk factors were not associated with fecal shedding of multiple-antimicrobial–resistant Salmonella spp in preweaned calves.

ABBREVIATIONS

ARC

Antimicrobial resistance cluster

GEE

Generalized estimating equation

a.

Copan swab, Copan Diagnostics Inc, Corona, Calif.

b.

Difco, Becton Dickinson & Co, Sparks, Md.

c.

Hardy Diagnostics, Santa Maria, Calif.

d.

Difco Laboratories, Detroit, Mich.

e.

ATCC, Manassas, Va.

f.

Proc Univariate, version 8.2, SAS Institute Inc, Cary, NC.

g.

Proc Cluster, version 8.2, SAS Institute Inc, Cary, NC.

h.

Proc Logistic, version 8.2, SAS Institute Inc, Cary, NC.

i.

Proc Genmod, version 8.2, SAS Institute Inc, Cary, NC.

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Contributor Notes

Supported by National Integrated Food Safety Initiative-Cooperative State Research, Education, and Extension Service (Grant No. 00-51110-9721).

Address correspondence to Dr. Berge.