Salmonella enterica is an important cause of disease in adult horses and foals. Clinical signs of salmonellosis in horses include diarrhea, fever, colic, dehydration, and manifestations of septicemia.1 Horses with peracute salmonellosis can die rapidly despite aggressive therapy. However, many Salmonella infections in horses remain subclinical or cause only mild signs of disease. During the period of 1998 through 1999, the prevalence of fecal shedding of Salmonella spp among US horses was estimated to be 0.8%.2 The estimated prevalence of fecal shedding of Salmonella spp ranges from 3% to 7% for hospitalized horses3–5 and from 9% to 13% for horses with signs of gastrointestinal tract disease.6–8 For hospitalized horses, risk factors for salmonellosis include colic as the reason for hospitalization,5 antimicrobial administration,5,8 and abdominal surgery.8,9 Risk factors for salmonellosis in the general horse population are poorly understood.
Over 2,500 Salmonella serovars have been identified to date, but few are responsible for most clinical infections in horses and other species.10 In 2013, only 4 Salmonella serovars (Typhimurium, Anatum, Newport, and Agona) were isolated from > 50% of specimens obtained from horses with clinical signs of salmonellosis, according to data from the National Veterinary Services Laboratories in Ames, Iowa.11 Those 4 serovars have also been most frequently associated with salmonellosis outbreaks in horses at veterinary medical teaching hospitals.12–16 However, the frequency distribution of Salmonella serovars can shift over time as new strains emerge.17,18
Use of antimicrobials for the treatment of salmonellosis in adult horses is controversial, with antimicrobial treatment generally restricted to horses with severe neutropenia, persistent pyrexia, or an indwelling IV catheter.19 Conversely, antimicrobial treatment is indicated for foals with salmonellosis because of their propensity for the development of septicemia and secondary joint infections.20 Antimicrobials typically used alone or in combination for the treatment of salmonellosis in horses include ceftiofur, enrofloxacin, and gentamicin.19,20 However, multidrug-resistant Salmonella isolates from horses have been frequently reported, particularly among those serovars that are most commonly associated with clinical disease.13–16,21 Antimicrobial-resistant Salmonella isolates limit treatment options and increase the risk for therapeutic failure in veterinary clinical settings.
Antimicrobial-resistant Salmonella isolates obtained from horses also represent a potential threat to public health. Zoonotic transmission of Salmonella can occur by direct contact with the feces of infected horses.13,22 Transmission through foodborne exposure is also possible. On 1 farm, the Salmonella strain that was previously isolated from a horse with salmonellosis was also isolated from soil samples obtained from a fruit and vegetable garden that was fertilized with raw horse manure from that farm.23 Humans have also developed salmonellosis following consumption of meat from Salmonella-infected horses.24 Although salmonellosis in humans is usually self-limiting, Salmonella spp can cause invasive infections that may be fatal. Children < 5 years old, elderly adults, and immunocompromised individuals are especially susceptible to severe salmonellosis10,25–27 and require antimicrobial treatment when affected, with the drugs of choice being ciprofloxacin in adults and ceftriaxone in children.28,29
Despite the implications of antimicrobial-resistant Salmonella strains for veterinary medicine and public health, elucidation of antimicrobial resistance among Salmonella spp and other bacteria isolated from companion animals has been hindered by limited surveillance in those populations. The objectives of the study reported here were to describe the antimicrobial resistance patterns of Salmonella isolates obtained from horses in New York and other northeastern states between 2001 and 2013 and to identify trends in resistance to certain antimicrobials during that time.
Materials and Methods
Samples
Data were collected retrospectively for all Salmonella isolates obtained from equine specimens that were submitted to the Cornell University AHDC between January 1, 2001, and December 31, 2013, and subsequently underwent antimicrobial susceptibility testing. It was assumed that specimen submission by practitioners was either prompted by a history of clinical signs consistent with salmonellosis or performed as routine surveillance within the context of hospital biosecurity. It was estimated that 75% of specimens were obtained from horses in New York and the remaining 25% from horses in other northeastern states on the basis of AHDC data on equine specimen submission for Salmonella testing over an overlapping time frame of comparable duration. For each Salmonella isolate, information extracted from the computerized database included the location from which the specimen was acquired, date that Salmonella organisms were isolated, serovar, antimicrobial susceptibility pattern, and whether the specimen was submitted by a regional veterinary practice or the CUHA. Isolates that originated from repeated sampling of Salmonella-positive horses hospitalized at the CUHA were excluded from the analysis when they were the same serovar. Two Salmonella isolates representing different serovars were obtained from 1 horse hospitalized at the CUHA, and both were included in the present analysis.
Microbiological procedure for Salmonella detection
Standard bacteriologic culture methods were used to isolate Salmonella organisms from specimens. Each fecal sample (approx 10 g) was enriched in 90 mL of tetrathionate brotha that contained 1.8 mL of iodine solution. Occasionally, a fecal swab specimen was submitted rather than a fecal sample; in those instances, the entire swab was enriched in 10 mL of tetrathionate broth that contained 0.2 mL of iodine solution. Individual swab specimens from nonfecal sources were processed in the same manner as the fecal swab specimens. The sample-broth mixture was incubated at 42°C for 18 to 24 hours. Then, 10 µL of mixture was streaked onto a plate of brilliant green agar with novobiocinb and another plate of xylose lysine tergitol 4 selective media. Both plates were incubated at 37°C for 18 to 24 hours. Red colonies (indicative of lactose-nonfermenting bacteria) on the plates with brilliant green agar with novobiocin and black colonies (indicative of hydrogen sulfide–producing bacteria) on the plates with xylose lysine tergitol 4 selective media were inoculated into Kligler iron agar slants and incubated at 37°C for 18 to 24 hours. Xylose lysine tergitol 4 selective media plates without suspicious colonies were incubated at 37°C for an additional 18 to 24 hours and reassessed for black colonies. Bacterial colonies cultured in the Kligler iron agar slants that exhibited the biochemical properties of Salmonella spp were serogrouped with slide agglutination by the use of standard protocols. Colonies with positive results on slide agglutination were identified as Salmonella by use of an automated microbial identification system.c Confirmed Salmonella isolates were sent to the National Veterinary Services Laboratories in Ames, Iowa, for serotyping by use of standard protocols.30
Antimicrobial susceptibility testing
The antimicrobial susceptibility of Salmonella isolates was determined by use of a broth microdilution method. For each isolate, the MIC for each antimicrobial within the National Antimicrobial Resistance Monitoring System gram-negative panel and an equine-specific paneld was determined. One or both of those panels included the following 13 antimicrobials: amikacin, amoxicillin–clavulanic acid, ampicillin, cefazolin, cefoxitin, ceftiofur, chloramphenicol, enrofloxacin, gentamicin, imipenem, tetracycline, ticarcillin, and trimethoprim-sulfamethoxazole. Clinical and Laboratory Standards Institute guidelines31,32 were used to interpret MICs when available; otherwise, MICs were interpreted by the use of breakpoints established by the National Antimicrobial Resistance Monitoring System.33 All MICs were interpreted by the same guidelines, regardless of isolation year. Each isolate was classified as either resistant or susceptible to each antimicrobial assessed; those few isolates that had intermediate susceptibility to an antimicrobial were classified as susceptible to that antimicrobial. Quality control was performed weekly with the following 4 strains of bacteria: Escherichia coli American Type Culture Collection 25922, Staphylococcus aureus 29213, Enterococcus faecalis 29212, and Pseudomonas aeruginosa 27853. The MIC ranges for quality control recommended by the Clinical and Laboratory Standards Institute were used, and results were considered acceptable if the MICs were within the expected ranges for those bacterial strains.
Statistical analysis
Data were imported into a commercial statistical software programe for variable coding and analysis. Variables of interest included locations from which specimens were acquired, serovars, and antimicrobial resistance patterns, and descriptive data were generated for each of those variables. The prevalence of antimicrobial resistance among Salmonella isolates submitted by regional practices versus those submitted by the CUHA was evaluated with a χ2 test. For each antimicrobial assessed, temporal trends in the prevalence of resistant Salmonella isolates between 2001 and 2013 were evaluated with the Cochran-Armitage trend test. Serovar was considered an important confounding variable; therefore, the Cochran-Armitage trend test was also used to evaluate temporal trends in the prevalence of the most common serovars for the duration of the observation period. For all analyses, values of P < 0.05 were considered significant.
Results
Salmonella isolates
Between January 1, 2001, and December 31, 2013, the Cornell University AHDC performed antimicrobial susceptibility testing for 502 Salmonella isolates obtained from horses; 40 of those isolates were excluded from the analysis because they were obtained by repeated sampling of horses hospitalized in the CUHA. Of the 462 isolates included in the analysis, 426 (92.2%) were cultured from fecal samples, 13 (2.8%) were cultured from gastrointestinal tract specimens, and the remaining 23 (5.0%) were cultured from specimens obtained from miscellaneous or unspecified locations. Two hundred fifty-six (55.4%) isolates were obtained from specimens submitted by regional veterinary practices, and 206 (44.6%) isolates were obtained from specimens submitted by the CUHA.
Antimicrobial resistance patterns and serovars
The number of antimicrobials for which the MIC was determined varied among Salmonella isolates. For each antimicrobial assessed, the MIC was determined for a median of 459 isolates (range, 341 to 462 isolates).
The percentage of tested isolates that were resistant to an individual antimicrobial ranged from 0% (imipenem) to 51.5% (chloramphenicol). The prevalence of resistance to amoxicillin–clavulanic acid, cefazolin, cefoxitin, ceftiofur, and chloramphenicol was significantly higher among isolates obtained from CUHA specimens, whereas the prevalence of resistance to gentamicin and trimethoprim-sulfamethoxazole was significantly higher among isolates obtained from regional practice specimens (Table 1). Of the 337 isolates for which the susceptibility to all 13 antimicrobials was determined, 138 (40.9%) were pansusceptible and 192 (57.0%) were multidrug resistant (resistant to ≥ 3 antimicrobial classes; Table 2).
Number (percentage) of Salmonella isolates from equine specimens submitted to the Cornell University AHDC by regional veterinary practices or the CUHA between January 1, 2001, and December 31, 2013, that were resistant to various antimicrobials.
| CUHA | |||||
|---|---|---|---|---|---|
| Antimicrobial | No. of isolates evaluated | No. (%) of resistant isolates | No. of isolates evaluated | No. (%) of resistant isolates | P value* |
| Amikacin | 254 | 1 (0.4) | 205 | 0 (0) | 1.0 |
| Amoxicillin–clavulanic acid | 200 | 58 (29.0) | 141 | 77 (54.6) | < 0.001 |
| Ampicillin | 255 | 116 (45.5) | 206 | 110 (53.4) | 0.090 |
| Cefazolin | 218 | 92 (42.2) | 174 | 97 (55.7) | 0.008 |
| Cefoxitin | 200 | 55 (27.5) | 141 | 73 (51.8) | < 0.001 |
| Ceftiofur | 255 | 95 (37.3) | 206 | 95 (46.1) | 0.050 |
| Chloramphenicol | 219 | 99 (45.2) | 177 | 105 (59.3) | 0.005 |
| Enrofloxacin | 222 | 1 (0.5) | 177 | 3 (1.7) | 0.300 |
| Gentamicin | 256 | 74 (28.9) | 206 | 26 (12.6) | < 0.001 |
| Imipenem | 254 | 0 (0) | 205 | 0 (0) | — |
| Tetracycline | 254 | 117 (46.1) | 205 | 111 (54.1) | 0.09 |
| Ticarcillin | 218 | 83 (38.1) | 175 | 57 (32.6) | 0.30 |
| Trimethoprim-sulfamethoxazole | 256 | 97 (37.9) | 206 | 55 (26.7) | 0.01 |
The antimicrobials included in the susceptibility testing panel varied among isolates; therefore, the number of isolates evaluated for susceptibility varied among antimicrobials.
Results for χ2 tests that compared the prevalence of resistance between isolates submitted by regional practices and those submitted by the CUHA.
— = Not calculated.
Resistance patterns for 337 Salmonella isolates obtained from equine specimens submitted to the Cornell University AHDC between January 1, 2001, and December 31, 2013, that were tested for susceptibility to all 13 antimicrobials assessed.
| Resistance pattern | No. (%) of isolates |
|---|---|
| Pansusceptible | 138 (40.9) |
| AUG-AMP-FAZ-FOX-TIO-CHL-TET | 34 (10.1) |
| AUG-AMP-FAZ-FOX-TIO-CHL-GEN-TET-TIC-SXT | 30 (8.9) |
| AMP-FAZ-TIO-CHL-GEN-TET-TIC-SXT | 27 (8.0) |
| AUG-AMP-FAZ-FOX-TIO-CHL-TET-SXT | 20 (5.9) |
| AUG-AMP-FAZ-FOX-TIO-CHL-TET-TIC-SXT | 19 (5.6) |
| AUG-AMP-FAZ-FOX-TIO-CHL-TET-TIC | 17 (5.0) |
| AMP-FAZ-CHL-GEN-TET-TIC-SXT | 13 (3.9) |
| AMP-CHL-TET-TIC | 7 (2.1) |
| AUG-AMP-FAZ-TIO-CHL-TET | 5 (1.5) |
| AMP-CHL-TET-TIC-SXT | 4 (1.2) |
| AMP-CHL-GEN-TET-TIC-SXT | 3 (0.9) |
| AUG-AMP-FAZ-FOX-TIO-CHL-ENRO-GEN-TET-TIC-SXT | 2 (0.6) |
| TET | 2 (0.6) |
| AMI-AMP-FAZ-TIO-CHL-GEN-TET-TIC-SXT | 1 (0.3) |
| AUG-AMP-FAZ-FOX-TIO-CHL-GEN-TET-SXT | 1 (0.3) |
| AUG-AMP-FAZ-FOX-TIO-CHL-GEN-TET | 1 (0.3) |
| AUG-AMP-FAZ-FOX-TIO | 1 (0.3) |
| AUG-AMP-FAZ-CHL-ENRO-GEN-TET-TIC-SXT | 1 (0.3) |
| AUG-AMP-FOX-TIO-CHL-TET | 1 (0.3) |
| AUG-TIO | 1 (0.3) |
| AUG-TET | 1 (0.3) |
| AMP-FAZ-TIO-CHL-ENRO-GEN-TET-TIC-SXT | 1 (0.3) |
| AMP-FAZ-TIO-CHL-GEN-TIC-SXT | 1 (0.3) |
| AMP-FAZ-CHL-TET-TIC | 1 (0.3) |
| AMP-TET-TIC | 1 (0.3) |
| FAZ-FOX | 1 (0.3) |
| FAZ-TIO-TIC-SXT | 1 (0.3) |
| FOX | 1 (0.3) |
| CHL-TET | 1 (0.3) |
None of the isolates were resistant to imipenem.
AMI = Amikacin. AMP = Ampicillin. AUG = Amoxicillin-clavulanic acid. CHL = Chloramphenicol. ENRO = Enrofloxacin. FAZ = Cefazolin. FOX = Cefoxitin. GEN = Gentamicin. SXT = Trimethoprim-sulfamethoxazole. TET = Tetracycline. TIC = Ticarcillin. TIO = Ceftiofur.
The serovar was determined for 444 S enterica isolates. The most common serovar identified was Newport (n = 121 [27.3%]), followed by Typhimurium (79 [17.8%]), Oranienburg (56 [12.6%]), Agona (40 [9.0%]), Typhimurium variant 5– (29 [6.5%]), Thompson (23 [5.2%]), 4,5,12:i:– (14 [3.2%]), Holcomb (14 [3.2%]), Muenster (7 [1.6%]), Cerro (6 [1.4%]), Anatum (5 [1.1%]), Give (5 [1.1%]), Ohio (4 [0.9%]), Paratyphi B variant L-tartrate+ (4 [0.9%]), Kentucky (3 [0.7%]), Mbandaka (3 [0.7%]), Orion (3 [0.7%]), Senftenberg (3 [0.7%]), Enteritidis (2 [0.5%]), Give variant 15+ (2 [0.5%]), Hartford (2 [0.5%]), Montevideo (2 [0.5%]), Muenchen (2 [0.5%]), Uganda (2 [0.5%]), and 4,12:i:–, 6,7:k:–, 6,8:1,2–, Altona, Anatum variant 15+, Barranquilla, Bovismorbificans, Dublin, Heidelberg, Johannesburg, Litchfield, Liverpool, and Rubislaw (1 [0.2%] each). Newport was the most common serovar identified among isolates obtained from specimens submitted by both the CUHA (61/200 [30.5%]) and regional veterinary practices (60/244 [24.6%]).
Of the 112 Salmonella Newport isolates for which the susceptibility to all 13 antimicrobials was determined, 28 (25.0%) were pansusceptible and 80 (71.4%) were multidrug resistant. Of the 74 Salmonella Typhimurium isolates for which the susceptibility to all 13 antimicrobials was determined, 50 (67.6%) were pansusceptible and the remaining 24 (32.4%) were multidrug resistant. All 47 Salmonella Oranienburg isolates for which the susceptibility to all 13 antimicrobials was determined were multidrug resistant.
Temporal trends
Results of the Cochran-Armitage trend tests indicated a significant (all P < 0.001) decreasing trend in the annual prevalence of resistance to amoxicillin–clavulanic acid, ampicillin, cefazolin, cefoxitin, ceftiofur, chloramphenicol, and tetracycline among Salmonella isolates obtained from equine specimens (Figure 1). Significant temporal trends in the annual prevalence of resistance to gentamicin, ticarcillin, and trimethoprim-sulfamethoxazole were not detected. There was minimal or no resistance to amikacin, enrofloxacin, and imipenem among the Salmonella isolates analyzed in this study. When the isolates were stratified by the source (regional veterinary practices or CUHA) of submission, the temporal trends for prevalence of resistance were identical to the overall trends for both groups.
Annual prevalence of resistance to amoxicillin-clavulanic acid and ampicillin (A); cefazolin, cefoxitin, and ceftiofur (B); chloramphenicol (C); and tetracycline (D) among Salmonella isolates cultured from equine specimens submitted to the Cornell University AHDC between January 1, 2001, and December 31, 2013.
Citation: American Journal of Veterinary Research 77, 5; 10.2460/ajvr.77.5.505
The Cochran-Armitage trend tests also revealed that the prevalence of Salmonella Newport among equine isolates decreased significantly (P < 0.001) during the observation period, whereas no significant trends were identified for the prevalence of Salmonella Typhimurium or Salmonella Oranienburg (Figure 2). Between 2001 and 2005, the median annual prevalence for Salmonella Newport was 40.4% (range, 30.0% to 61.9%) and the median annual number of Salmonella Newport isolates identified was 19 (range, 9 to 27), whereas between 2006 and 2013, the median annual prevalence for Salmonella Newport was 9.0% (range, 0% to 17.1%) and the median annual number of Salmonella Newport isolates identified was 2 (range, 0 to 11).
Annual prevalence of Salmonella enterica serovars Newport (solid line), Typhimurium (dotted line), and Oranienburg (dashed line) isolated from equine specimens submitted to the Cornell University AHDC between January 1, 2001, and December 31, 2013.
Citation: American Journal of Veterinary Research 77, 5; 10.2460/ajvr.77.5.505
To evaluate whether the temporal trend for Salmonella Newport was responsible for the observed trends for antimicrobial resistance, the Cochran-Armitage test was used to separately analyze trends in the prevalence of resistance among Salmonella Newport and non-Salmonella Newport isolates. Temporal trends in antimicrobial resistance were not identified for the Salmonella Newport isolates. For the non-Salmonella Newport isolates, there were significant trends for decreasing resistance to amoxicillin-clavulanic acid (P < 0.001), ampicillin (P < 0.001), cefazolin (P = 0.002), cefoxitin (P < 0.001), ceftiofur (P < 0.001), chloramphenicol (P < 0.001), tetracycline (P < 0.001), and trimethoprim-sulfamethoxazole (P = 0.003), but there was no significant change in resistance to gentamicin and ticarcillin during the observation period.
Discussion
Monitoring antimicrobial resistance trends among Salmonella isolates obtained from horses is useful for guiding antimicrobial use in equine practice and assessing the potential risk to public health. Although antimicrobial susceptibility among bacteria isolated from food animals is regularly monitored by various federal programs,33,34 similar targeted surveillance for bacteria isolated from horses and other companion animals is limited. The present study analyzed data retrieved from the Cornell University AHDC database over a 13-year period and included 462 Salmonella isolates that were obtained from equine specimens and subsequently underwent antimicrobial susceptibility testing. Although not all isolates were tested for susceptibility to each of the 13 antimicrobials assessed in this study, the number of isolates analyzed, duration of the observation period, and the fact that all isolates were obtained from the database of 1 diagnostic laboratory made this dataset valuable for evaluating temporal trends in the antimicrobial resistance of Salmonella isolates obtained from equine specimens within a specific geographic region. To our knowledge, no other studies have been conducted to investigate the antimicrobial resistance trends of Salmonella isolates obtained from horses in the same region of the United States over several consecutive years.
Some of the AHDC specimens likely originated from horses after antimicrobial treatment had been initiated, particularly among those specimens that were submitted by the CUHA. Thus, the study isolates are presumably not representative of the broad population of Salmonella isolates from horses in the northeastern United States, and the antimicrobial resistance patterns observed may be biased by selection pressure associated with prior antimicrobial administration.
The prevalence of resistant Salmonella isolates varied widely among antimicrobials. For some antimicrobials, the prevalence of resistant isolates varied significantly between specimens submitted by regional veterinary practices and those submitted by the CUHA, which might indicate a difference in antimicrobial use between primary care and referral settings. The prevalence of resistant isolates decreased significantly for several antimicrobials during the 13-year observation period and did not increase for any antimicrobial assessed. Those results suggested that current practices for antimicrobial use in horses are not promoting the emergence and dissemination of drug-resistant Salmonella strains in the region served by the laboratory. In the United States, antimicrobials frequently administered to horses include ceftiofur, enrofloxacin, penicillin, trimethoprim-sulfamethoxazole, and certain aminoglycosides (amikacin and gentamicin), macrolides (clarithromycin and azithromycin), and tetracyclines (doxycycline, minocycline, and oxytetracycline).35 When the isolates were stratified by the source (regional veterinary practices or CUHA) of submission, the temporal trends for the prevalence of resistance were identical to the overall trends for both groups. However, the majority (192/337 [57.0%]) of isolates for which the susceptibility to all 13 antimicrobials was determined were multidrug resistant (resistant to ≥ 3 antimicrobial classes), which underscores the need for continued efforts to develop and implement strategies for the mitigation of antimicrobial resistance among Salmonella and other bacterial isolates.
Currently, the antimicrobials most frequently used to treat horses with salmonellosis include ceftiofur, enrofloxacin, and gentamicin.19,20 Of the Salmonella isolates evaluated in the present study, only 1.0% (4/399) were resistant to enrofloxacin, whereas 21.6% (100/462) were resistant to gentamicin and 41.2% (190/461) were resistant to ceftiofur. Those results suggested that enrofloxacin is likely to be an effective choice for the empirical treatment of salmonellosis in adult horses. Although enrofloxacin is not approved by the FDA for use in horses in the United States, multiple studies36–39 have been performed to determine the pharmacokinetics of enrofloxacin in horses following IV and PO administration and provide a basis for dosing recommendations. As with enrofloxacin, the prevalence of resistance to amikacin (0.2% [1/459]) among the isolates analyzed in the present study was negligible. Therefore, amikacin may be a viable option for treating salmonellosis in foals; however, its high cost precludes its use in adult horses. In the United States, parenteral use of amikacin in horses has not been approved by the FDA, although the pharmacokinetics of amikacin in horses following IM and IV administration has been reported.40–43 Amikacin is an aminoglycoside, and administration to foals, especially those that are dehydrated or otherwise compromised, may lead to renal toxicosis; therefore, close monitoring of renal function of foals treated with amikacin is required. Although none of the Salmonella isolates analyzed in the present study were resistant to imipenem, administration of that drug should be restricted to horses that have become refractory to treatment with other antimicrobials. Given that the prevalence of resistance to particular antimicrobials among Salmonella isolates can change over time, general recommendations for the treatment of salmonellosis in horses should be based on susceptibility results for contemporary isolates in a relevant geographic area, clinical efficacy studies, and judicious use guidelines.
Although Salmonella Newport was the most frequently identified serovar in the present study, the annual prevalence of Salmonella Newport isolates decreased significantly during the 13-year observation period. That shift in the frequency distribution of Salmonella serovars had only a minimal effect on the observed antimicrobial resistance trends. The decreasing trends in resistance to amoxicillin–clavulanic acid, ampicillin, cefazolin, cefoxitin, ceftiofur, chloramphenicol, and tetracycline appear to reflect a broader pattern of resistance dynamics.
Whether the decrease in Salmonella Newport isolates was indicative of a declining prevalence of that serovar among horses in the northeastern United States depends on the extent to which the Cornell University AHDC caseload represented the general horse population in that geographic region. The frequency with which Salmonella Newport was isolated from bovine specimens also declined during a similar time frame in that region.44,45 Nevertheless, Salmonella Newport remains an important threat to equine health and veterinary hospital biosecurity.15,16 Although there were no overall temporal trends in the prevalence of other common Salmonella serovars in the present study, a Salmonella Oranienburg outbreak in horses hospitalized at the CUHA led to a sharp increase in the isolation of that serovar at the midpoint of the observation period.21 Consequently, Salmonella Oranienburg was the predominant Salmonella serovar isolated from equine specimens in 2007. All the Salmonella Oranienburg isolates that underwent susceptibility testing were classified as multidrug resistant, and the peak in Salmonella Oranienburg isolates in 2007 was likely responsible for the increase in the prevalence of resistant isolates observed for many of the antimicrobials that year.
Trends in antimicrobial resistance for Salmonella isolates obtained from equine specimens submitted to the veterinary diagnostic laboratory at Utrecht University between 1993 and 2000 were evaluated.46 In that study,46 232 isolates underwent susceptibility testing to 7 antimicrobials, and Salmonella Typhimurium was the predominant serovar isolated (> 70%). Similar to the present study, a decreasing trend in the prevalence of resistance was observed for several antimicrobials, including ampicillin, kanamycin, tetracycline, and trimethoprim-sulfamethoxazole, and the prevalence of resistance did not increase over time for any of the antimicrobials assessed in that study.46 However, contrary to the present study, none of the Salmonella isolates in that study46 were resistant to ceftiofur, and only 6% were resistant to gentamicin.
Horses that shed antimicrobial-resistant Salmonella spp represent a potential threat to public health. Particularly concerning was the fairly high proportion (41.2% [190/461]) of Salmonella isolates that were resistant to ceftiofur in the present study. Among Salmonella isolates, plasmid-encoded β-lactamase genes are the most important mediators of resistance to third-generation cephalosporins,47–51 and isolates resistant to ceftiofur are most likely to also be resistant to ceftriaxone, a third-generation cephalosporin used in human medicine. In other studies, 100% (n = 103) and 99% (737) of ceftiofur-resistant Salmonella isolates from dairy cattle45 and human patients,52 respectively, were also resistant to ceftriaxone. The World Health Organization considers third- and fourth-generation cephalosporins as critically important antimicrobials for human medicine.53 Ceftriaxone is one of the primary antimicrobials used for the treatment of severe Salmonella infections in human patients and is specifically recommended for the management of invasive salmonellosis among pediatric patients.28,29 An infection caused by an antimicrobial-resistant Salmonella isolate is more likely to result in hospitalization and death than is an infection caused by a pansusceptible Salmonella isolate.54–57 Although the overall prevalence of ceftiofur-resistant Salmonella isolates was fairly high in the present study, there was a significant decline in the annual prevalence of ceftiofur-resistant Salmonella isolates during the observation period. Nevertheless, the results of the present study emphasized the importance of routine hand hygiene following animal contact, especially for young children and others with increased risk for the development of invasive disease.
Results of the present study indicated that the prevalence of resistance to several antimicrobials decreased significantly over time for Salmonella isolates obtained from equine specimens submitted to the Cornell University AHDC between January 1, 2001, and December 31, 2013. Few of the Salmonella isolates analyzed were resistant to amikacin or enrofloxacin, which suggested that those 2 antimicrobials might be viable options for the empirical treatment of salmonellosis in foals and adult horses, respectively. However, over half (57.0% [192/337]) of the Salmonella isolates analyzed in the present study were multidrug resistant. Additional research is necessary to determine whether the results of the present study are consistent among horses in other geographic regions and ideally should include data on antimicrobial use within the studied population. Monitoring of antimicrobial resistance in other companion animals would likewise be valuable for guiding recommendations for antimicrobial use in veterinary medicine and assessing the potential risk to human health.
ABBREVIATIONS
| AHDC | Animal Health Diagnostic Center |
| CUHA | Cornell University Hospital for Animals |
| MIC | Minimum inhibitory concentration |
Footnotes
Difco Laboratories Inc, Detroit, Mich.
Becton, Dickinson and Co, Franklin Lakes, NJ.
Sensititre Automated Microbiology System A80 panel, TREK Diagnostic Systems, Cleveland, Ohio.
EQUIN1F equine species MIC plate, TREK Diagnostic Systems, Cleveland, Ohio.
SAS, version 9.4, SAS Institute Inc, Cary, NC.
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