Clindamycin resistance of skin derived Staphylococcus pseudintermedius is higher in dogs with a history of antimicrobial therapy.

BACKGROUND: In the Netherlands there is a lack of data regarding resistance of Staphylococcus pseudintermedius to the systemic antimicrobial drugs used for the treatment of superficial pyoderma.
OBJECTIVES: To assess antimicrobial resistance, with emphasis on resistance to clindamycin and meticillin, in clinical isolates of S. pseudintermedius isolated from dogs with superficial pyoderma. Results were compared between dogs with and without a history of systemic antimicrobial therapy during the previous year. ANIMALS: A retrospective study of 237 referral cases presented to an academic teaching hospital between 2014 and 2019, with the clinical and microbiological diagnosis of superficial pyoderma. METHODS AND MATERIALS: All clinical isolates were identified primarily by MALDI-TOF mass spectrometry. Antimicrobial susceptibility was tested either by an agar diffusion method (2014-2016) or by broth microdilution. Antimicrobial history in the preceding year was obtained from medical records.
RESULTS: Meticillin-resistant S. pseudintermedius (MRSP) was isolated from 8% of superficial pyoderma cases. Within the meticillin-susceptible S. pseudintermedius (MSSP) population, clindamycin resistance was significantly more common in isolates derived from dogs with histories of antimicrobial treatment (37.7%) compared to dogs with no histories of exposure (21.7%; P = 0.03).
CONCLUSIONS: Given the high prevalence of clindamycin resistance in MSSP isolated from dogs with prior antimicrobial exposure, it is recommended that bacterial culture and susceptibility testing be pursued before prescribing systemic antimicrobials. Clindamycin should be regarded as the preferred treatment option if susceptibility is confirmed, due to its narrow spectrum and reduced selective pressure for MRSP.

Introduction

The commensal and opportunistic pathogen Staphylococcus pseudintermedius is the most frequently isolated bacterial pathogen in canine pyoderma. Due to the worldwide emergence of meticillin‐resistant staphylococci, responsible antimicrobial use is of paramount importance to maintain clinical efficacy of current antimicrobials. According to international guidelines, clindamycin, lincomycin, trimethoprim‐potentiated sulphonamides (TMPS), first generation cephalosporins and amoxicillin‐clavulanate are indicated as empirical antibiotics of choice for systemic treatment. However, the Dutch national guidelines indicate a slightly different classification with only clindamycin listed as an empirical option (Table 1). Compared to β‐lactam antibiotics, clindamycin shows, along with a narrow spectrum for mainly Gram‐positive bacteria, a lower selective pressure for meticillin‐resistant S. pseudintermedius (MRSP) which is the fundamental reason that clindamycin is recommended for empirical therapy. However, according to an extensive review, the published resistance of meticillin‐sensitive S. pseudintermedius (MSSP) against clindamycin within Europe shows a wide range (7–98%).

Table 1

Classification of veterinary antimicrobial agents used for canine superficial pyoderma according to Dutch policy on antimicrobial use

Classification	Systemic antimicrobials	Reasoning
1st choice	Clindamycin	Empirical therapy
2nd choice	Amoxicillin, ampicillin, amoxicillin/clavulanate and first‐ and second generation cephalosporins	Antimicrobials not classified as 1st or 3rd choice antimicrobials. Do not use, unless there is a good clinical motivation to not use the 1st choice.
3rd choice	Fluoroquinolones and third‐ and fourth generation cephalosporins	By Dutch law restricted to use only after culture and susceptibility testing shows resistance to 1st and 2nd choice antimicrobials
Never allowed	Carbapenems, glycopeptide antimicrobials, oxazolidinone, daptomycin, mupirocin and tigecycline	By Dutch law restricted to use only as last resort antimicrobial in people

Surveillance of antimicrobial resistance in animal pathogens at a national level is urgently needed in order to formulate evidence‐based antimicrobial prescribing guidelines, which may vary by locality. A surveillance system for antimicrobial resistance in animal pathogens is not available in the Netherlands.

The main objective of this retrospective study was to assess the prevalence of resistance to systemic antimicrobial drugs, with an emphasis on clindamycin, in S. pseudintermedius isolates derived from cases of canine superficial pyoderma. A comparison between dogs that had and had not received systemic antimicrobial therapies within the past 12 months was sought, with the intention of making recommendations for refinement of the Dutch national guidelines. Such data are key to promoting prudent antimicrobial use within the Netherlands and also may contribute to international perspectives on responsible antimicrobial stewardship.

Methods and materials

Study design

Medical records of dogs referred to the Dermatology Service of the Department of Clinical Sciences of Companion Animals between January 2014 and January 2019 were retrospectively evaluated. Only dogs that were initially presented with clinical signs compatible with superficial pyoderma (papules, pustules and/or epidermal collarettes) and confirmed by bacterial culture showing moderate to profuse growth of S. pseudintermedius, were included. If multiple samples had been obtained from one dog, only the S. pseudintermedius isolate from the first presentation was analysed. Prior use of antimicrobial drugs was assessed using the medical record from the referring veterinary practice. Dogs were divided into two groups: those with no history of systemic antimicrobial use over the previous 12 months and those with documented exposure. Data were included in the analysis only for MSSP and if a dog had a known antimicrobial‐use history.

Specimen collection, microbe identification and antimicrobial susceptibility testing

A standardized laboratory protocol was used between 2014 and 2019. Specimen collection was performed using sterile transport swabs (Copan; Brescia, Italy) from pustules if these were present, or papules or epidermal collarettes according to the guidelines. Bacterial infection was confirmed based on bacterial culture at the Veterinary Microbiological Diagnostic Center (VMDC) of (Utrecht University). Samples were cultured on sheep blood agar (bioTRADING; Mijdrecht, the Netherlands) overnight at 37°C. Before August 2014, presumptive colonies were identified as S. pseudintermedius using phenotypic and biochemical tests. From August 2014 onwards, identification of presumptive colonies was confirmed by matrix‐assisted laser desorption ionization–time of flight mass spectrometry (MALDI‐TOF MS, Bruker; Delft, the Netherlands).

Up to August 2016, antimicrobial susceptibility was tested by an agar diffusion method using Neo‐Sensitabs (Rosco; Taastrup, Denmark). From August 2016 onwards, minimum inhibitory concentrations (MICs) were determined by broth microdilution using a commercially available automated MICRONAUT system incorporating a custom‐made panel for routine diagnostic testing at the VMDC (MERLIN Diagnostika GmbH; Bonn, Germany). Antimicrobial susceptibility testing was performed as recommended by the manufacturer for inoculum preparation, broth composition and incubation conditions. Staphylococcus aureus ATCC 29213 was used as a quality control strain. The results were read automatically using a photometer (MERLIN Diagnostika GmbH). Veterinary breakpoints were used according to the Clinical and Laboratory Standards Institute, when available. When veterinary breakpoints were unavailable for S. pseudintermedius, breakpoints for other Staphylococci or human breakpoints were used. , Screening for meticillin resistance was performed using oxacillin susceptibility testing and confirmed by mecA real‐time PCR. Antimicrobial agents used in the susceptibility analysis for S. pseudintermedius included ampicillin (representing also amoxicillin), amoxicillin/clavulanate, first‐ and third‐generation cephalosporins (cephalothin and ceftiofur), clindamycin, tetracycline (representing also doxycycline), trimethoprim‐sulfur (TMP/S) and enrofloxacin. Isolates (n = 7) showing erythromycin‐clindamycin discordance (erythromycin resistance and clindamycin susceptibility) were subjected to D‐zone testing. Inducible clindamycin resistance was not detected.

Statistical analysis

Cross contingency tabulations with Fisher’s exact test were used to compare percentages of resistance between groups. P‐values < 0.05 were considered significant. Ninety‐five percent confidence intervals were calculated based on the modified Wald method. Differences in baseline characteristics between dogs were calculated with a chi‐square test. All analyses were performed with prism v8.2.0 for Windows (GraphPad Software; San Diego, CA, USA).

Results

A total of 134 males (56 of which were neutered) and 103 females (77 of which were spayed), were included with a median age of 5.2 years (range three months to 16 years; Figure 1). From the 218 MSSP‐positive dogs, 40 dogs had an unknown treatment history and were excluded from the analysis of antimicrobial resistance in relation to the antimicrobial treatment history. From the 178 MSSP‐positive dogs with a known treatment history, 118 (66.3%) had received antimicrobials (mean 1.9 treatments: range 1–6 treatments). Of the prescribed antimicrobial treatments, 11% consisted of clindamycin and 75% consisted of a ß‐lactam drug.

Figure 1

Number of dogs in the study per group, indicating (meticillin‐resistant) Staphylococcus pseudintermedius and treatment history.

MSSP meticillin‐sensitive S. pseudintermedius; MRSP meticillin‐resistant S. pseudintermedius.

Nineteen dogs (8.0%) were MRSP‐positive. These dogs had received on average 1.8 antimicrobial treatments in the preceding year (range 1–4 treatments). High rates of resistance (>80%) to clindamycin, doxycycline and TMP/S were detected in the MRSP isolates. The antimicrobial resistance to fluoroquinolones was much lower (<40%).

Overall resistance rates of MSSP (n = 218) for systemic antimicrobials were: amoxicillin 76.7%; clindamycin 32.4%; cephalosporins and amoxicillin‐clavulanate 0%; doxycycline 30.1%; fluoroquinolones 3.9%; and TMP/S 6.3%. There were no significant differences in MSSP resistance rates for any drug other than clindamycin, when dogs with antimicrobial exposure histories were compared to those without prior exposure (Figure 2). Clindamycin resistance in isolates from dogs with a prior history of antimicrobial use (37.7%) was significantly higher than in isolates from dogs that had not received antimicrobial therapy within the preceding 12 months (21.7%; P = 0.03). There were no other significantly different patient‐level factors between dogs with clindamycin‐susceptible and clindamycin‐resistant MSSP.

Figure 2

Resistance percentage of meticillin‐sensitive Staphylococcus pseudintermedius (MSSP) against amoxicillin, clindamycin, trimethoprim‐potentiated sulphonamides (TMPS), doxycycline and fluoroquinolones.

No resistance of MSSP against amoxicillin‐clavulanate and first‐ and third‐generation cephalosporins was recorded in dogs with a known history of antimicrobial use (n = 178). Error bars illustrate 95% confidence intervals (*P < 0.05). White columns represent the dogs with no antibiotic history and grey columns represent the dogs who had received systemic antimicrobials within the past 12 months.

Discussion

This study has revealed a high rate of clindamycin resistance (37.7%) in MSSP isolates from dogs with superficial pyoderma after receiving systemic antimicrobial therapy within the 12 months before sampling. Prior studies from Denmark and Sweden reported clindamycin resistance of MSSP just below 30% in dogs with a systemic or unknown antimicrobial exposure histories, and resistance rates between 13% and 14% in dogs without histories of systemic antimicrobial exposures. , A higher prevalence of clindamycin resistance in dogs that had received antimicrobial drug therapy (42.9%) compared to dogs without prior antimicrobial treatment (20.6%) was first reported in the late 1970s from the USA.

The high rate of resistance to amoxicillin (76.7%) in MSSP isolates in this study are consistent with rates of resistance to the (amino) penicillins reported worldwide. An adjustment of the Dutch guidelines (to now exclude amoxicillin and ampicillin as possible treatment options) would be prudent. The overall prevalence of MRSP reported from continental European clinical laboratory samples has ranged from 7% (the Netherlands) to 14% (Finland) and 32% (Italy). , , The prevalence of MRSP reported in the current study (8.0%) is consistent with the prior Dutch report.

A primary limitation of this study is that it was performed within a referral dermatology clinic. Therefore, the authors recommend that surveillance for antimicrobial resistance should be performed in a primary care population to better represent community prevalence rates. Still, the key findings of this study are a high relative rate of MSSP resistance to clindamycin when isolated from dogs with a history of antimicrobial therapy and a rather low prevalence of meticillin resistance compared to other European countries.

In conclusion, based on our findings, we recommend that the empirical use of clindamycin in dogs with histories of prior antimicrobial therapy be tempered. We instead recommend that bacterial culture and susceptibility testing be performed before prescribing systemic antimicrobial therapy in this population. If antimicrobial susceptibility testing confirms susceptibility of S. pseudintermedius to clindamycin, this drug should still be regarded as a preferred treatment option due to its narrow spectrum and reduced selective pressure for acquisition of MRSP.