External Ear Canal Evaluation in Dogs with Chronic Suppurative Otitis Externa: Comparison of Direct Cytology, Bacterial Culture and 16S Amplicon Profiling

Antimicrobial Resistance and Risk Factors of Canine Bacterial Skin Infections

Bacterial skin infections are common in dogs and often secondary to underlying conditions like allergies or ectoparasite infestations. Untreated primary causes can lead to recurrent infections and an increased risk of antimicrobial resistance, including methicillin-resistant Staphylococcus pseudintermedius (MRSP), posing a substantial clinical challenge. Here, we analyzed 896 canine bacterial skin infection samples collected from the China Agricultural University Veterinary Teaching Hospital between 2018 and 2022. Species identification was confirmed by MALDI-TOF and 16S rRNA gene sequencing. Of the 896 samples, 722 (80.6%) yielded 1123 bacterial isolates, with Staphylococcus pseudintermedius (n = 421), Pseudomonas aeruginosa (n = 108), and Escherichia coli (n = 73) being the most prevalent. Antimicrobial susceptibility was evaluated using the broth microdilution method according to CLSI guidelines. Notably, resistance to florfenicol in S. pseudintermedius increased from 9.1% in 2018 to 20.0% in 2022, while resistance to ceftriaxone in E. coli rose from 30.0% to 72.7% over the same period. Among 305 reviewed cases, pyoderma (47.5%, 145/305) was the most common infection type, predominantly associated with S. pseudintermedius (n = 114), followed by otitis (25.6%, 78/305) primarily linked to P. aeruginosa (n = 24). Mixed infections occurred in 35.4% (108/305) of cases, with S. pseudintermedius as the most frequently isolated species in both single and mixed infections. The multivariable logistic regression model revealed that MRSP infections were correlated with a history of invasion (p <0.001) and prolonged disease duration (six months to less than one year: p = 0.005; one year or longer: p < 0.001). Core-genome SNP analysis showed that eight dogs were infected with identical S. pseudintermedius strains, in which one dog exhibited a shift from gentamicin susceptibility to resistance within nine days. Conversely, three dogs were infected by distinct S. pseudintermedius strains at two time points. To effectively manage MRSP infections and chronic skin infections in dogs, rigorous disinfection protocols in veterinary hospitals, control of disease duration, prevention of recurrent infections, and continuous monitoring of antibiotic resistance patterns are essential.

Brachycephaly, Ear Anatomy, and Co-Does Size Matter? A Retrospective Study on the Influence of Size-Dependent Features Regarding Canine Otitis Externa

This study, which involved 868 patients, focused on size-dependent features (body size, pinna formation, brachycephaly, overweight) and their associations with the occurrence of canine Otitis externa. Breed, sex, and neuter status were also included. Canine patients diagnosed with Otitis externa presented between 1 January 2019 and 31 July 2022 in a referral small animal hospital were included, while dogs with matching categories presented at the Unit for Reproductive Medicine in the same time period functioned as a randomized control group. Statistical analyses included Pearson-Chi2 test, Fisher's exact test, and Bonferroni correction. p-values of <0.05 were considered statistically significant. Rhodesian Ridgebacks, non-brachycephalic breeds, intact female dogs, dogs with semi-erect ears, and large-sized breeds had a significantly reduced risk of developing Otitis externa. Conversely, Pugs, French Bulldogs, Cocker Spaniels, medium-sized breeds, dogs with erect ears, brachycephalic breeds, neutered female, and neutered male patients were more frequently diagnosed with Otitis externa. The study concluded that large breed dogs showed a reduced risk of developing Otitis externa in our data set as well as meso- and doliocephalic breeds and dogs with semi-erect pinna formation. Particularly, the Rhodesian Ridgeback and intact female dogs showed a significantly low risk of suffering from Otitis in the current study.

Fungal dysbiosis following antibacterial monotherapy in canine otitis externa

OBJECTIVES

To evaluate evidence of Malassezia overgrowth following successful topical antibacterial monotherapy of refractory canine bacterial otitis using semi-quantitative cultures.

MATERIALS AND METHODS

Twenty-nine dogs with bacterial otitis were treated topically with either fluoro-quinolone [0.5% enrofloxacin (18 dogs, 19 treatment events, 25 ears) or 0.1% marbofloxacin (1 ear), with 0.1% dexamethasone] ("FQ") SID, or 143 mg/mL piperacillin/18 mg/mL tazobactam ("PT") BID (11 dogs, 14 treatment events, 19 ears) for 8 to 36 days (mean 20 days). At visits 1 (V1) and 2 (V2), ear swab tips were washed in PBS + Triton X-100 and serial dilutions spread-plated onto blood, MacConkey (37°C, 48 hours) and modified Dixon's agar (32°C, 14 days) to generate semiquantitative counts. Microbes were identified by phenotype and MALDI-TOF.

RESULTS

Prior to treatment, Pseudomonas aeruginosa was isolated alone or in combination with other bacteria in 14 FQ-treated ears and 18 PT-treated ears; the next most frequent bacteria were Streptococcus canis (8 FQ, 2 PT) and Staphylococcus pseudintermedius (8 FQ, 1 PT). The proportions of dogs' ears (excluding cross-over treatments and contra-lateral affected ears) from which bacteria were isolated were significantly reduced by treatment with both FQ (V1, 13/15; V2, 5/15) and PT (V1, 14/14; V2, 2/14). The proportions of dogs' ears from which yeasts (Malassezia pachydermatis, Candida spp.) were isolated were significantly increased by treatment in dogs treated with PT (V1, 1/14; V2, 14/14) but not FQ (V1, 3/15; V2, 6/15).

CLINICAL SIGNIFICANCE

Otitis cases that receive potent antibacterial monotherapy must be monitored for yeast overgrowth.

Molecular Epidemiological Characteristics of Staphylococcus pseudintermedius, Staphylococcus coagulans, and Coagulase-Negative Staphylococci Cultured from Clinical Canine Skin and Ear Samples in Queensland

Background/Objectives: Infections in dogs caused by methicillin-resistant staphylococci (MRS) present limited treatment options. This study's objective was to investigate the molecular epidemiology of Staphylococcus spp. cultured exclusively from clinical canine skin and ear samples in Queensland, Australia, using whole-genome sequencing (WGS). Methods: Forty-two Staphylococcus spp. isolated from clinical canine skin and ear samples, from an unknown number of dogs, were sourced from two veterinary diagnostic laboratories between January 2022 and May 2023. These isolates underwent matrix-assisted laser desorption ionisation- time of flight bacterial identification, minimum inhibitory concentration testing using SensititreTM plates and WGS. Phylogenetic trees and core genome multilocus sequence typing (cgMLST) minimum spanning trees (MSTs) were constructed. Results: The isolates included methicillin-resistant and -sensitive S. pseudintermedius (MRSP: 57.1%, 24/42; and MSSP: 19.1%, 8/42), methicillin-resistant and -sensitive S. coagulans (MRSC: 14.3%, 6/42; and MSSC: 2.4%, 1/42) and methicillin-resistant coagulase-negative staphylococci (MR-CoNS: 7.1%, 3/42). Thirty-nine isolates were included after WGS, where all MRS harboured the mecA gene. Eighteen sequence types (STs) were identified, including three novel MRSP and six novel MSSP STs. MRSP ST496-V-VII (23%; 9/39) and MRSP ST749-IV-(IVg) (12.8%; 5/39) were commonly isolated. Phylogenetic analysis of single nucleotide polymorphisms showed that MRSP, MRSC and MSSC were similar to globally isolated staphylococci from canine skin and ear infections. Using cgMLST MSTs, MRSP isolates were not closely related to global strains. Conclusions: Our findings revealed a genotypically diverse geographical distribution and phylogenetic relatedness of staphylococci cultured from clinical canine skin and ear samples across Queensland. This highlights the importance of ongoing surveillance to aid in evidence-based treatment decisions and antimicrobial stewardship.

The role of the microbiome in allergic dermatitis-related otitis externa: a multi-species comparative review

The external ear canal, characterized by species-specific structural and physiological differences, maintains a hostile environment that prevents microbial overgrowth and foreign body entry, supported by factors such as temperature, pH, humidity, and cerumen with antimicrobial properties. This review combines several studies on the healthy ear canal's structure and physiology with a critical approach to the potential existence of an ear microbiome. We use a comparative multi-species approach to explore how allergic conditions alter the ear canal microenvironment and cerumen in different mammalian species, promoting pathogen colonization. We propose a pathogenetic model in which allergic conditions disrupt the antimicrobial environment of the EEC, creating circumstances favorable for facultative pathogenic micro-organisms like Staphylococcus and Malassezia species, leading to otitis externa (OE). A better understanding of the underpinning mechanisms may lead to innovative approaches to disease mitigation.

Future Challenge: Assessing the Antibiotic Susceptibility Patterns of Staphylococcus Species Isolated from Canine Otitis Externa Cases in Western Romania

INTRODUCTION

Antimicrobial resistance (AMR) has surfaced as a critical challenge to public health on a global scale. The precise and swift identification of resistance to antimicrobial agents, along with timely and suitable antimicrobial therapy paired with effective stewardship practices, is crucial for managing the rise and dissemination of antimicrobial resistance. The objective of our investigation was to outline the antimicrobial resistance profile of Staphylococcus spp., a significant contributor to canine otitis, a prevalent condition in dogs, isolated in Western Romania.

METHODS AND MATERIALS

All data were collected from clinical cases of canine otitis externa which presented at the University Clinic of the Faculty of Veterinary Medicine in Timișoara/Romania. A clinical evaluation was conducted, from which era swabs are usually collected and sent for analysis at the laboratory. Laboratory analysis included the microbiological examination for identifying Staphylococcus spp. and determining antibiotic susceptibility phenotypes. Statistical analysis was implemented on all data that were collected. The ear swabs were processed with standard procedures for cultivating and identifying bacteria. The resulting subcultures were processed to determine the staphylococcal species on the GP ID Cards of the Vitek® 2 automatic system. The antimicrobial susceptibility profiles were detected by the Vitek® 2 system using an AST-GP80 card. These isolated Staphylococcus spp. strains were further processed by real-time PCR and PCR-RFLP.

RESULTS

Of all the auricular exudate samples analyzed, 76 were positive for Staphylococcus spp. (59.38%). Within these, in 82% of auricular samples, six distinct Staphylococcus spp. were identified (Staphylococcus (S.) pseudintermedius, S. intermedius, S. hyicus, S. delphiny, S. shleiferi, and S. aureus). Our data indicate that the PCR-RFLP assay is a practical approach to S. pseudintermedius identification, allowing for discrimination from the other Staphylococcus Intermedius Group (SIG) species and important staphylococcal pathogens of dogs. The highest frequency of resistant S. pseudintermedius isolates was detected against tetracycline (21/34; 61.76%; p-value 0.003), gentamicin (20/34; 58.82%), and kanamycin (20/34; 58.82%).

CONCLUSIONS

These results are essential to guide the prudent use of antibiotics in veterinary medicine. They will also help design efficient control strategies and measure their effectiveness.

Antibacterial effects of chitosan-based hydrogels containing Trachyspermum ammi essential oil on pathogens isolated from dogs with otitis externa

BACKGROUND

Growing antibiotic resistance has made treating otitis externa (OE) increasingly challenging. On the other hand, local antimicrobial treatments, especially those that combine essential oils (EOs) with nanoparticles, tend to be preferred over systemic ones. It was investigated whether Ajwain (Trachyspermum ammi) EO, combined with chitosan nanoparticles modified by cholesterol, could inhibit the growth of bacterial pathogens isolated from OE cases in dogs. In total, 57 dogs with clinical signs of OE were examined and bacteriologically tested. Hydrogels of Chitosan were synthesized by self-assembly and investigated. EO was extracted (Clevenger machine), and its ingredients were checked (GC-MS analysis) and encapsulated in chitosan-cholesterol nanoparticles. Disc-diffusion and broth Micro-dilution (MIC and MBC) examined its antimicrobial and therapeutic properties.

RESULTS

Staphylococcus pseudintermedius (49.3%) was the most common bacteria isolated from OE cases, followed by Pseudomonas aeruginosa (14.7%), Escherichia coli (13.3%), Streptococcus canis (9.3%), Corynebacterium auriscanis (6.7%), Klebsiella pneumoniae (2.7%), Proteus mirabilis (2.7%), and Bacillus cereus (1.3%). The investigation into the antimicrobial properties of Ajwain EO encapsulated in chitosan nanoparticles revealed that it exhibited a more pronounced antimicrobial effect against the pathogens responsible for OE.

CONCLUSIONS

Using chitosan nanoparticles encapsulated with EO presents an effective treatment approach for dogs with OE that conventional antimicrobial treatments have not cured. This approach not only enhances antibacterial effects but also reduces the required dosage of antimicrobials, potentially preventing the emergence of antimicrobial resistance.

Microbiological Survey and Evaluation of Antimicrobial Susceptibility Patterns of Microorganisms Obtained from Suspect Cases of Canine Otitis Externa in Gran Canaria, Spain

A retrospective study of microbiological laboratory results from 2020 to 2022, obtained from a veterinary diagnostic laboratory of the island of Gran Canaria, Spain, focused on canine otitis cases, was performed. The objective of this study was to analyze the pathogen distribution, antimicrobial susceptibility, prevalence of multidrug resistant phenotypes and the role of coinfections in otitis cases in order to provide up-to-date evidence that could support effective control strategies for this prevalent pathology. A total of 604 submissions were processed for the diagnosis of canine external otitis. Of the samples analyzed, 472 were positive for bacterial or fungal growth (78.1%; 95% CI: 74.8-81.4%). A total of 558 microbiological diagnoses were obtained, divided in 421 bacterial (75.4%; 95% CI: 71.8-79.0%) and 137 fungal (24.6%; 95% CI: 20.9-28.1%) identifications. Staphylococcus pseudintermedius, Malassezia pachydermatis and Pseudomonas aeruginosa were the most prevalent microorganisms detected in clinical cases of otitis. High level antimicrobial resistance was found for Pseudomonas aeruginosa (30.7%), Proteus mirabilis (29.4%), Staphylococcus pseudintermedius (25.1%) and Escherichia coli (19%). Multidrug-resistant phenotypes were observed in 47% of the bacteria isolated. In addition, a 26.4% prevalence of methicillin-resistant Staphylococcus pseudintermedius was detected. The high prevalence of antimicrobial resistant phenotypes in these bacteria highlights the current necessity for constant up-to-date prevalence and antimicrobial susceptibility data that can support evidence-based strategies to effectively tackle this animal and public health concern.

Pseudomonas spp. in Canine Otitis Externa

Canine otitis externa (OE) is a commonly diagnosed condition seen in veterinary practice worldwide. In this review, we discuss the mechanisms of the disease, with a particular focus on the biological characteristics of Pseudomonas aeruginosa and the impact that antibiotic resistance has on successful recovery from OE. We also consider potential alternatives to antimicrobial chemotherapy for the treatment of recalcitrant infections. P. aeruginosa is not a typical constituent of the canine ear microbiota, but is frequently isolated from cases of chronic OE, and the nature of this pathogen often makes treatment difficult. Biofilm formation is identified in 40-95% of P. aeruginosa from cases of OE and intrinsic and acquired antibiotic resistance, especially resistance to clinically important antibiotics, highlights the need for alternative treatments. The role of other virulence factors in OE remains relatively unexplored and further work is needed. The studies described in this work highlight several potential alternative treatments, including the use of bacteriophages. This review provides a summary of the aetiology of OE with particular reference to the dysbiosis that leads to colonisation by P. aeruginosa and highlights the need for novel treatments for the future management of P. aeruginosa otitis.

Antimicrobial Activity of Sempervivum tectorum L. Extract on Pathogenic Bacteria Isolated from Otitis Externa of Dogs

The present study investigated the antimicrobial activity, total phenolic content, and proanthocyanidin concentration of ethanolic extracts from fresh leaves of Sempervivum tectorum L. The extracts were phytochemically analyzed and evaluated for antimicrobial activity. The broth microdilution method was used to assess antimicrobial activity against pathogenic bacteria isolated from ear swabs taken from dogs with otitis externa. Many compounds were present in the ethanolic aqueous extracts, which exhibited a broad spectrum of antimicrobial activity. They showed strong antibacterial activity against standard clinical Gram-positive strains such as S. aureus and Gram-negative strains such as P. aeruginosa. In our study, the obtained quantity of total phenolic compounds in the ethanol:water extract of leaves was 126.17 mg GAE/g. The proanthocyanidin concentration in the tested Sempervivum tectorum L. extracts was 15.39 mg PAC/g material. The high contents of total phenolics and proanthocyanidin indicated that these compounds contribute to antimicrobial activity. The antimicrobial activity of the tested S. tectorum L. extracts ranged from 1.47 to 63.75 µg/mL, starting with 1.47 µg/mL and 1.75 µg/mL against S. aureus ATCC 25923 and P. aeruginosa ATCC 27853 strains, respectively. Likewise, S. tectorum L. ethanol extract demonstrated a bacteriostatic effect against S. aureus clinical isolate with a median MIC of 23.25 µg/mL and MBC of 37.23 µg/mL; and bactericidal against S. aureus ATCC 25923 with the median MIC of 20.33 µg/mL and MBC of 37.29 µg/mL. In the Gram-negative P. aeruginosa clinical and standard strains, the expressed MIC and MBC values were 24.234 and 20.53 µg/mL for MIC, and 37.30 and 37.02 µg/mL for MBC, respectively.