Microbiology of Otitis externa in dogs reveals wide variation in Staphylococcus species
DOI:
https://doi.org/10.12834/VetIt.3749.34672.3Keywords:
dogs, otitis externa, StaphylococcusAbstract
Bacterial infections are a major cause of otitis externa in dogs, with Staphylococcus species frequently implicated. This study analyzed samples from 24 dogs with otitis externa to identify the Staphylococcus species involved. The isolates included Staphylococcus pseudintermedius, Staphylococcus schleiferi, Staphylococcus simulans, and Staphylococcus haemolyticus. While S. pseudintermedius and S. schleiferi are well-recognized pathogens in canine otitis externa, S. simulans and S. haemolyticus are rarely reported. Given their zoonotic potential, these findings emphasize the importance of further investigations to clarify the microbiology of otitis externa and to
identify the pathogens of greatest clinical relevance.
Otitis externa is a common clinical condition in dogs. In hospital settings, prevalence has been reported to range from 7.30% (O’Neill et al., 2021) to 18.14% (Manju et al., 2018). Among dermatological cases, prevalence has been estimated at approximately 14% (Singh et al., 2024). The etiological factors of otitis externa include allergies, ectoparasites, bacterial and fungal infections, and endocrine disorders.
Among bacterial pathogens, Staphylococcus species are the most commonly isolated, particularly Staphylococcuspseudintermedius/intermedius (Lyskova et al., 2007; Tesin et al., 2023; Miszczak et al., 2023; Nocera et al., 2023). Other reported bacteria include Proteus spp., Klebsiella spp., and Escherichia coli. Notably, most bacteriological studies on otitis externa were conducted during the 2010s. To address this gap, the present study aimed to identify the predominant Staphylococcus species in dogs with otitis externa.
This study was conducted at College of Veterinary Science, Rampura Phul, Guru Angad Dev Veterinary and Animal Sciences University, Punjab, India between January and July 2024. Twenty-four dogs diagnosed with otitis externa were included. Ear swabs were collected and transported on ice to the laboratory within one hour. Samples were cultured on Brain Heart Infusion (BHI) agar and incubated at 37 °C for 24 h. Colonies were purified and identified using Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS; Bruker Daltonics Inc., Bremen, Germany) at the Department of Microbiology, College of Veterinary Science, Guru Angad Dev Veterinary and Animal Sciences University, Ludhiana, Punjab, India. Confirmed Staphylococcus isolates underwent antimicrobial susceptibility testing via the disc diffusion method (Kirby–Bauer). Results were interpreted according to the Clinical and Laboratory Standards Institute guidelines (CLSI Vet 01S Ed7:2024 and CLSI M100 Ed34:2024). Veterinary-specific standards were used where available; otherwise, human standards were applied. The zone size standards employed are summarized in Table I.
| Antibiotic (Concentration) | Staphylococcus species | Escherichia coli and Klebsiella pneumominae | |||||
| Sensitive | Intermediate | Resistant | Sensitive | Intermediate | Resistant | ||
| Gentamicin (10 µg) | All Staphylococci | ≥15 | 13-14 | ≤12 | ≥16 | 13-15 | ≤12 |
| Enrofloxacin (5 µg) | All Staphylococci | ≥23 | 17-22 | ≤16 | ≥23 | 17-22 | ≤16 |
| Ciprofloxacin (5 µg) | All Staphylococci | ≥21 | 16-20 | ≤15 | ≥26 | 22-25 | ≤21 |
| Tetracycline (30 µg) | All Staphylococci | ≥19 | 15-18 | ≤14 | ≥15 | 12-14 | ≤11 |
| Trimethoprim-Sulfamethoxazole (1.25/23.75 µg) | All Staphylococci | ≥16 | 11-15 | ≤10 | ≥16 | 11-15 | ≤10 |
| Cefoxitin (5 µg) | Other Staphylococci | ≥25 | - | ≤24 | ≥18 | 15-17 | ≤14 |
| Oxacillin (1 µg) | S. pseudintermedius and S. schleiferi | ≥18 | - | ≤17 | Not available | ||
| Penicillin (10 units) | All Staphylococci | ≥29 | - | ≤28 | Not available | ||
| Clindamycin (2 µg) | All Staphylococci | ≥21 | 15-20 | ≤14 | Not available | ||
| Amikacin (30 µg) | Not available | ≥20 | 17-19 | ≤16 | |||
| Amoxyclav (20/10 µg) | Not available | ≥18 | 14-17 | ≤13 | |||
From the 24 otitis externa cases, 26 bacterial colonies were isolated. Twenty-two samples yielded a single colony type, while two produced two distinct colonies. Of these, MALDI-TOF MS successfully identified 11 isolates: 9 Staphylococcus species, 1 Escherichia coli, and 1 Klebsiella pneumoniae. Among the Staphylococcus isolates, 4 were S. pseudintermedius, 2 S. schleiferi, 1 S. intermedius, 1 S. simulans, and 1 S. haemolyticus.
Antimicrobial susceptibility testing revealed that all S. pseudintermedius and S. schleiferi isolates were resistant to oxacillin. One S. pseudintermedius isolate showed additional resistance to penicillin, enrofloxacin, ciprofloxacin, and tetracycline. The S. simulans isolate was susceptible to all antibiotics tested. The results are detailed in Table II.
| Isolate No. | Name of bacterial Isolate | GEN | EX | CIP | TE | COT | CX | OX | P | CD | AK | AMC |
| 31 | Staphylococcus pseudintermedius | S | S | S | R | S | - | R | R | S | - | - |
| 33 | Staphylococcus pseudintermedius | S | S | S | S | S | - | R | S | S | - | - |
| 38 | Staph intermedius | S | S | S | S | S | - | R | S | S | - | - |
| 43 | Staphylococcus pseudintermedius | S | R | R | R | S | - | R | R | S | - | - |
| 44 | Staphylococcus pseudintermedius | S | S | S | S | S | - | R | S | S | - | - |
| 46 | Staphylococcus haemolyticus | S | R | R | R | S | R | - | R | S | - | - |
| 47 | Staphylococcus schleiferi | S | S | S | S | S | - | R | S | S | - | - |
| 49 | Staphylococcus simulans | S | S | S | S | S | S | - | S | S | - | - |
| 50 | Staphylococcus schleiferi | S | S | S | S | S | - | R | S | S | - | - |
| 37 | Escherichia coli | R | R | R | R | S | R | - | - | - | S | R |
| 45 | Klebsiella pneumoniae | S | S | R | S | S | S | - | - | - | I | S |
The predominance of S. pseudintermedius in this study aligns with previous findings. For instance, Tesin et al. (2023) reported that 54.72% of otitis externa cases in Serbia were due to S. pseudintermedius, with more than half of isolates resistant to tetracycline, amoxicillin, and penicillin. Miszczak et al. (2023) described similar findings in Poland, while in Italy, Nocera et al. (2023) reported S. pseudintermedius in 33.3% of diseased dogs and 46.1% of healthy dogs. Notably, the same study identified S. pseudintermedius in the nasal swabs of owners of three affected dogs, reinforcing concerns about its zoonotic potential (Carroll et al., 2021; Moses et al., 2023). Although humans are typically considered transient carriers (Guardabassi et al., 2003; Nocera et al., 2023), the risk of transmission warrants attention.
Two S. schleiferi isolates were detected, resistant only to oxacillin. This species has been reported in otitis externa and pyoderma cases of dogs (Lee et al., 2019; Nguyen et al., 2023). Zoonotic transmission has been occasionally documented, including a 2023 case in which S. schleiferi was isolated from a dog with otitis externa and its immunocompetent owner, who subsequently developed septic shock (Nguyen et al., 2023). Kumar et al. (2007) isolated S. schleiferi from an immunocompromised male with infective endocarditis (Kumar et al., 2007).
The detection of S. simulans, susceptible to all tested antibiotics, is noteworthy as it is rarely reported in veterinary medicine. To our knowledge, this may represent the first report of S. simulans associated with canine otitis externa. In humans, S. simulans is responsible for skin and urinary tract infections (Drobenuic et al., 2021).
Finally, S. haemolyticus was isolated and found resistant to fluoroquinolones, tetracyclines, cefoxitin, and trimethoprim-sulfamethoxazole. Reports of S. haemolyticus in dogs are limited (Ruzauskas et al., 2014). In humans, this species has been associated with infections of the skin, urinary tract, meninges, and bloodstream (Eltwisy et al., 2022).
Conclusions
In this study, Staphylococcus pseudintermedius, S. schleiferi, S. intermedius, S. simulans, and S. haemolyticus were isolated from dogs with otitis externa. While S. pseudintermedius, S. intermedius, and S. schleiferi are established pathogens in such cases, S. simulans and S. haemolyticus appear to be rare findings. Further investigations are warranted to better understand the microbiology of otitis externa and to identify the most clinically significant pathogens.
Acknowledgements
The authors acknowledge the financial support provided by Guru Angad Dev Veterinary and Animal Sciences University for the research work. -
Conflict of interest
None to declare
Authors contributions
Conceptualization: MS; Methodology: MS; Investigation: MS, JPY; Writing original draft preparation: MS; Writing, review and editing: MS, JPY; Project administration: MS, JPY.
All authors have read and agreed to the published version of the manuscript.
Data availability
The data generated is already analysed and discussed in the manuscript.
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