P-76 The effect of omega-3 fatty acid supplementation on cutaneous and plasma fatty acid concentrations in dogs with atopic dermatitis

The objectives of this study were to evaluate in vivo tolerance, and antimicrobial and clinical activities of a topical otic preparation containing EDTA tromethamine (Tris) and chlorhexidine digluconate 0.15% solution (Otodine^®) in dogs with chronic bacterial otitis externa. Eleven dogs were included. The affected ears were filled with the solution once daily during a 2-week period. Dogs were evaluated on days 0, 14 and 28. Three clinical parameters (exudate, erythema, pain) and three cytologic parameters ( Malassezia , cocci, rods) were scored (0–4 scale) by otoscopic and cytological examinations of otic exudate. Bacterial cultures were performed at each time point. If there were bacteria on cytological examination on day 14, the dogs were treated with the original product, with the addition of enrofloxacin (5%) applied 10 min after the original product, for a further 2 weeks. All 11 cases yielded isolates of resistant gram-negative bacteria; gram-positive bacteria were also isolated from six of 11 dogs. On day 14, six of 11 dogs were negative on culture examination; on day 28, 10 of 11 were negative and only one case had a positive culture. On day 14, clinical and microbial scores (cytology) were reduced by 54.6 and 71.1%, respectively, and by 85.7 and 94% on day 28. All cases reported good tolerance of the treatment. The results show that this ear solution was helpful in the management of chronic bacterial otitis externa in dogs and was well tolerated. There seems to be a synergistic effect of the combination of Tris-EDTA/chlorhexidine digluconate 0.15% solution, and an antimicrobial agent (enrofloxacin) against resistant gram-positive and gram-negative bacteria.
Funding: Self-funded.     

Ciprofloxacin as a representative of disk diffusion in vitro susceptibility of enrofloxacin for bacterial organisms from the middle-ear tissue of dogs with end-stage otitis externa

The purposes of this study were to determine whether ciprofloxacin disk diffusion susceptibility test (DDT) results could be used to assess the in vitro susceptibility of otic bacterial organisms to enrofloxacin and to determine the effect of concurrent enrofloxacin administration on the DDT results for enrofloxacin and ciprofloxacin. Thirty dogs with end-stage otitis externa undergoing unilateral total ear canal ablation were enrolled. The dogs were randomized to one of four enrofloxacin-treatment groups or to the control group. Each dog in the treatment groups received two intravenous doses of enrofloxacin prior to surgical removal of the middle-ear tissue while the control group did not receive any enrofloxacin. One dog was excluded from the study as no middle-ear tissue was removed during surgery. Twenty-four dogs were in the enrofloxacin-treatment groups and five dogs were in the control group. In 12 of 29 dogs (41.4%), 14 of 82 (17.1%) of bacteria had discrepancies in DDT results for enrofloxacin and ciprofloxacin. Discrepancies between the control group and treatment groups were not significantly different even though the percentage of discrepancies for the combined treatment group was 19.4% compared to 6.7% for the control group. In this study, ciprofloxacin DDT results were not an accurate indicator of the in vitro susceptibility of enrofloxacin for bacteria isolated from the middle-ear tissue of dogs with end-stage otitis.     

Otomycosis due to Aspergillus spp. in a dog: case report and literature review

This report describes the clinical findings, clinicopathology and treatment of otomycosis caused by Aspergillus spp. in an atopic dog affected by chronic unilateral purulent otitis externa unresponsive to topical and oral antibiotics and antifungal treatments. Cytology of otic exudate revealed neutrophils and septate fungal hyphae, and otic culture grew Aspergillus spp. and no bacteria. Treatments included allergen‐specific immunotherapy, topical and oral antifungal therapy and anti‐inflammatory steroid therapy. Final resolution occurred after treatment of the underlying hypersensitivity disorder, administration of topical ketoconazole and debridement of infectious ear exudate. Otomycosis due to filamentous fungi may, as in humans, occur in dogs with ear canals compromised by pre‐existing allergic or bacterial otitis, and possibly previous antibiotic therapy. Antifungal medications provided clinical improvement, but the key to successful treatment was the restoration of the normal physiology of the external auditory canal.     

Determination of enrofloxacin stability and in vitro efficacy against Staphylococcus pseudintermedius and Pseudomonas aeruginosa in four ear cleaner solutions over a 28 day period

Chemical stability and in vitro bactericidal efficacy of 0.9% enrofloxacin‐compounded solutions were evaluated following storage at room temperature for 28 days. Chemical stability of enrofloxacin was determined by high‐performance liquid chromatography (HPLC) in five compounded solutions, including sterile water. Bactericidal efficacy was determined by spiral plating serial 10‐fold dilutions of bacteria and solutions followed by colony counts. Tris–EDTA [TrizEDTA^® (TE)], Tris–EDTA and 0.15% chlorhexidine [TrizChlor^® (TC)], 2.5% lactic acid, 0.1% salicylic acid and 0.1% parachlorometaxylenol [Epi‐Otic (EO)], and 0.1% free salicylic acid, 0.1% parachlorometaxylenol and 0.5% EDTA [Epi‐Otic Advanced (EA)] were used. High‐performance liquid chromatography was carried out with one‐step liquid/liquid extraction to detect and quantify enrofloxacin stability. Mean recoveries for compounded samples run in triplicate at 28 days were 97.7% (TE), 99.9% (TC), 98.1% (EO) and 97.8% (EA). Kruskal–Wallis analysis showed no significant difference in the percentage recovery (H = 0.0539, df = 3, P = 0.9967). American Type Culture Collection strains of Staphylococcus pseudintermedius and Pseudomonas aeruginosa were used to evaluate in vitro efficacy following 30 min incubation on days 0, 14 and 28. Consistent in vitro bactericidal efficacy of all compounded solutions, indicated by killing >2.3 × 10⁷ colony‐forming units/mL, was seen; however, bactericidal efficacy decreased for compounded TC on day 14. Pseudomonas aeruginosa was more sensitive to the ear cleaners and enrofloxacin than S. pseudintermedius. The HPLC and in vitro data suggest that 0.9% enrofloxacin compounded with sterile water, TE, EO and EA maintains chemical stability and bactericidal efficacy for 28 days.     

Fluoroquinolone levels in healthy dog urine following a 20‐mg/kg oral dose of enrofloxacin exceed mutant prevention concentration targets against Escherichia coli isolated from canine urinary tract infections

A 3‐day course of oral enrofloxacin is effective for treating uncomplicated urinary tract infection (UTI) in dogs when administered 20 mg/kg Q24H. However, emergence of fluoroquinolone‐resistant mutants of uropathogens is a concern. Urine concentrations of enrofloxacin and ciprofloxacin were measured in six healthy dogs following dose of enrofloxacin 20 mg/kg. Mutant prevention concentrations of Escherichia coli isolated from canine UTI were also determined against ciprofloxacin. Urine AUC(24)/MPC ratios considering ciprofloxacin concentrations ranged 3819–7767, indicating that selection of resistant E. coli mutants in dogs with uncomplicated UTIs is unlikely in the bladder given that an AUC(24)/MPC = 39 is considered to be protective against mutant selection for ciprofloxacin. However, additional studies are required to evaluate the effects of this enrofloxacin treatment protocol on bacteria that colonize anatomic sites where fluoroquinolones achieve lower concentrations compared to the urinary bladder.     

Age‐associated changes to pathogen‐associated molecular pattern‐induced inflammatory mediator production in dogs

Objective – To determine whether older dogs will have a more pronounced pro‐inflammatory response and blunted anti‐inflammatory response to pathogen‐associated molecular patterns (PAMPs) compared with younger dogs. Design – Prospective. Setting – University teaching hospital. Animals – Thirty‐eight privately owned sexually altered dogs of various ages. Interventions – Blood was collected for HCT, WBC count, plasma biochemical analysis, and whole blood culture. Whole blood was stimulated with lipopolysaccharide (LPS) or, lipoteichoic acid or, peptidoglycan or, addition of phosphate‐buffered saline. Tumor necrosis factor (TNF), interleukin (IL)‐6, and IL‐10 production from whole blood were compared among young, middle aged, and geriatric dogs. Measurements and Main Results – LPS, lipoteichoic acid, and peptidoglycan stimulated significant TNF, IL‐6, and IL‐10 production from canine whole blood compared with phosphate‐buffered saline. Whole blood from geriatric dogs had a blunted IL‐10 response to LPS stimulation and middle‐aged dogs had increased LPS‐induced TNF production compared with the other groups. Conclusion – PAMPs from gram‐positive and gram‐negative bacteria stimulate TNF, IL‐6, and IL‐10 production from canine whole blood. The inflammatory mediator response to PAMPs from gram‐negative bacteria alters with age and may be one factor contributing to mortality in geriatric dogs with sepsis.     

Malassezia otitis externa in the dog: the effect of heat-fixing otic exudate for cytological analysis

This study was conducted on 32 dogs with Malassezia otitis externa to determine the effect of heat-fixing otic exudate on cytological analysis. Malassezia infection was confirmed by cytological examination of otic exudate. Otic discharge collected with cotton swabs was then rolled onto glass slides. One slide per dog was heat-fixed prior to staining; the other slide was not heat-fixed. The number of yeast in 10 oil-immersion fields (1000 x magnification) was counted for both slides from each dog. Heat-fixing did not systematically cause either increased or decreased numbers of Malassezia on cytology of otic exudate.     

Identification and antimicrobial susceptibility patterns of bacteria causing otitis externa in dogs

Bacterial agents are considered important pathogens causing external otitis in dogs. It is essential to carry out bacterial culture and antimicrobial susceptibility test in the case of otitis externa, particularly for chronic or recurring cases. Sterile swab samples were obtained from terminal part of vertical ear canals of 74 dogs with otitis externa for cytology, bacterial culture and antimicrobial susceptibility test. Cytologic smears were stained using Gram and Giemsa staining methods. Aerobic bacterial culture performed on blood agar and MacConkey agar. Among total number of 92 isolated bacteria, 68 were Staphylococcus intermedius. Other isolated bacteria included: Pseudomonas aeruginosa, Proteus mirabilis, Escherichia coli, Pasteurella canis, and six other species of coagulase-negative Staphylococcus. Antimicrobial susceptibility test were performed for all isolated bacteria using 14 antibiotics. Based on the results of this study, all isolated Staphylococcus spp. were sensitive to amikacin, enrofloxacin, and rifampin, and had low resistance to gentamicin, cephalothin and ceftriaxone. More than half of gram-positive isolates were resistant to penicillin and ampicillin. Generally, all isolated gram-negative bacteria, were sensitive to amikacin and enrofloxacin, and had low resistance to ceftriaxone and gentamicin. They were highly resistant to penicillin, eythromycin, and cephalothin. Regarding the results of this study, in cases of uncomplicated otitis externa, it is possible to select antimicrobial drugs merely based on cytology, but it is recommended to perform bacterial culture and antimicrobial susceptibility test. However, in complicated or refractory cases, antimicrobials should be selected based on bacterial culture and antimicrobial susceptibility test.     

Comparison of bacterial organisms and their susceptibility patterns from otic exudate and ear tissue from the vertical ear canal of dogs undergoing a total ear canal ablation

The purpose of this prospective study was to compare bacterial organisms and their susceptibility patterns from otic exudate and ear tissue from the vertical ear canal of six dogs with end-stage otitis undergoing a total ear canal ablation. Twenty-six organisms, 13 from the exudate and 13 from the tissue, were morphologically and biochemically similar, and their susceptibility patterns were compared. There were discrepancies for only 5 antibiotics for 4 organisms from 3 dogs. Culture of otic exudate from the vertical ear canal of dogs with chronic end-stage otitis externa provides an accurate reflection of the bacterial species present and the antimicrobial susceptibility patterns of these infections.     

Airborne Gram‐Negative Bacterial Flora in Animal Houses

The concentration and the species composition of airborne gram‐negative bacteria were studied in four cattle houses, one pig house and one poultry barn. On average only between 0.02 and 5.2 % of the total number of culturable aerobic bacteria were identified as gram‐negative bacteria. Obligate anaerobic gram‐negative bacteria were not isolated at all. In the airborne gram‐negative bacterial flora the following bacterial families dominated: the Enterobacteriaceae, the Pseudomonadaceae and the Neisseriaceae. Within the family of the Enterobacteriaceae the species Escherichia coli and Enterobacter agglomerans were predominant. In animal houses using straw as bedding material Ent. agglomerans was most frequent, whereas in animal houses without litter E. coli was mainly found. Airborne Neisseriaceae were isolated very frequently in cow barns with Acinetobacter lowffii as the primary species. Airborne Pseudomonadaceae were found in high concentrations during periods of high air humidity. The results presented may also give some indications on the origin and sources of airborne endotoxins in animal housing.     

Biofilm production by pathogens associated with canine otitis externa,and the antibiofilm activity of ionophores and antimicrobial adjuvants

Otitis externa (OE) is a frequently reported disorder in dogs associated with secondary infections by Staphylococcus, Pseudomonas and yeast pathogens. The presence of biofilms may play an important role in the resistance of otic pathogens to antimicrobial agents. Biofilm production of twenty Staphylococcus pseudintermedius and twenty Pseudomonas aeruginosa canine otic isolates was determined quantitatively using a microtiter plate assay, and each isolate was classified as a strong, moderate, weak or nonbiofilm producer. Minimum biofilm eradication concentration (MBEC) of two ionophores (narasin and monensin) and three adjuvants (N‐acetylcysteine (NAC), Tris‐EDTA and disodium EDTA) were investigated spectrophotometrically (OD_570nm) and quantitatively (CFU/ml) against selected Staphylococcus and Pseudomonas biofilm cultures. Concurrently, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of planktonic cultures were assessed. 16/20 of the S. pseudintermedius clinical isolates were weak biofilm producers. 19/20 P. aeruginosa clinical isolates produced biofilms and were distributed almost equally as weak, moderate and strong biofilm producers. While significant antibiofilm activity was observed, no MBEC was achieved with narasin or monensin. The MBEC for NAC ranged from 5,000–10,000 µg/ml and from 20,000–80,000 µg/ml against S. pseudintermedius and P. aeruginosa, respectively. Tris‐EDTA eradicated P. aeruginosa biofilms at concentrations ranging from 6,000/1,900 to 12,000/3,800 µg/ml. The MBEC was up to 16‐fold and eightfold higher than the MIC/MBC of NAC and Tris‐EDTA, respectively. Disodium EDTA reduced biofilm growth of both strains at concentrations of 470 µg/ml and higher. It can be concluded that biofilm production is common in pathogens associated with canine OE. NAC and Tris‐EDTA are effective antibiofilm agents in vitro that could be considered for the treatment of biofilm‐associated OE in dogs.     

Evaluation of enrofloxacin use in koalas (Phascolarctos cinereus) via population pharmacokinetics and Monte Carlo simulation

Clinically normal koalas (n = 6) received a single dose of intravenous enrofloxacin (10 mg/kg). Serial plasma samples were collected over 24 h, and enrofloxacin concentrations were determined via high‐performance liquid chromatography. Population pharmacokinetic modeling was performed in S‐ADAPT. The probability of target attainment (PTA) was predicted via Monte Carlo simulations (MCS) using relevant target values (30–300) based on the unbound area under the curve over 24 h divided by the minimum inhibitory concentration (MIC) (fAUC_0–24/MIC), and published subcutaneous data were incorporated (Griffith et al., 2010). A two‐compartment disposition model with allometrically scaled clearances (exponent: 0.75) and volumes of distribution (exponent: 1.0) adequately described the disposition of enrofloxacin. For 5.4 kg koalas (average weight), point estimates for total clearance (SE%) were 2.58 L/h (15%), central volume of distribution 0.249 L (14%), and peripheral volume 2.77 L (20%). MCS using a target fAUC_0–24/MIC of 40 predicted highest treatable MICs of 0.0625 mg/L for intravenous dosing and 0.0313 mg/L for subcutaneous dosing of 10 mg/kg enrofloxacin every 24 h. Thus, the frequently used dosage of 10 mg/kg enrofloxacin every 24 h subcutaneously may be appropriate against gram‐positive bacteria with MICs ≤ 0.03 mg/L (PTA > 90%), but appears inadequate against gram‐negative bacteria and Chlamydiae in koalas.     

Otitis externa induced with Malassezia pachydermatis in dogs and the efficacy of pimaricin.

Eight beagles were experimentally inoculated intraotally with Malassezia pachydermatis to induce acute otitis externa. Three or 4 days after the inoculation, the animals showed the symptoms of otitis externa. All ear canals were erythematous and the dogs were shaking their heads. A large number of M. pachydermatis was noticed in exudate taken from every ear canal. Clinical signs of otitis externa were reduced after treatment with 0.1 ml (per canal) of 1% pimaricin suspension twice a day for 3 days. The amount of exudate decreased gradually and 12 of the 16 ear swabs examined, thereafter, were found to be negative for M. pachydermatis within 10 days. No side effects were observed in all the treated cases. These results suggested that M. pachydermatis could induce the canine otitis externa, and that pimaricin is effective agent for M. pachydermatis infection in ear canals.     

Safety and tolerability of 0.1% tacrolimus solution applied to the external ear canals of atopic beagle dogs without otitis

Tacrolimus is a nonsteroidal alternative to treat noninfectious otitis externa (OE) in people. This 21‐day study investigated whether twice daily application (0.2 mL/dose) of sterile olive oil based 0.1% tacrolimus suspension in ears of atopic beagle dogs without OE was associated with adverse local reactions, development of OE, change in otic cytology, vestibular dysfunction, or hearing loss detected by brainstem auditory evoked response (BAER). The study was randomized, double‐blinded, and placebo‐controlled. Twenty‐two dogs matched for age and sex were randomized to tacrolimus or vehicle control treatment groups. Two investigators independently evaluated dogs for signs of adverse effects including OE the first 4 days of treatment, then every 3 days. A logistic regression model was fit for each investigator’s clinical scores (SAS, 9.2, 2008). Time (P = 0.0032) and group (P = 0.0167) were always significant for OE. Inter‐observer reliability of clinical scores was strong, measured using Kappa coefficients and proportion of agreement. All nine exclusions (7/10 control‐ and 2/12 tacrolimus‐treated dogs) were excluded for yeast OE. Inter‐observer agreement to exclude was 100%. All dogs not excluded had normal BAER assessments before treatment, weekly during treatment, and after 21 days of treatment. None showed vestibular abnormalities at these times. Tacrolimus blood concentrations (Abbott IMx Tacrolimus II) were below detection limits (3 ng/mL) at baseline and after 21 days of treatment. Results suggest otic application of olive oil based tacrolimus suspension to canine ears with intact tympanic membranes is unlikely to result in hearing loss or vestibular dysfunction but yeast OE is a possible risk.     

Morphologic measurements of the external horizontal ear canal of dogs

Microscopic anatomy of the horizontal part of the external ear canal was evaluated in 24 dogs. Sixteen dogs were from breeds known to have a predisposition to otitis externa. The remaining 8 dogs were from breeds that do not have a predisposition to otitis externa. Dogs were separated into groups according to predisposition to otitis externa: group 1-predisposed dogs without otic inflammation, group 2-predisposed dogs with otic inflammation, and group 3-nonpredisposed dogs without otic inflammation. Qualitative microscopic evaluation of distribution of hair follicles revealed hair within proximal, middle, and distal regions of the horizontal ear canal in all breeds. The degree of keratinization was directly proportional to the presence of otic inflammation and was excessive in group-2 dogs. Quality of sebaceous glands within the horizontal ear canal was similar among dogs with and without otitis externa, whereas the quantity of apocrine tubular glands was significantly increased (P less than 0.05) in dogs with otitis. Quantity of apocrine tubular glands was also greater in group-1 dogs than in group-3 dogs. Thickness of the soft tissue in the external ear canal increased in direct proportion to the progression of disease and was greatest in the proximal region of the affected ear canal. Soft tissue located caudally between nonopposing ends of the annular cartilage, within the proximal region of the horizontal ear canal, contained few glands and hair follicles in dogs without otitis externa. In dogs with otitis externa, this region was infiltrated by distended apocrine tubular glands.     

Airway microbial culture and susceptibility patterns in dogs and cats with respiratory disease of varying severity

Objective – To compare airway microbiological culture and susceptibility results in 2 groups of dogs and cats: 1 with respiratory failure requiring positive pressure ventilation (PPV) and 1 with respiratory disease. Design – Retrospective study. Setting – University teaching hospital. Animals – Fifty‐two dogs and cats requiring PPV that had an airway microbiologic culture submitted from October 1, 2003 to October 31, 2008 were included. One hundred and four airway microbiologic cultures from dogs and cats with respiratory disease not requiring PPV were randomly sampled for comparison. Interventions – None. Measurements and Main Results – Patients with respiratory failure were more likely to have a gram‐negative enteric isolate identified (P<0.001), while patients with respiratory disease were more likely to have a gram‐negative nonenteric isolate (P<0.001) or anaerobic isolate (P<0.001) identified. Aerobic bacterial isolates from patients with respiratory failure were less likely to be susceptible to ampicillin (P=0.006), amoxicillin/clavulonate (P<0.001), chloramphenicol (P=0.004), enrofloxacin (P<0.001), ticarcillin/clavulonate (P=0.004), and the combination of ampicillin with enrofloxacin (P<0.001) than were aerobic bacterial isolates from patients with respiratory disease. Conclusions – Canine and feline patients with respiratory failure severe enough to require PPV exhibit a different pattern of bacterial isolates cultured from their airways when compared with isolates from patients with respiratory disease that has not resulted in ventilator dependence. These isolates are more likely to be resistant to commonly used antimicrobials/antimicrobial combinations than patients in the respiratory disease group. These findings suggest that in canine and feline patients with infectious lower respiratory tract disease, consideration of the severity of the pulmonary insult may allow for better prediction of likely isolates and their antimicrobial susceptibilities. Further prospective studies with a standardized collection technique are warranted.     

A comparative study of two antimicrobial/anti-inflammatory formulations in the treatment of canine otitis externa

The efficacy and tolerability of a marbofloxacin-clotrimazole-dexamethasone otic suspension (MCD) was compared with a standard topical treatment using a phase III clinical trial protocol. In a total of 140 dogs with clinical signs of acute or subacute otitis externa, Staphylococcus, Pseudomonas, Enterobacteriaceae and Malassezia were isolated from samples taken at inclusion to identify the causative pathogen; a further sample was collected in the event of failure or relapse, and from dogs (at day 14) for which Pseudomonas species had been isolated at inclusion. One group received MCD (10 drops per affected ear) once daily and a second received Surolan (containing polymyxin B, miconazole and prednisolone) (5 drops per affected ear), twice daily. Each group received treatment for 7 or 14 days according to the clinical outcome on day 7. Efficacy and tolerability were evaluated on days 7, 14 and, if necessary, 28 for dogs treated for 14 days. The trial demonstrated equivalence of both treatments in terms of efficacy, with a cure rate of 58.3% for MCD and 41.2% for Surolan. Both medications were equally well tolerated by dogs, but MCD was superior in terms of pain relief, decrease in pus quantity and smell, response rate and investigator's assessment on day 14.     

Potentiating effect of EDTA-Tris on the activity of antibiotics against resistant bacteria associated with otitis, dermatitis and cystitis

Possible synergistic effects of the combination of EDTA-tromethamine (EDTA-Tris) and three antimicrobial agents (cephaloridine, kanendomycin and enrofloxacin) against resistant Gram-positive and Gram-negative bacteria are reported. Bacteria were isolated from eight cases of chronic otitis externa, five cases of chronic dermatitis and four cases of recurrent cystitis in dogs which had previously been treated with one of the three antibiotics without success. Animals exposed to EDTA-tromethamine plus the antibiotic recovered completely within 10 days, and were controlled clinically and bacteriologically for 180 days. Local irrigation with EDTA-tromethamine solution was well tolerated and no side effects were recorded.     

Development of resistant bacteria isolated from dogs with otitis externa or urinary tract infections after exposure to enrofloxacin in vitro

Minimum inhibitory concentrations for enrofloxacin were determined for 63 bacterial isolates from dogs with otitis externa or urinary tract infections. Development of resistant mutants was determined after exposing the isolates to enrofloxacin in vitro for up to five serial passages. Results indicated that Pseudomonas aeruginosa and Enterococcus spp isolates exposed to enrofloxacin developed resistance rapidly, whereas Klebsiella, Proteus, and Streptococcus spp were less likely to develop resistance. Despite the presence of enrofloxacin pressure, no resistant bacteria developed in the Escherichia coli and staphylococcal isolates. In many isolates, susceptibility patterns changed from susceptible to intermediate.     

In vitro antimicrobial activity of a commercial ear antiseptic containing chlorhexidine and Tris–EDTA

Minimum bactericidal concentrations (MBCs) of a commercial ear antiseptic containing chlorhexidine 0.15% and Tris–EDTA (Otodine^®) were determined by broth microdilution for 150 isolates representing the most common pathogens associated with canine otitis. The microorganisms were classified into three groups according to their levels of susceptibility. The most susceptible group included Staphylococcus pseudintermedius, Malassezia pachydermatis, Streptococcus canis and Corynebacterium auriscanis, which were generally killed by 1 : 64 dilution of the antiseptic product (MBC = 23/0.8 μg/mL of chlorhexidine/Tris–EDTA). The most resistant organism was Proteus mirabilis, which survived up to 1 : 8 dilution of the product (MBC = 375/12 μg/mL). Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus displayed intermediate MBCs ranging between 188/6 and 47/1.5 μg/mL. Interestingly, S. pseudintermedius was more susceptible than S. aureus, and no significant difference was observed between meticillin‐resistant and meticillin‐susceptible isolates within each species, indicating that antiseptic use is unlikely to co‐select for meticillin resistance. Although the concentrations required for killing (MBCs) varied considerably with microorganism type, the combination of chlorhexidine 0.15% and Tris–EDTA was active against all the pathogens most commonly involved in canine otitis.