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Otitis externa in a tertiary care hospital in Zagazig, Egypt: isolated pathogens and their antibiotic sensitivity patterns

Introduction: Recurrent otitis externa is a worldwide problem. This study aims to identify the different aetiological organisms isolated from otitis externa and their sensitivity to different antibiotics. Methods: A total of 27 patients with clinical presentation of otitis externa for a period of three weeks or more were enrolled for the study. Two swab samples collected from each infected ear were cultured for bacterial and fungi, and growth identified using standard microbiological methods including analytical profile index (API) system. Antibiotic susceptibility of isolated bacteria was performed by the disk diffusion technique. Results: Thirty one organisms were isolated from the 27 patients; 12 (38.7%) fungi and 19 (61.3%) bacteria species. Aspergillus spp was the most frequently isolated organism (35.4%) while Pseudomonas aeruginosa was the most frequently isolated bacteria (19.3%), and was most sensitive to amikacin. Four of 11 patients with Aspergillus infection showed clinical resistance to econazole local treatment but had complete clinical response to itraconazole oral treatment. Conclusion: Otitis externa in Egypt is caused by antibiotic resistant bacteria or fungi, and the most causative organisms are Aspergillus spp and Ps. aeruginosa.


Introduction:
Otitis externa, also known as external otitis or swimmer's ear, is an inflammation of the pinna and external ear canal (1). Swimming in polluted water is a common means by which swimmer's ear is contracted but it is also possible to contract the disease from water trapped in the ear canal after a shower, especially in a humid climate. In addition to the presence of microorganisms, impairment of the integrity of the skin of the ear canal is required for external otitis to develop (2). Acute otitis externa is usually caused by bacteria while fungal involvement is commoner with chronic otitis externa.
Otitis externa is one of the most commonly encountered diseases by otorhinolaryngologists, and is common in certain parts of world with hot humid climate. Due to humid climate of Egypt, there is a high occurrence of otitis externa, especially in the summer. A number of patients are seen without evidence of eczematoid-type chronic otitis externa or other clear-cut causes of their recurrent bouts of otitis externa (3).
Although, the course of the disease is usually self-limited and responds quickly to basic treatment, some cases of otitis externa become resistant or are associated with multiple recurrences. This study aims to identify the microbial pathogens of otitis externa and determine their susceptibility to different antibiotics.

Clinical features of subjects
Each patient had full medical history including duration of illness, symptoms and history of previous treatment, and a general examination. The patient ears were then examined after thoroughly dry-mopping with cotton pads, and signs such as tenderness, erythema, scales, exfoliation and ear discharge were recorded.

Specimen collection and culture isolation
Two swab samples were collected from each infected ear, one swab for bacterial culture and antibiotic susceptibility, and the other for fungal culture. The swabs were rapidly transferred to laboratory within one hour. For bacterial isolation, swab specimens were cultured on Blood, Chocolate and MacConkey agar plates. The Chocolate agar plate was incubated in candle extinction jar while the Blood and MacConkey agar plates were incubated in air. All media were incubated at 37 o C for 48 hours. For fungi isolation, swab specimens were inoculated on two plates of Sabouraud Dextrose agar (SDA), one plate was incubated at 25 o C (room) temperature and the other at 37 o C for 14 days.

Microbial identification from culture
Isolated bacteria were identified according to standard bacteriology techniques of Gram stain, colony morphology and biochemical reactions including the API system (4). Antibiotic susceptibility test was performed by the disc diffusion method and interpreted according to Clinical and Laboratory Standards Institute (CLSI) guidelines (5). Filamentous fungi isolated were identified using modified slide culture technique (6) while Candida albicans was identified by the germ tube test.

Modified slide culture technique:
A 1 by 1 cm block of SDA was cut with sterile scalpel and transferred to a plate of SDA. The isolated fungus was inoculated using sterile wire needle into the four sides of the agar block. Sterile cover slip was put over the block with slight pressure to ensure adherence and the plate cover replaced afterwards. The plates were incubated at 25 o C (room temperature). The growth of the fungus was examined periodically. The closed Petri dish was placed on the microscope stage and the slide culture examined with the low power. When reproduced structures had developed, a forcep was used to remove the cover slip and put on a second slide for examination. The agar block was removed using flamed wire needle and put into a container of antifungal disinfectant. A drop of Lactophenol cotton blue stain was put on the remaining slide and was then covered with a new cover slip (6).

Therapy and follow up of patients:
Patients were treated with appropriate antibacterial and antifungi agents, and followup visit was at the seventh day after the baseline visit. Treatment was continued for patients whose symptoms persist with followup visits at 2 and 3 weeks. At each follow up, clinical information such as pain, itching, otorrhoea, stuffy feeling, burning, irritation and compliance with treatment were obtained from patients and recorded.

Results:
The study was conducted on 27 patients with clinical otitis externa, 18 males and 9 females (M: F ratio of 2:1). The age range of the subjects was 15-64 years with mean of 42.37±14.01. Table 1 summarizes the demographic characteristics and clinical manifestations of the patients. Table 2 shows that 31 organisms were isolated from the patients; 12 (38.7%) fungi and 19 (61.3) bacteria species. Aspergillus spp was the most frequently isolated organism (35.4%). Bacteria isolated were 9 (29%) Gram positive and 10 (32.3%) Gram negatives. Pseudomonas aeruginosa was the most frequently isolated bacteria (19.3%), followed by Staphylococcus epidermidis (16.1%), Klebsiella spp (9.7%) and Staphylococcus aureus (9.6%). As shown in Table 3, there was no significant association between the types of microorganism isolated (bacteria or fungi) and symptoms of otalgia (p=0.8137), itching (p=0.7347) or ear discharge (p=0.2522).
The antibiotic susceptibility of the Gram positive bacteria as depicted in Table 4 shows that they were mostly sensitive to ciprofloxacin, levofloxacin and linezolid, and resistant to clindamycin, tetracycline and ceftriaxone. The Gram negative bacteria were mostly sensitive to imipenem, levofloxacin, ciprofloxacin and colistin, and largely resistant to chloramphenicol, gentamicin, tobramycin and ceftriaxone.
Patients with Aspergillus infection (n=11) were treated with econazole but only 7 had complete clinical response to econazole local treatment while the remaining 4 had complete clinical response to itraconazole oral treatment. The patients with Candida infection had clinical response to terbinafine cream.

Discussion:
Otitis externa is a common disease and its severity varies from mild, self-limited condition to severe malignant otitis externa with involvement of cartilage and bone of the external auditory canal, and a tendency to break through to the brain (7). In the current study, Ps. aeruginosa was the most frequently isolated bacteria pathogen and constituted 19.5% of the isolates, which agrees with previous studies that established this pathogen as the most frequent in acute otitis externa (8,9). The rate for Ps. aeruginosa in this study is however lower than 41.7% reported from a study conducted on ear discharge of malignant otitis externa in Jordan (10) and 52.9% in India (11), but the rate is higher than 12% reported from a study in Iran (12).
Among the fungi, Aspergillus spp are the predominant organisms implicated in otomycosis (7). In the current study, Aspergillus spp was the most frequently isolated organism with 35.4%, which agrees with studies from Turkey (13) and Iran (14), where Aspergillus spp constituted 78.6% of otomycosis agents, followed by Candida species (6.8%) and other saprophytic fungi (4.7%). The high prevalence of fungal otitis externa may be secondary to over use of broad spectrum antibiotics and increased topical use of fluoroquinolones (13), which encourages fungi overgrowth.
Fungal otitis externa is often asymptomatic, with discomfort being the most frequent complaint, but there may be pruritus and a feeling of fullness in the ear. Aspergillus species, particularly Aspergillus niger may grow in the cerumen and desquamated keratinaceous debris in the external auditory canal, sometimes forming a visible greenish or blackish fluffy colony (6). In this study, there was no significant difference between the symptoms of otalgia, ear itching or discharge between bacteria and fungi otitis externa (p>0.05).
In the current study, the Gram negative bacteria were mostly sensitive (90%) to imipenem, laevofloxacin, ciprofloxacin and colistin, but largely resistant to chloramphenicol, gentamicin, tobramycin and ceftriaxone. The Gram positive bacteria on the other hand were totally sensitive (100%) to laevofloxacin, ciprofloxacin and linezolid, and 89% sensitive to imipenem, but were largely resistant to clindamycin, tetracycline and ceftriaxone. The most frequently isolated Gram negative bacteria, Pseudomonas spp was mostly sensitive to amikacin, imipenem, laevofloxacin, ciprofloxacin and colistin, but largely resistant to amoxicillin-clavulanate, gentamicin, tobramycin and chloramphenicol. All S. aureus and S. epidermidis isolates were resistant to oxacillin, while the only diphtheroid was sensitive to the antibiotic. The issue of bacterial resistance to antibiotics is a global challenge (15). In developed countries such as the United States, contemporary ear culture isolates at quaternary care center show higher rates of methicillin resistant S. aureus (MRSA) compared to historical reports in the literature (16). However, the challenge of antibiotic resistance is more in developing countries such as Egypt (17), as a result of low socioeconomic state and behavioral pattern of the populace regarding antibiotic use (18). In Egypt, antibiotics can be purchased from pharmacies without medical authorization or prescription.
In this study, fungi were isolated in 38.7% of cases with half of them having bacteria pathogen concurrently involved, which explains the chronicity of the lesion. These cases were previously treated as pure bacterial infection while the fungal aetiology was hidden by the discharge and erythema usually seen in bacterial infection. On the other hand, fungal infection is usually associated with itching as seen in 11 out of 12 (91.7%) cases in this study.
Treatment of acute otitis externa should include an active antimicrobial agent and not just an acidifying agent such as acetic acid (19). Moderate to severe cases will require ototopical agents in addition to antiseptic solution (20). Ciprofloxacin ear drop was the commonest agent used in the initial treatment of our patients followed by gentamicin with or without corticosteroid, and then econazole antifungal agent. Our study observed that many of the locally used drugs such as gentamicin, chloramphenicol and tetracycline have poor activity against most pathogens. This may be due to the injudicious use of these agents which has resulted in the emergence of more resistant strains, especially among the troublesome pseudomonads.
The resistant otitis externa cases could also be associated with contact dermatitis (7), therefore antiinflammatory corticosteroid combination in the ear drops may be required. In the case series by Smith et al., (21) in 1990, 23.5% of their patients with otitis externa had contact dermatitis, and this was even less than what Holmes and Johnson (22) had reported in 1982. Infestation by Demodex species can also cause persistent itching in resistant cases especially in immuno-compromised patients (23) or with prolonged use of local steroid which may increase the frequency of the parasite in the external ear canal of affected patients (24). In this study, local treatment of candida otitis externa was totally responsive to terbinafine cream but only 7 out of 11 patients with Aspergillus infection had complete clinical response to econazole local treatment. The four patients resistant to econazole had complete clinical response to itraconazole oral treatment. This observation agrees with a study in India (25), which reported that 5 days course of itraconazole treatment was very effective for recurrent otomycosis in diabetic patients.

Conclusion:
Our study reports that otitis externa in Egypt is caused by antibiotic resistant bacterial and fungi, the most common pathogens being Aspergillus spp and Ps. aeruginosa. Oral itraconazole was effective for recurrent fungal otitis externa. We recommend that in cases of otitis externa resistant to antibacterial agents, fungi aetiology should be investigated by culture on Saboraud Dextrose agar.