Otoscopy and aural cytological findings in a population of rescue cats and cases in a referral small animal hospital in England and Wales

Abstract

Objectives

Otitis externa is seen clinically in cats, although studies investigating this condition within the UK are lacking. The objective of this study was to investigate the prevalence of Otodectes cynotis mites and microbial infection in the ear canals of cats in various rescue centres and a referral hospital.

Methods

Otoscopy was performed in 332 cats. Otoscopic findings were noted, including the gross visualisation of Otodectes species. A sample of cerumen was collected for cytological evaluation and a cerumen smear for detection of Otodectes mites if there was a large amount of aural exudate present.

Results

O cynotis infestation was noted in 3/341 cats (0.9%, 95% confidence interval [CI] 0.3–2.6). A total of 129/341 (37.8%; 95% CI 32.7–43.0) cats were found to have Malassezia species within one or both ears. Bacteria were found unilaterally in 9/341 (2.6%; 95% CI 1.4–4.9) cats. Analysis of the cytological findings showed an increased likelihood for Malassezia species to be present as age increased (n = 293; Pearson r = 0.204, P <0.001). There was also an increased likelihood of finding Malassezia species in both ears if found within one ear (n = 327; r = 0.499, P <0.001). There was a positive correlation between the number of Malassezia organisms and the quantity of aural exudate (n = 338; r = 0.778, P <0.001). Cats in which Otodectes species infestation were noted (n = 3) had moderate or large quantities of cerumen.

Conclusions and relevance

This study shows that there was a low prevalence of O cynotis in this cohort of cats. In normal cats it was not unusual to find Malassezia microorganisms upon aural cytology, bacteria were noted far less frequently and in two cats this was associated with underlying anatomical pathology.

Introduction

Otitis externa is seen more frequently in dogs than cats.^1–3 Many studies have investigated the prevalence of otitis externa in cats, although studies in the UK are lacking.

Malassezia species are known to be part of the normal aural microflora in cats.^4–7 Many studies have used ear swabs for bacterial and fungal culture to investigate the aural microflora of cats, with and without otitis externa, but fewer studies have used cytology for investigating the normal feline aural microflora. Malassezia yeasts were cultured from 95.1% and 48.4% of cats in Iran with and without otitis externa, respec-tively. ⁸ In a study performed in Brazil, Malassezia species were isolated (also using fungal culture) in 75% and 28% of cats with and without otitis externa, respectively. ⁹ A study performed in Belgium examined a stray population and reported 74% of cats to have Malassezia species in one or both ears based upon cytological examination alone. ¹⁰ Fifty-five percent of cats were found to have Malassezia species upon aural cytological examination and Otodectes cynotis were found in 29.4% of cats in an Italian study also examining stray cats. ¹¹ In a study performed in France investigating pet cats, 15 healthy cats were examined and no Malassezia yeasts were detected; bacteria were isolated from a single ear. ¹² In a study performed in the USA, 52 privately owned cats were examined using aural cytology; yeasts were detected in 83% and coccoid-shaped bacteria were detected in 71% of cats. ⁶ The median number of microorganisms per high-power dry field was 0.2 and 0.3 for Malassezia species and coccoid-shaped bacteria, respectively. Far higher numbers of Malassezia organisms and bacteria were found in a study performed in Spain, where 16 normal cats were examined; ⩾12 Malassezia organisms and ⩾15 bacteria per high-power dry field were found. ⁵

There is marked variation in the reported prevalence of O cynotis in cats, ranging from 0.9% in Australia ¹³ to 83.7% in the UK. ¹⁴ Many of these studies have examined cats from a feral population, which may not be representative of the population seen in primary veterinary care or referral practice. A study from the UK published in 1955 examined 153 cats at post mortem and the incidence of O cynotis was reported to be 51%. ¹⁵

The aims of this study were to examine the external ear canal otoscopically and evaluate cytological findings in a large population of cats in a non-feral environment from rescue centres, and in cats presenting to a referral Small Animal Hospital and first-opinion practice, from centres in England and Wales.

Materials and methods

Sampling and data collection

Three hundred and forty-one cats were included in this study. Ethical approval was obtained. Cats were recruited from across six rescue centres in the south west of England and south Wales, London and Birmingham (total n= 288, range per centre = 13–82). Cats were also recruited from Langford Small Animal Practice and Small Animal Referral Hospital (n = 53). Owners of the rescue centres and pet cats gave written or verbal telephone consent for cats to be enrolled in the study. The centre, age, sex, reproductive status, reason for examination, whether there were ‘in contact’ animals, use of ectoparasite control and frequency, lifestyle (indoor/outdoor) and concurrent medication were recorded for each cat. If treatment was recommended based upon the aural and cytological findings, this was also noted.

Cytological and microscopic evaluation

A clean, non-sterile cotton bud was inserted and rotated into the vertical ear canal to obtain a sample of cerumen for cytological examination. The same person collected the sample and characterised the colour of the cerumen. The sample was rolled onto a clean microscope slide in two lines to distribute the exudate evenly over the slide. The microscope slide was stained with a modified Wright’s stain (Diff-Quik; Atom Scientific), with five 1 s dips in each of the component three solutions and then the slides were washed and allowed to air dry.

If there was a sufficient quantity of aural exudate present consistent with that described in O cynotis infected cats,^15,16 an extra sample was taken and mounted in paraffin oil on a microscope slide and a cover slip was applied. This was examined under a low power using × 40 or × 100 magnification and the presence of Otodectes or Demodex adult mites, or their immature life cycle stages (eggs, larvae and nymphs), was noted.

Each stained microscope slide was examined by the same operator using the same microscope (Olympus), blinded to the previously noted otoscopy findings. Ten fields were examined using immersion oil. Each slide had the total number of Malassezia organisms recorded (the sum of all 10 fields) and the average number per oil immersion field (OIF) was calculated.

The number of bacteria were classified using a previously reported method (Table 1). ¹⁷

Table 1.

Classification of the quantitative scale used to assess bacteria (based on a previous study) ¹⁷

Classification	Description
0	No bacteria/yeast/inflammatory cells
1+	Occasional bacteria/yeast/inflammatory cells present, but slide must be scanned carefully for detection
2+	Bacteria/yeast/inflammatory cells present in low numbers but detectable rapidly without difficulties
3+	Bacteria/yeast/inflammatory cells present in larger numbers and detectable rapidly without any difficulties
4+	Massive amounts of bacteria/yeast/inflammatory cells present and detectable rapidly without difficulties

Inflammatory cells, saprophytes, squamous cells and melanin granules were noted as being present or absent for the whole of the slide.

If otitis (defined as aural discomfort, erythema or abnormal exudate) was noted upon otoscopy while examining a cat, cytology samples were evaluated on the same day so that medication could be prescribed.

Otoscopy

Each external ear canal was examined using a Heine veterinary hand-held otoscope (HEINE Optotech-nik) with a small otoscope head if cerumen sampling was well tolerated. A small number of cats were examined under sedation or general anaesthetic if they were undergoing a procedure at Langford Small Animal Hospital or Small Animal Practice. Table 2 shows the scale used for otoscopic assessment, which is an adaptation of a previously reported method of aural clinical scoring. ¹⁸ The presence of a space-occupying lesion, such as a polyp or mass, was noted. Assessment also included the gross presence of Otodectes mites (yes/no) and whether it was possible to visualise the tympanic membrane (yes/no). Any other dermatological lesions (ears or whole skin) were noted.

Table 2.

Clinical parameters and scoring system

Grade	Quantity of cerumen	Degree of ulceration	Erythema
0	None	None	None
1	Small	Mild	Mild
2	Moderate	Moderate	Moderate
3	Large	Severe	Severe

Data were entered into a Microsoft Excel spreadsheet and statistical tests were performed using IBM SPSS Statistics v24 (IBM). Overall prevalences are reported as a percentage of cats, together with a 95% confidence interval (CI) of the estimate calculated using Wilson’s method.

Results

Population

Three hundred and forty-one cats were included in this study, aged from 3 weeks to 18 years. Two hundred and ninety-one cats were reported to have had contact with other cats or dogs. Two hundred and seventy-five cats had an indoor/outdoor lifestyle, 45 cats were indoor only, one cat was outdoor only and for 20 cats their lifestyle was unknown.

One hundred and forty (41.1%) cats were male and 198 (58.1%) were female, with data missing for three cats. One hundred and fifteen (33.7%) were entire, 224 (65.7%) cats were neutered and data were missing for two cats. Twenty-seven (7.9%) cats were receiving systemic therapy or topical ear medication at the time of sampling. Fifteen different breeds were sampled (see Table S1 in the supplementary material); however, the majority (94.7%) were classified as domestic longhair, mediumhair or shorthair, with other breed classifications poorly represented.

Eight of 341 (2.3%) cats were noted to have focal-to-generalised signs of dermatological disease, including moist and crusting dermatitis, abscessation, pinnal comedones, hypotrichosis of the ventrum, miliary dermatitis, chin acne, pododermatitis, paronychia, over grooming (see Figure 1) and exfoliative dermatitis (see Figure S3 in the supplementary material).

Figure 1.

Concurrent dermatological disease found in some cats. (a) Erythema of the muzzle and chin along with mild feline acne; (b) moderate feline acne over the intermandibular region; (c) ceruminous cystomatosis

Otoscopic examination

Otoscopy was generally well tolerated, although it was not possible in 9/341 (2.6%) cats in either one or both ears. The tympanic membrane was visualised partially or completely in 306/332 (91.2%) cats in one or both ears. Three cats (0.9%, 95% CI 0.3–2.5) were found to have O cynotis adult mites visible upon otoscopy within one or both ears (confirmed using microscopy).

Cerumen smear and cytological examination findings

An extra sample of aural exudate for low-power microscopy (× 40 and × 100) was taken in 13 cats (3.8%), 11 of these cats had excessive aural exudate bilaterally and two had unilateral presentation; therefore, 24 exudate samples mounted in paraffin oil were examined for microscopic evidence of mites. Cytological findings are shown in Table 3. Demodex gatoi was noted unilaterally in one cat. Otodectes cynotis was noted in 3/341 (0.9%; 95% CI 0.3–2.5) cats using microscopy (see Figure S1 in the supplementary material). Two of the three cats had bilateral O cynotis infestation. One cat with bilateral infestation microscopically only had gross otoscopic evidence in one ear.

Table 3.

Cytological findings

	Malassezia species	Coccoid-shaped bacteria	Rod-shaped bacteria	Coccoid- and rod-shaped bacteria	Otodectes cynotis	Demodex gatoi	Melanin granules	Saprophytes
Number of cats with cytological findings (out of 341 cats)	62 (bilateral) 67 (unilateral)	7 (unilateral)	1 (unilateral)	1 (unilateral)	2 (bilateral) 1 (unilateral)	1 (unilateral)	212 (bilateral) 85 (unilateral)	311 (bilateral) 26 (unilateral)

Table S2 in the supplementary material shows the otoscopic and cytological findings of four cats with evidence of Otodectes and/or Demodex species. Neither bacteria nor inflammatory cells were noted. Some of the cytological findings that were noted are shown in Figure S2 in the supplementary material.

There was an increased likelihood of Malassezia species being present with increasing age (n = 293; Pearson r = 0.204, P <0.001) and an increased likelihood of finding Malassezia species in both ears if found within one ear (n = 327; r = 0.499, P <0.001). There was a significant correlation between the number of Malassezia organisms and the quantity of aural exudate (n = 338; r = 0.778, P <0.001).

Thirty-nine cats were found to have otitis externa based on either having presented for otitis, or incidental findings upon otoscopy (aural discomfort, erythema, abnormal exudate, presence of a mass or O cynotis) or O cynotis visible microscopically. Four cats presented to the dermatology service at Langford Small Animal Hospital with otitis as a presenting complaint; in 35 cats it was an incidental finding. A two-sided exact Mann–Whitney test showed there to be a significant difference in the number of Malassezia organisms per OIF between the two groups; the mean number for the otitis group was 0.687 (95% CI 0.153–1.380) vs 0.169 (95% CI 0.114–0.228) in the group of cats without clinical signs of otitis.

Those cats with otitis are shown in Table 4 with the underlying aetiology of the otitis (if known).

Table 4.

Cats with otitis and the underlying aetiology

	Demodex gatoi	Otodectes cynotis	Aural mass/polyp	Allergic skin disease	Ceruminous cystomatosis	Generalised skin disease	Unknown
Number of cats	1	3	3	4	2	2	24

Bacteria were found unilaterally in 9/341 (2.6%) cats. Six of these cats were in the non-otitis group and three were from the otitis group. Seven of these cats had coccoid-shaped bacteria only, one cat had both rod- and coccoid-shaped bacteria and one cat had rod-shaped bacteria only. Those cats with higher numbers of bacteria (3 or 4+) were within the otitis group. Two of these cats (one with rod-shaped bacteria) were found to have a space-occupying lesion, documented using CT, within the ear where bacterial infection was found. Table S3 in the supplementary material shows the otoscopic and cytological findings of cats where bacteria were found upon cytology. Mites were not detected in any of these cats.

Some form of ectoparasite control had been used in 278/341 (81.5%) cats at the time of enrolment into the study. Thirty-eight/341 (11.1%) cats received regular ectoparasite control at the manufacturer’s recommended frequency of application.

Discussion

The primary aims of this study were to investigate both the prevalence of O cynotis in a large cohort of cats and to examine the ear cytology of clinically normal cats from both a rescue centre and veterinary practice setting within the UK. Those cats presenting for otitis or with disease noted incidentally were removed when analysing the data for normal ear cytology values. To the best of our knowledge, there have not been any recent studies investigating the prevalence of O cynotis in a large cohort of cats in the UK, and there have been only three studies that have evaluated the normal external ear cytology in cats.^5,6,12

This study found that the prevalence of O cynotis was low, recorded as 0.9%. This result is in agreement with a Belgian study (2%), ¹⁰ an Australian study (<0.1%) ¹³ and a Portuguese study (2.2%). ¹⁹ Far higher numbers were reported in a Greek study (25.5%), ²⁰ an Italian study (29.4%) ¹¹ and in a study from the USA (37%). ²¹ Climate differences between countries could also account for these differences. A far older study from 1955 in the UK showed that the prevalence was 55%, ¹⁵ although this was during a time period where preventative acaricidal products were not available and therefore may have influenced the findings in the population studied.

The prevalence may have been underestimated in this study as low-power microscopy was only performed in samples from those cats with a large amount of black or brown aural exudate on otoscopy. In a previous study, ²¹ otoscopic examination was normal in eight cats that were positive microscopically (in total, 74/200 cats were found to have Otodectes species microscopically), which suggests that all ears should have a cerumen sample taken for paraffin oil microscopy, even if otoscopy does not reveal a large amount of the classical brown/black exudate seen in otodectic ascariasis. ¹⁵ In one study, ²⁰ the ear canal was flushed with 1–2 ml of mineral oil along with vigorous massaging to determine the presence of O cynotis as there was a concern that the cotton swab technique was less efficient than flushing. Anecdotally, the risk of ototoxicity and discomfort to cats with this method was deemed unacceptable for use in our study. An alternative method of detecting O cynotis infection is the use of PCR, ²² which could be evaluated in future studies. However, this may be cost prohibitive in clinical practice and therefore trial treatment may be elected in the first instance. The life cycle stage of the O cynotis mite seen upon microscopy was not noted in this study.

Another reason for a low prevalence in this study compared with investigations on stray populations could also be attributable to owned and rescue cats receiving ectoparasite control (many of which have acaricidal activity), albeit not necessarily at the manufacturer’s recommended application frequency. Most rescue centres tend to apply ectoparasite control routinely when cats are admitted to help prevent flea infestation. Many owners also use ectoparasite control for their pets; therefore, it would have been challenging to enrol a large number of cats into this study that had not received any form of ectoparasite control. Future studies are required investigating UK stray cats in order to remove ectoparasite control as a potential cause for the low prevalence of O cynotis reported in this study. However, this information may be less valuable to veterinary surgeons practising in the UK who generally treat pet cats receiving regular prophylactic ectoparasite treatments.

Two of the three cats were found to have live O cynotis mites, despite having received one application of ectoparasite control (Stronghold: Selamectin and Broadline: eprinomectin, fipronil, S-methoprene and praziquantel). One of these cats was a 7-week-old kitten that had received Stronghold within 4 weeks of enrolment in the study; therefore, clinicians should not discount O cynotis based on previous acaricidal treatment alone. Unfortunately, the exact date of Broadline application for the other cat was not recorded; therefore, the acaricidal application may been several weeks to months prior to sampling. One single application of eprinomectin, fipronil, S-methoprene and praziquantel has been shown to be effective in treating otoacariasis where one treatment corresponded to 96% preventive efficacy at day 28 based on ear mite counts. ²³ A single application of selamectin was found to be 100% effective in resolving infestation 30 days after the treatment application in another study. ²⁴

Previous studies have found very different values for aural Malassezia organism counts in normal cats.^5,6,12 Two studies used the × 40 objective for examining each high-power field.^5,6 In our study, similar to a previous study, ¹² we used the × 100 oil immersion objective. Cytological methods have several limitations when compared with fungal culture. It is a method that is readily available to clinicians and gives semi-quantitative, immediate results. Limitations include inaccuracies in both cellular and microbial counts, operator dependency and reproducibility. Sometimes stain artefact was seen on slides, which could easily be misinterpreted as infection if microorganisms were incorrectly noted (see Figure S2 in the supplementary material). Some Malassezia organisms did not take up the stain so well, therefore appearing as very faint structures that could easily be missed (see Figure S2 in the supplementary material). Seven species of Malassezia have been identified in the cat; of these, most are lipid dependent. Therefore, if fungal culture alone is used to detect Malassezia species in feline cerumen, lipid-dependent Malassezia species may go undetected as many laboratories only use mycological culture media without lipids. ⁹ In this instance, cytology may be more sensitive in detecting yeast infection.

Despite these limitations, those cats with otitis had five times as many Malassezia organisms per OIF than those with normal ears. The mean number for the otitis group was 0.687 (95% CI 0.153–1.380), which equates to approximately one Malassezia organism per two OIFs. The mean number of Malassezia organisms per OIF was 0.169 (95% CI 0.114–0.228) in the group of cats without otitis, which equates to one Malassezia organism per six OIFs. It is important to note that some cats without clinical signs or otoscopic evidence of otitis externa had in excess of 10 Malassezia organisms per OIF. Therefore, if Malassezia species are noted, this should be interpreted along with otoscopy findings and clinical signs of otitis. The presence of aural Malassezia species in healthy cats in this study corroborated previous studies.^4–7

One cat with O cynotis and another cat with D gatoi isolated, were found to have >10 and 7.8 Malassezia organisms per OIF, respectively, which is not surprising given that it may be an opportunistic microorganism, as well as being part of the normal microflora. Interestingly, the ear with D gatoi infestation had previously undergone a pinnectomy of the same ear. However, one ear with O cynotis detected did not have any Malassezia species found upon cytology.

One cat from the otitis group referred to the Langford Small Animal Hospital with various comorbidities along with generalised exfoliative disease (Malassezia species exfoliative dermatitis), was found to have very high numbers of aural Malassezia organisms bilaterally (>10 per OIF; see Figure S3 in the supplementary material). Unfortunately, this cat presented to the cardiology service at the Small Animal Hospital for congestive heart failure and further investigation, including dermatohistopathology, was not taken; therefore, the underlying aetiology for the severe exfoliative dermatological disease was unknown. Other than echocardiography, further thoracic imaging was not performed; therefore, a thymoma could not be excluded. Previous studies have documented increased Malassezia organisms in cats with concurrent illness.^12,25

Two cats with large numbers (4+) of bacteria on cytology were associated with underlying aural pathology (bilateral otitis media and polyps in one cat and a unilateral aural mass in the other cat) documented using CT. One other cat with large numbers (4+) of bacteria unilaterally was found to have primary otitis externa and the underlying cause was not found. Only 6/341 cats were found to have low numbers of bacteria (1+ or 2+), which is very different from previous studies, where higher numbers of cats were found to have bacteria within the external ear canal.^5,6,10,11 These six cats with low bacterial counts were part of the non-otitis group (n = 6/302). As bacteria were only noted cytologically in nine cats and two of these had a space-occupying lesion present, mean bacterial values were not calculated.

A link between acne and O cynotis has been reported. ²⁰ The three cats identified as having Otodectes species in this study did not have acne-like lesions documented.

Only low numbers of saprophytes were found compared with a previous study, ¹⁰ most likely because most of the rescue cats were mainly housed indoors at time of sampling. The cats in this study were sampled throughout the spring and summer. All nine of the cats in which saprophytes were detected upon cytology had an indoor/outdoor lifestyle.

Conclusions

In this study, only a small number of cats were found to have O cynotis. If cats present for otitis, it is important to rule out ectoparasitic disease and to consider other causes of otitis in cats including allergic skin disease (non-flea, non-food-induced feline hypersensitivity dermatitis, cutaneous adverse food reaction), and space-occupying aural lesions such as a polyp, neoplasia and otitis media (especially in cases of bacterial otitis). New mean values of Malassezia organism counts in the external ear canals of cats were documented in this study, which may be a useful benchmark for clinicians routinely performing ear cytology in cats.