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© 2006 The Authors. Journal compilation © 2006 European Society of Veterinary Dermatology. 17; 175–181 175

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Otoscopic, cytological, and microbiological examinationof the equine external ear canal

Sandra J. Sargent, Linda A. Frank,

Benjamin R. Buchanan*, Robert L. Donnell

and Federica Morandi

University of Tennessee College of Veterinary Medicine,

Department of Small Animal Clinical Sciences, C247 VTH,

Knoxville, TN 37996-4544, USA

Correspondence: Sandra J. Sargent, University of Tennessee

College of Veterinary Medicine, Department of Small Animal

Clinical Sciences, C247 VTH, Knoxville, TN 37996-4544, USA.

E-mail: [email protected].

*Present address: Brazos Valley Equine Hospital, 6999 Highway

6 North, Navasota, TX 77868, USA.

Abstract

Otoscopic examination and cytology of the equine ear

would be beneficial in diseases such as head trauma,

headshaking, otitis externa secondary to otitis media,

vestibular disease, aural neoplasia and aural pruritus

secondary to parasites. In practice, otic examinations

of horses are rarely done due to the perceived diffi-

culty in visualizing the equine external ear canal and

tympanic membrane, as well as the need for chemical

restraint. In this study, the proximal external ear canal

was examined in live horses using a handheld oto-

scope and in cadaver heads using video otoscopy.

Visualization of the proximal ear canal of the sedated

horse could be done with a handheld otoscope, but

more sedation or general anaesthesia and a video

otoscope would be required to adequately visualize

the tympanic membrane in the live horse. The proximal

ear canals of 18 horses were examined cytologically

and cultured aerobically. In three horses, both ears

were sampled. No cells or organisms were seen on

cytological examination of 11/21 ears. Nine of the 21

ears were sterile when cultured. Ten of the 21 ears had

mixed growth with low numbers of organisms

(

Corynebacterium

sp. being most common). Two of

the 21 ears had heavy growth of a single organism

(

Corynebacterium

sp. and Staphylococcus intermedius

,

respectively). Equine cadaver heads were examined in

cross-section by computed tomography (CT) imaging

and histopathology in order to further understand the

anatomy of the equine external ear canal. Equine

practitioners should be aware that otic examination

is possible and may provide important diagnostic

information.

Accepted 6 March 2006

Introduction

Sparse information is available about examination of the

equine external ear canal. Descriptions of the normal anatomy

and appearance of the external ear canal and tympanic

membrane of the horse are rare and may be misleading.

1

There have been no published studies on the microbial

flora and cytology of the external canal in normal horses.

Otoscopic examination and cytology would be beneficial

in diseases such as head trauma, headshaking, otitis

externa secondary to otitis media, vestibular disease, aural

neoplasia and aural pruritus secondary to parasites. Otherrare disorders such as muscle spasms associated with ear

tick infestations have been reported and may go undiag-

nosed without an otic examination.

2

However, in practice,

otic examinations of horses are rarely done. This may be

due to the perceived difficulty in visualizing the equine

external ear canal and tympanic membrane as well as the

need for chemical restraint. The tympanic membrane of

the horse is considered to be difficult, if not impossible, to

visualize by most equine practitioners. Previous references

state that general anaesthesia is required for examination

of the tympanic membrane and myringotomy in the horse.

3

Headshaking is an increasingly recognized syndrome

and presents as uncontrollable, spontaneous, rapid andrepetitive head movements without apparent cause.

Although the majority of cases of headshaking in the horse

are idiopathic, several different underlying diseases

including otitis externa, media or interna, otic foreign

bodies or ear mites can be present.

4

The importance of

eliminating any definable cause by thorough examination

is emphasized due to the poor prognosis for treatment of

idiopathic headshaking.

5

A thorough otic examination is

needed to rule out aural disease as an underlying cause of

this debilitating condition. However, there is little discus-

sion in the literature of techniques that enable visualization

of the proximal ear canal and tympanic membrane. In a

review of 20 cases of headshaking, a standard routine ofdiagnostic tests was described to allow for identification

of any underlying cause prior to making the diagnosis of

idiopathic headshaking.

5

This included aural endoscopy

using a 4-mm fibreoptic paediatric endoscope and radiog-

raphy of the head. However, further details regarding this

instrumentation and the technique utilized for aural endos-

copy were not mentioned. As this equipment is not readily

available to most equine practitioners, the use of more

commonly available instruments that are often used to

visualize the external canal and tympanic membrane of

other species needs to be explored.

Diseases of the external ear canal of the horse are con-

sidered to be uncommon and few reports of otitis externaas a primary disease can be found in the literature. Otitis

externa secondary to otitis media is more commonly



176

© 2006 The Authors. Journal compilation © 2006 European Society of Veterinary Dermatology.

SJ Sargent et al.

reported.

6

Otitis media is an insidious disease in the

equine. The early clinical signs of otitis media include ear

rubbing, head tossing or head weaving. These signs are

frequently attributed to parasites in the ear canal or behav-

ioural problems and may go unnoticed.

7

Rupture of the

tympanic membrane and presence of external ear drain-

age is reported to be uncommon with the majority of

cases of otitis media.

8,9

However, in a report describing

endoscopy of the auditory tube diverticula (guttural pouch)

in four horses with otitis media/interna, three of the four

horses had otic examinations and in all of these horses the

tympanic membrane was ruptured.

3

The authors did not

examine the ear canal in one of the horses due to the risk

associated with general anaesthesia and it was not

mentioned whether otic examination was attempted with-

out general anaesthesia. Knowledge of the anatomy and

appearance of the normal tympanic membrane could

result in earlier diagnosis of otitis media, thus avoiding the

serious neurological complications that occur months to

years after onset. Development of a transtympanic

membrane tap was mentioned by one author as an aid toearlier diagnosis but would be difficult without excellent

visualization of the proximal canal and tympanic membrane.

7

The purpose of this study was to determine whether the

proximal external ear canal and the tympanic membrane

of the horse can be visualized using available equipment

such as a handheld or a video otoscope, and to document

the microbial flora of the normal equine external ear canal

through cytology and culture of samples collected from

live horses. Cross-sectional imaging using computed

tomography (CT) and histopathology of equine cadaver

heads were also performed to allow further understanding

of the anatomy of the equine external ear canal.

Materials and methods

Animals

Four adult equine cadaver heads (unknown breeds and ages) were

utilized to determine the best method of visualizing the proximal

portion of the external ear canal and to determine whether the tympanic

membrane could be consistently seen. Pictures of the proximal and

distal canal were obtained using video otoscopy.

Eighteen clinically healthy horses (unknown ages, different breeds

and sexes) with no history of ear disease were randomly selected

from the teaching herd at the University of Tennessee. The horses

were restrained in individual stocks and sedated with intravenous

xylazine, and a nose twitch was applied to supplement the restraint.

To minimize stress, only one ear was examined and sampled in allbut three horses where both ears were examined and sampled. This

protocol was approved by the Institutional Animal Care and Use

Committee.

Visualization of the ear canal and sample collection

A handheld otoscope (3.5v Halogen Pneumatic Otoscope; Welch

Allyn Medical Products, 4341 State Street Road, Skaneateles Falls,

NY, USA) with a 7-mm cone and a video otoscope (Vetcam XL Otoen-

doscope System; Karl Storz Veterinary Endoscopy, 175 Cremona

Drive, Goleta, CA, USA) with a tip diameter of 5 mm and length of

8.5 cm were used to visualize the canal in the four cadaver heads. In

the 18 live horses, the handheld otoscope containing a gas-sterilized

plastic 7-mm otoscope cone was introduced into the external ear

canal of each horse. Each ear examined was determined to be normal

in appearance before proceeding. An attempt to pass the cone into

the proximal portion of the ear canal as atraumatically and aseptically

as possible was made. In those horses where visible cerumen was

present and partially obstructing the tip of the cone after introduction

into the proximal canal, a second attempt was made with a new ster-

ile cone in the effort to obtain a sample free of cerumen from the outer

portion of the canal. Once the proximal portion of the ear canal was

visualized, a sterile microtip swab (BBL CultureSwab with Amies gel;

Becton, Dickinson and Co., Sparks, MD, USA) was passed through

the sterile cone. The swab was immediately placed into transport

media upon removal from the canal. A second swab was then intro-

duced in the same manner to collect a sample for cytological exami-

nation. This swab was immediately rolled onto a new glass slide and

labelled with each horse’s identification number.

Cytology

The glass slides were briefly heat fixed. They were then stained with

a commercial staining solution (3 Step Stain; Richard-Allan Scientific,

Kalamazoo, MI, USA) and allowed to air dry. Each slide was first exam-

ined under ×

10 for the obvious presence of debris, cells or organisms

before proceeding to examine under the ×

100 oil immersion lens. The

entire slide was examined and numbers of organisms were recorded.

Bacterial cultures

The microtip swabs were submitted to the microbiology laboratory of

the University of Tennessee for routine aerobic culture. Each specimen

was inoculated onto one quadrant of Columbia agar with 5% sheepblood, Columbia agar with colistin nalidixic acid and MacConkey agar

and streaked for isolation. The Columbia agars were incubated at

35 °

C in 7% CO

2

. The MacConkey agar and thioglycolate broth were

incubated aerobically at 35 °

C. The plates and broth were examined

daily for a total of 5 days. All colonies were enumerated, gram stained

and identified to genus level.

Computed tomography

The head of a 15-month-old Arabian horse that had no clinical signs of

otitis media or externa and was euthanized for reasons other than

neurological disease was disarticulated at the atlanto-occipital joint

and prepared for CT. Using a fourth-generation CT scanner (Marconi

PQ 6000, Universal Medical Systems, Inc., Solon, OH, USA), axial

overlapping images at 3 mm collimation and 2 mm intervals were

acquired through the tympanic bullae with a sharp (edge-enhancing)

algorithm. Images were viewed in both bone (window width = 2500

HU, window level = 500 HU) and soft tissue windows (window

width = 350 HU, window level = 50 HU).

Histopathology

The external ear canal of one cadaver head was cross-sectioned at the

area of the proximal canal and tympanic membrane for gross visuali-

zation. The head was then sectioned for histological evaluation of the

proximal canal and tympanic membrane. Segments of the temporal

region of the skull were fixed in 10% buffered neutral formalin for

5 days and then transferred into fixative decalcifier (Formical-4, Decal

Chemical Corp. Talman, NY, USA) until selected areas were suffi-

ciently softened to be trimmed with a knife. The segments were

routinely processed for paraffin embedding, sections cut at 6 µ

m andstained with haematoxylin and eosin (H&E) for microscopic examination.

Results

Visualization of the ear canal and sample collection

The ears from four cadaver heads were examined with

handheld and video otoscopes. In those ears with small

amounts of cerumen in the proximal canal, the tympanic

membrane could be visualized using the video otoscope.

It was frequently necessary to flush the ear with normal

saline through the video otoscope port to achieve optimal

images. Some ears had accumulations of cerumen, as

well as wood shavings from bedding, in the osseousexternal acoustic meatus adjacent to the tympanic

membrane. The lighting and magnification provided by




177

Examination of equine ear

the video otoscope were necessary as neither the hand-

held nor video otoscope could reach deep enough into thecanal to allow adequate visualization without it. The canal

narrowed significantly at the junction of the cartilaginous

and osseous portions of the external acoustic meatus and

prevented both instruments from reaching the tympanic

membrane (Fig. 1). This junction was grossly identifiable

by the abrupt change from pigmented to nonpigmented

epithelium (Fig. 2). It was possible to visualize adequately

the semitransparent tympanic membrane and the stria

mallearis where the manubrium of the malleus attaches to

the medial surface (Fig. 3).

A total of 21 ears from 18 horses were evaluated

otoscopically and samples for cytology and culture were

collected. No horses suffered damage or subsequentlydeveloped clinical signs of ear disease following collection

of samples from the external ear canals. The procedure

was well tolerated but sedation supplemented with twitchrestraint was necessary to allow visualization and sample

collection from the proximal portion of the canal. The desired

landmark was the obvious junction between pigmented

(cartilagenous external acoustic meatus) and nonpigmented

canal (osseous portion of external acoustic meatus) that

occurs distal to the tympanic membrane. An attempt to

consistently sample the nonpigmented area adjacent

to the tympanic membrane was made. In all 18 horses, the

tympanic membrane could not be adequately visualized

through the handheld otoscope due to the length of the

canal and presence of copious cerumen as well as the

minimal magnification provided by the instrument.

Cytology

Eleven of the 21 samples (52%) were negative for cells

and organisms on cytology. Ten of the 21 samples (48%)

contained few to moderate numbers of epithelial cells,

many containing melanin pigment granules. One sample

(4.7%) contained low numbers (0–10 per HPF) of diploco-

cci and rods. Two samples (9.5%) contained Malassezia

sp. on cytology, one had low numbers (0–2 per HPF) while

the other had too numerous to count. One cytology sample

contained many red blood cells likely due to iatrogenic

trauma during collection of the samples.

Bacterial cultures

Bacteria were isolated from 12 ears of 11 horses. Cultures

from nine of the 21 ears (43%) were sterile.Corynebacterium

sp. was the most frequent bacteria isolated. In only one case

was it in pure culture with more than 50 colonies isolated.

Culture from one ear yielded 100 colonies of Staphylococcus

intermedius

. The rest exhibited low numbers (< 10–20

colonies each) of Corynebacterium

sp., Staphylococcus

sp., Streptomyces

, Bacillus and unspeciated gram nega-

tive and positive rods and gram-positive cocci. Culture of

the ear with cytological evidence of rods and cocci yielded

low numbers of Corynebacterium

sp., Staphylococcus

sp.

and Streptococcus

sp. Cultures of the two ears with

cytological evidence of yeast were negative for growth. Inthe three horses where both ears were cultured, 2/3 had

negative cultures in both ears and 1/3 grew small numbers

Figure 1. Photo: Adult horse, cross-section at the level of the left

proximal ear canal and tympanic membrane. The junction of the

cartilaginous (pigmented) and osseous (nonpigmented) portions of

the external ear where the canal narrows significantly is apparent. SC:

spiral canal, cer: cerebellum, CN V: cranial nerve V, O: occipital bone,

GP: guttural pouch, BS: basisphenoid bone, B: tympanic bullae, P:

petrous temporal bone, TM: tympanic membrane, E: external ear canal.

Figure 2. Photo: Adult horse, right ear canal. Junction of the

cartilagenous (pigmented) and osseous (nonpigmented) portion of

the equine external ear canal as visualized with a video otoscope.

Figure 3. Photo: Adult horse, right ear canal. A video otoscopic image

of the tympanic membrane of the horse.



178


SJ Sargent et al.

(<20 colonies) of Corynebacterium

sp. in both ears. Sapro-

phytic fungi were isolated from four ears. These were

thought to be contaminants.

Computed Tomography

The CT images obtained from the single cadaver head

provided exquisite anatomical detail of the structures of

the external and middle ear. The tympanic bullae, petrous

temporal bones, external (osseous) acoustic meatus, sty-

lohyoid bones, guttural pouches, as well as the external

(cartilaginous) ear canal were all clearly identified (see

Figs 4 and 5). The tympanic bullae were proportionally

small compared to other species such as carnivores andruminants, measuring only approximately 15 mm in

maximum diameter. The osseous acoustic meatus had an

hourglass shape, measuring 9.4 (right)

−

9.8 (left) mm in

diameter at the lateral aspect, narrowing to 4.6 (right)

−

4.7

(left) mm in diameter in the medial aspect and widening

slightly to 6.7 (right)

−

6.3 (left) mm at the level of the tym-

panic membranes

.

Measurements of the length of the

osseous portion of the ear canal vary by a few tenth of

milimetre depending on the slice imaged. The maximum

length of the osseous portion of the right ear canal was

26.5 mm and the left was 25.2 mm. Measuring the entire

length of the ear canal would be difficult due to its angulation

and because it cannot be imaged in a single slice, butrather in several contiguous slices due to its orientation.

The osseous acoustic meatus was not horizontal, but

rather angled in a ventromedial to dorsolateral direction,

and the cartilaginous ear canal continued laterally following

very similar angulation. The structures of the inner ear

were not clearly visible; however, the malleus and cochleawere identified on both sides.

Histopathology

Histopathology of the proximal canal of a single cadaver

head showed an abrupt change at the level of transition

from underlying cartilagenous to osseous ear canal sup-

port. Overlying the cartilagenous portion of the canal was

a moderately thick, pigmented epithelial layer with an

irregular profile, and prominent rete ridge/peg interdigita-

tion, hair follicles and sebaceous and tubular ceruminous

glands. The epidermis overlying the portion supported by

bone had an absence of glands, a lack of melanocytes and

the epithelium had a marked reduction in thickness andinterdigitation (Fig. 6).

Discussion

In the present study, the authors were able to visualize the

proximal external ear canal in 18 horses without evidence

of otitis using handheld otoscopy, although sedation and

manual restraint were necessary. However, the handheld

otoscope did not provide good visualization of the tym-

panic membrane due to inadequate magnification and

lighting. Video otoscopy would have allowed the tympanic

membrane to be examined in detail, but would have

required more sedation or general anaesthesia to be safelyperformed. Although many small animal practitioners

currently have access to video otoscopy in their practices,

Figure 4. Photo: Axial CT image of the head of a 15-month-old male

Arabian foal obtained at the level of the tympanic bullae, obtained

at 3-mm collimation and 2-mm increment and displayed in a bone

window (width = 2500 HU; level = 500 HU). E: external (osseous)

acoustic meatus, P: petrous temporal bone, B: tympanic bulla,

C: cochlea, cer: cerebellum, O: basilar part of the occipital bone,

J: jugular foramen, S: stylohyoid bone, L: lateral compartment of the

guttural pouch, M: medial compartment of the guttural pouch. The

soft tissue density in the left external acoustic meatus (*) represents

fluid accumulation, likely a postmortem change, and terminated at the

level of the tympanic membrane. The tympanic membrane itself is not

visible. The gas surrounding the cerebellum is a post-mortem change.

Figure 5. Photo: Axial CT image of the head of a 15-month-old male

Arabian foal obtained just rostral to the level of Fig. 1. Collimation:

3 mm, increment: 2 mm, displayed in soft tissue window (width =

350 HU; level = 50 HU). Notice the hypoattenuating triangular shaped

cartilage (open arrowheads) at the articulation between the stylohyoid

bone and the lateral margin of the bulla. Notice the small amount

of hypoattenuating material in the dependent portion of the right

tympanic bulla, representing fluid, likely a post-mortem change.




179


it is unlikely that equine practitioners would have this

specialized equipment. Many equine practitioners do have

access to flexible endoscopes; however, the typical endo-

scopes used in equine practice are 7.9–9.8 mm in diame-

ter.

1

These endoscopes are too large to enter the osseous

portion of the equine external ear canal and therefore

would provide limited visualization of the proximal canal

and tympanic membrane. Many equine surgical special-

ists use 4–5 mm rigid endoscopes for arthroscopy and

these endoscopes could be useful for visualization of the

proximal canal and tympanic membrane. Specialty flexibleendoscopes with a shaft diameter of only 2.9 mm and mini

rigid endoscopes of 2–2.7 mm are available. These endo-

scopes would be small enough to visualize the equine

tympanic membrane and may be useful for performing a

myringotomy. Unfortunately, endoscopes of this size are

uncommonly available even to veterinary specialists.

In one reference, a picture obtained via endoscopy of

the previously mentioned junction between pigmented

and nonpigmented epithelial lined canal found at the start

of the osseous part of the external acoustic meatus was

identified as the tympanic membrane.

1

This may lead to

confusion as to the normal appearance of the equine

tympanic membrane. The present study confirmed that theobvious junction between pigmented and nonpigmented

epithelial lined canal found at the start of the osseous

part of the external acoustic meatus was found in all

horses examined. However, it was necessary to bypass

this region in order to visualize the tympanic membrane.

All instruments used in this study were too large to reach

the level of the tympanic membrane. CT confirmed that

the osseous portion of the canal might narrow to 4.6 mm.

Since the video otoscope utilized in this study has a tip

diameter of 5 mm, this explains the inability to pass the

instrument to the level of the tympanic membrane. It

is possible that horses of different ages, breeds or with

larger head sizes may have larger canals as CT was onlydone on a single 15-month-old Arabian colt. However, four

adult cadaver heads of different breeds and ages were

examined with the video otoscope. The otoscope seemed

to reach the same portion of the canal before meeting

resistance in all four heads leading the authors to theorize

that there is little difference in the canal size of horses of

different breeds and ages. Although the authors feel that

the manubrium of the malleus where it attaches to the

medial surface of the tympanic membrane could be seen

through the tympanic membrane using the video oto-

scope, it is difficult to determine the accuracy of this belief

as no pictures of the equine tympanic membrane could be

referenced.

There have been no published studies on the microbial

flora and cytology of the external ear canal in normal

horses. All but three cytology samples taken from the

proximal canal in this study were negative for organisms.

The diplococci, rods and Malassezia

sp. found in these

samples were likely contaminants from the distal canal or

the environment. This is supported cytologically by the

presence of epithelial cells with melanin granules, most

likely obtained from the pigmented cartilagenous portion

of the external acoustic meatus. There have been a fewreports of Malassezia

sp. being isolated from the skin

of normal horses.

10,11

It is possible that Malassezia

sp.

may be part of the normal microbial flora in the cerumen

commonly found in the distal canal. In hindsight, sampling

of the distal canal for cytology may have been useful in

determining whether this is the case, and as a comparison

to the samples taken from the proximal canal. However,

the purpose of this study was to assess the microbial flora

of the proximal canal.

In this study, 43% of the cultures taken from the proximal

canal were negative for organisms. Of the positive cultures,

83% had low numbers of organisms that are considered

to be normal residents and transients of equine skin andhair or environmental contaminants,

12

indicating that the

proximal canal of most normal horses may be relatively

sterile. In only two ears were large numbers of pure isolates

found. These isolates were S. intermedius

and Coryne-

bacterium

sp., both of which have been isolated from the

skin and hair of normal horses.

12

In comparison, multiple

studies have looked at the microbial flora present in the

ear canals of normal dogs. The most commonly isolated

organisms in the dog include Staphylococcus

sp. and

Malassezia

sp. with 47.6% and 37.9% isolated, respec-

tively, in one study.

13

Environmental organisms are likely

to contaminate the hairs of the external pinna, as well as

the distal ear canal, due to the environment in which thehorse lives. Because of the difficulty in sampling the prox-

imal canal in live horses, it is possible that those ears with

positive cultures were due to contamination from the distal

canal or the environment during the sampling process.

While otitis externa was possible in these horses, it is

unlikely due to the lack of clinical and cytological evidence.

Cultures were not obtained from the distal canal for com-

parison due to the difficulty in interpreting results that

would likely contain a large amount of organisms from the

environment.

The origin of the infectious process of otitis media is

presently unknown. Extension from otitis externa and

tympanic membrane rupture as occurs commonly in the dog

13

is considered to be unlikely in the horse. Haematogenous

spread of bacteria to the inner ear is highly suspected.

14

Figure 6. Photomicrograph: Adult horse, decalcified cross-section

at the junction of the cartilaginous and osseous portion of the equine

external ear canal. The epidermis overlying the osseous portion is

nonglandular and lacks melanocytes. There is marked reduction in

thickness and interdigitation of the epithelium compared with the

outer cartilaginous canal. EC: ear canal, G: glands, B: bone,

C: cartilage. H&E. Bar = 2.0 mm.



180


SJ Sargent et al.

Since the guttural pouch is an outpouching of the

mucosal lining of the auditory tube seen only in the

equine,

15

guttural pouch infections have been implicated

as an initiating cause.

14

However, otitis media has rarely

been associated with concurrent guttural pouch infection,

and microorganisms isolated from the guttural pouch in

one study did not correlate with microorganisms obtained

from the middle ear of normal horses.

7

In the same study,

organisms isolated from the middle ear of recently eutha-

nized horses not affected by neurological disease included

Actinobacillus

, Salmonella

, Enterobacter

, Pseudomonas

,

Streptococcus

, Staphylococcus

and Aspergillus

. With the

exception of S. intermedius and

Streptococcus

sp., dif-

ferent bacteria were isolated from the proximal ear canals

of the normal horses in the present study. The middle ears

were not sampled in the present study; therefore, populations

of bacteria cannot be compared.

The external acoustic meatus is described as having

both distal cartilagenous and proximal osseous portions.

15

In carnivores and pigs, the cartilagenous part is relatively

long and curved with its initial part directed downward(vertical canal), followed by a horizontal part that is

directed medially (horizontal canal). The osseous part of

the external acoustic meatus in these species is relatively

short and found at the level of the tympanic membrane.

15

Descriptions of the cartilagenous portion of the canal in

the horse could not be found. In the present study, utilizing

cross sections of a gross specimen and CT, the cartilagen-

ous part was found to have a gradual slope in a dorsola-

teral to ventromedial direction. This slope was continued

in the osseous portion of the canal, which was consider-

ably longer than in carnivores, measuring up to 26.5 mm

on the right and 25.2 mm on the left. Unlike carnivores, a

distinct vertical and horizontal canal was not found. Thesedifferences need to be considered when doing an otoscopic

exam in a horse.

Changes secondary to otitis media can be identified

radiographically but only after moderate bony proliferation

has occurred.

7

The exquisite anatomical detail of this

region seen on CT contributes substantially to understand-

ing how the horse’s unique anatomy results in adverse

sequelae associated with otitis media.

14,16

Previous reports

describing the head anatomy in the horse were carried out

using thicker slices, therefore sacrificing detail of the

middle ear,

17,18

and did not focus on description of the

auricular structures. It is possible that CT may be a more

sensitive diagnostic tool in the early diagnosis of otitismedia and in the diagnosis of early changes in the tem-

porohyoid joint before fractures and neurological disease

has occurred. Evaluation of the external and middle ear in

live horses can be carried out using CT; however, this

technique requires general anaesthesia and a dedicated

large animal table that is available only at few large referral

practices and universities. More studies of affected horses

need to be done to determine the value of this diagnostic tool.

In horses and ruminants, sebaceous and tubular cerumi-

nous glands, which secrete cerumen, are reported to be

found only within the cartilagenous part of the external

acoustic meatus, while in carnivores, they are located

along the entire external acoustic meatus.

15

This was con-firmed in the present study in the single cadaver head

where histopathology was done. Histopathology was not

done on multiple heads due to the difficulty in sectioning

the head at the level of the tympanic membrane and the

time required for decalcification. The difference in number

and location of glands as well as the presence or absence

of melanocytes results in the abrupt transition noted

grossly at the junction of the cartilagenous and osseous

portion of the equine external ear canal. This may explain

the difference in the appearance of the epithelium of the

equine external ear canal as opposed to that of the dog

and cat. It is possible that the lack of glands as well as the

relatively sterile environment in the proximal portion of

the canal may be in part why horses do not appear to suffer

from primary otitis externa. In dogs, the species most

commonly affected by otitis externa, excessive cerumen

production by the glands lining the entire ear canal is a

common predisposing factor.

13

Horses also rarely suffer

from other common primary causes of otitis externa in the

dog such as atopy and food allergy.

12

In conclusion, although clinical signs of ear disease are

uncommon in the horse, otic examination may be an

important diagnostic tool. An understanding of the normalanatomy of the equine external ear canal is necessary.

Equine practitioners should be aware that otic exami-

nation is possible and may provide important diagnostic

information.

References

1. Slovis NM. Miscellaneous endoscopic procedures. In: Atlas of

Equine Endoscopy. St. Louis, MO: Mosby, 2004: 239–40.

2. Madigan JE, Stephanie J, Valberg SJ et al. Muscle spasms associated

with ear tick (

Otobius megnini

) infestations in five horses. Journal

of the American Veterinary Medical Association 1995; 207: 74–6.

3. Hassel DM, Schott HC 2nd, Tucker RL et al. Endoscopy of the

auditory tube diverticula in four horses with otitis media/interna.Journal of the American Veterinary Medical Association 1995;

207: 1081–4.

4. Newton SA. Idiopathic headshaking in horses. Equine Veterinary

Education 2005; 17: 83–91.

5. Newton SA, Knottenbelt DC, Eldridge PR. Headshaking in horses:

possible aetiopathogenesis suggested by the results of diagnos-

tic tests and several treatment regimes used in 20 cases. Equine

Veterinary Journal 2000; 32: 208 –16.

6. Blythe LL, Watrous BJ, Schmitz JA et al. Vestibular syndrome

associated with temporohyoid joint fusion and temporal bone

fracture in three horses. Journal of the American Veterinary Med-

ical Association 1984; 185: 775–81.

7. Blythe LL. Otitis media and interna in the horse. Its relationship

to head tossing and skull fractures. Proceedings of the 7th Annual

Meeting of the American College of Veterinary Internal Medicine1989: 1015–18.

8. Power HT, Watrous BJ, de Lahunta A. Facial and vestibulocochlear

nerve disease in six horses. Journal of the American Veterinary

Medical Association 1983; 183: 1076– 80.

9. Montgomery T. Otitis media in a thoroughbred. Veterinary Medi-

cine, Small Animal Clinician 1981; 76: 722–4.

10. Crespo MJ, Abarca ML, Cabanes FJ. Occurrence of Malassezia

spp.

In horses and domestic ruminants. Mycoses 2002; 45: 333–7.

11. Nell A, James SA, Bond CJ et al. Identification and distribution of

a novel Malassezia

species yeast on normal equine skin. Veterinary

Record 2002; 150: 395–8.

12. Scott DW, Miller WH (eds). Structure and function of the skin. In:

Equine Dermatology. Philadelphia: WB Saunder Co., 2003: 1– 58.

13. Scott DW, Miller WH, Griffin CE (eds). Diseases of eyelids, claws,

anal sacs, and ears. In: Muller and Kirk’s Small Animal Dermatol-

ogy, 6th edn. Philadelphia: WB Saunders Co., 2000: 1185–235.

14. Rush BR. Vestibular disease. In: Reed SM, Bayly WM, Sellon DC,




181


eds. Equine Internal Medicine, 2nd edn. Philadelphia: WB Saunders

Co., 2004: 583– 5.

15. Liebich HG, Konig HE. Vestibulocochlear organ. In: Liebich HG,

Konig HG, eds. Veterinary, Anatomy of Domestic Mammals. New

York: Schattauer Stuttgart, 2004: 569– 83.

16. Mayhew IG. Large Animal Neurology: A Handbook for Veterinary

Clinicians. Philiadelphia: Lea & Febiger, 1989.

17. Morrow KL, Park RD, Spurgeon TL et al. Computed tomographic

imaging of the equine head. Veterinary Radiology and Ultrasound

2000; 41: 491–7.

18. Smallwood JE, Wood BC, Taylor WE et al. Anatomic reference

for computed tomography of the head of the foal. Veterinary

Radiology and Ultrasound 2002; 43: 99–117.

Résumé

La réalisation d’un examen otoscopique et cytologique de l’oreille du cheval devrait être

bénéfique dans certaines maladies comme les traumatismes faciaux, le prurit facial, l’otite externe conséc-

utive à une otite moyenne, le syndrome vestibulaire, les plaques aurales et le prurit auriculaire secondaire

à une parasitose. En pratique, l’examen otoscopique est rarement réalisé, à cause d’une difficulté ressentie

par le praticien pour visualiser le conduit auditif et le tympan, mais également à cause de la nécessité d’une

sédation. Dans cette étude, l’extrémité proximale du conduit auditif externe a été étudiée avec un otoscope

chez des chevaux vivants, et avec un vidéootoscope sur des cadavres. La visualisation du conduit a été pos-

sible avec l’otoscope sur les chevaux tranquillisés, mais l’observation de la mebrane tympanique n’a été

possible qu’après anesthésie ou sédation poussée, avec le vidéootoscope seulement. Les conduits auditifs

de 18 chevaux ont été prélevés pour examen cytologique, et culture aérobie. Pour trois chevaux, les deux

oreilles ont été prélevées. Aucune cellule ni microorganisme n’ont été observés pour 11/21 oreilles. 9/21

oreilles étaient stériles à la culture bactériologique. 10/21 présentaient une pousse de plusieurs espèces

bactériennes, en faible nombre (

Corynebacterium

sp. Notamment). 2/21 présentaient une pousse importanted’une seule espèce (

Corynebacterium

sp. et Staphylococcus intermedius

). Des têtes de cadavre ont été

étudiés par scanner et histopathologie pour mieux comprendre l’anatomie du conduit auditif externe du

cheval. Les praticiens équins doivent savoir que l’examen otoscopique est possible chez le cheval et peut

permettre d’obtenir des données diagnostiques importantes.

Resumen

El examen otoscópico y citológico del oído equino podría ser beneficioso en enfermedades

como el trauma cranial, temblores de la cabeza, otitis externa secundaria a otitis media, enfermedad

vestibular, neoplasia auricular y prurito aural secondario a infestaciones parasitarias. En la práctica, el examen

ótico en caballos raramente se realiza debido a la percepción de la existencia de dificultades para visualizar

el canal auditorio externo equino y la membrana timpánica, así como por la necesidad de tranquilización con

fármacos. En este estudio, el canal auditivo externo proximal se examinó en caballos vivos utilizando un

otoscopio manual y en cadáveres utilizando un video otoscopio. La visualización del canal auditivo proximal

en caballos sedados se pudo realizar con el otoscopio manual, pero mayor sedación o anestesia generaly un video-otoscopio fueron necesarios para obtener imagenes adecuadas de la membrana timpánica

en caballos vivos. El canal auditivo proximal de 18 caballos se examinó mediante citología y con cultivos

aeróbicos. En tres caballos se tomaron muestras de ambos oídos. No se observaron células ni organismos

en 11/21 oídos. 9/21 oídos fueron estériles en cultivo. 10/21 desarrollaron cultivos mixtos con bajo número

de organismos (

Corynebacterium sp. fue el más común). 2/21 desarrollaron elevado crecimiento de un solo

organismo (

Corynebactirum sp

. y Staphylococcus sp., respectivamente). Las cabezas de los cadáveres se

examinaron en sección transversal mediante tomografía computerizada (CT) e histopatología con el fín de

entender mejor la anatomía del canal auditivo externo equino. Los clínicos veterinarios de equinos deben

saber que el examen del oído equino es posible y puede proporcionar información diagnóstica de importancia.

Zusammenfassung

Die otoskopische Untersuchung und Zytologie des Pferdeohres wäre vorteilhaft bei

Krankheiten wie Kopftrauma, Kopfschütteln, Otitis externa sekundär auf Otitis media, Vestibulärsyndrom,

aurale Neoplasie und auralem Pruritus sekundär auf Parasiten. In der Praxis werden Untersuchungen desOhres bei Pferden selten durchgeführt aufgrund der anerkannten Schwierigkeit der Darstellung des equinen

äußeren Gehörganges und des Trommelfells, sowie wegen der Notwendigkeit der chemischen Sedierung.

In dieser Studie wurde der äußere Gehörgang bei lebenden Pferden mittels manuellem Otoskop untersucht

sowie bei Köpfen von Kadavern mittels Videootoskop. Der äußere Gehörgang konnte beim sedierten Pferd

mittels manuellem Otoskop sichtbar gemacht werden, aber eine stärkere Sedierung oder Vollnarkose sowie

die Verwendung eines Videootoskops waren notwendig, um das Trommelfell beim lebenden Pferd ausre-

ichend darzustellen. Die äußeren Gehörgänge von achtzehn Pferden wurden zytologisch untersucht und

unter aeroben Bedingungen kultiviert. Bei drei Pferden wurden von beiden Ohren Proben genommen.

Weder Zellen noch Organismen waren bei 11/21 Ohren bei der zytologischen Untersuchung zu sehen.

Neun/21 Ohren zeigten kein Wachstum. Zehn/21 zeigten ein gemischtes Wachstum mit einer kleinen

Anzahl an Organismen (

Corynebacterium

sp. war der häufigste Erreger). Zwei/21 zeigten starkes Wachstum

eines einzigen Organismus (

Corynebacterium

sp. bzw. Staphylokokkus intermedius

). Die Köpfe von toten

Pferden wurden im Querschnitt untersucht mittels Computertomographie (CT) und histopathologisch, umdie Anatomie des äußeren Gehörganges besser zu verstehen. Die Pferdepraktiker sollten sich bewußt sein,

dass die Untersuchung des Ohres möglich ist und wichtige diagnostische Informationen liefern könnte.

otoscopic, cytological, and microbiological examination of the equine external ear canal (pages...

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