A range of diagnostic techniques can be employed to investigate ear disease in the dog and cat. Many quick basic tests can be undertaken by the primary care veterinary surgeon within the confines of a normal practice laboratory. Other more advanced techniques such as radiography, CT, and MRI may also be useful in some circumstances, but the latter two are generally confined to referral institutions.
Both the ear pinna and the ear canal can yield diagnostic information and samples should be taken from both sites where possible. On the ear pinna the type of test that is suitable will depend on the clinical presentation, particularly the type of primary lesion that is present. Where samples are taken from the ear canal and middle ear a more formulaic approach is possible.
2.1 EAR PINNA
2.1.1 Basic Diagnostic Tests
For a quick summary see Table 2.1.
|Most suitable tests
|Major differential diagnoses
|Tape strips, diascopy
|Allergy, malassezia, haemorrhage
|Crust and scale
|Superficial skin scrapes, tape strips, dermatophyte culture
|Sarcoptes, dermatophytosis, endocrine disease, immune-mediated disease
|Pustule cytology, culture, excisional biopsy
|Pyoderma, sterile pustular diseases (esp. pemphigus foliaceus)
|Papule cytology, impression smear
|Fine needle aspirates, impression smear, excisional biopsy
|Neoplasia, hyperplasia, infectious pyogranulomatous disease
|Impression smears, biopsy of periphery of ulcer
|Vasculitis, immune-mediated disease
|Skin scrape, hair pluck, dermatophyte culture, biopsy
|Demodex, dermatophytosis, endocrine disease
This is a useful and simple technique to assess the difference between vasodilatation and haemorrhage. A glass slide is placed over an erythematous lesion and gentle pressure is applied.
- Erythematous lesions blanch when pressure is applied, as they are caused by dilated blood vessels.
- Haemorrhagic lesions do not blanche when pressure is applied as they are caused by red blood cell leakage out of vessels (Fig. 2.1).
Acetate Tape Impression Smears
This technique can be performed on the skin or hair.
The tape is pressed repeatedly on to the coat to pick up such things as louse eggs from the hairs as well as parasites on the surface of the skin, e.g. lice, trombicula. This is a useful technique to trap some of the fast-moving parasites which can often be seen with the naked eye.
The tape is pressed firmly on to an alopecic area of skin on the ear pinna (Fig. 2.2). The author usually rubs the tape gently with a thumb nail to ensure good contact of the tape with the skin. The tape is then everted sticky side up on a microscope slide (Fig. 2.3). A modified Wright’s stain such as Diff-Quik can then be applied to the tape (Fig. 2.4) which is then carefully inverted with the adhesive slide downwards on to a microscope slide for examination (Fig. 2.5). As the tape acts as a cover clip. the sample can be examined under high power (100× oil immersion) if necessary. This method can be used to identify surface bacteria; yeast, especially malassezia (Fig. 2.6); and parasites, especially surface-living demodex mites – Demodex corniei in dogs (Fig. 2.7) and Demodex gatoi in cats.
Scrapes may be taken into either liquid paraffin or 10% potassium hydroxide. When liquid paraffin is used a few drops can be gently added to the sample before a cover slip is added. When potassium hydroxide is used the sample is best left to stand for 10–15 minutes and may be gently warmed before examining as this allow the potassium hydroxide to digest keratin to clear the field. There are advantages and disadvantages of both mounting materials (see Table 2.2).
|Not irritant to skin
|Potential skin irritant
|Will not clear the sample
|Will clear the sample by digesting keratin
|Does not kill the mites which can thus be identified by movement across the slide
|Mites will curl up if sample not examined soon after taking; identification may be difficult
|Mites well preserved and observation of body parts easier
Superficial Skin Scraping
A scalpel blade should be blunted by gently rubbing it on the microscope slide before use (Fig. 2.8). It is moistened with either 10% potassium hydroxide or liquid paraffin to provide better collection of material and scraped through the coat and superficial layers of the skin. Hair may be gently clipped away if the coat is very thick. The scalpel blade is normally held perpendicular to the skin and the scraping should be in the direction of the hair coat. Material should be spread thinly on to the slides (Fig. 2.9). Multiple samples should be taken from non-excoriated areas. When potassium hydroxide is used then the slides may be gently warmed to break up keratin and/or left to stand for 10–15 minutes before examination. This technique is best employed for superficial parasites such as lice (Fig. 2.10) and trombicula (Fig. 2.11).
Deep Skin Scraping
The technique is identical to that for superficial scrapings except that scrapings should be to a taken to a depth where the skin is erythematous and mild capillary ooze is seen (Fig. 2.12). Where scrapings are taken to investigate sarcoptes they should be taken from the periphery of the ear pinna from areas of crusting (Fig. 2.13). Typical round-bodied mites, eggs (Fig. 2.14), and faeces can be found. Where follicular demodex (Demodex canis or Demox cati) is suspected then scrapes should be taken from the glabrous areas of the pinna, if possible from areas of comedone formation. The skin may be pinched to express the mites from the follicles.
Hair plucks are a useful way to assess a variety of different factors including the presence of fungal infection, shaft defects, and hair growth phase. To take a sample, hair should be grasped firmly between the finger and thumb and sharply pulled out (Fig. 2.15). A small pair of artery forceps may be used to grasp the hairs but can cause crush artefact on the shaft of the hair. Hair can then be mounted in liquid paraffin or 10% potassium hydroxide.
Examination of the tip is most useful where traumatic hair loss is suspected. Tip damage suggests self-inflicted trauma (Fig. 2.16). In hair loss through endocrine disease the tip remains finely tapered (Fig. 2.17).