The CSI approach to pruritic pets: dermatology due diligence (Proceedings)


Once you have finished interviewing the witnesses, the next step is to collect the evidence. A minimum database ("derm–due-diligence") for any pruritic pet should include skin scrapings, close examination for external parasites (fleas, ticks, and lice), acetate tape tests, cytology and possibly coat brushings.

"Collecting the evidence" – dermatology-due-diligence

Once you have finished interviewing the witnesses, the next step is to collect the evidence. A minimum database ("derm–due-diligence") for any pruritic pet should include skin scrapings, close examination for external parasites (fleas, ticks, and lice), acetate tape tests, cytology and possibly coat brushings.

Other (secondary) tests that may be indicated include bacterial culture fungal culture (Woods lamp evaluation?), dietary trials, allergy tests, isolation and provocation (for contact sensitivity), hematology, biochemistry, hormone panels (rare in the pruritic patient), skin biopsy, and a variety of therapeutic trials.

Remember, at some point the owners may want to "lawyer-up", so it is important to describe and quote for the tests before they are performed.

It is important to remember that you are a diagnostician – don't treat the lab report – treat the patient. One can find the techniques in standard texts, but here are a few hints, pointers and suggestions:

     • Flea combing

          ⇒ This is a procedure that can be done by the veterinary assistant or technician and can be an invaluable aid to the veterinarian. Tell them to concentrate the search along the back and at the top of the tail.

          ⇒ Remember, not finding fleas means they weren't found (as opposed to not there). Never tell a client that their pet does not have fleas! It is very difficult to find fleas on some patients, especially those suffering from flea bite hypersensitivity. A parasitic idée response trial is almost always recommended, even in strictly indoor pets. I have diagnosed fleas in 19th story apartment cats that never left the building.

     • Coat brushing and Acetate Tape impressions

       (Coat brushing: collecting fallen debris after vigorous rubbing of the coat)

          ⇒ Collect debris for closer examination with CLEAR acetate tape; acetate tape can also be applied directly to the skin for direct examination when representative lesions are sampled (parasites are found close to the skin, so scissors may be needed to clip the hair )

          ⇒ Sandwich the tape between two layers of mineral oil and drop the condenser

     • Cytology – this test is an inexpensive test that is part of the dermatologist's SOP

          ⇒ Do it often and take lots of samples

          ⇒ Use a variety of techniques to collect the samples (impression, superficial scraping, acetate tape, fine needle aspirate)

               o Tape can be stained with the third stain of a Diff-Quick stain and examined directly. This technique is particularly good for hard to reach areas. Either place the tape on a drop of stain or dip the tape into the stain. It takes a bit of practice to differentiate debris from organisms on the stain. For cats, I will often use the last two stains

               o Greasy areas and erosions can be sampled with direct impression, heat fixed and stained, and then examined as you would a blood smear.

               o Superficial scrapings can be sampled as well. The scales are collected, placed on a microscope slide and pressed while heat fixing. Then the slide is stained and read as you would do a blood smear.

               o Masses and indurate tissue may be best examined by fine needle aspirate. Gently express the tissue from the needle – don't "fire it" onto the microscope slide – in order to decrease the "splash" effect. These samples can be dried and sent to the pathologist (better stains available there) or stained as you would a blood smear and examined

          ⇒ Look for:

               o Organisms (bacteria – mostly cocci, rods, yeast). Identify whether they are "targeting" epithelial cells and estimate what numbers are present

               o Inflammatory response (neutrophils, macrophages, eosinophils) and whether these cells are degenerate or not or have engulfed organisms

               o Unusual cells

     • Skin Scrapings

When performing skin scrapings, remember to dull the blade and scrape with the blade at right angles. This will reduce the likelihood that you will slice the dog which is considered to be bad form! Most importantly, advise the owner that the lesion may look worse after the visit than before. Superficial scrapes may be all you need to diagnose Cheyletiella and scabies but be sure to get deep enough to get a significant amount of capillary bleeding and collect samples from multiple sites if you are looking for Demodex. You need enough of a sample to make the procedure worthwhile.

          ⇒ After smearing on a microscope slide, add a cover slip to minimize oil slicks and aid visualization

          ⇒ Lower condenser and look for motility

Scabies and Notoedres

These mites induce a hypersensitivity reaction and may be difficult to identify, necessitating a parasiticide therapeutic trial. The hypersensitivity reaction may be prolonged in some cases, so identification is always best, whenever possible. Interestingly, there appears to be an increase in house dust mite sensitivity in patients with a previous history of scabies, a consideration in the case of persistent pruritus in the face of apparent resolution of the mite.


This parasite can sometimes be elusive as well. Most of our pruritic pets start to become scaly – don't forget to look for "walking dandruff!"


Multiple species of Demodex have been identified in the dog and cat. In the dog, the most common one is Demodex canis, but more recently, Demodex injai and the short tailed Demodex of the dog have been identified. Cats have long been known to have Demodex cati but regionally, many of us are now identifying and treating the short bodied Demodex of cats, Demodex gatoi.

     1. Demodex canis is the cigar shaped parasite that we all fondly remember from school. The adult measures approximately 170-225 microns (Hillier, 2002).This mite is transferred from the bitch to the neonate within the first 3 days of life and are kept in relatively low numbers by the host's immune system (cell mediated immunity). Clinical signs of demodicosis can range from focal or multifocal alopecic and oftentimes hyperpigmented patches associated with comedones, to a generalized scaling dermatosis which can be markedly alopecic and can be quite erythematous (the condition has been called "red mange").

     2. Demodex injai is a relative newcomer to the scene. The parasite is 330-370 microns long (approximately 2 times the size of D. canis). (Hillier 2002). These patients present with a seborrheic dermatitis on the dorso-lumbar area. Dogs older than 2 years and terrier breeds appear to be overrepresented, though the parasite has been identified in other breeds as well (such as the Dachshund and Lhasa Apso). Excessive glucocorticoid therapy and hypothyroidism have been reported as predisposing causes. Secondary bacterial folliculitis and Malassezia dermatitis may be seen (Carlotti, 2006).

     3. A short-bodied Demodex mite that may be more of a surface dweller (stratum corneum) similar to Demodex gatoi of the cat has been described in the dog (Carlotti, 2006). It is often found in conjunction with D. canis and measures approximately 50% the length of the D canis (Gortel 2006) Since it co-infects with the more common D. canis, it can be missed on skin scrapings.

     4. Demodex cati is similar to D. canis – The adult mite is about 200 μ m in length. The ova are more oval compared to the "pregnant banana" appearance of D. canis. Localized feline demodicosis is a rare disease that most often affects the periocular region, head, neck and eyelids and presents as a variably pruritic, patchy alopecia, scaling and crusting. Localized lesions can self heal, especially if you can find and resolve an underlying cause. There is good evidence that lime sulphur will work. Generalized demodicosis is usually associated with an important underlying disease such as diabetes, hyperadrenocorticism, FIV or FeLV. Infestation by Demodex cati was identified at lesional sites of multicentric squamous cell carcinoma in situ (SCC in situ).(Scott, 2001; Guagiere 1999) Demodex can also present as a ceruminous otitis externa. Siamese and Burmese cats may be predisposed. Differential diagnosis includes dermatophytosis (which can occur simultaneously), bacterial pyoderma, allergic skin disease and, in fact, all causes of feline seborrhea and crusting in cats.

     5. Demodex gatoi is a pruritic skin disease in usually young, short-haired cats with alopecia (broken hair), erythema, scaling, excoriations and crusting, particularly on the head, neck and elbows and/or flanks, and the ventrum and rear legs. Hyperpigmentation can occur, and the disease may be symmetrical. History may also be suggestive, given the apparent geographical predilection (esp. southern United States) and/or a history of contagion. There may be also an association with allergic skin disease (steroid treatment?).

Key clinical diagnostic points regarding demodex:

     1. Both superficial (surface dwellers) and multiple deep scrapings have their respective benefit in the diagnosis of demodicosis. (Of course you'll get the superficial guys with the deep scrapings, so that is the test that I perform most often). Squeeze the skin before the scraping (it's what we all do – but there is no actual evidence that it helps!) and be sure to get deep enough to place a good volume of red sludge on the microscope. Use of a cover slip and lowering the condenser on the microscope to look for motility helps immensely.

     2. Location - location - location – It is important to choose the right sites. Using an opti-visor was routine for me, even before my eyes deteriorated! Have a good close look at the skin. Good places to scrape:

          • red scaly areas

          • hyperpigmented regions (actually look "blue" and with the opti-visor multiple, nearly coalescing comedones are apparent)

          • regions of follicular casts (these can also be good areas to do hair plucking in those hard to reach areas such as the interdigital region)

          • cati is most commonly found at the head and neck and can be a cause of ceruminous otitis externa

          • Demodex gatoi – interscapular superficial scrapings (cat can't reach there); scrape another cat in the household that may be less affected, can be found on tape preps in some cases. Perform superficial interscapular scrapings for D. gatoi but do deep ones as well – it is possible to have dual cati-gatoi infections

     3. Deep skin scrapings under sedation or biopsy may be needed in the case of pododemodicosis

Take home message: How to maximize the likelihood of a diagnosis? Look for hyperpigmented areas, regions of follicular casts, comedones, squeeze the skin and scrape until it bleeds; lower the condenser on the microscope and check every field at 10 power. Be patient! (technicians seem better at that)

Ancillary tests – other tests to consider with a diagnosis of Demodex


     • Juvenile Onset (<12 mo in sml breeds, <18 mo in lge breed and <2 yr in giant breed dogs)

          ⇒ Localized cases

               o Fecal exams - the idea is to create a "stress–free" life for these patients and so SAF fecal evaluation and appropriate deworming is an important part of care.

               o Assess diet for nutritional value

               o No additional diagnostic tests need be performed

          ⇒ Generalized cases

               o Fecal, nutrition, stress as above

               o Pre-treatment heartworm test, +/- MDR1 genetic testing

          • Adult onset

          ⇒ Localized

               o Fecal examination

               o Assessment of nutrition

               o Close scrutiny of concurrent medications (steroids – even chronic use of potent topical steroids)

               o Hematological and biochemical profile

               o Heartworm

               o +/- MDR1 genetic testing

               o Additional tests may be warranted based on the results of above and clinical examination

          ⇒ Generalized As above plus

               o Thyroid evaluation

               o Evaluation for hyperadrenocorticism

               o Abdominal ultrasound (tumor search)


          ⇒ Treatment history (steroid)

          ⇒ CBC, FIV, FeLV (especially if different species are identified)

          ⇒ Biochemical profile – especially looking for diabetes

          ⇒ Consider adverse food reactions?

          ⇒ Abdominal ultrasound?

     • Parasiticide response trial

Like any test, one must keep in mind that a negative scraping means the test is negative, not the patient. Sarcoptes mites are notoriously difficult to find and Cheyletiella is sometimes surprisingly elusive. However, be logical. Demodex canis is usually found on good, deep skin scrapings and daily treatment for a suspected Demodex case with ivermectin is ill advised (yes, I've seen this).

Fleas may be extremely difficult to demonstrate in pets with flea bite hypersensitivity. Many of these cases respond well to a topical adulticide enabling a diagnosis by response to treatment. There are many new products on the market from which to choose. Nevertheless, some of the older products still have their place.

     • SAF fecal (SAF Stands for Sodium acetate, Acetic acid and Formalin- this helps stabilize both external and internal parasites)

          ⇒ The test is run as a centrifugation technique that concentrates the sediment and increases the percentage of positive findings; remind the lab to report both internal and external parasites. There have been numerous occasions at our clinic where fleas or mites were diagnosed on fecal exam (the pet swallows them) and missed on all other tests

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