Challenges in Clinical Diagnosis

Recognizing the Lesions

Staphylococcal skin infection (commonly referred to as pyoderma) arises very commonly in dogs, typically as a secondary complication of some primary underlying condition. Pyoderma often creates discomfort for the pet, ranging from mild to severe pruritus. Because of this - and because secondary infection often obscures lesions of the primary disease - one of the first steps in evaluating a patient with a skin condition is to recognize and treat any infection present.

Superficial pyoderma (also called bacterial folliculitis) is the most common form and is recognized by the typical lesions of papules, pustules, and epidermal collarettes. Note that grossly recognizable pustules filled with exudate may be absent - even though the tiny papules, if examined histologically, would reveal that they are actually pustular. Epidermal collarettes (which are, in effect, "old" pustules) are often the most easily found lesion in superficial pyoderma. If the infection becomes widespread, the collarettes may coalesce to form large, "geographic" regions of alopecia and scaling. When many collarettes have formed in a dog with a thick coat or long hair, the edges of the collarette continually flake off into the hair coat, often creating very diffuse scaling that can be mistaken for seborrhea. In a dog that appears to be seborrheic, it is important to examine the skin surface carefully to see if the scale appears to originate from multifocal lesions with intervening areas of normal skin (more consistent with pyoderma) or originate very diffusely (more consistent with seborrhea).

Deep pyoderma is characterized by the appearance of furuncles, draining lesions, and ulceration; it is typically more painful than pruritic. There is generally no problem in finding a pus-filled lesion on which to perform cytology or culture.

Some patients with pyoderma exhibit rather unusual or atypical lesions. Mucocutaneous pyoderma, most common in German shepherd dogs, appears as weeping, superficial erosions or ulceration often most apparent on the lips and face. Some dogs get a rapidly advancing, erythematous, highly pruritic eruption termed "superficial spreading pyoderma" that some associate with possible hypersensitivity against bacterial components.

Confirmation with Cytology

Skin cytology continues to be one of the easiest and most valuable, yet most underutilized diagnostic procedure in evaluating a patient with skin disease. One need not have any special skills in clinical pathology to look at skin cytology!

It's obvious where to take a sample if there are draining lesions or pustules. It is ideal in this situation to rupture an intact pustule or furuncle with a needle and press a microscope slide gently to the exudate. Examination of exudate from intact lesions reveals more the true characteristic of the infection, as opposed to sampling open or draining lesions where surface contamination may complicate the picture. If the appearance is mainly papular or erythematous, pressing the slide to a moist area of skin surface may suffice; alternatively, a small amount of surface debris can be gently scraped off with a tiny metal spatula and applied to the slide. If the main lesions present are epidermal collarettes, use a small forceps or spatula to peel back the edges of the rim of scale, and sample the area exposed. Some clinicians prefer the "cellophane tape method" to take the sample if obvious exudate is not present.

After the sample has dried, it can be stained with any routine 3-component quick hematology stain. Tape strips can be likewise stained (a clothespin is helpful to hold the tape strip while dipping), but remember to use only stains #2 and #3 if staining tape. The first stain component (fixative) will dissolve many types of cellophane tape! The tape strips can then be mounted on glass slides, sticky-side down, into a drop of mineral oil.

One of the keys to easy cytology is to have a quality, well-maintained and adjusted microscope. In recent years, the quality of Asian microscopes of lesser-known brands has dramatically improved, while prices have declined. The cost of a quality microscope can easily be recovered in a short time through revenue from skin or ear cytology! The slides should be examined briefly at low magnification to find areas of interest, then more thoroughly using a 1000x, oil-immersion lens. With skin cytology, it is too difficult to discriminate between organisms and debris without high-power magnification.

What are you looking for? The main question is "cocci, rods, or yeast?" Of course, more than one of these may be present. A secondary question is "inflammatory cells or not?" With staphylococcal pyoderma, the typical finding will be inflammatory cells with cocci both within and outside of the cells. The finding of many cocci without inflammatory cells, when the sample has been taken from a lesional area, may suggest "bacterial overgrowth," which in itself can create a rather extensive and very pruritic erythematous rash.

Culture and Susceptibility: When, How, Where?

When

As more and more multidrug-resistant strains of Staphylococcus and other organisms appear, it is becoming more and more necessary to perform a bacterial culture and susceptibility test. Though unnecessary for many routine cases of skin infection, the following situations should prompt greater consideration of culture:

 Recurrent pyoderma that has not responded well to antibiotics and/or topical therapies

 Pyoderma in a dog that has been treated with multiple courses and/or multiple antibiotics in the recent past

 The finding of rod organisms plus inflammatory cells cytologically; particularly if the rods can be seen within inflammatory cells, this is a clue that the infection may not be staphylococcal.

How and Where

As with cytology, the ideal lesion to culture is an intact pustule or furuncle, ruptured with a needle and then touched with a culture swab. Try to sample multiple lesions with your swab. Here are some additional guidelines:

 Avoid culturing "open" lesions, which may be contaminated with nonpathogenic skin surface organisms.

 Very tiny pustules, when ruptured with a needle, often yield only a tiny drop of blood; this blood is exactly what you want to touch with the swab!

 To culture an epidermal collarette, position the swab at the outer scaling margin of the lesion, in the "crevice" under the scale, and rub gently so as to peel back the margin of the lesion with the swab.

 When lesions consist primarily of broad areas of erythema and papules, and you have cytologic evidence of bacterial overgrowth or pyoderma, use a dry culture swab and rub it vigorously over a small area of the lesion for 10 seconds or so.