Scooby: Puzzling Pruritic Pustules

Patient

Scooby is a 6-year-old, spayed female, West Highland White Terrier

History

Scooby has a history of seasonal pruritus that is worse in the summer months. The itch started when she was 2 years old; it returns every summer and disappears over the winter. Historically, the itch has been mild and satisfactorily treated with some oral antihistamines along with more frequent bathing. However, the itch does seem to be getting worse every year. In some years, especially recently, during the worst times some pustules have appeared on her trunk and the itch becomes severe. These lesions (and some of the pruritus) disappear if treated with an antibiotic and 2% chlorhexidine baths. Scooby receives monthly routine parasite control with topical fipronil.

This year, Scooby's seasonal allergies flared up as usual. By early summer, her itch was severe, and pustular lesions again appeared. These were numerous and prominent on her trunk and face, including the ears which were especially pruritic. Treatment with the usual antibiotic and shampoo were only slightly helpful, and the itch persisted. Finally, the veterinarian prescribed a 1-month course of oral prednisone, which immediately helped the itch. The itch and lesions both gradually faded over the summer. The treatment was discontinued, and she remained normal over the next 3 months.

However, in late autumn, the allergies returned rather dramatically. Scooby was very pruritic, again accompanied by a pustular eruption. The owner became very concerned due to this apparent worsening, and the veterinarian decided to refer Scooby to a dermatologist for additional diagnosis and treatment of her allergies.

Clinical Signs

On physical examination, Scooby appeared generally healthy other than her skin, was afebrile, and of normal weight. She was obviously very pruritic in the examination room. No abnormalities were detected other than her skin lesions. Numerous pustules, ruptured pustules with dried crusty exudate, and epidermal collarettes were found over large areas of the trunk and head. In particular, lesions were prominent and easily visible bilaterally on the ear pinnae.

Initial Workup and Laboratory Results

An obvious consideration here was superficial pyoderma that had become antibiotic-resistant. Intact pustules were readily found and ruptured with a needle. The pustule contents were swabbed for culture, and an impression smear of the exudate was made for cytology. Perhaps the prednisone administration had precipitated demodicosis, already a prominent disease in this breed? Deep skin scrapings and trichograms were prepared for examination.

The results of these examinations were as follows:

 Skin scrapings and trichograms were negative for parasites.

 The culture revealed very light growth of Staphylococcus epidermidis, broadly susceptible to all antibiotics tested.

 Cytology of pustule contents revealed many neutrophils, with no organisms seen. At low power, admixed with the neutrophils were many large, rounded, deeply basophilic cells that sometimes occurred in clusters.

Next Steps

The culture result was interpreted as only light contamination with normal skin flora. The absence of organisms on cytology, along with the finding of many acantholytic cells, was suggestive of Pemphigus foliaceus. Based on this suspicion, punch skin biopsy was performed and Scooby was sent home with bathing in chlorhexidine shampoo.

Final Diagnosis and Case Discussion

Histopathology results were received 4 days later and were diagnostic of Pemphigus foliaceus. Scooby was started on immunosuppressive doses of prednisone, and responded well to treatment.

We believe that Scooby began life as an atopic dog, as evidenced by seasonal occurrence of mild and easily controlled pruritus. Even worsening over time, and appearance of lesions consistent with secondary pyoderma, are typical of atopic dermatitis. However, the appearance of her pustular eruption was unusual in that:

 It came on rapidly and severely and was very widespread, with many lesions; it was prominent on the facial area, including the ear pinnae. Most pyoderma comes on gradually and is primarily truncal. Lesions on the pinnae would be unusual for pyoderma.

 Culture results were unconvincing for a bacterial etiology; even so, she had been recently treated with an antibiotic to which the cultured bacteria were susceptible, and was not responsive.

 On cytology, it was impossible to find any cocci bacteria, despite long searching. On the other hand, there were very large numbers of acantholytic cells - an unusual finding that would not be seen with pyoderma.

 Why did her pustular eruption clear with treatment during the summer, yet recur in the autumn? Here are two possibilities:

 The dose of prednisone used for her "allergies" was sufficient to cause some resolution of her Pemphigus lesions.

 More likely, the Pemphigus lesions resolved spontaneously. Almost always, Pemphigus foliaceus begins as a condition that waxes and wanes spontaneously, often over a period of 6–12 months, before it becomes fully evident. During this period, owners often seek veterinary treatment, which is usually effective - not because the treatment works, but because the disease wanes on its own. Several months later, a new breakout of lesions occurs. The same treatment may this time appear ineffective because this breakout is worse. It is very typical for both the owner and the veterinarian to become confused and frustrated by this series of events.

In evaluating patients with a pustular, pruritic eruption, it is well to remember that "all that is pruritic is not allergy" but also "all that is pustular is not pyoderma!"