Dermatology Challenge: Acutely pruritic eruptions on a dog's extremities and trunk

A 1-year-old 66-lb (30-kg) intact male Labrador retriever was presented to the University of Wisconsin School of Veterinary Medicine's Dermatology Service for evaluation of severe pruritus and skin eruptions on its trunk and distal extremities. The dog had no previous history of skin disease, and the presenting condition had developed acutely. The dog had just returned from a five-day camping trip with its owners to northern Wisconsin (in mid-August). The dog had appeared normal before the camping trip. While on the trip, the dog had wandered off for several hours on numerous occasions. After returning home, the owners had bathed the dog with an over-the-counter flea shampoo containing pyrethrins. The owners typically bathed the dog after excursions into wooded areas or after hunting trips, and they had used this shampoo on the dog many times before. But this time, within several hours after being bathed, the dog had become agitated and had started biting at its trunk and extremities. The owners had rinsed the dog's coat with water again, assuming residual shampoo had caused the pruritus. This extra rinse did not alleviate the dog's signs, and the lesions seen on presentation had developed during the previous 24 hours. The dog's vaccination status was current, it was receiving a monthly heartworm preventive, and it was not receiving any other medication.

Figure 1. The skin of the dog's lateral thorax. Note the raised, red lesions on the trunk.

Clinical signs

The dog's physical and dermatologic examinations revealed a severely pruritic dog: Unless restrained, the dog would mutilate its extremities. Palpating the skin on the trunk revealed firm, nodular masses. When the dog's coat was clipped, diffuse, raised serpiginous lesions were found (Figure 1). On palpation, the lesions felt edematous. Similar lesions could be felt under the dog's coat. The carpal and metatarsal regions were edematous and swollen and had raised, exudative, proliferative eruptions (Figures 2A & 2B), and the dog exhibited pain on palpation of these areas.

Figure 2A. The carpal region. Note the marked proliferative and exudative lesions.

Differential diagnoses

The differential diagnoses focused primarily on diseases that cause acute nodular eruptions. Although the time frame suggested that the dog's lesions developed within the last 24 hours, it was possible that the lesions had started to develop during the camping trip and that the owners noticed them only after returning home. It was also possible that the underlying cause had been going on for even longer, and the timing of the eruptions' appearance was only coincidental. Another consideration was that the urticarial lesions on the trunk were the primary lesions and that the lesions on the legs were secondary to self-mutilation. This possibility seemed less likely given the lesions' proliferative nature.

The acute nature of the dog's condition was compatible with an urticarial eruption, most likely an allergic contact reaction to the shampoo. Because of the dog's young age and the lesions' exudative nature, a deep pyoderma secondary to demodicosis was also considered. Conditions that could have been acquired during the camping trip, including insect bite hypersensitivity, irritant or allergic contact reaction (e.g. shampoo), cutaneous infestation with Pelodera strongyloides, and schistosomiasis (swimmer's itch), were considered as well. A fungal kerion reaction was compatible with the lesions on the extremities but not with those on the trunk, raising the possibility that the lesions on the two areas had separate etiologies. Although considered unlikely, other differential diagnoses included infections such as deep or intermediate mycoses (e.g. blastomycosis, sporotrichosis), foreign-body reactions, and neoplasia.

Figure 2B.The metatarsal region. Note the marked proliferative and exudative lesions.

Diagnostic testing

The results of a microscopic examination of numerous deep skin scrapings were negative for Demodex mites and Pelodera strongyloides. Cytologic examination of impression smears of the lesions revealed an inflammatory exudate that was predominantly eosinophilic with neutrophils and rare extracellular cocci. A bacterial culture and sensitivity test of a tissue section grew Staphylococcus intermedius that was sensitive to all tested antibiotics. A dermatophyte culture was performed; the culture results were finalized 21 days later and were negative. Histologic examination of skin biopsy samples from the lesions on the trunk revealed angioedema, and samples from the lesions on the legs revealed diffuse superficial and deep eosinophilic dermatitis, eosinophilic folliculitis and furunculosis, dermal edema, and vascular dilatation. Special staining (periodic acid-Schiff for fungi) of biopsy samples revealed no infectious agents. The results of a thoracic radiographic examination to rule out systemic fungal infection and neoplasia were normal, as were the results of a complete blood count and serum chemistry profile. These findings led to a preliminary diagnosis of eosinophilic dermatitis with a secondary bacterial infection caused by an unknown trigger.

Figure 3. The skin lesions 48 hours after administration of a glucocorticoid. Note the marked reduction in swelling and exudation.

Treatment and follow-up

Pending the results of the bacterial culture and sensitivity testing, histologic examination of the skin biopsy samples, and special staining techniques, cephalexin (30 mg/kg orally b.i.d.) was given. Whole-body hydrotherapy every eight hours and diphenhydramine hydrochloride (2 mg/kg orally t.i.d.) were also initiated. The dog showed little response to this therapy. When the biopsy results became available, prednisone (0.5 mg/kg orally b.i.d.) was initiated. At this time, the diphenhydramine was discontinued and the hydrotherapy was continued. To prevent self-mutilation, the dog wore an Elizabethan collar until the prednisone relieved the dog's discomfort. During the next 48 hours, the lesions rapidly decreased in size (Figure 3) and the pruritus began to resolve; therapy was continued for five days.

Several weeks later, the owners bathed the dog in the same flea shampoo that had been used just before the lesions developed. The dog's condition recurred, and the dog was again treated with hydrotherapy and oral prednisone. The dog continued to develop urticarial lesions whenever the owners bathed it using an over-the-counter flea shampoo. If left untreated, the dog developed lesions similar to those seen at the original presentation. Avoiding flea shampoos prevented further episodes. After numerous trial-and-error episodes, the owners determined that diphenhydramine therapy was not beneficial even when the lesions were discovered early.

Discussion

The histopathologic diagnosis was eosinophilic dermatitis. This diagnostic pattern can result from many causes, including allergic or parasitic diseases. Several eosinophilic dermatoses in dogs have been described, including canine eosinophilic proliferative otitis externa, sterile eosinophilic pinnal folliculitis, canine eosinophilic pustulosis, eosinophilic dermatitis and edema, canine eosinophilic granuloma, and canine nasal folliculitis and furunculosis.1-3 No single unifying or underlying trigger is noted. Identified causes include drugs, new diets, insect bites, concurrent allergic disease, immune-mediated disease, and possibly stress.

The definitive diagnosis in this case was an eosinophilic drug reaction caused by a topical flea shampoo. It is unknown if the offending agent was the insecticide or a common ingredient in the shampoos. Diagnosing drug reactions in animals is complicated because a wide range of signs can occur and no classic histologic pattern is found in skin biopsy samples. In addition, many patients receive multiple drugs concurrently, and the trigger could be a drug combination, not a single agent. Finally, it is often difficult to confirm the diagnosis of a drug reaction because it involves identifying the causative agent and challenging the patient to the drug. This last step is generally not recommended because subsequent exposure can cause increasingly serious reactions, and owners may be unwilling to risk causing additional pain and distress to their pets.

In my experience, immediate adverse reactions to topical shampoos usually are mild and consist of pruritus, erythema, flushing, and rare urticarial episodes. Many of the patients needing long-term shampoo therapy are patients with pruritic diseases (e.g. atopy). A worsening of the pruritus by the shampoo may be missed or attributed to the underlying disease. I recommend diluting medicated shampoos before application and triple rinsing afterward. To identify a suitable shampoo in a shampoo-sensitive dog, perform an open patch test (unlike in the traditional patch test, the test compound in the open patch test is not under an occlusive bandage). This test involves applying a small amount of diluted shampoo to the dog's abdomen, allowing five minutes of contact time, and rinsing. Then watch the dog carefully for any signs indicating intolerance (e.g. erythema, pruritus). If the dog shows signs of intolerance, do not use the shampoo.

An interesting question is why the dog in this case tolerated the shampoo previously. Two possible reasons are that allergic contact reactions can take months to years to develop and that the formulation of the shampoo may have changed.

REFERENCES

1. Scott, D.W. et al.: Skin diseases. Muller and Kirk's Small Animal Dermatology, 6th Ed. W.B. Saunders, Philadelphia, Pa., 2001; pp 1125-1183.

2. Holm, K.S. et al.: Eosinophilic dermatitis with edema in nine dogs compared with eosinophilic cellulitis in humans. JAVMA 215 (5):649-653; 1999.

3. Moriello, K.A.: Eosinophilic dermatoses. BSAVA Manual of Small Animal Dermatology, 2nd Ed. (A. Foster; C. Foil, eds.). British Small Animal Veterinary Association, Gloucester, Engl., 2003; pp 233-241.

The photographs and information for this case were provided by Karen A. Moriello, DVM, DACVD, Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53706.