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


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

Nov 01, 2004

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

Clinical signs

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

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.

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