Canine histiocytic diseases

Histiocytic diseases in dogs are well recognized. They are a frustrating group of diseases because it may be difficult to differentiate them from granulomatous, reactive inflammatory or lymphoproliferative diseases. The clinical presentation and behaviour as well as response to therapy vary greatly among the syndromes.

There are at least four well-defined canine histiocytic diseases:

Histiocytoma is a common, benign, cutaneous neoplasm in dogs. Histiocytomas usually occur as solitary lesions which undergo spontaneous regression. The age specific incidence rate for histiocytomas drops precipitously after 3 years, although histiocytomas do occur in dogs of all ages. Reports of recurrence of histiocytomas at the same or other sites are rare; and the occurrence of multiple tumors is considered unusual. Epidermal invasion by cells of histiocytoma frequently occurs and intra-epidermal nests of histiocytes resemble Pautrier's aggregates, characteristically found in epidermotropic lymphoma (Mycosis Fungoides or MF). Epidermal invasion in histiocytoma, or presence of simultaneous multiple histiocytomas especially in aged dogs can present a diagnostic dilemma and distinction from MF and non-epidermotropic cutaneous lymphoma (NECL) is difficult on purely morphological grounds. Multiple histiocytomas are also readily confused with cutaneous histiocytosis on clinical appearance, although morphologically, histiocytomas are consistently epidermotropic and commonly epidermally invasive, these are not features of cutaneous histiocytosis. Multiple histiocytomas appear to be more common in Shar Peis (Moore, unpublished data), but can occur in any breed. Delayed regression of multiple histiocytomas can occur and lesions can persist for up to 10 months.

Recently, several cases of histiocytoma were observed in which histiocytes had migrated to draining lymph nodes and completely obliterated them. Pathologists diagnosed histiocytic sarcoma in these instances and prognosis was reported as poor. In 3 instances regression of these lesions occurred spontaneously within 3–4 weeks. In other instances the metastatic lesions of histiocytoma failed to regress and dogs were euthanized. The disease course in these cases extended over several months. Spread beyond lymph nodes to lung has also been observed in some of these cases. These complications are rare.

The presence of multiple histiocytomas is now a well recognized syndrome. However, there is yet another presentation in which widespread cutaneous lesions histologically identical to histiocytoma are observed. Clinically, the lesions are almost confluent in affected regions. Rapid internal spread is observed and the affected animals have all been euthanized. There is one published account of such a case, and we have data on 3 dogs with what appears to be an identical syndrome.

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