Histiocytes are tissue based leukocytes (white cells) in which 2 lineages, macrophages and dendritic cells, are recognized. Histiocytes (especially dendritic cells) function in the immune system to ensure that appropriate responses are made to foreign molecules. Also, histiocytes (especially macrophages) ensure that tissue debris and foreign agents (eg certain pathogenic organisms) are inactivated or degraded. There are at least 4 well defined histiocytic proliferative diseases that have been recognized in dogs. Histiocytic diseases are frustrating because it may be difficult to differentiate some forms from inflammatory disease in which appropriate infiltration and proliferation of histiocytes has occurred in response to an infectious agent (so called granulomatous, reactive inflammatory disease).

The clinical presentation and behavior and responsiveness to therapy vary tremendously between the various histiocytic disease syndromes observed. Canine cutaneous histiocytoma is a benign neoplasm which usually occurs as a single lesion in young dogs of all breeds, and spontaneously regresses (there are rare exceptions to this rule). Cutaneous histiocytosis (CH) presents with single or multiple lesions, which tend to wax and wane, and may even spontaneously regress. Few cases respond to corticosteroids, the remainder persist and may require more aggressive immunosuppressive therapy. Systemic histiocytosis (SH) is a familial disease of Bernese Mountain Dogs and also occurs sporadically in other breeds. SH presents with prominent skin manifestations identical to those seen in CH, but mucous membranes (ocular and nasal) and a variety of other organ systems, including lymphoid organs, lung, and bone marrow may also be involved. Although the lesions may wax and wane early in the course, SH is a progressive disease that often requires continuous immunosuppressive therapy. Histiocytic sarcoma and malignant histiocytosis (MH) occur with high incidence in Bernese Mountain Dogs, Rottweilers, Flat Coated Retrievers, Golden Retrievers and sporadically in many other breeds. Histiocytic sarcomas occur as localized lesions in spleen, lymph nodes, lung, bone marrow, skin and subcutis, brain, and periarticular tissue of large appendicular(limb) joints. Histiocytic sarcomas can also occur as multiple lesions in single organs (especially spleen), and rapidly disseminate to involve multiple organs. Hence, disseminated histiocytic sarcoma is difficult to distinguish from MH, which is a multi-system, rapidly progressive disease in which there is simultaneous involvement of multiple organs such as spleen, lymph nodes, lung, bone marrow, skin and subcutis. Response of histiocytic sarcomas and MH to chemotherapy is at best brief.

It is vitally important that the correct diagnosis by a pathologist experienced with the full spectrum of histiocytic disease is obtained, so that it is clear which of these histiocytic diseases has occurred in a given case. Only then can the likely outcome be predicted and appropriate treatment (if one exists) instituted. More detailed descriptions with images of the individual histiocytic diseases are available on this site. This information is targeted at clinicians and pathologists who seek to understand the finer points of distinguishing the various diseases, and wish to learn about treatment options. Feel free to point your veterinarian to this site for this purpose.

Some breeds have a higher incidence of histiocytic disease. Bernese Mountain Dogs, Rottweilers, Flat Coated Retrievers, and Golden Retrievers experience a high incidence of the malignant histiocytic diseases (histiocytic sarcoma/malignant histiocytosis). Bernese Mountaind Dogs also have a high incidence of cutaneous and systemic histiocytosis. In the case of Bernese Mountain Dogs, a clear familial relationship has been established for SH and histiocytic sarcoma/MH. Shar Peis have an apparent high incidence of multiple cutaneous histiocytomas with or without delayed regression.