A rare case of histiocytic proliferative disease in a cat

Abstract

A 7-year-old intact female domestic medium hair cat was examined at a veterinary clinic for a scabbed nodule over the right shoulder. Multiple nodules recurred at the same site after the first surgical excision, and a second surgical excision was performed. Histopathology demonstrated high-mitotic-rate neoplastic cells and therefore a histiocytic proliferative disease was initially suspected. The condition progressed rapidly within a 5-month period and the cat was euthanized due to sudden onset of severe dyspnea. Necropsy showed diffuse metastatic nodules in the lungs, confirming a histiocytic proliferative disease, with histiocytic sarcoma being the most likely differential diagnosis.

A 7-year-old intact female domestic medium hair cat was examined at a veterinary clinic and a firm, circular, 1-cm in diameter, well-demarcated cutaneous lesion just caudal to the right shoulder was identified. The veterinarian suspected the lesion to be a sebaceous cyst or healing wound since some crusts and a central opening were present. The cat appeared to be clinically healthy otherwise. It was recommended to the owner to use warm compresses on the lesion and the option to excise the mass was mentioned.

After administration of warm compresses, no material could be expressed from the nodule; therefore, surgical excision was conducted a month after first presentation. The nodule had grown to 2 cm × 1.8 cm and was white, uniform, and slightly gritty in texture on cut surface. A biopsy sample was submitted for histopathological examination. Microscopically, the lesion appeared to be situated within the dermis and extending into the subcutaneous fat and skeletal muscle. Round cells with eccentric round nuclei were closely packed, forming sheets. Cytoplasm was abundant, eosinophilic, and faintly granular. The cells had up to threefold anisocytosis and anisokaryosis, with 23 mitotic figures in ten 400× high power field, which is highly suggestive of a neoplastic nature. Some binucleations and multinucleation were also present. From these observations, a mast cell tumor with high mitotic rate was suspected. A special stain was ordered for confirmation, but surprisingly the neoplastic cells did not stain, ruling out a mast cell tumor. After discussion among the pathologists, the differential diagnoses were narrowed down to a cutaneous round cell tumor of either plasma cell or histiocytic origin. An immunohistochemistry test was recommended to differentiate between the two, since a plasma cell origin tumor has a better prognosis than a histiocytic origin tumor. However, further diagnostic examinations were ultimately declined by the client.

Three months after the surgery, the cat was returned for a follow-up examination and 2 new nodules were discovered during the examination. A chest radiograph was performed, and no abnormal radiographic findings were noted. A second lumpectomy was performed 1 wk later, and a total of 5 nodules were identified, removed, and submitted for histopathological examination. One of the nodules was an axillary lymph node, the other 4 were located around the previous excision site. Histological appearance was identical to that of the first nodule and the lymph node appeared to have been effaced by the neoplastic cells (Figure 1). Due to the rapid progression, a diagnosis of histiocytic proliferative disease was made, with feline progressive dendritic cell histiocytosis (FPDCH) and histiocytic sarcoma (HS) as the main differentials. Since the treatment options and prognosis of these 2 diseases are similar, no further diagnostic testing was pursued.

Figure 1.

Microscopic examination of histological slides of the axillary lymph node. A — Low power microscopic image. Small amount of normal lymph node tissue present over the top right area, most of the lymph node was effaced by histiocytes. B — High power (40×) microscopic image. Numerous highly mitotic cells of histiocytic origin present, demonstrating eccentric and irregular nuclei, with some multinucleation shown (black circles).

The cat started to demonstrate labored breathing within 13 d after the second lumpectomy. Her condition rapidly deteriorated, and she was euthanized about a month after the second surgery. A necropsy showed a random multifocal pattern in the entire lung consisting of numerous small 1-mm sized, smooth, white, round, raised nodules (Figure 2 A). The lung texture was rubbery, and the edges were slightly rounded. In addition, severe hemothorax was present. There was an enhanced lobular pattern present throughout the liver (Figure 2 B). An in-house impression smear cytology from a cross section of the lung revealed several cells of macrophage origin, likely histiocytes, with varying degrees of anisokaryosis (Figure 3). Metastasis of the primary skin neoplasia was suspected due to the random multifocal pattern of small nodules in the lungs.

Figure 2.

Necropsy images. A — Thoracic cavity of the cat opened. Small, white, smooth, raised, round nodules were present throughout the entire lung. Hemothorax was also seen. B — Abdominal cavity of the cat. Enhanced lobular pattern present throughout the entire liver.

Figure 3.

In-house lung impression smear cytology. Cells from macrophage origin in the lungs, showing some degree of anisokaryosis, suggesting a metastatic process from a primary skin lesion.

Discussion

Histiocytic proliferative diseases can stem from either proliferation of dendritic cells or macrophages (1). Several histiocytic proliferative diseases have been identified in dogs and cats, but they are considered rare in cats compared to dogs (1). To the author’s knowledge, only a few case reports have been published on histiocytic proliferative diseases in cats (2,3).

Feline progressive dendritic cell histiocytosis is the most common histiocytic proliferative disease reported in cats (1). It often presents as single or multiple non-painful, non-pruritic intradermal nodules, especially around the head, neck, and lower extremities, measuring up to 1.5 cm in diameter (2,4). Lesions are often hairless and ulcerated on the surface (4). Typical signalment includes middle to older aged cats (range: 2 to 13 y old), with no sex or breed predilections (4). Initially, the disease is often limited to the skin and subcutis, for as long as 3 y (2). At later stages, lymph node involvement and internal organ infiltration are seen, with lungs, liver, spleen, and kidneys being the most likely affected (1). Initially, FPDCH tends to have benign cytological features during the early stages, but then slowly progresses to more malignant cytological features with high mitotic figures, at which point it is histologically indistinguishable from HS, which has a much more rapid and aggressive disease process (1,2). Thus, FPDCH cases are often seen as an indolent form of HS (1).

Histiocytic sarcoma is also a rare disease in cats (3,4), with most cases arising in an internal organ or in the skin, which then rapidly spreads to other locations in the body (1). Most cats with HS present with nonspecific signs, such as anorexia, weight loss, and lethargy; some may have signs more specific to the organ system targeted (3,5). There is 1 report of localized HS on the tarsus of a cat; however, localized HS is extremely rare (6). Once the disease has spread, it is termed disseminated HS (1). Cytologically, neoplastic cells in HS show malignant features very early in the disease course, which is the main difference from FPDCH (1). In the case herein, no definitive diagnosis was confirmed. However, given the rapid nature of the progression of disease process within 4 to 5 mo, HS would be higher on the differential diagnoses list than FPDCH. If this were true, it would be an unusual case of HS, since the cat was first seen with cutaneous lesions, whereas most cats with HS are brought in for examination because of signs of general illness.

Currently, there are no medical treatment options for either FPDCH or feline HS (3). Corticosteroids may help intermittently with FPDCH, but this treatment protocol does not slow down the progression of disease (3). Surgical excision is recommended if only a few nodules are present, but this does not prevent recurrence of the disease (3). Both diseases have an ultimately poor prognosis since they both behave aggressively at some point during the disease process (3).

In the case presented, it was suspected that the neoplasia had metastasized to the lungs, as suggested by the impression smear result and the random multifocal pattern of the nodules in lungs. This likely decreased pulmonary compliance and led to the severe dyspnea seen in the cat. A buildup of pressure in the pulmonary vessels led to a backup of blood into the heart and subsequently the liver, resulting in congestion and an enhanced lobular pattern seen at necropsy.

Due to the rarity of the histiocytic proliferative diseases in cats and the unusual presentation, this case serves as an addition to the literature for primary care veterinarians, students, and researchers.