Reactive Histiocytosis in Dogs: Multiple Skin Nodules and Systemic Spread

Based on peer reviewed veterinary research · For educational purposes only · Updated September 2026 · 8 min read

What Is Reactive Histiocytosis in Dogs?

Reactive histiocytosis in dogs is a non-cancerous disorder in which immune cells called histiocytes (dermal dendritic cells and macrophages) accumulate in the skin, and sometimes in internal organs, forming firm nodules. It belongs to a group of histiocytic diseases that range from benign to malignant, and it is distinct from histiocytic sarcoma, which is a true cancer (Moore, 2014).

Veterinary pathologists recognize two reactive forms. Cutaneous histiocytosis (CH) is confined to the skin and subcutaneous tissue. Systemic histiocytosis (SH) involves the skin plus other sites, commonly the nasal planum, eyelids, lymph nodes, and occasionally the lungs, spleen, or liver (Affolter & Moore, 2000). Both arise from activated dermal dendritic cells and are considered a reactive, immune-regulation problem rather than a neoplasm.

Systemic histiocytosis has a well-documented breed predisposition in Bernese Mountain Dogs, and it is also reported in other breeds such as Rottweilers, Golden Retrievers, and Labrador Retrievers (Moore, 2014). Because the nodules can wax and wane, owners often first mistake them for insect bites, cysts, or a harmless lump.

What Causes Reactive Histiocytosis?

The exact cause is not known, but the current evidence points to immune dysregulation. The proliferating cells express markers of activated dermal dendritic cells (CD1, CD11c, MHC class II) and are accompanied by many T lymphocytes, which is why the disease is treated as an antigen-driven, immune-mediated process (Affolter & Moore, 2000).

A hereditary component is suspected in Bernese Mountain Dogs, where the systemic form clusters in certain families. The trigger for the immune activation is unidentified. Unlike infectious causes, cultures from the nodules are typically negative unless secondary infection is present.

This distinction matters clinically. Because the disease behaves like a relapsing immune disorder, lesions may regress spontaneously or on immunosuppressive therapy and then return, particularly in the systemic form (Palmeiro et al., 2007).

How Do Veterinarians Diagnose Reactive Histiocytosis?

Diagnosis requires tissue. Fine-needle aspiration can show a mixed population of histiocytes and lymphocytes, but a skin biopsy with histopathology is needed to separate reactive histiocytosis from histiocytoma, histiocytic sarcoma, lymphoma, and granulomatous infections (Moore, 2014).

Pathologists typically add immunohistochemistry to confirm a dermal dendritic cell phenotype and to look for the mix of histiocytes and T cells that characterizes the reactive form. Special stains and cultures help rule out fungal and mycobacterial disease, which can produce similar nodules.

For dogs with multiple nodules, veterinarians commonly add bloodwork, imaging, and lymph node assessment to establish whether the disease is cutaneous or systemic. If you are still trying to work out what a skin change might be, the Dog Skin Condition Checker can help you organize what you are seeing before your appointment, though it cannot replace a biopsy.

Signs and Symptoms to Watch For

Nodules are the hallmark sign. They are usually firm, dome-shaped, and 0.5 to several centimeters across, and they may be haired, hairless, or ulcerated. The table below compares the two forms as described in the veterinary literature (Affolter & Moore, 2000; Palmeiro et al., 2007).

Treatment Options for Reactive Histiocytosis

Treatment is directed by a veterinarian, often a dermatologist, and centers on immunomodulation. Reported options include corticosteroids, cyclosporine, leflunomide, and azathioprine, with the choice depending on whether the disease is cutaneous or systemic. Cutaneous cases sometimes respond well, while systemic cases usually need long-term management and monitoring for relapse (Palmeiro et al., 2007).

Skin care plays a supporting role only. Nodules that ulcerate or that sit in warm, moist areas can become secondarily infected, and immunosuppressive drugs raise that risk. Your veterinarian may confirm infection with cytology and recommend topical antimicrobial support. Vetified's Chlorhexidine Shampoo contains chlorhexidine gluconateand ketoconazole, an antibacterial and antifungal combination used on the skin surface to address secondary bacterial or yeast overgrowth. It does not treat histiocytosis itself, and it is an FDA-registered OTC veterinary drug listed on DailyMed, so use should follow your veterinarian's guidance.

For background on how secondary infections are recognized and handled, see our page on bacterial skin infections in dogs.

Prevention and Long-Term Management

There is no proven way to prevent reactive histiocytosis, so management focuses on early detection and consistent follow-up. Photograph and measure new nodules, note any nasal or eyelid changes, and report them promptly, especially in Bernese Mountain Dogs.

Expect periodic rechecks, since relapse is common when medication is tapered. Keep lesions clean and dry, prevent licking and scratching, and ask your veterinarian before starting any new topical product on broken skin. Responsible breeders of predisposed breeds may also avoid repeating pairings that produced affected offspring.

Frequently Asked Questions

Is reactive histiocytosis in dogs cancer?

No. Reactive histiocytosis is a non-cancerous, immune-driven accumulation of histiocytes, whereas histiocytic sarcoma is a true malignancy. A biopsy with immunohistochemistry is required to tell them apart.

What is the difference between cutaneous and systemic histiocytosis?

Cutaneous histiocytosis stays in the skin and subcutis, while systemic histiocytosis also involves sites such as the nasal planum, eyelids, and lymph nodes, and sometimes internal organs.

Which dog breeds get systemic histiocytosis?

Bernese Mountain Dogs are the classic predisposed breed, and cases are also reported in Rottweilers, Golden Retrievers, and Labrador Retrievers.

Can reactive histiocytosis go away on its own?

Some cutaneous lesions regress spontaneously, but the disease often waxes and wanes, and systemic cases usually relapse without ongoing veterinary management.

Sources

1. Affolter VK, Moore PF. Canine cutaneous and systemic histiocytosis: reactive histiocytosis of dermal dendritic cells. American Journal of Dermatopathology. 2000;22(1):40-48.
2. Moore PF. A review of histiocytic diseases of dogs and cats. Veterinary Pathology. 2014;51(1):167-184.
3. Palmeiro BS, Morris DO, Goldschmidt MH, Mauldin EA. Cutaneous reactive histiocytosis in dogs: a retrospective evaluation of 32 cases. Veterinary Dermatology. 2007;18(5):332-340.
4. Miller WH, Griffin CE, Campbell KL. Muller and Kirk's Small Animal Dermatology. 7th ed. Elsevier; 2013.

Related Reading

• Dog Skin Lumps and Bumps: Types, Causes & When to Worry
• Histiocytoma in Dogs: Benign Bump That Regresses
• Canine Eosinophilic Granuloma: Unusual Raised Skin Lesions Explained

E. Maddens

Founder of Vetified. Develops topical antifungal and antimicrobial formulations for companion animals. Vetified products are listed on DailyMed and manufactured through FDA-registered facilities in the United States.

Research basis: All Vetified content references peer reviewed research published in journals including Veterinary Dermatology, JAVMA, and Journal of Small Animal Practice.