What are histiocytes and what do they do?
The histiocyte group of cells are part of the body's immune surveillance system. They take up and process foreign materials (called antigens), such as pollens and microorganisms. Then they migrate to the local lymph nodes where they present the antigens to other immune system cells (T lymphocytes) to stimulate them into a variety of activities to protect the body (immunity).
What is cutaneous histiocytosis?
Cutaneous (reactive) histiocytosis is an uncommon condition of dogs. The condition is poorly described in the scientific literature and has various grades all of which may be found in one animal. Other names for the disease include sterile granuloma/pyogranuloma syndrome. It is a non-cancerous increase in number of mixed reactive cells, mostly histiocytes but also may include white blood cells (neutrophils) and lymphocytes.
Sometimes, cutaneous histiocytosis is caused by an infection. Because of this variation, histiocytosis does not have a predictable response to treatment, but some cases resolve with therapy to suppress the immune system. Some cases are recurrent and very occasionally they are pre-cancerous.
What do we know about the cause?
Cutaneous, reactive histiocytosis is an immune dysfunction, mainly of young dogs and probably due to persistent antigenic stimulation by a variety of antigens (foreign proteins). Although some specific infections will cause this reaction, the cause of most cases remains undetermined.
It has been suggested that some cases of reactive histiocytosis are vector (tick or insect) transmitted infections that provide antigenic stimulation. Often, however, no organism is detectable by routine microbiology. Treatment reduces the harmful immune reaction, so there is clinical improvement but not cure.
Possible microorganisms include Bartonella, Borrelia, Coxiella, Ehrlichia, Hepatozoon, and Rickettsia species, all of which are infections transmitted by ticks. In some cases, there is a history of contact with rodents, which may be a source of infection.
In areas where the microorganism Leishmania is regularly found (e.g., Mediterranean coasts), Leishmania antigens have been found in about half the cases of cutaneous histiocytosis.
In older dogs, cutaneous histiocytosis is rare but may be a sign of underlying diseases such as diabetes, liver dysfunction, or cancer.
In puppies, juvenile sterile granulomatous dermatitis and lymphadenitis (also know as puppy strangles) is clinically similar. See handout “Juvenile Cellulitis (Puppy Strangles)” for further information on this condition.
Are these common conditions?
These are uncommon to rare conditions. In temperate regions of the world, cutaneous reactive histiocytosis affects mainly young dogs, often Collies, Springer Spaniels, and Golden Retrievers.
Puppy strangles is rare. A hereditary or genetic cause is supported by an increased incidence in some breeds, including yellow Labradors and Golden Retrievers, Dachshunds and Lhasa Apsos. Breeding from affected pups is therefore not advisable.
A cancerous version of this condition, called histiocytic sarcomas, are predominantly found in middle-aged to older animals. The tumors are more common in the Bernese Mountain Dog, Rottweiler, and Golden Retriever breeds. Systemic histiocytosis of the Bernese Mountain Dog has a polygenic (many genes involved) mode of inheritance. Tumors are rare in cats.
How will reactive histiocytosis affect my pet?
Reactive cutaneous histiocytosis is noticed as lumps under the skin. Often there are multiple lumps and they grow rapidly. The nodules may also occur inside the mouth and nose, and around the eyes, particularly on the white part of the eyes (sclera). This disease is difficult to distinguish from nodular fasciitis, which has similar clinical features and response to therapy.
The reaction may last for weeks to months, spreading outwards from the initial lesions, which may sink in the center. Ulceration and secondary infection are significant problems. The lesions may wax and wane. Many are on the face or ears although Springer Spaniels have them more often on their flanks. The disease is sometimes severe enough to consider euthanasia. Very rarely, it may progress to malignancy (cancer). The local lymph nodes may swell. This may be because the histiocytes have migrated and proliferated there or because there is a reaction to secondary infection.
Puppy strangles has massive enlargement of the lymph nodes of the neck followed by pustules and nodules on the face and neck, which weep purulent (pus) material.
Systemic (body-wide) histiocytosis is a chronic, debilitating disease usually in young Bernese Mountain Dogs. It has a fluctuating course with lumps affecting the skin, lymph nodes, eyes, and other tissues. It is rarely fatal although it is a progressive disease. Benign (harmless) and malignant forms are probably extremes of a single disease. Malignant disease is characterized by single to multiple lumps that often progress and multiply rapidly. It occurs in older dogs and commonly involves many areas: spleen, liver, lung, kidney, lymph nodes and bone.
How is reactive histiocytosis diagnosed?
Your veterinarian may suspect one of these conditions when your pet is examined, but accurate diagnosis relies upon microscopic examination of tissue. To obtain the appropriate samples, your veterinarian may recommend one or more sampling techniques such as fine needle aspiration, punch biopsy, and full excision. The sample will be prepared and examined by either cytology or histopathology.
Cytology is the microscopic examination of cell samples obtained by needle aspiration. This is useful for rapid or preliminary screening and may show malignant cells where there is malignancy. Cytology is not able to distinguish reactive histiocytosis from inflammatory reactions.
Histopathology is the microscopic examination of tissue samples that have been specially prepared and stained. Histopathology will give the most accurate diagnosis, including prediction of behavior (prognosis) of the lesions. Histopathology also rules out the possibility of cancer. Your veterinarian will submit the samples to a specialized laboratory for examination and diagnosis by a veterinary pathologist. The piece of tissue may be a small part of the mass (biopsy) or the whole lump.
"Histopathology of a single lump does not indicate whether there is systemic disease or the extent of it."
The information obtained by examining the whole lump will be more informative and also may indicate whether the lesion has been fully removed. However, histopathology of a single lump does not indicate whether there is systemic disease or the extent of it.
What treatments are available?
Surgical removal of lumps is the first line of treatment if possible.
Microbiological investigation of underlying causes of histiocytosis should, in theory, be next but can be expensive, is complex, and may not assist in diagnosis. Serology (looking for antibodies) for Leishmaniasis is helpful in endemic areas. To date, serology for organisms such as Borrelia has not proved helpful.
Cutaneous histiocytosis may respond to immunosuppressive (high dose) drugs such as steroids. This may have unpleasant side effects. Other drugs that have been used include other immune system modulators, some of which can also promote killing of infectious agents. No single treatment is successful in every case. The condition may worsen on maintenance therapy. In some cases, the sebaceous glands, which produce the skin's protective oils, are destroyed by this disease. If this happens, symptomatic treatment for dry skin may help reduce secondary skin problems.
Leproid granulomas usually regress but they can be removed surgically. Leishmania needs specific treatment.
Juvenile sterile granulomatous dermatitis and lymphadenitis (puppy strangles) responds permanently to high (immunosuppressive) doses of glucocorticoids. Secondary bacterial infection usually needs treatment.
If diagnostic tests indicate that histiocytic sarcoma (cancer) is present, they may spread rapidly in many cases. Cases with multiple tumors are uniformly fatal with involvement of many organs. Attempts at chemotherapy have not been successful and there is no known treatment.
Can these lesions disappear without treatment?
Histiocytosis is often a disease that comes and goes. Sometimes it disappears completely without treatment, presumably because the stimulus has gone. The body's own immune system may cause it to regress and there is often spontaneous death of central areas (necrosis).
How can I nurse my dog?
Preventing your dog from scratching, licking, or biting the lumps will reduce itching, ulceration, secondary infection, and bleeding. Any ulcerated area needs to be kept clean.
After any surgery, you need to keep the incision site clean and dry and prevent your pet from interfering with it by rubbing, licking, biting, or scratching. Report any loss of stitches or significant swelling or bleeding to your veterinarian. If you require additional advice on post-surgical care, please ask your veterinary team.
How or when will I know if this is permanently cured?
The histopathology report will give your veterinarian the diagnosis that helps to indicate how the condition is likely to behave. The veterinary pathologist usually adds a prognosis that describes what is likely to happen.
Histiocytosis often recurs so it is difficult to be certain when it is cured.
Histiocytic sarcomas spread rapidly in many cases. Multiple tumors are uniformly fatal with involvement of many organs. Newly developing masses may need investigation and treatment as outlined above.
Are there any risks to my family or other pets?
Although cutaneous histiocytosis may be due to infections, there are no records of spread by close contact between animals and people. The lesions do not occur in clusters in a household or neighborhood unless an infection such as Leishmania is involved. Routine hygienic precautions (such as washing your hands after handling your pet) are always advisable.