Histiocytomas in Dogs: Pictures & What to Do

A histiocytoma is a benign (non-cancerous) growth found on the skin of young dogs. These growths form when a type of immune cell in your dog’s skin over-replicates. The over-replication causes tissue build-up and inflammation (histiocytoma). While they occur commonly in dogs under three years old, they can affect any dog. However, specific breeds are more likely to get histiocytomas. Dog breeds most commonly affected by these tumors are:

• English bulldog
• Scottish terrier
• Greyhound
• Boxer
• Boston terrier
• Dachshund

Most histiocytomas will heal without treatment in 2-3 months. However, in some cases, they can get worse with time. They can look pretty scary to the untrained eye, and it can be hard to tell them apart from dangerous growths in dogs. In this article, we will review what histiocytomas look like on the different body parts of a dog. We will also look at other skin conditions in dogs that look similar. 

What does a Histiocytoma look like on a dog [with pictures]

Many owners think histiocytomas appear suddenly, but they usually take about 1-4 weeks to grow. They appear as a single, dome shaped growth and they typically don’t cause pain itching, or irritation. Let’s look at some pictures of their appearance.

1. Button like histiocytoma on a dog:

2a. Ulcerated histiocytoma on a dog

The picture below shows a red, ulcerated histiocytoma:

3b. Ulcerated histiocytoma on a dog’s leg.

Other pictures of a histiocytoma on a dog

1. Picture of a histiocytoma on a dog’s paw:

2. Picture of a histiocytoma on a dogs eye:

Pictured below is a small histiocytoma in a dog’s eye:

3. Histiocytoma on a dog’s leg/foot

Pictured: Histiocytoma on a dog’s foot:

4. Dog histiocytoma turning black

Pictures of a histiocytoma healing

Most histiocytomas will start to heal within 1-3 months. Here is an example of what it might look like if your dog’s growth starts to heal.

Healing stages

Below is a picture showing stages of a histiocytoma slowly disappearing (regressing). Most histiocytomas will heal within three months:

Signs that your dog has a Histiocytoma

Lumps need to be checked by a veterinarian. However, there are some tell-tale signs your dog may have a histiocytoma. A histiocytoma typically appears as a single dome-shaped bump that is moveable, and does not usually cause any itching or pain. Look for a lump with any of the following symptoms:

• Hair loss
• Redness
• Ulceration
• Scabbing
• Bleeding
• Swollen lymph nodes near the growth. 

How do you know it’s a Histiocytoma and not something else?

There is no way to tell what a lump is by looking at it. The best way to know what type of lump your dog has is to visit a veterinarian for testing. However, it is good to know what you might be up against. Let’s look at some photos of skin issues in dogs that look similar to histiocytomas.

1. Ringworm

Ringworm is a fungal infection found in dogs, cats, and humans. It is very contagious and spreads quickly. Ringworm rashes are usually flat, but in some dogs, nodules called kerions form on the skin. Kerions are round, raised, red, and hairless and can look very similar to a histiocytoma. Some common symptoms of ringworm are:

• One or multiple ring-like rash(s)
• Flaking 
• Itching
• Hair loss and regrowth in the center of the rash
• Scabbing
• Kerion Infection (round nodule-like growths).

2. Mast cell tumor

A mast cell tumor is a cancerous tumor that forms when mast cells (immune cells) over-replicate. These tumors are known for their aggressive nature. They come in all shapes and forms, but can sometimes look like histiocytoma (read our article: histiocytoma or mast cell tumor? to view more pictures)

While they commonly appear in older dogs, younger dogs can also get them. Common symptoms of mast cell tumors are:

• A lump that suddenly appears; or grows fast.
• Itching
• Scratching
• An ulcerated lump
• Diarrhea 
• Lethargy
• Vomiting
• Poor appetite 
• Enlarged lymph nodes

Steps you can take at home to help your dog with histiocytoma

If your dog has a histiocytoma, you should see the vet before you attempt at-home treatment. However, there are a few things you can do at home while waiting to visit the vet.

1. Prevent your dog from licking or rubbing the lump by putting an E-collar (Elizabethan collar) on them. Most histiocytomas become irritated because your dog is able to reach them. Putting an e-collar on your dog will prevent ulceration, infection, bleeding, and scabbing.
2. Keep the lump clean by bathing and flushing it with a mild saline solution. You can make this at home by mixing iodized salt with distilled water. Mix 1 cup of distilled water with ¼ tsp of salt. Warm the mixture in a saucepan to medium heat (not so hot that you can’t stick your finger in it.) Then, let it cool, and use a clean cotton towel or gauze sponge to wipe the histiocytoma clean. 

When to call your veterinarian

When your dog has a new lump, it is always a good idea to call your vet. In the best-case scenario, your vet will prescribe a three-month wait to see if the tumor regresses (goes away with time). In the worst-case scenario, your dog will need surgery to remove it. While histiocytomas are typically harmless, your dog could have a more dangerous type of tumor. And the only way you can get an accurate diagnosis is by visiting your vet and getting a biopsy done.

In rare cases, histiocytomas can be severe (see example picture here on UC David website). For example, in a condition called reactive histiocytosis, affected dogs will have multiple histiocytomas. While this is very rare, occurring in less than 1% of dogs, it is a fatal condition. These histiocytomas can spread throughout the body, causing multiple ulcerated lesions. Dogs with severe histiocytic disease are euthanized if the lesions become unmanageable. If your dog has a histiocytoma and any of the following symptoms, you should call the vet and book an appointment immediately: 

• Bleeding 
• Itching
• Pain
• Multiple growths that won’t heal.
• Weight Loss
• Vomiting
• Diarrhea
• Loss of appetite
• Lethargy