Cancer in horses: the risks and recovery rates

Cancer is uncommon in horses compared to dogs, cats and humans. The reasons for this are not fully understood, but it may be due to horses having slower cell division and an effective immune system. Genetic factors also play a role, and horses are generally less exposed to environmental factors linked to cancer. When horses do develop tumours, they are most often benign.

Can horses get cancer?

More than 200 horses insured with Agria in Sweden are diagnosed with some form of cancer every year. In most cases, horses recover after treatment, particularly when a tumour is benign and detected early. However, some forms of cancer can be difficult to treat and carry a poorer prognosis.

"The most common tumours we see in horses are skin tumours such as melanomas and sarcoids, but there are also more unusual and serious internal cancers," says Linda Dunbäck, an Agria veterinarian.

Early diagnosis is one of the most important factors in successful treatment, which is why regular health checks are essential.

Signs of cancer in horses

Many cancers first appear as lumps or changes to the skin, although internal cancers can cause a wider range of symptoms.

Lumps and skin changes

Check your horse regularly for:

New lumps or bumps
Dark or black growths
Areas of swelling
Tenderness around a lump
Bleeding or oozing growths
Changes in the size or appearance of an existing lump

Not every lump is cancerous, but any new or changing growth should be assessed by your vet.

Signs of internal cancer

Internal cancers, such as lymphoma, can present with a wide range of clinical signs depending on which organs are affected. If your horse develops unexplained symptoms or persistent illness, speak to your vet.

Common types of cancer in horses

Melanomas in grey horses

Melanomas are the most common skin tumours in grey horses, especially those over 15 years of age.

They often appear as black or dark grey lumps around:

The anus
Under the tail
The base of the ears
Behind the jaw
The base of the mane

Larger tumours around the anus can make it difficult for a horse to pass droppings normally.

Research from the Swedish University of Agricultural Sciences, supported by the Agria Research Fund, has shown that up to 80% of grey horses over the age of 15 develop melanomas. This is thought to happen because melanin accumulates in the body as the horse's coat fades with age.

Although many melanomas are benign, it's important to monitor any new growths and seek veterinary advice if you notice any changes.

Sarcoids

Sarcoids are the most common type of skin tumour found in horses. They are caused by a virus and most commonly appear:

Inside the thighs
Behind the elbows
Under the belly
Around the sheath

Depending on their location, they can be both unsightly and physically troublesome. They may interfere with tack or equipment and can become prone to bleeding or oozing.

Penile tumours in geldings and stallions

Tumours on the horse's penis are often discovered at a late stage, making treatment more extensive and challenging.

Make it a habit to inspect your horse's penis while he is urinating or when he has been sedated. This may help identify potential problems earlier, when treatment is often more straightforward.

Lymphoma

Lymphoma is rare in horses but is the most common type of internal malignant tumour.

The disease can affect multiple organs and presents with a wide range of clinical signs depending on which parts of the body are involved.

Equine lymphoma most commonly occurs throughout multiple sites in the body (multicentric), although forms affecting the skin and gastrointestinal tract are also among the most frequently diagnosed.

How is cancer in horses treated?

Surgery

Today, most horses with cancer are treated with surgery to remove the tumour along with a margin of surrounding tissue.

Other treatment options

More advanced treatments, including radiotherapy and chemotherapy, are available in some countries.

When treatment may not be necessary

Benign tumours can sometimes be left untreated if they are not causing discomfort or affecting the horse's quality of life.

What is the prognosis for a horse with cancer?

In many cases, horses recover well following surgical or medical treatment, particularly when a tumour is benign and diagnosed early. This is one of the reasons it is important to check your horse regularly for lumps and bumps, so that anything unusual can be identified as soon as possible. Many skin tumours, including melanomas and sarcoids, can be removed without affecting a horse's quality of life or ability to work and enjoy normal activities.

However, some tumours can return after treatment, making ongoing monitoring and follow-up important.

Thanks to Agria's unique lifetime equine insurance, veterinary fees for treatment relating to recurrent tumours may be covered, provided the policy was in place before the initial cancer diagnosis.

In more unusual cases, such as internal cancers like lymphoma, the prognosis can be poorer because these conditions can be difficult to treat and may carry a higher mortality rate.