Eventing fans were stunned this autumn when Olympic prospect Billy Shannon lost her short, sharp battle against lymphoid leukaemia. Andrea Oakes and Karen Coumbe MRCVS investigate this aggressive — and thankfully very rare — blood condition

Pippa Funnell remembers vividly the last morning that Billy Shannon went out to graze. After unexplained weight loss in the preceding weeks, the 10-year-old mare’s condition had deteriorated rapidly over the weekend and further blood tests had been taken for urgent laboratory analysis.

“It was a beautiful sunny Monday and she loved being outside,” says Pippa. “She perked up a bit in the field, but she quickly went downhill again.”

Later that day tests confirmed the worst — Billy Shannon was suffering from acute lymphoid leukaemia. That night Pippa and the mare’s owners, Barbara and Nicholas Walkinshaw, made the heartbreaking decision to have her put down.

Worrying signs

It was the end of June this year when some subtle signs first alerted Pippa that all was not well with Billy Shannon.

“She’d had a foot abscess that took an unusually long time to sort out,” explains Pippa. “She then scratched her eye in the field and it was slightly odd that the injury didn’t respond very quickly to treatment.”

By August the mare was gradually losing weight. Coupled with the eye injury, this prompted Pippa and vet John Hennessy to decide to wean her filly foal.

“There were legitimate reasons why she was starting to lose a little condition,” explains Pippa. “It was the end of the summer and she’d had a big foal who was doing well

“She was a bit depressed, but that could be attributed to the fact that she’d just been separated from her foal — and that she was on antibiotics for the eye injury, which was really quite sore.”

Over the next two weeks, however, Billy Shannon’s continuing weight loss started to ring alarm bells.

“Every sign had an explanation, but I sensed that something was wrong,” says Pippa. “A blood test showed that she had probably been fighting a bit of an infection, yet the results didn’t seem as bad as what we were seeing with our own eyes.

“I was away competing in Ireland for 5 days so I asked John to come out every day to monitor her.

“That weekend she went off her food and developed a slight nasal discharge. When I got back on Sunday night I couldn’t believe how much she had deteriorated.

“The following morning a further blood test revealed loads of rogue white cells.”

Abnormal cells

John explains that Billy Shannon had developed acute lymphoid leukaemia.

“The condition originates in the bone marrow and the body then produces abnormal white blood cells,” he says. “The chronic form is more common in the older horse, but the acute form tends to hit younger horses around 8 to 10 years old.

“The disease process is not lengthy — just a few weeks after starting to lose weight the horse becomes very ill. There may also be some filling in the legs or along the midline of the tummy, but the early signs are not specific. A lot of conditions cause similar signs, but you don’t automatically start thinking of lymphoid leukaemia.”

While the number of white blood cells may be normal, John explains that a more detailed examination of the nature of these cells will reveal abnormalities.

“Without blood cell examination you wouldn’t know,” he says. “Billy Shannon’s weight loss wasn’t dramatic at first and there were a number of reasons as to why she might have been running a bit light. But when she came in from the field after weaning and should have gained condition, she started to lose more.”

The kindest end

So why might a horse like Billy Shannon develop the disease?

“It’s cancer,” says John. “It’s a lottery. She was not under undue stress or strain and the condition is extremely rare.

“The chronic form tends to take a more gradual course, but the acute form is particularly aggressive. In younger horses it all happens a lot faster and you have less chance of controlling it.”

Sadly, prognosis is poor once diagnosis is confirmed.

“With intensive treatment you might keep the horse alive a little longer, but treatment is usually disappointing,” explains John. “Once the process has started there’s only going to be one end result.

“With Billy Shannon we made the diagnosis before things were too bad. She was still grazing and not unhappy, but the condition was clearly troubling her. Her body was completely overwhelmed by cancer. It’s a really sad case.”

“She was a sick horse,” agrees Pippa, who remains devastated by the loss. “Cancer is cancer and things were bleak. We felt it was our duty to Billy Shannon to put her to sleep at home, in the field, before she started to suffer.”

Understanding leukaemia

Why does lymphoid leukaemia run such a rapid and devastating course? Karen Coumbe MRCVS explains

As other equine diseases become more manageable, life expectancy increases — as a result, difficult to treat or incurable cancers become more common. Even so, lymphoid leukaemia is still rare in horses. There have been just 3 confirmed cases out of nearly 5,000 medical in-patients we’ve seen at our practice in the last 5 years.

Leukaemia means a cancer of blood cells and it arises within the bone marrow, which produces the body’s blood cells. Lymphoid refers to the fact that it involves the white blood cells that become lymphocytes.

These are the type of cells that would normally fight infection and confer immunity. In leukaemia they fail to function properly and, as a result, infections end up becoming a complication of the disease. One cell type multiplies excessively in the same way that a solid tumour grows from one cancerous cell type.

The term “acute” relates to the stage of development within the bone marrow when the cancer occurs. With acute leukaemia the cell type involved is immature, more like the stem cells in the bone marrow and less like proper blood cells found in the circulation.

Knowing the enemy

Typically, acute leukaemia has a short and rapidly-progressive clinical course. It is important to try to make a prompt and accurate diagnosis, so at least everyone knows what they are dealing with.

Diagnosis can be surprisingly difficult, but leukaemia should be suspected if there is an increased white blood cell count and/or abnormal types of blood cell present. If the cancer cells are not detectable in the peripheral blood, then a bone marrow biopsy — which can be uncomfortable for the horse and involves potential complications — may be required to confirm the condition. The only good news in Billy Shannon’s case was that blood tests were diagnostic and showed up the cancerous cells.

A losing battle

With any unexplained, vague signs of ill health, especially continuing weight loss despite good management, it makes sense to investigate.

There are no classical clinical clues for leukaemia, but common observations include lethargy, reduced appetite, weight loss and fever, usually over a period of a few weeks. Since the disease affects the white blood cells, which are part of the body’s immune system, immunosuppression may occur and infections can be difficult to resolve.

The cancerous blood cells may also disrupt the circulation and other organs. This can cause problems such as peripheral oedema, which may be noticeable as fluid accumulating under the belly, filled legs or an enlarged sheath in the case of a gelding or stallion. The proliferation of cancerous cells within the bone marrow means that the production of other blood cells such as red blood cells and platelets is reduced, leading to anaemia and blood clotting problems.

Chemotherapy in horses is expensive, unpleasant and largely unsuccessful. It is fortunate that equine acute leukaemia is so rare — sadly, there is little to no effective treatment available.

This veterinary feature was first published in Horse & Hound magazine.