Leukemia

Leukemia is a type of cancer, and like other cancers there are relatively benign forms and highly aggressive types. The term leukemia means an excess of white blood cells in the blood. There are many forms of leukemia affecting different types of white bloods and some forms are more serious than others. Although, it might sound very frightening if your pet is given a diagnosis of leukemia it may not be as bad as you think.

White blood cells help the body fight infection. Some white cells effectively eat up foreign material in the blood such as bacteria, and others are responsible for making antibodies to help fight infectious diseases. White blood cells are mostly produced in the bone marrow (the soft tissue found in the center of bones) where they mature until they are ready to be released into the blood. Some specialized white cells travel to the thymus gland (in the neck) after release where they mature to become T cells. These are important in fighting viral infections and some cancers. Other cells produced in the bone marrow are platelets which have an important role in control bleeding, and red blood cells which carry oxygen around the body.

In healthy animals there are only a few white blood cells circulating in the blood at any time. These are ‘patrolling’ for infections whilst others wait in stores in the body until they are needed. If any infections or diseases are detected the number of white blood cells in the blood will increase dramatically. The type of white cell that is increased will depend on what ‘invader’ the body is trying to fight. However, as soon as the body recognizes any danger, a large number of cells are released from the bone marrow and production of white cells increases. So, within hours the number of white cells in the blood increases dramatically.

In leukemia there is an uncontrolled proliferation of one type of white cell. These cells divide and grow with no regard to any of the normal checks and balances in the body. As the number of these cells increases in the marrow they start to crowd out the other cells that are being produced there and so in leukemia there is an excess of one type of white blood cell and a deficiency of other cell types. The cancerous white blood cells that are being produced often do not function completely normally.

Leukemia can be either lymphoid (arising from lymphocyte precursors in the bone marrow) or myeloid (arising from precursor of white blood cells other than lymphoid, in the bone marrow).

Leukemias can be characterized as chronic (arising from late precursors of white blood cells) or acute (arising from early precursors of white blood cells). Typically chronic leukemias have a better prognosis than acute leukemias.

In some forms of leukemia there is a slow and steady increase in white cells. Signs of this can be hard to spot because the changes are slow, over weeks or months, and your cat may adapt better to them. General signs of ill health might be present including depression and inappetence or there might be other changes such as swollen lymph glands or a tendency to bruise or bleed more easily. Some animals with leukemia have an unexplained lameness – possibly due to the cancer in the bones. Sometimes chronic forms of leukemia are only picked up at a routine veterinary examination or during investigation for something completely unrelated.

In acute leukemia the white cell numbers rise quickly and the bone marrow production of other cell types (including red cells) stops to make room for producing more of the cancerous cells. Cats with acute leukemia become sick very suddenly, within days or weeks. The signs of disease often relate to the lack of normal cells of other types rather than to the effect of the cancerous cells. So cats with too few red blood cells (anemia) may be pale, lethargic and have rapid breathing and heart rate. Those with too few white cells of other types may have sudden severe infections with a fever and anorexia. Sometimes the first sign is a tendency to bruise or bleed more easily, particularly nose bleeds (because of the lack of platelets).

If you notice any abnormal signs it is important to ask your veterinarian for advice.

Your veterinarian may have a suspicion of leukemia from the history you report – but the signs are generally so vague that there are many other, more common, conditions that can be causing them. This means that leukemia is commonly diagnosed after routine blood screens. A blood sample can be taken and examined under a microscope to identify the different types of cell present and to count the numbers of each type of cell. If there are abnormally high number of one type of white cell (and usually a low number of other types of cell) then your veterinarian may suspect leukemia. The findings of blood tests have to be considered alongside other information (for example, whether there is a bacterial infection or an abscess present) to decide whether the change in white cell numbers is a normal response or something more sinister. A large number and variety of tests may be required; tests are often repeated frequently to assess progress and response to treatment.

The diagnosis is made on examination of bone marrow tissue. It is a relatively simple procedure to collect a sample of bone marrow under general anesthetic. A needle is inserted through the bone to its core and a small sample of the cells is sucked out.

Another special test called flow cytometry can be carried out on the blood to characterize the abnormal cells by measuring them and staining them with special markers. This is a non-invasive test because it only requires blood sampling. In some cases, this test is sufficient enough to diagnose a leukemia and bone marrow examination is not required.

Your veterinarian may also want to perform some other screening tests to look for disease elsewhere in the body or other effects of the leukemia. These generally include imaging tests such as X-rays, CT and ultrasound.

As for many other types of cancer no-one really knows why some animals (or people) develop the disease and others do not. The cancer develops from a mutated cell in the bone marrow which grows uncontrollably. This mutation can happen for no apparent reason or can be triggered by exposure to radiation, some viruses and toxins – but why this happens in some animals and not others is poorly understood. In cats there is a known association with a specific viral infection (feline leukemia virus (FeLV) which can be related to the development of cancer in the lymph nodes (lymphoma) which then spreads to affect the bone marrow as well.

In a few cases the leukemia may not need to be treated and your veterinarian may advise just monitoring the condition to see how it progresses. This can be the case for chronic leukemia when cats are still well in themselves with no symptoms.

In cats which are sick a lot of supportive care may be needed before any treatment can start. This can include intravenous fluids or a blood transfusion and antibiotics if the immune response is affected. This can be the case for acute leukemia when the disease has progressed rapidly and lack of other cells result in infections and spontaneous bleeding.

There are anticancer treatments for many forms of leukemia which can slow the cancer or put it into remission in some cases. Sometimes these can be in the form of tablets that you give your cat, but your cat may need to attend regular visits to the veterinary hospital for intravenous chemotherapy. If the leukemia can be controlled your pet may have a good quality of life for some period of time.

Sadly, for the very aggressive types of cancer, treatment is not likely to be successful and the kindest option may be to have your cat put to sleep.

Leukemia is a very varied disease which can range from barely affecting your pet’s life to being fatal. It is important to seek help from your veterinarian if you are concerned about any aspect of your pet’s heath as early diagnosis and treatment is beneficial in most diseases.