Chemotherapy In Pets: What To Expect

Written by Shula Berg BVSc CertAVP(GSAS) GPAdvCert(SASTS) MRCVS
Clinically reviewed by Elizabeth McLennan-Green BVM&S CertAVP(SAM) MRCVS

- Overview
- Treatment Process
- Side Effects
- Outlook

What is Chemotherapy?

Chemotherapy refers to the administration of cytotoxic drugs (drugs that harm or kill cells). Chemotherapy is commonly used in veterinary practice, and tends to be given as a primary cancer treatment in conditions such as lymphoma, leukaemia and multiple myeloma. Chemotherapy can also be given after surgical treatment to help manage secondary tumour spread (metastases), such as with osteosarcoma and haemangiosarcoma.

Veterinary chemotherapy uses lower doses of each drug than commonly given to humans, so we rarely see the drastic side effects seen in human chemotherapy use. Doses are calculated specifically to each patient’s size, and may be adjusted if they do not tolerate them well.

Chemotherapy is given as several treatments, referred to as a protocol. Often, multiple protocols can be used for a single cancer type, so your vet may wish to consult a specialist oncologist before starting treatment. Chemotherapy can be comprised of a single drug, or several drugs in combination. Using a single drug can be significantly cheaper, however, this may be less effective than a multi-drug protocol, in which drugs with different modes of action are used in combination. Your vet will explain the available options, and which they feel is best for your pet.

What happens during Chemotherapy treatment?

Chemotherapy is most often given as an intravenous injection or infusion. We aim to give doses often enough for maximum effect on the tumour cells, but not so frequently that the body’s normal cell population can’t recover between doses. Typically, this results in chemotherapy being administered every 2-4 weeks. It is important that the process is performed correctly, so pets who are very stressed or uncooperative in the vets may require sedation. Rarely, a pet’s temperament may mean that they are not suited to receiving chemotherapy.

For injectable chemotherapy to be given, your pet will be admitted to the practice for the day. Giving the chemotherapy itself is mostly a relatively quick process, but often specific members of staff will do the treatment due to the potential human risk. Chemotherapy is only ever administered in a quiet, designated area of the practice. Everybody involved will wear full PPE and dispose of any waste carefully. Patients may be kept in the clinic for a few hours after treatment to monitor for side effects, or give intravenous fluids.

Less commonly, oral chemotherapy drugs are administered at home. This may be part of a protocol in combination with injectable treatments. For some cancers, it is possible to give only oral treatment, which is usually administered daily. Giving low doses of chemotherapy every day is known as metronomic chemotherapy. If giving oral chemotherapy, always wear gloves and wash your hands afterwards. You must never split or crush chemotherapy tablets.

What are the side effects of Chemotherapy?

Although chemotherapy in animals has far fewer side effects than in humans, we cannot give drugs strong enough to kill cancer without causing some effect on the rest of the body. The most common side effects are mild, while the more serious risks are very rare.

Bone marrow suppression

Bone marrow is responsible for producing red and white blood cells, and platelets. Marrow has a high rate of turnover, which makes it more susceptible to damage by the chemotherapy. The most common side effect of chemotherapy is a reduction in the circulating white blood cell count. White blood cells fight infection, and this is why both human and animal patients receiving chemotherapy are at increased risk of infection. A blood test is usually performed before every treatment to make sure the number of white blood cells is acceptable, and treatment doesn’t pose an unnecessary risk. If the white cell count is too low, it may be recommended to delay the treatment. Often, if white blood cells drop after a treatment, the maximum effect is seen 7-10 days later. Depending on the drugs used, your vet may wish to check a white cell count in between treatments as well. If white cell counts drop too low, the patient is at risk of sepsis. This is uncommon, but requires hospitalisation for intensive treatment with antibiotics. If your pet is receiving chemotherapy and seems unwell, or has a fever, you must contact the practice immediately.

Gastrointestinal toxicity

The lining of the intestines is replaced almost constantly. Similar to with bone marrow, this high turnover of cells makes the intestines more vulnerable to adverse effects of chemotherapy. Gastrointestinal toxicity usually causes vomiting, with or without inappetence, within 48 hours of chemotherapy administration. Diarrhoea may also occur, but takes slightly longer to develop. Often, anti-emetic medication is given at the time of chemotherapy to try and reduce side effects. Most cases of gastrointestinal toxicity are mild, and will resolve with supportive care alone. If signs are not improving within 12-24 hours, please contact the clinic as more aggressive treatment may be required.

Alopecia

Alopecia (hair loss) is a common concern when discussing chemotherapy. We do not tend to see complete alopecia in veterinary patients, however, shaved patches may take longer to grow back than normal and the hair coat may become thinner. Some breeds are at greater risk of hair loss due to their coat type, including Poodles, Old English Sheepdogs and some Terriers. Hair usually grows back once treatment is stopped.

Extravasation

Some chemotherapy drugs are extremely irritant to the tissues. They are always administered through a secure cannula, but sometimes the drugs can leak around the cannula or out from the vein (known as extravasation). Chemotherapy requires cannulas to be placed regularly and repeatedly, which becomes more challenging over time due to scar tissue. Unfortunately, this leads to a slight increase in the risk of extravasation. If irritant drugs do leak into the surrounding tissues, this can cause an extreme reaction including pain and swelling. The drug can cause death of the local tissues within the following 10 days. This is usually treatable, but may require regular dressings for several weeks depending on the extent of the reaction. Some drugs are significantly more harmful if extravasation occurs, and rarely can cause significant damage to the affected limb.

Allergic reaction

As with any drug, allergic reactions to chemotherapy drugs are possible. They are very uncommon, however patients are always monitored very closely during and after treatment. Acute Tumour Lysis Syndrome is an extremely rare condition in which chemotherapy administration causes a large number of tumour cells to rupture and leak into the circulation. This causes signs similar to an acute allergic reaction, and can be very difficult to manage. It is very rarely reported in veterinary patients, but tends to happen more in those pets with a very large tumour burden, advanced disease, and pre-existing organ dysfunction.

Others

Some chemotherapy drugs have specific effects on certain organs, such as the heart, bladder, kidneys and nervous system. If these drugs are recommended for your pet’s protocol, your vet will discuss specific risks in detail. Further tests, such as a heart scan, may be advised prior to starting treatment and repeated to monitor for changes.

What is the outlook for pets receiving Chemotherapy?

The prognosis for pets receiving chemotherapy depends on the type of cancer being treated, and the individual response to treatment. Some pets do not respond as well as we would like, and may get worse in spite of treatment. Other pets respond well, and may continue receiving chemotherapy for months or years. The best possible response to chemotherapy is to enter remission, where the cancer is no longer detectable. Not all cancer types can achieve remission, however, this can significantly extend a pet’s lifespan if it is achieved.

Disclaimer

Please note that the content made available on this webpage is for general information purposes only. Whilst we try to ensure that at the time of writing all material is up to date and reflects industry standards, we make no representation, warranties or guarantees that the information made available is up to date, accurate or complete. Any reliance placed by yourselves is done so at your own risk.

Page last reviewed: 21^st May 2024

Next review due: 21^st May 2026