A cancer diagnosis is a stressful event for anyone, including pet owners. Decisions regarding chemotherapy can be difficult, and pet owners often worry about the prospect of their pet becoming sick from the treatment, particularly if they think it may be how they spend their remaining time. Most of these fears are unwarranted however, and knowing what to expect can help in making appropriate decisions. 


What is cancer?

Found in animals as well as humans, cancer is an uncontrolled growth of abnormal cells. There are various types, and it is one of the leading causes of death in pets particularly those that are more than 10 years old. Symptoms are often similar to those in humans, such as unexplained weight loss, abnormal swelling, and lethargy. In cases where treatment is appropriate, it may include surgery, radiotherapy, and chemotherapy.


Which types of cancers are most common?

Some of the more commonly found cancers in dogs and cats include:

  • Cutaneous mast cell tumours, which are among the most commonly reported canine skin tumours.
  • Lymphoma - a cancer of white blood cells, which circulate through the blood and the lymphatic system. In dogs, it is a rapidly progressive disease, and if left untreated can result in death within a relatively short time.
  • Osteosarcoma which is a malignant cancer of the bone and most commonly affects one of the bones of the limbs such as the shoulder, wrist, or knee. 
  • Oral tumours such as oral squamous cell carcinoma (SCC) and malignant melanoma (MM). SCC is the most common oral malignancy in cats, MM the most frequent in dogs.  They both can be aggressive cancers which may not diagnosed until the tumour is advanced.
  • Haemangiosarcoma, which is a malignant tumour of the cells that line blood vessels. Much more common in dogs than other species, it occurs most frequently in large breed dogs, particularly Golden Retrievers, Labradors, and German Shepherds.


When is chemotherapy used?

Generally, chemotherapy is used to either control cancer or ease some of its symptoms. In veterinary medicine it is usually not used to attempt to cure cancer, as this may cause unacceptable side effects.


Some cancers are treated only with chemotherapy while others are treated with a combination of modalities that may also include radiation and surgery. When cancer has spread from its primary point to other parts of the body (metastasised), chemotherapy is more likely to be recommended. It is also commonly used for tumours that cannot be completely removed with surgery.  Chemotherapy can also be used to help eliminate any remaining cancer cells after surgery, and occasionally to shrink large tumours prior to other treatments such as surgery.



How is chemotherapy administered?

The type of chemotherapeutic drug used, the number of treatments, and the frequency of them, will vary from case to case depending on the general health of the patient, the type of cancer, and the extent of disease.  Some chemotherapy drugs come as oral medications which can be given by pet owners at home. Some can be administered by injection during a brief visit to the vet. Others require hospitalisation for multiple treatments over the course of the day, and for slow infusions.  Repeat treatments are usually required which typically occur in cycles of one to several weeks. Between treatments blood tests may be conducted to monitor the patient, as well as physical checks to assess progress and effectiveness of treatment.

The frequency or type of chemotherapy treatment may change over time, or be discontinued if the cancer is in remission, however some patients will require it for the rest of their lives. Two cycles of chemotherapy are commonly administered before an evaluation is made. In cases of relapse, chemotherapy that has been discontinued may be resumed. Once a decision has been made to proceed with the treatment, pet owners must adhere as closely as possible to the schedules that have been set for their animal’s chemotherapy.


How does it work?

Chemotherapy drugs are designed to attack cells during the process of growth and division, however different drugs work in different ways. Some of them damage the cancer cells while others prevent them from dividing.  Rapidly dividing cells, regardless of whether they’re cancerous or not, are likely to be affected by chemotherapy because the drugs cannot distinguish between normal healthy cells and malignant cancer cells. This is the reason for side-effects from chemotherapy treatment, however the damage caused by the drugs is rarely permanent as normal cells continue to repair themselves and grow. Bone marrow, hair follicles, and the intestinal lining, are generally the most sensitive.


What side-effects can chemotherapy cause?

Most of our chemotherapy patients experience very mild or no side effects from their treatment. Lower doses of chemotherapy drugs are used in the treatment of animals compared to humans therefore animals experience fewer side effects. As well as this, fewer drug combinations are used which also contributes to less severe side effects. Whilst most side-effects will be mild, some can still range from moderate to severe, and animals undergoing chemotherapy may experience decreased appetite, vomiting, and diarrhoea. Animals that experience severe diarrhoea may require the administration of fluids in hospital, however this is unusual. These symptoms are caused by the effects of chemotherapy drugs on the gastrointestinal tract and may not occur until several days after the treatment.


Bone marrow is the spongy tissue inside bones that produces red and white blood cells and platelets. Chemotherapy can cause a drop in the white blood cell count thereby increasing susceptibility to infection. Hospitalisation may be required for severe infections, so that fluid therapy and antibiotics can be administered. Blood counts are checked after chemotherapy treatments when there is a high risk of bone marrow suppression. Whilst rarely caused by the chemotherapy, low red blood cell count (anaemia) can be a complication of cancer.

Are some animals more susceptible to chemotherapy side-effects than others?

Hair loss, mostly around the face and tail, may be experienced by certain breeds of dogs such as poodles and terriers. Cats on the other hand, may experience the loss of whiskers and the long hairs over their eyes. Hair regrows when chemotherapy has stopped and may initially have a slight change in texture or colour. The hair of non-shedding dogs and cats, or those with wiry hair, may be more affected.


Very few chemotherapy patients experience severe side effects, and when chemotherapy drugs are known to cause side effects, medications are prescribed to help counter them. With appropriate treatment of side-effects, most animals recover within a few days without any further issues.


Will chemotherapy cure my pet’s cancer?

Unfortunately, chemotherapy alone will not cure cancer in most patients, unlike surgery in those cases where the cancer can be completely removed. However remission (i.e. reduction of the cancer to non-detectable levels) is often achieved. As such, the goal of chemotherapy is to improve quality of life, avoid pain and discomfort, and allow our patients to continue enjoying a full life (not unlike treating many chronic diseases such as heart failure or kidney disease).


What if I’m exposed to my pet’s chemotherapy drugs?

Chemotherapy drugs remain active in the patient’s waste for a few days after treatment has been given. Caution should therefore be exercised when cleaning up after them, and gloves should be worn. Specific advice is given to owners of chemotherapy patients at the time of treatment.


Hands should be washed after administering oral drugs, and people with weakened immune systems such as the elderly, and pregnant women should be particularly careful.  Chemotherapy drugs should be clearly marked and kept away from any other medication. If by accident you swallow any of your pet’s medication call your doctor rather than your vet. It is safe for other pets to share food and water bowls with animals who have cancer or are being treated for it.


What alternatives are there to chemotherapy for treatment of cancer?

In addition to surgery and radiation as mentioned previously, immunotherapy which is a vaccine used to stimulate the immune system, may be used to treat cancer.  It is currently used mainly for treatment of melanoma in dogs. Other therapies may sometimes be integrated into the treatment plan for chemotherapy patients too.


What is chemotherapy likely to cost?

Chemotherapy can be costly. The cost, as with any medical treatment, will vary depending on the type, frequency, and duration. If your vet suggests chemotherapy for your pet they will estimate the costs for you. If you have pet insurance, check whether the policy provides any cover.


Everyone is touched by cancer in some way, and because of the human experience with it, the thought of chemotherapy often strikes fear into the hearts of pet owners. Animals’ experience, in many ways, will be different from a human’s experience yet it doesn’t make the decision of whether to proceed with it any less difficult. For most types of tumours – but not all – veterinary oncologists will provide advice on average life expectancy, and the decision of which road to take is ultimately a very personal one, often based on a variety of criteria. If you need to talk-over your concerns and ask any questions about your pet’s cancer treatment, whether it involves chemotherapy or not, get in touch with any of the Vets4Pets hospitals. We appreciate that it is a decision not made lightly.

Dr Brendan Maguire who leads the Mawson Lakes veterinary team has a particular interest in oncology, and treats many cancer patients with chemotherapy.