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Vaccine-Associated Sarcoma in Cats


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Preventing disease is always preferable to treatment once it has developed. Vaccines are one of the best means of prevention and they have saved the lives of untold numbers of cats over the years. However, every medical procedure has the potential to result in unwanted side-effects and vaccines are no different. One of the most serious side effects associated with feline vaccination goes by the name vaccine-associated sarcoma or VAS.

"With early and aggressive treatment, some cats with VAS can essentially be cured of their disease."


What is VAS?

Vaccine-associated sarcomas are cancerous tumors that develop at a vaccination site months to years after a vaccine has been given. Fibrosarcomas are the most common type of vaccine-associated sarcomas, but other forms of cancer (i.e. malignant fibrous histiocytomas, osteosarcomas, rhabdomyosarcomas, liposarcomas, and chondrosarcomas) are also possible. These cancers can all develop in cats that have not been vaccinated, especially if they are infected with the feline sarcoma virus, but this is quite rare. Therefore, when a sarcoma develops at the site of a previously given vaccine, it is generally assumed to be caused by that vaccine.

What Vaccines Are to Blame?

Vaccines are manufactured in different manners; one type is a "killed" vaccine. This means that the microorganism that causes a particular disease is dead before it is included in the inoculation. This allows it to stimulate a pet's immune system but not to produce the disease that normally results from infection. Unfortunately, dead viruses tend to not stimulate a long-lasting immunity so adjuvants have been added to many of these vaccines to irritate the tissues surrounding the vaccination site. The resulting inflammation increases the cat's immune response to the vaccine but is also thought to play a role in the development of VAS.

Killed rabies and feline leukemia vaccines that contain adjuvants are thought to be responsible for most cases of VAS. Thankfully, newer methods of developing vaccines (i.e. recombinant DNA technology) have resulted in the availability of non- adjuvanted vaccines that provide good immunity against rabies and feline leukemia with a much lower risk of VAS.

Diagnosing and Treating VAS

When presented with a cat with a suspicious lump, a veterinarian will usually biopsy it (surgically remove a piece of tissue) and send the sample to a pathologist for identification. If the pathologist's report comes back as a fibrosarcoma or one of the other types of cancer associated with vaccination and the tumor is located at a previous injection site (typically between the shoulder blades or low down on the hind legs), VAS will be the likely diagnosis. Vaccine-associated sarcomas do not tend to spread throughout the body, but they grow very aggressively at the vaccination site. Wide surgical excision is needed to completely remove as much of the cancer as possible, and this is often best performed by a board-certified veterinary surgeon. When the tumor is located on a leg, amputation provides the best prognosis. In fact, this is the reason some veterinarians now try to vaccinate in this area rather than between the shoulder blades. Chemotherapy and radiation treatments may be recommended in conjunction with surgery.

With early and aggressive treatment, some cats with VAS can essentially be cured of their disease. Unfortunately, other individuals experience a recurrence of the tumor at the surgery site, and in many of these cases the cancer grows even more quickly than it did initially. If surgery is not an option for a particular cat, pain-relief (Metacam), infection control and other forms of hospice care should be utilized until euthanasia is in the cat's best interest.

Preventing and Monitoring for VAS

A cat's vaccination protocol should be individually tailored to his or her risk factors. For example, adult indoor-only cats generally do not need to be vaccinated against feline leukemia virus. When vaccination is in order, using a non-adjuvanted vaccine will reduce the risk of VAS. Owners should monitor their cats for new lumps and bumps. Most of these will not be cancerous, but they should all be brought to the attention of your cat's veterinarian. If they persist or grow, a biopsy should be performed so appropriate treatment can begin ASAP.


The above is provided for information purposes only and should not be used for the diagnosis or treatment of any condition. This information does not cover all possible variables, conditions, reactions, or risks relating to any topic, medication, or product and should not be considered complete. Certain products or medications may have risks and you should always consult your local veterinarian concerning the treatment of your pet. Any trademarks are the property of their respective owners.


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