Vaccines have prevented illness in and saved the lives of untold numbers of pets, but like all medical procedures, vaccination is not without some risk. The inflammation that is caused by some types of rabies and feline leukemia vaccines very infrequently causes a cancerous growth to develop at the injection site. Unfortunately, although this side effect is quite rare, the resulting cancer grows aggressively and is very difficult to completely remove with surgery. Vaccine-associated sarcomas are a concern for cat owners, but dogs are much more resistant to the condition.
Diagnosis
Not every fibrosarcoma is caused by vaccination. Cats can develop the cancer after being infected with the feline sarcoma virus, and both dogs and cats are at risk for the spontaneously developing form. Fibrosarcomas can be identified by sending a tissue sample to a veterinary pathologist. If the tumor is located in an area where injections are frequently given, it was likely caused by a vaccine. In the past, veterinarians typically injected vaccines under the skin between the shoulder blades. Many veterinarians have now switched to giving rabies vaccines low on the right rear leg and feline leukemia vaccines low on the left rear leg to aid in identification of side effects and make treatment of vaccine-induced fibrosarcomas easier.
Treatment
Wide surgical excision of fibrosarcomas is the treatment that is most likely to have the best effect. Amputation if a leg is involved is frequently recommended and carries the best prognosis. Chemotherapy and radiation treatment may also be helpful. Tailoring immunization protocols to the needs of individual cats to reduce the administration of unnecessary vaccines and using certain types of vaccines that are not associated with fibrosarcoma development can decrease the chances that a cat will develop this type of cancer.
Written by: Jennifer Coates, DVM
Last reviewed: October 2, 2008