Vaccine-associated sarcoma

Vaccine-associated sarcoma (VAS) is a type of tumor that develops at the site of vaccination in cats. Initially recognized in the early 1990s, these sarcomas were observed to form at the sites where vaccines were administered, leading to a significant concern among veterinarians and pet owners. The exact cause of VAS is not fully understood, but it is believed to be related to inflammation caused by the vaccine, which then leads to abnormal cell growth and cancer development. This condition is also known by other names, including injection-site sarcoma and feline injection-site sarcoma (FISS).

Etiology

The etiology of vaccine-associated sarcomas is complex and multifactorial. It is hypothesized that the inflammatory response induced by vaccines, particularly those containing adjuvants, plays a critical role in the development of these tumors. Adjuvants are substances added to vaccines to enhance the immune response, but they can also cause chronic inflammation at the injection site, which may lead to the formation of a sarcoma. Genetic factors may also predispose some cats to develop VAS.

Clinical Presentation

Cats with vaccine-associated sarcoma may present with a firm, often rapidly growing mass at the site of a previous vaccination. These tumors can occur anywhere vaccines are administered, but are most commonly found on the trunk, particularly between the shoulder blades. The mass may not become apparent until several weeks, months, or even years after vaccination.

Diagnosis

Diagnosis of VAS typically involves a combination of physical examination, medical history (including vaccination history), and diagnostic imaging such as X-rays or ultrasound to assess the extent of the tumor. Definitive diagnosis is usually achieved through biopsy and histopathological examination of the tumor tissue.

Treatment

Treatment options for vaccine-associated sarcoma include surgical removal of the tumor, radiation therapy, and chemotherapy. Complete surgical excision is challenging due to the invasive nature of these tumors and their tendency to recur. Radiation therapy and chemotherapy may be used as adjunct treatments to surgery or when surgery is not feasible.

Prevention

Preventive measures for VAS include reducing the number of vaccines administered to cats, using non-adjuvanted vaccines when possible, and altering the sites of vaccination to areas where surgical removal of a potential sarcoma would be more feasible, such as the limbs. The American Association of Feline Practitioners (AAFP) has issued guidelines recommending that vaccines be given in specific locations to facilitate easier identification and treatment of VAS should it occur.

Prognosis

The prognosis for cats with vaccine-associated sarcoma varies depending on the size and location of the tumor, as well as the treatment options pursued. Early detection and treatment can improve outcomes, but these tumors often have a high rate of recurrence and can metastasize, leading to a guarded to poor prognosis.