Feline Leukemia Vaccine Side Effects

Feline leukemia vaccine is used to protect pets against the feline leukemia virus (FeLV). This virus is transmitted to pets through contaminated sources such as food and water bowls. Infected pets can also transmit the virus to healthy cats through their saliva and nasal discharge. Feline leukemia virus attacks the pet's immune system which in turn makes the pet more susceptible to bacteria and secondary infections. Although all pets are at risk of contracting FeLV, cats with weak immune system are at greatest risk.

Vaccinating High Risk Pets

The feline leukemia vaccine is recommended for all pets that are at risk of contracting the virus. Pets that are frequently lodged at boarding facilities and kittens living with infected mothers are administered the vaccine. However, pets have to be tested for feline leukemia virus before the vaccine is administered. It's important to consider the cat's current health condition and the side-effects of the vaccine before vaccinating pets.

Side Effects of Feline Leukemia Vaccine

  • Fever
  • Lethargy
  • Swelling and pain at the injection site
  • Granulomas
  • Vaccine associated sarcoma

Vaccine Associated Sarcoma

This is the most adverse reaction to the FeLV vaccine. It develops due to a chronic inflammation caused by the vaccine adjuvant. Although all pets don't develop vaccine associated sarcoma, several pets suffer from granulomas and skin lesions that form tumors. Research suggests that specific intracellular oncogenes in some pets cause malignant tumors or cancer in the pets body after vaccination. Cats suffering from sarcoma or fibro sarcoma develop mutated tissues or cells in any part of the body. Malignant tissues are capable of spreading to different parts of the body and infecting various organs and surrounding tissues.

Treatment of Vaccine Associated Sarcoma

Vaccine associated sarcomas aren't easily cured with chemotherapy. The best treatment is surgical removal of tumors or cancer cells. Radiation therapy is also used to prolong the cat's life. Pet owners should palpate vaccination sites to detect any lumps or growths. Prompt treatment is also necessary to prevent metastasis and death.


Pets shouldn't be administered several vaccines at the same injection site. Vaccines are available as both modified live vaccines and killed vaccines. Although FeLV was available as a killed vaccine that contained adjuvants, newer vaccines that don't contain adjuvants are safer for use in pets. It's best to administer the FeLV vaccine on the left hind leg. An intra-dermal vaccine is preferred as it's easy to determine any changes that occur at the injection site. Apart from fibro sarcomas, cats may develop other types of vaccine associated sarcomas that include malignant myosarcomas, osteosarcomas and histiocytomas.

Is the FeLV Vaccine Recommended?

Although pets are susceptible to various side effects from the vaccine, most cats are at greater risk of contracting the virus itself. Newer forms of the FelV vaccine are recommended for cats as early as 8 to 12 weeks of age. Since there isn't any established treatment for pets that are FeLV positive, prevention of the disease is of utmost importance. Cats may also develop cancer associated with FeLV and require aggressive treatment protocol.

FeLV vaccine is a non-core vaccine and is only administered to pets that are at risk. Pet owners should discuss with the vet vaccines and preventive measures that are essential to the cat's health and long life.