Cancer in cats post-vaccination: what is the likelihood of occurrence, causes, and recommended actions.

Post-injection or post-vaccination sarcoma in cats is rare, but not so uncommon that veterinarians overlook this side effect. Veterinarian Wendy Brooks shares everything you need to know about post-injection sarcoma in cats on the Veterinary Partners website.

What Post-Vaccination Sarcoma Looks Like in Cats

Typically, a soft lump appears at the injection site, which grows rapidly and may eventually become a benign or malignant tumor. The most common type of malignant tumor following vaccination is fibrosarcoma.

Fibrosarcoma resembles an octopus, says Wendy Brooks. It presents as a large lump with tentacle-like extensions. Additionally, it can be loosely attached and move under the skin. If fibrosarcoma is not removed in time, it can develop into a solid mass of inflammation that is no longer amenable to surgical treatment.

The primary issue with fibrosarcoma is its "tentacles." They can extend deep into soft tissues and are difficult to detect. Consequently, complete removal is challenging. An incompletely removed fibrosarcoma can regrow and often in a more aggressive manner.

Why Post-Vaccination Fibrosarcoma Occurs in Cats

Since the 1990s, when veterinarians began to notice cancer development in cats following vaccination, this issue has been under constant investigation. It has been established that most cases are reported after the use of inactivated vaccines—those where the disease-causing agent is "killed." According to Wendy Brooks, the problem likely lies not with the "killed" virus itself, but with the adjuvants in the vaccine.

Inactivated vaccines primarily include those for rabies and feline leukemia, which are lethal diseases, and avoiding these vaccinations is dangerous for the animal and its surroundings.

After the injection, such a vaccine acts in one specific location, familiarizing the cat's immune system with the pathogen. This area can develop local inflammation, leading to a lump, and subsequently, a tumor.

How Often Cancer Develops After Vaccination in Cats

According to Wendy Brooks, veterinarians report varying statistics—from one case in a thousand vaccinations to one case in ten thousand. Some other data even suggest one case in thirty thousand vaccinations.

“To explain more concretely, let’s consider your cat. Excluding vaccinations in early childhood, a cat that lives an average of 15 years receives three vaccinations per year, totaling 45 vaccinations over its lifetime. A case of cancer occurs once in a thousand vaccinations,” says Wendy Brooks.

Indeed, the chances of dying from rabies or leukemia may be higher for a cat, especially if the animal is not fully indoor and spends a lot of time outside.

Treatment of Post-Vaccination Fibrosarcoma in Cats

As soon as you notice a lump at the injection site that is larger than 2 cm in diameter or if the lump does not go away within three months after vaccination, contact a veterinarian immediately. The lump should be urgently removed and biopsied.

The challenge often lies in the necessity to excise not only the tumor itself but also several centimeters of soft tissue surrounding it. For a small animal, this is quite substantial.

Since fibrosarcoma is dangerous because of its "tentacles," post-surgical treatment may also require radiation therapy or chemotherapy. Some veterinarians believe radiation should be performed before surgery to shrink the tumor, while others advocate for it after surgery to eliminate any remaining tumor remnants. However, there should be at least three weeks between surgery and radiation, during which time the sarcoma may begin to grow again.

How to Prevent Post-Vaccination Sarcoma in Cats

In addition to fibrosarcoma occurring in the soft tissues of the neck or between the shoulder blades, sarcomas can also develop in bones, joints, fatty tissues, and more.

Considering the localization of the disease and other risk factors, Wendy Brooks recommends the following:

• Whenever possible, choose live or recombinant vaccines—the pharmaceutical market is now much more advanced than in the 1990s;
• Avoid unnecessary vaccinations;
• Request that the vaccine be administered in designated areas, such as the leg or even the tail (different vaccines have different designated sites)—in the event of a disease, it will be easier to excise the tumor, or the limb can be amputated;
• Avoid giving multiple injections in the same area of the body—this can exacerbate inflammation;
• Remember the 3-2-1 rule—if a lump does not go away within three months after vaccination, is larger than 2 cm in diameter, or does not decrease in size within a month after detection, take your cat to a qualified veterinarian.

The veterinarian adds that lumps after vaccination occur quite frequently in cats, but there is no need to panic. Most of them naturally resolve within a short period. It often takes many months or even years for a lump to progress to fibrosarcoma.

Wendy Brooks emphasizes that awareness of the potential development of post-vaccination sarcoma in cats should not lead to panic and refusal of vaccinations, but rather to attentiveness and a serious approach to your pet's health.